FlightAware Discussions

Antenna questions

I have a lousy antenna in a good place and now it is time to replace it with a better antenna in that same good place. Which, due to my total lack of knowledge in this area, raises questions that I hope some of you might be able to answer.

I plan to start with a spider. Does the thickness of the wire matter? The internet says yes, but then all the information I found was about transmitting, not about receiving. And no word about frequencies. Would it make any noticeable difference whether I use 1mm or 2mm diametre copper wire? My utterly unscientific gut feeling is that thicker is better, but thicker is also a lot harder to work with.

Naked copper oxidates superficially. With time it gets a greenish cover of “rust”, which blocks oxygen and prevents further inwards oxidation. Does a rusty copper antenna have significantly worse reception than a polished one, or is the difference negligible? If rust matters, would a thin layer of epoxy glue over the wires be better or worse than rust?

The radials of a spider for 1090 MHz are supposed to be ~69 mm long, but is that with or without the horizontal part that gets soldered to the base?

All answers appreciated, even if they raise more new questions than they answer. In fact, especially if they do.

Thicker wire gives you a broader band antenna - as this is not helpful, any wire you’ve got will do fine.
As for corrosion, tin plated copper (or silver plated copper if you can get it) will last longer, but if you have it inside, it’s not a big problem.

But broader from where? I mean, to have a “broader band” there must be a “centre frequency” of reception from which “broader” expands. Do you mean that the length of the wire determines the frequency and its thickness determines the width from that frequency?

Sorry if I sound a bit clueless; it is because I am totally clueless.

Ignoring other aspects of antenna design, yes, that’s how it behaves.

Not clueless - you know the right question to ask!

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Just a quick comment from a physics perspective: Antenna characteristics are reciprocal. What’s good/bad for transmission is good/bad for reception. (Except in reception, you don’t care about sustaining power.) A highly directional transmission antenna will make a highly directional reception antenna, etc.

I used a cheap coax TV cable to get the copper wire pieces for my spider - even left the white center conductor insulation on.

I use a RF impedance analyzer to fine-tune the spider’s antenna length as well as to fine-tune the angle of the spider’s ground plane radials (length not critical). The antenna length is critical for matching the 1090MHz freq, and the angle of the ground radials is critical for getting the VSWR value as close to 1.0 as possible (usually end up around 1.1). I’ll start with a 45-degree angle and then make slight adjustments with each ground radial until the VSWR value is close to 1.1 - a slight bend up or down can be the difference between 1.3 and 1.1 .

I’ve made 6 DIY spiders (3-1090, 3-978) using this tuning approach

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0.8mm or 14 AWG is a common wire thickness. Using galvanized steel wire sold commonly as fence wire, bailing wire, etc. is a good alternative to copper because it’s stiffer at the same thickness. It holds its shape better.

The oxidation is fine because it’s so thin. Any amount of externally applied sealer, insulation, etc. will change the velocity of propagation of the wire and will change the length calculation. That’s not bad but you do have to figure that in when cutting the wire.

The critical distance is from the center element to the tip of the radial. The exact distance is much less critical than the length of the radiating element.

Thank you to everyone who contributed advice and knowledge. A couple of hours ago an 8-legged spider replaced my previous stump of cable. Range went up by 100nm.

There’s one more lesson I got out of this: if you have to solder on thick metal such as a panel mount N-connector, do not buy the cheapest soldering iron you can find, even if you only need a soldering iron for this one-off job. It’s not enough to heat the metal properly and fast enough, so you end up struggling and swearing and with a rather mediocre result.

QUICK SPIDER - No Soldering, No Connector

 

 

found this simple and easy to make antenna from another forum.
The Range mentioned below by the antenna maker is remarkable for such a simple antenna, made with cutting edge technology :wink:

“Mine is made with 33cm unshielded TV coax and as a ground plane the bottom of a thunafish tin. More than 200nm.”

I had it bookmarked weeks ago and actually I tried it first. There was no way I could insert one radial between the outer insulation and the inner, let alone eight. Far too tight; I completely mangled the radials and I also hurt myself. I guess we had different cables, your outer insulation much more flexible than mine.

Anyway, I’m happy I managed to put together a functioning antenna and a working 18-metre cable with three connectors along the way without messing it up. My actual range is now pretty close to my theoretical (inner ring is 7.500 ft, outer ring 40.000 ft), so the marginal cost-vs-gain of further improving the antenna is getting fairly high:

It might still be worth it making better antennas just for the fun of it, but not really for improved range. I’ve got a connector at the top of the cable, so switching antennas is quick and easy and will not jeopardise the performance of the rest of the rig.

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Warm the coax so that its outer jacket softens.

Aaah, that was the missing step 4½. I still have some left-over coax bits and failed radials on my desk that I meant to throw away, so I can try and see if it works. And if you can still edit your original posting, it would make sense to add this instruction.