Graves radar heard from the moon – EME

Barry G8AGN has stimulated my interest in this.

I listened for the Graves radar on 143.050 MHz this afternoon (Wed-30-Aug-2017). I could hear meteor pings & see them on the P3’s SVGA display.

When the moon had risen I could see & hear pulses of about 4 or 5s, ranging from 2dB over noise to 8dB over noise, maybe 100 or 200 Hz away from 143.050 MHz. I believe this is moon reflection of Graves radar.


My notes on Wed were poor. On Thursday eve (31-Aug), I listened again & heard meteor pings. One at 19:33 ish GMT was fat on the screen and lasted about 30s.

At 19:54 GMT I took this picture:


The blue-dotted horizontal lines are at 2dB intervals (I’ve never calibrated this!). The dial frequency in CW mode is 143.050,030 (so 30 Hz up from Graves). The green area is the listening passband, maybe 650Hz. If I’ve read correctly, the moon would have been at 16.6 el 176.6 az. The picture shows a trail of pulses going down the waterfall screen which I believe are reflections from the moon. About half way down the moon trace is a horizontal dash. This was a very audible ping from a meteor.

I was beaming about 170deg on the rotator controller and using the 9el (2m band) Tonna yagi.

3cm SHF UKAC 22-August-2017

I managed to lash up the 80cm dish minutes before the contest in a patch of decent weather. There was quite a bit of insulation tape & bungie cord holding the bits together. A Blue Peter job really!


Last month I managed to work Denis G3UVR over the Pennines. I was using just the LNB horn (10dB gain maybe?). He was quite easy copy, but he struggled to hear me on my 0.25W approx. We aimed at the direct path.

Tonight we were Q5 to each other on CW. I was still using about 250mW and probably with greater losses in the coax couplings.

The dish looks more robust on it’s new roof pole – Peter G3PHO has given me a section of thick-walled aluminium scaffold pole. The upper part of the pole is steadied by some chimney-bracket extensions, custom-made by Blakes of Rutland Road, Sheffield. The azimuth rotator has some play in it, so the dish can waggle a bit.

At the end, I even managed to hear Peter G3PHO off a reflection. I could read him sending my callsign. He struggled to hear me however. He is tucked away behind a couple of hills roughly south of me, so a very difficult path. Mandy M0MDY is also a tricky path from here and she was obviously struggling to hear me, but we completed OK.

South is difficult! I am a bit down from the Crosspool hill-top. The top of the Crosspool tower looks a nice reflection target however, maybe with a bit of elevation.



K3’s K144V output

I assembled the K3 towards the end of 2013. I set the input to 1mW & just took the 10W output on trust. Everything else on the K3 worked. Why check it? I didn’t check the output.

Today I was setting up 2m drive for transverters. I checked the output using a 6460/1 power meter: 4W, and not the 10W I expected. Surprised, I tried the SX-1000 power meter on it’s 5W range, using the 20dB attenuator & the 6460/1 as the load. It agreed!

0.1 mW drive yielded 1W output, so 40dB gain as expected, but the gain tails off up to 1W drive: 36dB.

The SWR to the antenna looks high, so I hope I haven’t damaged the PA transistors.

Something else to sort out. I had been thinking 2m had been a bit of a struggle.

Experiment with circular waveguide/feed for 3cm

I have loosely assembled a copy of Peter’s G3PHO’s plumbing fittings implementation of a W2IMU dual-mode horn. At the moment, it is stuck together with insulation tape. To this I have added my own experimental design of a waveguide crook. Behind the horn is a 135 deg bend, followed by a 180 deg sweep bend up to the main bit of waveguide. Then there is a wide 90 deg sweep into the box. So it is a bit like a shepherd’s crook, but bendier.

This has probably all been done before for an off-set dish, so it is probably not a “G3YJR Crook” at all, it which case, here it is again!

I’ve used 22mm copper pipe as circular waveguide. I’m not using any rectangular waveguide. Rectangular waveguide seems popular and there are probably very good reasons for using it, but as a newcomer to microwaves, I don’t know them.

There is a twist to the polarisation along the waveguide. I haven’t measured this yet, but it looks like about 45deg at the coax feed point. My SMA-coax transition accounts for this. The twist seems stable. I’ve mounted a weather-protective plastic box behind the dish. The SMA feed transition sits inside the box along with the electrickery.

The idea is to move the pre-amp/transverter from the end of the off-set arm into the box behind the dish in order to minimise the weight on the off-set arm, whilst keeping the losses low by using rigid waveguide. My coax transition has two SMA feeds at right-angles in order to allow for switching polarisation.

With the dish in the terrestrial position, looking at the horizon, the waveguide slopes gently down to the horn so that any condensation should dribble down & escape from the horn.

Feeding the crook/horn from the DB6NT shows the same monitor output as I got using my ex-LNB horn with my home-brew SMA-to-rectangular transition: 5 “units” on the moving-coil meter scale. This looks encouraging. I need a way to measure the insertion loss.

I added a bracket under the box so as to not distort the dish with the weight of the box (not in this picture). I bought the box from Toolstation.

I tested the horn straight into the SMA transition using GB3FNY as a source. And then with the crook in between.

The results seemed comparable with the old LNB horn, so that seems reasonable. One surprise was that when I removed the back-short (the 22mm plumbing cap) from the SMA transition and left the waveguide open, I couldn’t see any difference in the signal strength.


Using the vertical polarisation SMA feed resulted in about a 20dB drop in signal (as predicted by Bryan G8DDK).




















13cm Bandpass filter

I bought a 2.5GHz bandpass filter recently in the hope that I could re-tune it for use on 13cm where I intend to use more power than the current 2W & maybe add a pre-amp. The label says “S.E.T P/N:SJCB-2549R106F S/N:08C00005”.

I took it along to the Martlesham Round Table & Martin had a look at it on the equipment in the lab. It had a lovely textbook BPF attenuation shape with steep sides & remarkably little ripple on the top. The transmission loss was about 0.5dB. Out of band attenuation was about 50dB.

The filter looked fairly symmetrical & of solid construction with about 80 screws holding the lid down on 10 active cavities. It was sold as a “diplexer”, but there are only 2 SMA connections.

At home I had a go at retuning it, using my Tiny VNA. I had managed to get it down to cover the range 2300 to 2350 MHz with a loss of about 0.6dB, but without the nice steep sides. Reasonable though. The downside was that most of the screw-in adjusters were no longer proud enough to be locked by their locking-nuts.

Then in an absent-minded moment, I did what I was trying to be careful not to do: I screwed one probe in so far that it dropped into the cavity. This meant that I had to remove the eighty-odd lid screws in order to remove the lid to retrieve the probe.

But this was also an opportunity! The photos show what I found inside. It looks beautifully designed & constructed; a high-quality unit. Perhaps it is silver-flashed aluminium? It is probably optimally sized for the design bandwidth, but I am optimistic that I can produce a decent 13cm BPF from it. I may have to glue the probes in place. It was probably air-tight, so I hope I haven’t let in moisture that will cause a condensation problem.


Of course, now I have the lid back on the tuning has gone to pot. I’ll have another go.