FlightAware Discussions

Three Easy DIY Antennas for Beginners

Which post are you referring to?

The length should be measured to a central point, where the radials and whip meet at the coax.
That is the length that is relevant.

These directions for “Your First ADS-B Antenna”. Author mentions ADS-B wavelengths are 10.3 inches, and then has you cut copper wire to 1/4 of that length and solder them to an SO 239 adapter. I think his calculations may have been incorrect, but it could be he was compensating for attachment to the adapter?

Just for a reality check here, in the real world the speed of light (in a vacuum) is about 300 thousand km/s (yes, I know your number is more accurate). Now dropping a load of zero’s, we have:
300/1090 = 0.2752 m (not m/s)

Now you can decide how much effort you’re going to spend on that 0.2mm and I’d suggest you don’t bother.
Why? because you are not working in a vacuum and you aren’t using perfect materials.
The rule of thumb is that your 1/4λ antenna needs 5% shaved off to compensate for inconvenient differences between theory and practice.

The other point to consider is that a 1/4λ driven element over some bent pieces of wire, is not a high performance antenna.
Build one for sure, but don’t get hung up on irrelevant precision. Put the effort into an antenna that will give you a better reward.

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Thanks for the perspective. Just a newbie here, but wanting to be as accurate as possible. Really just hoping that my homebuilt antenna will give me better performance than the whip that came with the SDR.

Yeah that pdf is not quite factual. But i don’t think those numbers are in this thread. (maybe it is linked somewhere but probably was linked by someone running the numbers themselves in mm so they didn’t notice the inch discrepancy)

Anyway your calculations are correct and using that length as the distance from the tip of the radial/whip to the point where the coax begins is going to work fine.

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@mwshell

No need to be so accurate.The 1/4 wavelength with ground-plane is dimensionally very tolerant.

An error of upto +/- 5% (about 3mm) wont cause any appreciable/noticeable difference in performance.

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Sure, I get the point and I assure you I’m not trying to you off - quite the opposite. The first one doen’t need to be perfect. You’ll learn a lot altering your technique on your second and third!
The fact that the standard mag base works at all is good indicator for how bad an antenna can be wile still performing reasonably.

The difficult thing thing is determining if one antenna is better than another and for that you need two receivers so you can run them side by side watching range differences, signal strengths.

I appreciate the encouragement. I actually do have two Piaware units, but they’re separated by about 330 miles. I also have enough materials and was planning to make two antennas, but was going to give the second to a friend if the build was reasonably successful. I’ve found this is a good tinker hobby, and even if the antenna doesn’t work, each one was about $4 to construct, so it’s not a terrible waste.

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These two are about $1 to construct :slight_smile:

Cantenna

Quick Spider

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UPDATE: I was able to assemble an antenna today. Assembly was easy, but when it came time time to connect to PL 259 to the SO 239, the plug would not fit into the antenna. Initially, I thought I had ordered the wrong item, but a little internet research showed the plugs were compatible. So I broke out the sand paper and sanded the end of the PL 259 and got it to fit. It’s snug, but it fits. Once I connected to my Piaware, I immediately saw more aircraft come up on the map display and greater range as well. This made me smile. :grinning: Successful assembly and do recommend! Now to go get some PVC to make a stand…

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Yesterday i built the quick spider just for testing purposes if it performs better then my improved whip antenna.

It’s not a beauty, looks like made by a three year old kid. But it works pretty well.
The average gain is 10-15% in number of messages, seen aircraft, range.

Based on @abcd567 suggestion in a different forum, i will rebuild it with eight arms instead of only four i’ve used for testing.

As i am not able to mount it outside, i am still impressed about the range/coverage indoor. (up to 170NM)

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The 1/4 spider is my reference antenna, to which I compare my other antennas. Have built a few collinears too, but what works the same as my commercial active antenna, is a 3-element 5/8 collinear antenna. Just keep in mind that if you need the antenna outside, you will need to put a cover, like a PVC sleeve over it, which will detune it a lot!!! For indoor usage, the calculated lenghts are fine, but once you put it in a sleeve, things are totally different, and you have to rebuild the antenna bit-by-bit, tune one length with sleeve on, etc. that is where a cheap network analyzer, like a NanoVNA is worth gold.

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Any dimensional details / drawings / photos of this winner will be appreciated. Thanks.

Will do, drawings and measurements are in my shack outside, but it is based on an antenna thread on this site (or maybe flightradar’s) from a few years ago,

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This is one area where I believe I have erred and pitched out a few antennas that might have been fine had I fully covered them.

Does the PVC effectively shorten then antenna element or lengthen it ?

 

 

Is it this one?
https://radarspotting.com/forum/index.php?topic=3943.0

 

That is the one. Here are the dimensions I used and a photo of it on the mast. As you can see, I used 8 radials instead of a solid ground plane.

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PVC sleeving lengthens the antenna electrically, so resonance point shifts lower

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what kind of program to measure?

thanks a lot for this info ! :+1:
now I understand why my colinears performed better indoors, as I always blamed the length of
the cable for poor performance when I put the antenna outdoors.