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

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1 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 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. Why 6cm? Figure 1. The Rise and Rise of 6cm EME activity 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:

2 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. EA6/HB9COG 1.5m, 0.4 f/d dish with 6cm feed Signal Characteristics 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. 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 1/10 th of the moon s surface and the effect of this can be clearly seen here, my echo is about twice as

3 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. Figure 3. Libration spreading of 6cm signals 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 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.

4 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

5 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 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.

6 Figure 6. VE6TA dish and feed unit Figure 7. WA9FWD dish and feed unit

7 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. Figure 8. W5LUA 5m dish 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 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 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. My first contacts on 5760 EME were with OE9PMJ, OE9YTV, and VE4MA on July 16, SM4DHN, OK1KIR, I6PNN, OE9ERC were worked in short order. At the time, this was about the extent of the activity on 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.

8 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

9 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 pp Predicting Libration Fading on the EME Path. Charlie Suckling G3WDG, 3. The Rise and Rise of 6cm EME. Peter Blair G3LTF, 2016 RSGB Convention. ME_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 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