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oxxyfx

Just a curiosity about the GP Patch...

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I have built 4 GP patch antennas. 2 of these are with metal backplate and copper director, one is aluminum backplate and copper director, and the one I built first is metal backplate and metal director.

I used them all in different situations and they all worked well giving me about the same performance - at least what I could detect by looking at the videos I made or the goggles.

About two weeks ago, one of the two metal backplate and copper director broke at the SMA connector - pretty strange I didn't even forced when I removed it. About the same time I started having quality problems with the other same antenna - and I did not realize it was the GP patch. I took the receiver circuit apart, rebuilt it replaced parts - etc - and still got horrible video quality. About a week ago desperatly I build another receiver - no extra components - just the straight Airwave module and connected a whip to it - it worked like a miracle. That it still did not ring the bell, but yestedy it hit me - what if the antenna is the issue...

I replaced the PAtch with a whip - and see miracle - there is nothing wrong with the receiver - it was all along the GP Patch. Today I started experimenting - from the one with the broken SMA connector I removed the SMA and replaced it, also replaced the little balsa pieces which keep the director at distance. Soldered the whole thing back - and got the same problem.

So I put on the one with the aluminum backplate, that has no problem, it works well. Also tried the one with metal and metal - that also works well...

What is the conclusion? I don't know - but something happened that two exactly the same construction GP Patch antennas failed just about the same time...

I know when I originally posted a question about antenna problems caused by different type of metals - there were a few people laughing at me - they never heard about these issues. However I doubt that would be it, because the metal is not in any contact with the copper.

None of the antennas are painted in any way, the SMA connectors are good quality from Digikey - and these are soldered to the back of the metal plate, so it cannot be any contact problem there. Measuring the antenna with a multimeter shows no short so I am puzzled...

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When you say you use copper, do you mean copper laminate (PCB) material? If so, then that may explain part of the problem. Can you post clear closeup photos of the antennas that don't work?

Edit:

Also, how is your SMA's body attached to the reflector? That is, is it soldered, screwed, or ? Can you post a photo of this area too?

Edited by Mr.RC-Cam

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Here are the photos. The reflector is 0.65mm (I think that is 22 or 24 gauge - not sure) metal and the director is pure laminate copper, 0.5mm thick.

The SMA is soldered.

Funny thing that I made several perfect videos with these....

post-7-1188958548_thumb.jpg

Edited by oxxyfx

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Fourth picture - the space between the reflector and director. It may seem that the balsa behind the pin is ver close - that is an optical illusion - in reality is about 10mm apart. The teflon (or plastic) which is around the middle pin is cut level with the reflector.

post-7-1188958883_thumb.jpg

Edited by oxxyfx

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Things look fine to me. However, I'm still confused about the copper. Is it copper laminate (PC Board) material or flat copper sheet (pure copper, no laminate)?

The teflon (or plastic) which is around the middle pin is cut level with the reflector.

It is probably best to leave it as-is as much as possible. On the next one you build, try not to trim the plastic material.

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Your sudden lack of performance is a mystery. My only thought at this point is that the balsa dielectric spacers have absorbed moisture. Perhaps a one hour antenna bake in a "cool" 150° F oven would drive out most of any that is there. At least long enough to perform another side-by-side test with a working antenna.

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Yep, the only possable cause I can see is the balsa and the glue holding it. I gave up useing spacers like that and just use a metal screw through the centre that shorts the plates. This has no ill effect on the RF as it is in the 'dead' part of the element and also gives protection to the receiver against static.

Just as a test try removing the balsa blocks and see what you get. I know it will be a bit fragile but it will solve the miystry if you can do it.

Terry

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Same here. Just as puzzled as you are...

I replaced the balsa spacers on the one which I changed the SMA connector on - and still - got the same problem - before I re-soldered it, i sanded the hole thing down, even the remains of the old glue, so technically it is brand new...

These were never in closesness of extreme moisture - unless the often 90% in the air (outside) had effect on it - but even though mostly it was stored in the basement where usually is cooler and less humid than in the rest of the hose.

I will try to remove the spacers today to see if that makes any difference. Mr. Rc-cam, would you be interested in getting one of these two so you can test it? I can ship one to you till I am experimenting on the other...

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The only other possible area there could be a problem is the SMA connector. Has the centre pin moved in the connector ?

I would be happy to check one out for you but I don't know what the cost would be to the UK.

Terry

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would you be interested in getting one of these two so you can test it?

I am buried in projects, so I would not be able to squeeze in another. But the cost to mail it to the UK should not be too bad, so Terry's offer is brilliant.

The only other possible area there could be a problem is the SMA connector.

I too thought about the center pin in the SMA. There is a chance that the mating receptacle on your Rx has been stressed and no longer adequately grabs the pin on some of the connectors. It needs to be better than a basic DC connection (contact surface area is important).

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Terry,

No problem, it is little use to me like this, send me an E-mail or PM with your address and I send it out to you.

I understand the suspicion about the SMA connector - in this case I have to check the other side - the receiver. The funny thing there is, if I put a whip on the receiver I get the image perfectly - no problems...

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The funny thing there is, if I put a whip on the receiver I get the image perfectly - no problems...

The center pin on the mass-produced whip could easily be slightly fatter or longer than the standard SMA. That would allow it to work, and at the same time, stress the Rx's mating receptacle so that other connectors won't.

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Probably that's what happened. I changed the Rx module and now it seems to work fine with the antenna.

Nevertheless, I will send the other one to Terry so he can put it on his test equipment and maybe give me back some reference points.

Thank you both.

Terry, I'll send the other antenna tomorrow, I'll let you know when will it arrive.

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OK, got the patch today so got down to business.

I think this was built from the pdf supplied by Thomas S so I checked it against that.

The sizes of the 2 plates is very close to the drawing so no problem there. There are 2 measurments that bother me though as I have found them critical in the past. The first is the centre pin connection point which should be 19.5mm from the edge of the plate but is only 19.1mm, the other is the air gap. The air gap is different at every point which may be due to postage ? But I checked it at the centre pin connection point and it is 0.27mm too big, this dose not sound much but it is critical for a good match.

I checked the sma connector and it is 100% ok.

On the swr bridge it was reflecting 2/3 of the power, so way way bad. A bit of pressure with a balsa stick to close the air gap a fraction brought this down so confirming the gap is too large.

I then tested it on a receiver pointed at a test transmission and it performed way better than I expected from the swr bridge results. Compared to an 8.5dbi patch and a standard whip it gave an RSSI signal only 20% lower than the patch campared to the whip.

So in short I found no big problems and as it stands would be a big improvment on a standard whip but with a bit of tweeking could be even better.

Hope this answers some questions for you oxxyfx ?

Terry

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Thank you Terry.

This patch actually was built based on The GP atch but whith Thomas Sherrer's modifications. So the gap should be more like 4mm, and the center pin is probably off because of the design modification.

Thank you for the measurements, I have to put my hand one an SWR meter to see how that works...

Any suggestions where should I buy one?

Thanks again.

Ox.

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Your best bet is to find a local radio ham as it's not cheap to buy. If you realy want one then you can search ebay for parts and build your own as I did but the price of any test gear for 2.4Ghz has gone up.

The parts to look out for are:

a tunable oscilator

an isolator

a directoinal coupler

an RF detector diode

a 3db attenuator

a 50 ohm load

various connectors

a frequency counter

All these need to work over the 2.4Ghz band and with the help of a good multimeter you should be able test all you aerials and cables :)

Good luck, Terry

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Hi

I am reading with avid interest as I am interested in building a GP patch. I have no knowledge about antenna design and theory. I read this post and was struck by the strict need for tolerances such as it was noted that the dimension should have been 19.5 mm instead of 19.1 mm.

How are you guys cutting the metal pieces and working with such tight tolerances? I only have a pair of snips to cut the metal sheets and a plastic ruler. I am looking at a 900 Mhz patch so I will have to multiply all of the specs by 2.6667 but the impossible part will be cutting the sheets to the exact size. I can probably only aim for 1 mm accuracy. I guess that will still be much better than the stock antennae.

Does anyone sell the 2 metal sheets cut to size by a cnc machine?

TIA

Vic

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Hi Vic, don't worry, if you read that post you will see that I said that even with the measurments out it still performed very well. With simple tools you should be able to buid within 0.5mm and that will still give good results :)

The design is called the GPP for a reason ;)

Terry

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Thanks Terry,

That is a nice re-confirmation that this project is doable for me. Just have to order the connectors and find some metal sheets. I have been disappointed (as I am sure everyone else on this forum) with the stock dipoles and looking forward to the GP Patch.

Thanks to all the folks who keep forums like this open to the public and willingly share their knowledge. :)

Cheers,

Vic :)

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