Circularly polarised antenna

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Circularly polarised antenna

Postby Mark Hitchman » Fri Mar 23, 2012 12:54 am

Hi Rf gurus, I am about to make a VTX antenna for my Tricopter and was wondering if stainless steel is ok to use instead of copper coated steel. I have s/s but I would have to go to an engineering firm to get some mig wire. I will silver solder/easy flow the joins apart from the coax to the s/s join which I will solder.
Good or bad?, welcome any comments
Thought I would take your advice X-jet on my tri video but as you will have noticed I was flying next to a steel shed which is ideal for multipath interference. I have used a 6 turn helical for a long time but never went circular on the VTX because of the size on 1010mhz but I think it is time to play around with them now. I used 1/4" copper tube for the helical :)


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Re: Circularly polarised antenna

Postby RCModelReviews » Fri Mar 23, 2012 7:30 am

I wouldn't use stainless steel for a couple of reasons...

1. it's a poor conductor when compared to copper.
2. it's probably not going to be possible to make it with sufficient precision if you're planning to braze or weld the wire

You don't say what frequency your video gear is on -- you might get way with it on 900MHz or 1.2/1.3 GHz but they are not my favorite frequencies anyway.

I make my 5.8GHz ones from 1mm copper wire and they work great -- plus, they're easy to make using copper.

I did make some with 0.8mm MIG wire but once the copper plating corroded their performance was significantly reduced. That's probably because at GHz frequencies, the current actually flows on the skin of the wire, not through the core of it. If the skin becomes corroded then its resistance goes up and the antenna efficiency can drop markedly.
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Re: Circularly polarised antenna

Postby Mark Hitchman » Fri Mar 23, 2012 8:30 am

I fly 1010mhz as stated in my post :)
Thats why copper is no good to me, a little bit soft at the size of the antenna. I see people use piano wire and I thought stainless steel would be better but I may just use mig wire and spray it with varnish of some sort, 1010mhz is a very forgiving frequency at least. Thanks Bruce.

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Re: Circularly polarised antenna

Postby RCModelReviews » Sat Mar 24, 2012 8:14 pm

Yes, the problem with circularly polarized antennas is that they get pretty big and unwieldy at the lower frequencies.

Are you using 1010MHz for a reason? Planning some very long-range stuff perhaps?

I'll be doing a series on "antennas with gain" soon and I'll see just how far you can go with 5.8GHz if you use the right antennas. That's one of the reasons I built the 4-channel diversity controller.

I've heard reports of some folks getting 8 miles or more -- which seems pretty good to me.
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Re: Circularly polarised antenna

Postby rogueqd » Sat Mar 24, 2012 11:43 pm

I looked up a few conductivity charts before I made mine (5.8Ghz). Silver or copper are great, aluminum, brass and bronze are also ok. Steel is bad and stainless steel is the worst.
I've flown to 1km on roughly made 1mm copper antennas. My second set I made with 0.5mm brass and they work much better. The second set was made much more precisely than the first with a much smaller feed gap. I'm planning a third set with silver plated copper jewelery wire.
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Re: Circularly polarised antenna

Postby RCModelReviews » Sun Mar 25, 2012 5:45 am

How about gold? Talk about blinging-out your FPV model :-)
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Re: Circularly polarised antenna

Postby Mark Hitchman » Sun Mar 25, 2012 6:13 am

Looks like stainless steel is out then, my linear antennas are copper but those circular ones look a bit flimsy for copper at 1010mhz.
Bruce, if you show me a video of 5.8ghz 10km out and 12 meters off the deck then i'm in :) Thats why the 900mhz band is so good, none of this flying at 1km high to go long distance.

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Re: Circularly polarised antenna

Postby RCModelReviews » Mon Mar 26, 2012 6:51 am

I've got some interesting plans with FPV. For instance -- you can fly 10Km out just a few meters above the deck -- if you set a model to act as a relay by orbiting at the 5Km mark. It would receive the signal from the model then retransmit it on a different channel to the pilot.

With gear like the FY31AP, you just program it to fly to the half-way point and orbit for half an hour or so then launch your FPV model and away you go.
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Re: Circularly polarised antenna

Postby Mark Hitchman » Mon Mar 26, 2012 7:41 am

Interesting concept but flying 5kms with omis, whether circular polarised or not would be a mission on it's own. I take it the relay plane couldn't have directionals on it. I am sure you have a way that I have not thought of and I am keen to hear about it.

https://vimeo.com/22184151
This is my 10km flight where I got down to 12 meters above launch altitude which is 17 meters above the ground according to google earth. I know you have seen it before on parkflyers because everyone gave me a hard time about exceeding the 400 foot altitude limit,I hope it's less PC here:), so I don't post my long flights anymore, some on here may not have seen it before though.

The thread on the Lab
http://fpvlab.com/forums/showthread.php?506-10-kilometers-at-17-meters-high
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Re: Circularly polarised antenna

Postby RCModelReviews » Mon Mar 26, 2012 7:56 am

Don't worry Mark -- this is an international forum and very relaxed -- you can post almost anything you want here :-)

Are you going to come to our FPV fly-in in Tokoroa later this year? (don't know when yet)
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