- 1 EME on the higher bands, Sam Jewell, G4DDK and John Worsnop, G4BAO
- 1.1 Introduction
- 1.2 1296MHz (23cm) and 2300MHz - 2400MHz (13cm)
- 1.3 2.3GHz (13cm)
- 1.4 System Engineering for GHz Bands EME
- 1.5 Power Amplifier considerations
- 1.6 10GHz (3cm)
- 1.7 Moving the dish
- 1.8 Equipment requirements
- 1.9 Feeds and Switches
- 1.10 Linear vs circular polarisation
EME on the higher bands, Sam Jewell, G4DDK and John Worsnop, G4BAO
Above the amateur radio 144MHz band low sky noise means EME takes on a different character.
Low sky noise (noise temperature) means that very weak signals can be heard against the
background noise that would otherwise be swamped by galactic and manmade noise on 144MHz
and below. Sensitive receivers, using very low noise amplifiers (LNA,) are even able to detect 'noise'
from the moon. Being a 'black body radiator' at a physical temperature of between 200 and 240k,
depending on the phase of the moon, its noise temperature can be readily detected against the
much lower background sky temperature of 2.7k. This is only possible when the beamwidth of the
receiving antenna is small and the noise temperature of the LNA is very low. In practice it is very
difficult to detect moon noise at 432MHz with an 'amateur size' antenna, but at 1296MHz and
above it becomes increasingly easy up to 10GHz within purely amateur radio means. Above 10GHz
atmospheric gases contribute noise due to absorption and it again becomes increasingly difficult to
detect moon noise. The frequency range between 1GHz and 10GHz is commonly known as the
microwave low noise window due to the prevalent low sky noise temperature. There are five
amateur radio bands between these frequency limits and all of them are exploited by EME
enthusiasts in order to make DX EME contacts. 432MHz also exhibits low sky noise, but it is still
higher than the five 'microwave bands'. It is this ability to detect weak signals against a low sky
noise that makes the microwave bands attractive to many EME enthusiasts.
It is not critical that the EME operator is able to detect moon noise except on the higher of these
bands, only that the ability to do so shows that the receiving system is working as expected.
Detecting moon noise on 1296MHz is not essential and usually only possible with larger dish
Note that I said that the beamwidth of the antenna must be small in order to detect moon noise.
What if the beamwidth is not narrow?
Then the antenna will see more cold sky than 'warm' moon. That also means that signals reflected
from the moon will be weaker since the moon fills less of the aperture that is the receive antenna.
Ideally, the beamwidth of the receive antenna will be exactly the same as the beamwidth that the
moon subtends on the surface of the earth (about 0.5°).
However, it is not quite that simple, as you might expect.
As radio operators we are interested in achieving enough signal to noise ratio (SNR) to be able to
communicate. The signal part is provided by the reflected signal and the more power that is directed
at the moon, the bigger the reflected signal received back on earth.
The noise part of the equation is the total noise contribution from a number of sources.
The total noise power is given by
𝑃𝑟 = 𝑘𝑇𝐵
Where 𝑃𝑟 = 𝑅𝑒𝑐𝑒𝑖𝑣𝑒𝑑 𝑛𝑜𝑖𝑠𝑒 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛 𝑊𝑎𝑡𝑡𝑠
𝑘 = 𝐵𝑜𝑙𝑡𝑧𝑚𝑎𝑛𝑛'𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 = 1.38 ∗ 10-23
The total noise temperature detected by the receiver is made up of three main parts. These are
𝑇𝑠𝑘𝑦 = 𝑛𝑜𝑖𝑠𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑘y in Kelvin
We have already seen that the sky noise temperature can be as low as 2.7k between 1 and 10GHz
𝑇𝑎𝑛𝑡𝑒𝑛𝑛𝑎 = 𝑁𝑜𝑖𝑠𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑎𝑛𝑡𝑒𝑛𝑛𝑎 in Kelvin
𝑇𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑟 = 𝑁𝑜𝑖𝑠𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑟𝑒𝑐𝑒𝑖𝑣𝑒 in Kelvin
𝐵 = 𝐵𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑟 in Hertz
The bandwidth of the receiver is the noise bandwidth, pre-detector, and will depend on the modulation mode to be received. i.e. 2.5kHz for SSB, ~200Hz for CW and a few Hz for some digital modes.
In order to improve SNR we need to reduce something on the noise power side of the equation if we cannot increase transmit power.
Not much can be done to reduce Boltzmann's constant and the bandwidth is set by the mode of modulation in use. That leaves us with reducing the total system noise temperature in order to achieve better SNR. Sky temperature is already as low as we can expect (post big bang and 14 billion years on!). Antenna noise temperature is something we can do something about and is the subject of much antenna design and optimisation. Likewise, receiver noise temperature is also something we can do something about. Modern Gallium Arsenide FET ( GaAs FET) technology has developed a great deal in the last twenty to thirty years. Variants on the humble GaAs FET, such as the High Electron Mobility Transistor ( HEMT), have enabled LNA designs to reach as low as 14k at room temperature (without cooling) and 20k LNAs are already relatively low cost items.
Where does this leave the EME operator?
Using dish or yagi antennas, HEMT LNAs and affordable solid state or valve amplifiers of 100W up to
500W rating there are currently many hundreds of EME stations operational on 1296MHz using SSB,
CW or digital (JT65C). The number of operational stations on the intermediate bands of 2.3, 3.4 and
5.7GHz is a bit lower and the reasons for this are covered later in this chapter. 10GHz has recently
become a very popular band, mainly because it has been shown possible to engineer a small but
effective system that can be used from home or from portable locations. The achievements of those
EME amateurs using small systems is remarkable and seems set to become even more popular in
The following sections of this chapter cover equipment and techniques, as well as give guidance, to operating on the microwave EME bands up to 10GHz.
1296MHz (23cm) and 2300MHz - 2400MHz (13cm)
The very first successful amateur radio EME contacts took place, not on VHF, but on 1296MHz back
Without doubt the 1.3GHz band is the most popular EME band above 144MHz. This is probably
closely followed by 10GHz and then 2.3GHz. The possible reasons for this will be examined later in
this chapter. Activity on 3.4 and 5.7GHz tends to be limited to contests and Activity Weekends (AW).
Since the techniques used for 1296 and 2300MHz are very similar it makes sense to group them
together for the purposes of this book. Where 2300MHz does differ is that there is no common
allocation for EME across all regions of the world due to differing requirements for mobile radio
The following table shows the mean EME, return, path loss for the three bands.
|Band||Mean path loss dB|
1.3GHz is by far the most popular microwave EME band. The vast majority of EME operation takes
place between 1296.000MHz and 1296.100MHz, with CW tending to occupy the lower end of this
sub band and digital modes the upper end. SSB is not that common (except between bigger stations
and during the DUBUS SSB contest, where it tends to be a mixture of SSB and CW in cross mode
Circular polarisation is almost always used although the big attraction for the beginner to microwave
EME may be the ability to make digital (JT65C or JT4) QSOs with medium and large stations using
just a single high gain Yagi, with elevation, and 50 to 100 Watts of RF power. Ground gain is virtually
non-existent on these bands, so elevation is desirable.
Several yagi antennas can be phased for circular polarisation, but this introduces undesirable
phasing line losses, so yagi equipped stations tend to use linear polarisation and accept the 3dB
penalty when working circularly polarised stations. In practice there is usually enough margin when
working medium to large dish equipped stations, that the polarisation mismatch is not a big
problem. Multiple yagi arrays can also be used, but care needs to be exercised to keep combining
excess loss to a minimum. Often a single long yagi will outperform an array. With a single yagi
system, the masthead preamp can be kept as close as possible to the antenna feed point to
minimise losses and hence keep the noise figure low.
Long helix antennas would seem to offer a solution to the problem of circular polarisation. A signal
reflected from the moon or any other object in front of the antenna will suffer a reversal of phase
(polarity). This means that the helix must be equipped for both right hand and left hand circular
polarisation. The added complication of doing this is often not worth the effort.
Using a small (~2 metre) dish, CW QSOs are also possible with the larger stations and home echoes can be detected with waterfall displays. JT QSOs have been made with dishes as small as 1.2 metres. Dish sizes of 3 metres and above and 100 or more Watts will enable you to hear your own echoes and easily make CW QSOs. Even SSB QSOs will be possible with some of the bigger stations.
Small dishes are more difficult to feed without overspill and side lobes. As you have read earlier, this
makes the antenna noisy and the system noise figure are poorer. The consequence of this is that a
QRO station with a small dish will tend to be "alligators" i.e they are heard rather better than they
hears others. This can be mitigated by careful feed optimisation to minimise overspill and side lobes.
Practical 1.3GHz EME systems
The bare minimum 1.3GHz system needed to make JT QSOs (as used by the Camb-Hams from the
Isle of Lewis in 2014) is shown in Figure 1. It consisted of a TS2000X with a remote PA and masthead
preamp. The antenna used was a single 55 element Yagi with an AZEL rotator. With hindsight, the
Tonna 55 element has a poor side lobe pattern (and hence inferior noise performance to something
like a 67 element Wimo or a Powabeam). While such a system is not recommended for a permanent
installation, it makes a very good way of taking a small EME system out for holiday operation on
1.3GHz and will produce a few QSOs.
For a more permanent and CW-capable system for use from home, a dish with a minimum size of
2.5 to 3 metres should be used, and a proper circular polarised horn or patch feed. The only
difference with this system is the way that the antenna equipment is configured. Once again,
separate TX and RX feeders are used, but this time, one is connected to the Left Hand circular
polarisation feed port for RX and the other to the Right Hand circular feed port for TX. Remember
that the sense of the polarisation reverses at moon reflection, meaning it "goes up Right Hand,
comes down Left Hand"
As these two ports are part of the same feed system, there is much less isolation between them;
typically only 26dB for a well made CP feed compared to 80 dB for a good relay. With only 26dB
isolation between transmit and receive ports, 400Watts on transmit will produce 1 Watt OUT of the
receive port, destroying any preamp connected to it. While no high power TX RX changeover relay is
required, a relay must be used to isolate the receive port during Transmit. This configuration is
shown in Figure 1Figure 2. During transmit, the relay disconnects the input to the preamp from the
feed port and terminates it in 50 ohms. The coaxial relay need only be a low power one as all it has
to do is to isolate and terminate the preamplifier input during transmit, keeping any fed-through
transmit power from the sensitive LNA input.
The characteristics of 2.3GHz EME are very similar to 23cms. System design is fundamentally the
same except that Yagis are rarely used on this band as they are more difficult to construct and
optimise. Combine this with the fact that the accepted polarisation is circular, dishes become the
only sensible option.
Based on calculations with VK3UM's EMECalc and the availability of cheap surplus solid state high power amplifiers, 2.3GHz is the band likely to give the best results with a very small (ultra light) system such as a 1.5m dish, a preamp noise figure of 0.35dB, and a TX power of 100 to 200 Watts. Despite this, 13cm is still a less popular band than one might expect based solely upon technical constraints, and activity tends to be much lower than at 23cm. The main reason for this seems to be the "fractured" allocation of the band throughout the world.
The available 13cm band is not consistent. Table 1 Worldwide 13cm band allocations used for EME
shows the various 13cm allocations used throughout the world. This makes both equipment and
operating slightly more complicated in that a means is needed to listen on a different frequency than
you are licensed to transmit on. Some form of cross band operation is needed to work certain
Table 1 Worldwide 13cm band allocations used for EME
|Location||13cm band EME usage (MHz)|
|UK||2320 - 2320.15 and 2301.9 - 2302 (by NoV)|
|Australia||2301.9 - 2302|
|Most of Europe||2320 -2320.15 (and 2304-2304.1 in some countries)|
|Japan||2400.0 - 2405 & 2424 *|
In Japan, 13cm EME was first licensed in 1993 for JA4BLC. The band was then 2424.0-2424.5. On Jan 2015, the post authority in Japan allowed the operation on 2400-2405 for amateurs for EME and all Japanese moonbouncers moved to 2400. The 2424.0-2424.5 segment is still allowed but suffers badly from QRM from Wifi. Therefore all Japanese operation is on 2400 - 2405.
From Europe it is accepted practice to work stations in the USA at 2304MHz via "cross-band." Europeans call on 2320.xxx MHz and listen for stations that are 16 MHz lower than their 2320MHz frequency. This can be achieved using a 2320 to 144 MHz transverter with a separate 128MHz IF receive converter to receive on 2304MHz. Calling “CQ down” or announcing on a reflector that you are looking on 2320.X MHz lets the USA station know that he should transmit on 2304.xxx MHz after adjusting for Doppler. A similar approach can be used to work Japan with a separate 2424MHz. receive converter. UK 2300MHz NoV holders can work Australians directly in their band.
System Engineering for GHz Bands EME
Dish feed optimisation
Low Noise Amplifiers and losses
With EME above 1 GHz , we have to look at antennas in a slightly different way. First, we need to
remind ourselves that the noise power radiated by a "hot body" (in our case, what the antenna is
looking at) is given by kTB Watts, where k is Boltzmann's constant, T is its temperature in Kelvin (not
"degrees Kelvin!") and B is the bandwidth of the noise measurement in Hz. In terrestrial systems
your antenna is pointing at the horizon, so it is "looking at" some combination of "cold" sky at
around 10K and ground, trees and houses at 290K. Typically for 1.3 and 2.3GHz this will total around
170K. Elevating the antenna above the horizon will mean it is just looking at "cold" sky at 10K, so
there will be a theoretical reduction in received noise power of 10log(10/170) or 12dB. Compare
looking at cold sky to looking at JUST the ground at 270K and the reduction in noise will be 14dB.
These numbers don't take account of two factors. The constant noise added by the receiver, and any
back or side lobes of the antenna. These two factors will contribute to increasing that temperature
and hence decreasing the difference between ground and cold sky measured by the antenna. Both
these factors are under our control. We can use low noise amplifier designs to minimise the noise
contribution of the receiver, and we can design our antenna systems to have "clean" patterns with
very low side lobes. The effect of horizon vs cold sky on system sensitivity and the results are
summarised in Table 2 below.
Table 2 An example of the difference in sensitivity between elevated and horizon-pointing systems
|Antenna points to||RX Noise figure(dB)||System NF(dB)||System sensitivity (dBm)|
|Cold sky (10K)||0.81||1.06||-158.8|
Two things are interesting about these results. For the same receiver noise figure, the overall system noise figure (including the antenna) is degraded by the antenna's temperature and an improvement in overall system noise figure of about 1.5dB improves your sensitivity by nearly 5dB. Similarly, small losses between the antenna and the preamplifier degrade your system sensitivity drastically. This explains why microwave EME systems and antennas are optimised, not for maximum gain, but for minimum side lobes and hence noise temperature. In the case of Yagis this means improving the front to back and side lobe performance by careful design and placing the preamplifier as close as possible to the antenna feed point. For a dish, it's all about making sure that the feed only illuminates the dish, minimising "overspill" where the feed looks at the hot ground behind the dish, and of course mounting the preamplifier close to the feed.
Power Amplifier considerations
A 100W PA using a pair of G4BAO PA modules
On 23cm and to a lesser extent 13cm, valve PAs are still used. However, in more recent years the
move to solid state amplifiers has gained a lot of momentum. Valve PAs are still the preferred way to
generate powers in excess of 1000W with the popular TH series of vales being used in a number of
converted ex-TV transmitter amplifiers at 23cm. Some Russian valves have also been used to great
effect where these higher powers are required.
Solid state power amplifiers, with 23cm band outputs up to about 1000W, are available as kits as
well as built units. The most popular kit designs are currently those from W6PQL and PE1RKI with
built amplifiers available from Kuhne Electronic, Beko and SM4DHN.
In many cases the amplifier is available as a module and requires a control board to ensure it is not
over driven and is protected against overheating and antenna problems causing a high SWR. In
addition the use of sequencing ensures that the amplifier is not enabled until the preamplifier is
disconnected from the antenna.
The popular W6PQL control board https://www.w6pql.com/amplifier_control_board.htm is recommended. At least one manufacturer of complete power
amplifiers uses the W6PQL control board in their own product. Definitely high praise!
13cm power amplifiers
One useful side effect of the rapid expansion in mobile communications has been the availability of
surplus high power ( 200W+) power amplifiers suitable for use in the 2300MHz - 2320MHz amateur
allocations. These amplifiers originally saw service as base station multichannel power amplifiers in
the 210-2170MHz '3G' allocation. Being multichannel the amplifiers were rated at, maybe, 30W.
However, because the base station combined a number of separate carriers, the peak envelope
power (PEP) rating was often well in excess of 200W. Most of the amplifiers have required some
minor modifications, but usually only in the control or power supply. Amateur use often meant that
the manufacturers ratings get abused and the protection circuits would not allow more than a few
seconds of power output at 200W. Disabling the protection circuits rarely seemed to cause problems
as long as sufficient cooling air was blown across the amplifier heatsink.
Other possible problems are the ratings of the inevitable isolator at the amplifier output. Most of
those tested seemed quite happy at the elevated power levels, but a few have failed and required
replacing with an isolator with a higher rating, or removing completely. Sometimes the PCB output
tracks have been seen to burn up and these have also needed replacing. Usually with a short coaxial
For the very low cost of these surplus amplifiers they are a bargain and often worth combining in
pairs for even higher output. A 250W amplifier connected close to the feedpoint of a 2m diameter
dish, can be expected to produce audible echoes. However, attention to the efficiency of the dish
feed and the lowest noise figure preamplifier is probably necessary most of the time.
9cm and 6cm power amplifiers
In these frequency bands the choice used to be a TWTA (Travelling Wave Tube Amplifier) but now solid
state is much more affordable with a surprising number of GaAs FET and GAN amplifiers have appearing on the surplus market in the last few years. These are usually capable of 50w to 100W output and can often be combined for even greater power output. Perhaps the best known GaAs FET power amplifier is the UM2683B manufactured by Toshiba. DL7YC has detailed mods for this unit.
These are rated at 40W-50W output and have extremely high gain,making them suitable for mounting near the dish feed but driven from a transverter located in the shack. The loss of the connecting coaxial cable can often be tolerated by the low drive requirement of the PA. Unfortunately, as standard, the Toshiba amplifiers are quite inefficient and draw quite a lot of current (~18A at 12.6V) which might make them difficult to cool due to the high dissipation. Cooling fans are usually necessary. They can also be modified to reduce quiescent current.
6cm band solid state amplifiers are often found on the surplus telecommunications market. These
are usually relatively low powered, with 10W to 15w being quite common. Higher power amplifiers
are occasionally seen on the surplus market, but combining two or more lower power amplifiers is
TWTAs are often used where higher power or multiple band use is required. Higher power TWTAs
use some very high helix voltages and unless you know what you are doing may be best avoided.
3cm power amplifiers
Again, these can be found on the surplus market but in general solid state ones are limited to,
maybe, 20W. TWTs like the RW1127 can be made to operate on both 10 and 24GHz.
TWTAs delivering up to 300W can also be found on the surplus market, but 20W to 50W is more likely unless someone is parting with their higher power amplifier. The same comments regarding care with high voltages applies with these amplifiers.
A 14GHz RW1127 TWT modifed by G4NNS for 24GHz
Location for the PA
Generating any significant power output costs money. It is a shame to waste any power in coaxial
feeder losses. Depending on how far the dish is from the shack it may be practical to locate the PA
inside the shack by using low loss coaxial cable for the transmit feeder. The largest size cable ought
to be used, consistent with remaining within the cable's maximum frequency. If low loss cable is
used then 1296MHz, 2300MHz and 3400MHz power amplifiers can be located inside. In general it is
better to mount 5760MHz and 10368MHz amplifier close to the dish feed as coaxial losses can
quickly consume valuable power.
Heliax® coaxial cable such as LDF5-50 or even 7-50 can be used on the lower three microwave bands
but as these cables cannot be rotated with the dish unless special precautions are taken, such as
rotary coaxial joints, it is usual to use a 'flexible' cable such as FSJ4-50 or LMR400UF® for the final
From a large dish mount to the feedpoint, around the rim of the dish, can be a significant distance
and should be taken into account. Opinions vary on whether it is better to take the coaxial cable
through the centre of the dish, direct to the feed, or to route via the rim. The danger with taking the
cable direct is that it introduces the possibility of stray reflection of ground noise to the feed. A cable
around the rim and then routed along one of the feed supports has the advantage that any reflection will be lower as the reflection occurs out towards the edge of the dish where the illumination is lower.
Undoubtedly mounting the power amplifier near the feedpoint, such as in the 'cage', allows much
lower losses but in turn means that power supplies need to be brought out to the amplifier (usually
along the feed support). In addition the extra weight of the power amplifier and its cooling
arrangement, if using a prime focus dish, may distort the dish shape, leading to unwanted gain loss
and maybe destructive beam lobes. Offset dishes are often easier in this respect because the weight
is carried on an arm below or above the dish and often not directly part of the dish structure.
A possible compromise is to mount the power amplifier immediately behind the dish with the
minimum of cable to the feedpoint, bearing in mind the comments about routing via the dish rim or
through the centre of the dish. A heavy power amplifier can act as a useful counterweight to the
dish. Whether the power amplifier is located at the feedpoint or behind the dish, it will need to be
weatherproofed, of course.
A further consideration is the power amplifier power supply. If using a TWTA then there are extra high voltages to consider and dampness is a real enemy here. Even solid stage amplifiers can need up to around 50V (currently) and although this is generally considered safe it is wise to ensure that you do not come into accidental contact with the supply.
The power supply could be in the shack and suitably rated DC connections brought out to the
amplifier, preferably in a suitable duct. However, the current drawn by many solid state amplifiers
will mean either using a very large gauge DC cable conductor to minimise voltage drop or using a
power supply with remote DC sensing to maintain the supply voltage at the amplifier end of the DC
an interesting idea, suggested by GM4PMK, and implemented by Roger in his 23cm EME system, is
to use a 'building site transformer'. These are the yellow, sealed, transformers used on building sites
to power drills etc. Suitable transformers are readily available from many tool stores at very
reasonable cost. Long, pre-terminated, yellow 'mains leads are also available.
The principle here is that the transformed take 230V AC mains in and supply 55-0-55V output. The
mains earth is continued through from the AC input side to the end of the 'yellow' cable. In the
event of accidental contact with one of the two 'live' connections the maximum voltage to ground is
limited to 55V (mean). Whilst this is still dangerous it is regarded as a little safer than full mains.
At the remote end of the 110V AC power lead a dual voltage switch mode power supply unit
(SMPSU) receives the 110V AC and supplies the required DC output voltage. Suitable surplus power
supplies are available with DC output voltages ranging from less than 12V to 50V from either 110 or 230V AC input. Usually the SMPSU accepts any AC voltage between these two extremes (and
occasionally down to about 85V).
Anyone using this technique to bring mains voltages out to a dish is recommended to seek advice from a professional electrician before doing it. The author is not responsible for your careless actions!
TBA Photo of SMPSU and 110V transformer
Frequency allocations at 10GHz
By far the most commonly used frequency allocation in the 10GHz EME band is 10368MHz to
10370MHz, with most EME activity taking place around 10368.100MHz.
In Japan the 10368MHz allocation is not permitted and the Japanese amateurs tend to use 10450.100MHz. Working Japanese amateurs using in-band cross-band between 10368 and 10450MHz is quite common and is easily achieved in a number of ways including dedicated equipment for both bands as well as (outside Japan) frequency down converters from 10450MHz to 10368MHz. Obviously a frequency up converter can be used in the same way, in Japan, to listen to 10368MHz EME signals.
A simple, straightforward, converter does not need to use an SHF conversion oscillator as shown in fig AA TBA
Contrary to what some may believe, modern 10GHz systems tend to use coaxial cable
interconnections with waveguide being consigned mainly to the antenna connections and high power
stages. to be slotted in somewhere
Tracking is without doubt the key to success in 10GHz EME. Unless very small dishes are being used the narrow beamwidth of most EME dishes make it more difficult to track the moon across the sky than on the lower bands. The moon subtends an angle of approximately 0.5° at the earth's surface. Of course, this varies slightly depending on whether the moon is at apogee or perigee. A 2.5m diameter EME dish has a half power (3dB) beamwidth of approximately 0.6°. Even small movements of the dish can cause several dB of signal loss. When a signal is marginal that can be the difference between success and failure.
Finding the moon can also be a big problem. An absolute tracking system (one that points exactly
where directed) needs to have an accuracy of around 0.5° and a resolution of 0.1°. That is a difficult
demand because although the tracking encoders may have sufficient resolution, play in the rotator
bearings, distortion of the dish as it elevates or sag in the feed cage can all lead to significant errors.
It is also worth noting that at moon rise and moon set the position of the moon may not appear to
be correct because of signal refraction in the earth's troposphere. Locations close to large bodies of
water, such as the North Sea or the Great Lakes of North America are especially prone to this effect.
Even with accurate absolute position tracking it is often necessary to 'nudge' the dish position to
maximise signal levels. The problem is how do you know where to aim the dish if there are no
signals to peak on? The first paragraph of this chapter gives a clue.
Because the moon subtends a roughly similar angle to a medium size EME dish beamwidth, an
accurately aimed dish sees mainly the moon and little of the cold sky that surrounds it. The moon is
much warmer than the cold sky, averaging several hundred k (Kelvin) compared to, perhaps, 2.7 -
10k for the sky. As long as the receiver (more specifically, the front end low noise amplifier or LNA) is
sufficiently sensitive it will see the moon as a bright beacon of noise in a sea of cold sky. In practice,
with larger dishes, an LNA noise figure of less than 1dB is often sufficient and 0.6 - 0.7dB is enough
with smaller dishes (<3m). Noise figures below about 0.6dB are difficult to achieve at 10GHz. You
should not be fooled by the claims of 0.1dB noise figures for some Ku band (12GHz) satellite TV LNBs
(down converters). These are marketing men's numbers. They are not real! This also means that
bigger dishes cannot benefit from greater sensitivity as they have reached close to their limit once
the moon fully fills the dish receiver aperture. For those using smaller dishes, such as the popular
1.8 and 2.4m offset dishes, selecting a good LNA is still very important.
Some form of noise amplifier and detector can be used to find, and then keep the dish antenna
pointed accurately at the moon as it moves across the sky. Noise amplifiers will be described more
fully later in this chapter.
The recent availability of a high power moon-directed 10.368.025GHz beacon transmission from the
DL0SHF 7.6m diameter earth station dish has considerably eased the problem of finding and then
tracking the moon, although in practice the beacon is not always on when the moon is 'up' and at
the time of writing this most useful signal is also suffering from some problems with varying signal
Chris Bartram G4DGU produced a good paper on small dish EME, you can read it here
Moving now to how the dish is moved and able to track the moon. This section discusses rotators
including screw jacks and other methods of moving the dish.
Moving the dish
To move a dish of any size requires motors. These could be electric, hydraulic, pneumatic or manual.
Electric motors are by far the most popular. A suitable gearbox, with minimal backlash, is required to
reduce the speed of rotation of most electric motors. Some very small portable systems use manual
adjustment based on the use of a camera or surveyor's tripod.
There are two main types of mount and depending on the type different types of rotator can be used.
Polar and equatorial mounts
Polar mounts are popular but not always well understood. The polar or equatorial mount principle is
Although this mount seems to be an easy way to track the moon across the sky and appears to need only one drive motor for the GHA movement, in practice the declination of the moon changes sufficiently in a short period of time because of the narrow beamwidth of the dish and this means that a declination drive is also desirable. Possibly the biggest drawback to the polar mount is that it really requires the use of circular polarisation. The reason for this is that as the mount moves from side to side so the angle of the feed changes (longitudinal skew) with respect to the moon. If linear polarisation is used then there could be a serious misalignment of the polarisation between two stations with one in, say, Europe and one in North America, as the polarisation skew is approximately 90° if both are using linear polarisation. This would lead to a serious loss of signal due to the cross polarisation. If both stations use circular polarisation then there is no problem as there is no change in polarisation due to the 90° difference. However, a significant number of stations still use linear polarisation. The simple expedient of using vertical linear polarisation in Europe and horizontal in North America and far Asia means that the skew is automatically taken care of. Simple and effective. A linearly polarised station can still work a circularly polarised station, but there will be a theoretical 3dB loss in signal due to the polarisation. Due to depolarisation of the reflected signal, the loss is often less than 3dB.
No doubt you are wondering why anyone would want to continue to use linear polarisation when
effective circular polarised feed designs are readily available? The answer is complicated and
involves arguments about depolarisation of moon-reflected signals, additional losses in circular
polarisation feeds and the easy availability of commercial (or the manufacture ) of linearly polarised
feeds. The disagreements over circular vs linear have waged for many years and show no sign of
being resolved as yet!
The polar mount Right Ascension (RA) movement is often driven by a simple linear actuator (jack), whilst either a motorised car jack or even a manual car jack is used to change the declination.
Elevation over Azimuth mounts
There is also an azimuth over elevation type of drive, but these are not covered here. They are not often used by EME operators. A two motor drive is used with azimuth/ elevation often known as AZEL mounts. A very popular AZEL drive is the RAS made by SPID in Poland. These are available as either the standard RAS or the larger BIG RAS. The BIG RAS is better suited to dishes over about 2.4m in diameter, whereas the RAS is perfectly suited to smaller dishes as long as the elevation drive is well balanced. Such mounts are widely available in Europe as well as in the USA and Canada. Many EME operators have chosen to build their own drives based on local availability of motors and gearboxes. As well as electric motor drive it is possible to use Hydraulic motor drives. This subject is rather more specialised than can be covered in this short Wiki.
Whichever route the EME operator takes, either acquiring or building a suitable drive, it will be
necessary to use position encoders to indicate the position of the dish and provide positional
feedback to the tracking system. These encoders can be of various sorts with the best probably
being the USDigital absolute encoders or the MAB absolute encoders. Both of these can be used to
indicate the absolute position of the dish in both azimuth and elevation when connected to suitable
position read outs.
Non-absolute Pulse counting position encoders rely on the dish moving a known amount for each
output pulse. However, the actual position depends on knowing the starting position and whether
the dish is moving clockwise or anti-clockwise as well as up or down. A non-volatile memory of some
sort is required to store the last position of the dish for when power is removed or the last position
information is lost and recalibration will be required before tracking can resume. Old digital satellite TV dish positioners can be used as the necessary up/down count and non-volatile memory are
already provided, together with some form of digital position read-out.
TBA Moon noise tracking explained
Transverting is the most common way to reach the higher microwave bands. A wide range of
transverters are available for all the popular bands and even into the millimetre wave frequency
range. At 10GHz the most popular transverter designs are those manufactured by Kuhne Electronic
GmbH (DB6NT) https://www.kuhne-electronic.com/funk/en/ and Down East Microwave Inc. (N2CEI) https://www.downeastmicrowave.com/. Some years ago Charlie Suckling (G3WDG)
together with Petra Suckling (G4KGC) produced a very cost effective 10GHz transverter. These are no
longer commercially available but are sometimes found on the surplus market. Similarly, GW4DGU
produced a very nice 10GHz transverter system for a number of years. Although no longer available,
used examples are sometimes found at microwave events.
Transverter intermediate frequencies (IF) of either 144MHz or 432MHz are both in use. Care must be
taken when using a 144MHz IF that the image frequency 288MHz below the wanted 10368MHz is
well suppressed or noise at the image frequency may make the transverter noise figure appear 'too
good' by up to 3dB. As well as potentially causing unintended interference to any commercial system
operating in the image frequency band.
Whilst a transverter local oscillator may use a 'free running' crystal oscillator, with temperature
stabilisation it is highly preferable that the local oscillator is frequency locked to either a GPS
disciplined reference, Oven Controlled Crystal Oscillator ( OCXO) or Rubidium reference, at 10MHz.
The Kuhne Electronic GmbH MKU10G3 transverter uses a 10MHz reference input.
A transverter, on its own, is unlikely to have a low enough noise figure at 10GHz to give acceptable EME results. A low noise amplifier (LNA) is usually required. Both homemade and commercial LNAs are used. Noise figures tend to be from around 0.6dB up to around 1dB, with HEMT transistors the most common active device used in LNAs. Popular homebuilt designs are the G3WDG and W5LUA preamps. Both of these are single stage designs and at 10GHz it is often necessary to have a little more gain to overcome inter-stage cable losses. Multistage LNA designs, by a number of amateurs, have appeared in the pages of DUBUS and VHF Communications over the years.
These are usually either TWTA (Travelling Wave Tube Amplifier) or SSPA (Solid State Power
Amplifier). TWTA are available surplus with power output levels of over 300W. At these power levels
the high voltage requirements of the TWT (Travelling Wave Tube) is very high and safety is very
much a concern, with voltages of over 3kV being common. Since transmission line losses are high at
10GHz, it is common to mount the TWTA close to the feed-point to minimise power loss and this
implies the power supply being mounted outside, usually in a weatherproof container or
occasionally in the open, but only installed when required and the weather is kind. In turn this
implies a mains power outlet close to the dish. It is beyond the scope of this book to cover the
regulations governing provision of outdoor mains supplies. These will vary from country to country
and are updated at regular intervals.
SSPA amplifiers are available both commercially and on the surplus market. Power output
requirements will depend on which modes the user intends to operate. Digital modes like JT4 can be
effective with small dishes and power levels of as low as 10W. In general a power level of between
25 and 50W would be desirable but inevitably more expensive.
The use of GaN (Gallium Nitride) devices has become more popular recently [xx]due to the (slowly) falling prices of GaN power transistors. GaN is an excellent semiconductor material for power devices as it exhibits high gain, tolerance to high junction temperatures and operates with, typically 48-50 volts, and therefore requires lower power supply current capability for a given output power compared to GaAs (Gallium Arsenide) devices. Photo XX shows A 30w GaN amplifier. One possible solution to the high voltage requirement of a GaN amplifier is to use a 48V SMPSU (Switch Mode Power Supply Unit) supplied from a 110V supply. 110V supplies are common on building sites as the familiar yellow encased isolation transformers that take in 230V and output 50 - 0-50V with the centre grounded. This way the AC voltage is a maximum of 50v (mean) with respect to ground. At the dish end of the external supply a suitable 48V output SMPSU with 110V input can be used to derive the wanted operating voltage. Such SMPSUs are common on Ebay and surplus for sale at many radio rallies. It should be added that this is a suggestion only and you use the idea at your own risk. Suitable transformers are available for around £56-£70 at many electrical discount outlets in the UK. The yellow 2.5mm cable, of around 14m length with suitable plugs and sockets fitted, can also be bought for around £30. Photo xy shows a 110v transformer.
Feeds and Switches
In order to effectively illuminate the dish reflector, whether prime focus or offset elliptical, requires
a suitable feed antenna. The feed antenna is usually known as a the 'feed'. The feed can be a horn or
some other arrangement such as a patch. Whatever type of feed is chosen will depend on the focal
length or dish diameter (f/D) and whether circular or linear polarisation is required. The purpose of
the feed is to launch the signal, impose the polarisation required and then illuminate the dish with
the optimum spread of signal across the reflector. It is usual to arrange the transmit signal level to
be around 10dB lower at the edge of the dish (edge illumination) compared with at the centre of the
dish. However, this may not be optimum for receive, where the edge illumination is probably better
set at around 14dB down on the centre. This reduces 'spill over' and makes the dish 'quieter'. Most
EME operators tend to prefer the quieter dish than that giving maximum transmit gain. The
exceptions are those who like to 'hear their own voice' They are often referred to as 'alligators'!
Linear vs circular polarisation
Up to 432MHz linear polarisation, whether vertical or horizontal, is used. Circular is not easy to
arrange due to the need to provide for reverse circular polarisation switching due the moon
reflection (polarisation reverses on reflection). On 1296MHz and above the use of dish reflectors
makes dual polarisation feeds much easier to manufacture. Circular polarisation is almost exclusively
used on the 1296, 2300/2304/2320/2400, 3400 and 5660MHz EME bands. At 10368/10450MHzthat
is not always the case. Why?
The nature of the moon at 10GHz is such that a significant amount of depolarisation takes place
when signals undergo reflection. The size of surface obstructions, including boulders, can cause
multiple scattering of signals and some areas of the moon permit significant subsurface reflection.
All of this can make the use of circular polarisation less effective than it might otherwise be.
Until recently the availability of low loss circular polarisation feed designs was poor. That has now
changed and feeds based on the use of squeezed waveguide and septum polarisers are in common
use. However, many fine examples of linear polarisation feeds, such as the once commonly used and
available Chaparral feed, are available and still in common use.
It happens that the majority of intercontinental activity takes place between Europe, North America, Australasia and Asia. These places are (approximately) separated by 90°. A signal transmitted to the moon at zenith will (mainly) return with the same polarisation. The same, vertically polarised, signal sent to the moon from, say, Europe, and received in North America with be received as horizontal. A standard has evolved whereby North American stations transmit to the moon using horizontal polarisation and Europeans use vertical polarisation. Australasia also uses vertical as it is 180° from Europe and 90° from North America. This system has been shown to work well and although there is theoretically up to a 3dB penalty when working any circular polarised station, depolarisation of the reflected signal often reduces this by several dB. For the beginner there is much to recommend the simplicity of linear polarisation. No doubt in the future circular polarisation will predominate, but it won't happen overnight.