A Survey of 6cm EME in 2018 Peter Blair G3LTF

Introduction and some history

The first 6cm EME contact was made in 1987 between the North Texas Microwave Society

WA5TNY, and W7CNK in Oklahoma. Activity after this was sporadic until the 1990s when

stations like VE4MA, W5LUA, SM4DHN, OK1KIR and OE9ERC established permanent

installations. The European (DUBUS) EME contest attracted only a few 6cm entries in the

1990s and the ARRL EME contest only introduced a microwave section in 2004. However,

the game changed as SSPAs became available to replace TWTs, and with a dedicated

weekend for each of the microwave bands in the DUBUS EME contest and the re-invention

of “activity weekends” the 6cm activity really started to grow. The 8m reflector in Brittany

activated as TM8PB from 2012 also increased interest. An additional factor is the recent

increased use of digital formats which have facilitated more DXpeditions on 6cm. The chart

below illustrates this showing the total of calls active in the DUBUS EME contest and the

growth rate of OK1KIR’s 6cm CW initials.

Figure 1. The Rise and Rise of 6cm EME activity

Why 6cm?

So, why is 6cm getting this attention? If you are finding 23cm a bit tame and possibly even

tedious, then the next band up, 13cm, is interesting (and actually the best band of all for

EME) but there is no single world-wide allocation. 9cm is also a great EME band but with

limited allocations and beginning to suffer from the same problems as 13cm. 6cm is widely

available with a common allocation, and good results are available with a small to medium

dish. For many it’s an experimenter’s playground and a nice engineering challenge.

Components are not too hard to find, and still at a size that’s easy to handle. 6cm is also the

highest band where you can use coaxial components without excessive losses, but care is

needed as every fraction of a dB counts if you want to build a good system. Note however

that all of these bands, 13, 9 and 6cm, are under severe and continuing threat from mobile

phone and Wi-Fi, and in all three there is the possibility of interference from Wi-Fi and other

short range systems and sources such as microwave ovens. This means two things:

1. “Use or Lose” these bands; and

2. Build your system anticipating that there will be some sources of interference.

EME signals are stronger as you go up in frequency. If you write the radar equation for the

moon reflected signal, for a fixed area antenna and the same transmitter power, then the

return signal increases as frequency-squared. That means that for the same antenna area

and power, and the same system noise temperature, Tsys, the signals will be 13dB stronger

on 6cm compared to 23cm.

Things don’t quite work out exactly like that,

of course – for example, if you have a

mesh dish rather than a solid one. Also the

losses in the receive chain and the noise

figure are a bit higher than on the lower

bands. However, that ‘6cm advantage’ is a

fact and my recent CW QSOs with EA6/

HB9COG with his 1.5m dish on 23, 13 and

6cm showed this nicely. His best signal

was on 6cm, even though my dish profile

errors and mesh loss degrade my G/Tsys

quite significantly.

In the following sections we will discuss

some of the issues that need to be dealt

with to make a success of 6cm operation.

They apply to other bands as well, of

course, but if you are making the move

from 23cm for example then they are

probably significant.

EA6/HB9COG 1.5m, 0.4 f/D dish with 6cm feed

Signal Characteristics

The Doppler shift at 6cm can be as high as 13 kHz (see Figure 2, next page) which is

typically well beyond the RIT range of many transceivers, so split-frequency operation is

usually needed. Many stations now use automatic correction from a software program,

especially when operating on digital modes. For CW and SSB, netting your echo on the

signal you want to call works fine and is easy to do, especially if you use an SDR. For a

detailed discussion on EME Doppler shift issues, see Reference 1. Libration and signal

spreading can become really apparent at 6cm with the note sounding quite ‘auroral’ at times.

The level of libration varies considerably over a moon pass and the location of your QSO

partner. The curve in Figure 2 is for echo Doppler, and you can see that close to moon-rise

and moon-set the rate of change goes through a minimum. At or near these points the tone

is often pure T9 with very little spreading and low fading rates. Figure 3 (next page) is a

recording of my echoes at 6cm together with the signal from PI9CAM with typical spreading

evident. Spreading is also affected by the size of the area illuminated on the moon. A large

dish like the 25m Dwingeloo reflector has a beamwidth which illuminates only 1/10th of the

moon’s surface and the effect of this can be clearly seen here, my echo is about twice as

wide as that from PI9CAM. For a more complete understanding of libration and how to

calculate it, see the excellent paper by G3WDG at Reference 2.

Figure 2. Typical echo Doppler shift at 6cm

Some Antenna Issues at 6cm.

When we talk about the “beamwidth”

of an antenna, we usually mean the

angle between the -3dB points of the

main lobe and so if we are off the

moon by half of that amount then our

echo is -6dB.

We really need to be interested in the

-1dB beamwidth and up-dating our

pointing in the time that the moon

moves through our -1dB beam width.

Figure 4 (next page) illustrates the

problem. For a 3m dish the -1dB BW

is about 0.6 degree, the moon moves

Figure 3. Libration spreading of 6cm signals at about 0.25 degrees per minute so

we should update about every minute.

The other pointing problem that many of us encounter at the higher bands, especially with

larger dishes, is backlash in the dish drive gearbox. Michael, DL1YMK, described his

solution, which uses a slewing gear, at the 2015 Swedish EME meeting.

http://moonbouncers.org/A%20semi%20professional.pdf

If you are moving up from

23cm with a mesh dish then

another issue to be aware of is

mesh transparency. When the

dish is elevated then the feed

‘sees’ the hot ground, at

approximately 290K, through

the mesh. Suppose the loss

through the mesh is 13dB then

there will be approximately

15K added to Tsys together

with a gain loss of about

0.2dB.

Figure 4. Dish beamwidths at 6cm

Mesh loss is determined by the hole dimensions and the wire thickness and W1GHZ has an

excellent spreadsheet for estimating it in his online Microwave Antenna Book. Figure 5

shows a range of results from this resource. If mesh loss is a problem and you can’t re-cover

the dish with finer mesh, then think about covering the centre 50%, at least, with adhesive

aluminium tape.

Figure 5. Potential losses through a mesh dish

(from http://www.w1ghz.org/antbook/conf/Mesh_Reflector_Loss_Calculator.xlsx)

Mounting your 6cm system at the focus

Transmitter power is hard to get and expensive, so you can’t afford feeder losses. For most

people this means you need to put the whole system at the feedpoint, although at least two

stations have waveguide runs to the focus. (I did say that 6cm is an experimenter’s

playground and an engineering challenge!) Things to think about early on are weight, and

minimising the blockage in prime-focus dishes from both feed and supports. The scattering

of ground noise into the dish from these structures can be a bigger problem than the gain

loss from the blockage, especially in small size dishes. Offset-fed dishes do not have this

problem of course. The feed support legs need to be strong enough to take the weight

without distorting the dish profile as the elevation changes. I recommend investing in

weather-proof plugs and sockets at the focus, with a cable that will handle the DC power and

a multiway cable for switching etc. You will also need cables for the IF, usually 144MHz

up/down, and a frequency reference which is much better sited in the shack than at the

focus. I use a single cable for this with HB diplexers and simple (non-coax) relays.

I strongly recommend building the feed system so that you can easily change bands. If you

try 6cm then almost certainly you will want to try other microwave bands too. My feed system

has standardised plugs and sockets and wing-nut fixings so I can change bands in 15-20

minutes. This is also the place to emphasise that you must have a safe and stable access to

the feedpoint – it is just too easy to have an accident here that can be life-changing.

I also recommend that you have really good screening and DC decoupling on all the units at

the feed point. I was caught out with my 9cm system by signals leaking from the LO chain

into the dish and then back into the LNA. When I moved the feed to maximise sun noise, I

was also changing this effect and so could not find the correct position.

Some Current Systems, Dishes and Feeds

We will now take a look at some of the systems being used today. At the RSGB Convention

in 2016 I made a presentation (Reference 3) featuring systems used by HB9Q, TM8PB,

OZ1LPR, SM6PGP, OK1DFC, LX1DB, PA3DZL, G4NNS, DL7YC, JA4BLC, PA7JB,

W7/VE4MA, and WA6PY. These covered dish sizes from 1.8m to 10m and with varying

degrees of detail. In this paper I want to feature some more systems, mainly from North

America where microwave EME activity is on the rise again.

VE6TA The pictures in Figure 6 show Grant’s system which uses a HB 5m dish, 0.45 f/D,

covered with 3 x 3 x 0.7mm mesh. The feed tray consists of a Kuhne 432/5.7 GHz

transverter, a Kuhne 0.7 dB NF preamp with HB pipe cap filter, a pair of Stealth Microwave

SSPAs giving 37W combined, and a W2IMU dual mode feed. The entire tray as well as the

feed can be moved to find the optimum focal point.

WA9FWD John’s system is shown in the pictures in Figure 7. It uses a 3.7m 0.5f/D TVRO

dish with about 80W at the feed and a Kuhne LNA. Note the multi-pole filter following the

LNA which helped eliminate interference (from WiFi?) in his original setup.

Figure 6. VE6TA dish and feed unit

Figure 7. WA9FWD dish and feed unit

W5LUA Al is without doubt one of the most experienced microwave operators and in the

following paragraphs he relates his experience on 6cm where he has worked 96 initials with

36 countries and WAC.

“The 6 cm (5760 MHz) EME

station at W5LUA in EM13qc

consists of a 5 m fiberglass dish

which was originally used as a 3.7

to 4.2 GHz downlink antenna.

Besides working very well on 5760

it also works well on 10368 MHz I

heard my first CW echoes on 5760

MHz on June 26, 1995 during

apogee while running 15 watts in

the shack from a Siemens RW-89

TWT. My preamp was an Avantek

MGA-86576 LNA with a noise

figure of 2.5dB. We started out

with linear polarization. North

America was horizontally polarized

and Europe was vertically

polarized due to the nearly 90

degree spatial offset between the

two continents.

Figure 8. W5LUA 5m dish

“My first contacts on 5760 EME were with OE9PMJ, OE9YTV, and VE4MA on July 16,

1995. SM4DHN, OK1KIR, I6PNN, OE9ERC were worked in short order. At the time, this was

about the extent of the activity on 5760. With the increase in world-wide activity on 5760, and

the varying spatial offsets between different continental areas, it was clear that circular

polarization was the best way to go. My system today is the same 5 m dish with a WD5AGO

circular polarized septum feed with a 3-ring scalar ring. My LNA at the feed is a Down East

Microwave with an NE3210S01 measured at 0.7 dB noise figure. I use a surplus solid state

PA which puts out 150 watts in the shack and runs through some rigid and flexible WR-137

waveguide providing a measured 110 watts at the feed. My transverters from 5760 to

144 MHz and down to 29 MHz are all homebrew. I use the new DEMI/Q5signal digiLO at

5616 MHz for my local oscillator. CW is still the most popular mode on 5760 followed by the

WSJT modes of JT-4F and QRA-64D and some SSB when conditions are good.”

W5LUA uses a multiband feed cluster for 23, 13, 6 and 3cm (see Figure 9).

G4BAO John uses a 1.9m RFHam Design dish which he has re-covered with 2.7mm x

2.7mm x1mm mesh to reduce feedthrough noise (Figure 10). The transmitter uses two

Ferranti 18W units combined with hybrids to give 30W at the RA3AQ circular aperture feed.

The HB LNA, based on a "Franco" surplus LNB board, uses a NE325 with 0.9dB NF. John

has made several QSOs with both CW and digital modes.

Figure 10. G4BAO dish and feed unit Figure 9. W5LUA feed system

KL6M Mike has a 9.6m dish 0.4f/D and a 3-ring Chaparral feed shown in Figure 11. There

is a 45m run of EW52 elliptical waveguide from the shack with a waveguide switch at each

end so it doubles as feeder for both tx and rx with a loss of 1.5dB. There are two cascaded

LNAs at the feed and the TWTA runs 60W.

Figure 11. KL6M dish and feed unit

Equipment

Reference 2 contains some material on designs for 6cm and I hope to update this in the

presented paper. Reference 5 shows where to go for feed and dish information.

A word of caution on transverters and preamps. Recent measurements on a transverter

design with only minimal filtering until the mixer show that when a 2-stage LNA, typically

1GHz wide, is put in front then the noise power at the final stage before the filter may be only

20dB or so below the P1dB. When such a system encounters Wi-Fi at high levels then

intermodulation results. The answer is always to fit a band limiting filter after the LNA.

System Optimisation

As with EME systems on other bands, the Sun is a very useful source for initial calibration.

The Sun at present is a fairly stable and predictable source with solar flux around 80 SFU.

Most systems will also see Moon noise which provides a more accurate calibration. Cold

sky to ground measurement for both the complete system and the feed separately can also

give useful measurements that help analysis. Reference 4 is rather old but still may be

useful for those starting on the higher bands. EMECalc was updated very significantly since

that was written but the principles still apply; it is important to read the Help sections.

Conclusion

I hope that this paper, by illustrating the interesting aspects of 6cm and the wide variety of

system designs being used to make contacts, will have inspired a few more to come and

work on what is a real experimenter’s band, open to all.

References

1. Understanding Doppler Shift: Critical Knowledge for Successful EME on the Higher

Bands. Al Katz K2UYH, Proceedings of 2014 EME Conference pp17-21.

2. Predicting Libration Fading on the EME Path. Charlie Suckling G3WDG,

http://www.vk3um.com/G3WDG_libration%20paper%20revised.pdf

3. The Rise and Rise of 6cm EME. Peter Blair G3LTF, 2016 RSGB Convention. http://moonbouncers.org/onewebmedia/The%20Rise%20and%20Rise%20of%206cm%20EME_2.pdf

4. Practical Optimisation of 432MHz and Up EME Systems using VK3UM’s EMECalc Programme. Peter Blair G3LTF, Proceedings of 2010 EME Conference, pp 163-174. http://www.ntms.org/eme/presentations/VE4MA/G3LTF%20eme2010presentation_v5.pdf

5. Much useful material on antennas and feeds can be found on the websites of W1GHZ (in particular his on-line antenna book), OK1DFC and SM6FHZ and also at www.moonbouncers.org