Page 2 of 2

Re: Hints on tuning the G0MJW feed patch

Posted: Sat May 04, 2019 6:14 am
by g0mjw
I don't think you need it. Just to bend that corner up of down slightly. If that is FR4 it might catch fire as the field strength is very high at the edges.


Re: Hints on tuning the G0MJW feed patch

Posted: Tue Jun 11, 2019 11:48 pm
by Nick
Decided trying to set up with a spectrum analyser was not a good idea as I could not obtain repeatable results, so bought a miniVNA Tiny.

Results are repeatable and around the correct frequency, but I can't obtain two dips, and the graph display does not seem to be as smooth as I think it should be.

Have to admit I'm not sure if the VNA is set or working correctly as the Load option in the Calibrate menu does not always seem to work.

Any comments on the results or the way the VNA is displaying these would be appreciated.


Nick - G4NKV
Image 1.
Patch.jpg (357.93 KiB) Viewed 704 times
Image 2.
VNA_190611_235507.jpg (246.63 KiB) Viewed 704 times

Re: Hints on tuning the G0MJW feed patch

Posted: Wed Jun 12, 2019 6:34 am
by g0mjw
That loop shown on the smith chart is exactly what you want to see. A really nice result that looks really really good. Noise is normal. You have got two dips but its not showing if you just look at the return loss and not the phase.

Adjustment would need to get that loop looping around the centre of the chart and 2400-2410 MHz (almost point 2) all in the loop - but its already good. I would argue good enough. You are very slightly HF. Is this with the PCB? Bending a little may be better.

Amateurs seem to have this fixation with VSWR, I think from HF where narrow bandwidth short antennas have to match to work at all. Its not healthy. If the return loss is good enough, it is good enough. Don't aim for perfection, aim for good axial ratio, which you will be hard pressed to measure without an antenna range. A VSWR of below 1.5 is fine. I would advise you stop messing with tuning it and get on the amplifier to get air with it.


Re: Hints on tuning the G0MJW feed patch

Posted: Wed Jun 12, 2019 12:34 pm
by radiogareth
Thats a nice toy to add to the test gear. £350+ though on a quick search, I suppose you get what you pay for....

Re: Hints on tuning the G0MJW feed patch

Posted: Wed Jun 12, 2019 4:33 pm
by G4WIM
Hi Nick,

I have the same VNA and attempted the same measurement but had trouble calibrating such that the measurement plane was at the feed point.

I found that even a small change of distance shifted the plot significantly - as you might expect 2.4GHz.

Indeed by having the right length of connector I could 'fudge' the result such that it looked right !

I wonder what your process was to calibrate to ensure the measurement plane was at the feed point ?

Regards Tim

Re: Hints on tuning the G0MJW feed patch

Posted: Thu Jun 13, 2019 12:57 am
by Nick

I got rid of the PCB material and used a piece of copper shim. I did try bending the plates but this did not work very well. I could easily get the resonance down to 2405, but the RL went down to around -11dB, and the loop on the smith chart all but disappeared.

The frequency on the 'as built' patch was only about 10-15MHz high. The small shim got this down and improved the loop. It took a little time to get this looking OK as even a tiny change in position had a large effect on the results. The attached photos show the 'as built' and adjusted results.

Quite happy with the results now.


I carried out a calibration at the end of the cable to take that into account. This was a short length of RG136. I checked the results with another slightly different length of cable and as expected the calibration was well off with that. All OK when I swapped back to the original cable. I do have a issue with loading the calibration files though - quite often when I select one the OK button remains ghosted and has no effect. What version of software do you have and what is the firmware version on your analyser ?

I have noted that the firmware download link on the mRS website does not work and gives a 404 error, so it looks like the firmware downloads have been removed.

As Built.jpg
Results as built.
As Built.jpg (371.05 KiB) Viewed 586 times
Final test results.
Final.jpg (364.79 KiB) Viewed 586 times
Patch Adjusted.jpg
Copper shim fitted.
Patch Adjusted.jpg (786.87 KiB) Viewed 586 times

Re: Hints on tuning the G0MJW feed patch

Posted: Fri Jun 14, 2019 5:56 pm

This picture looks perfect. The centre of the balloon is almost in Z=1 (50 + j0 Ohm).

You also see that the response 'broadens'.

Very good job, congrats!

Remco PA3FYM

Re: Hints on tuning the G0MJW feed patch

Posted: Fri Jun 28, 2019 9:33 am
by Nick
Patch is working OK.

Got through the satellite with 6W at 250 SR using a cheap 1.8Mtr dish from e-Bay.

Nick - G4NKV