FlightAware Discussions

Disappointing results with my setup. What's wrong?

Well… another round of failures.

Realized that the cable I’m using is in fact a RGC213. AFAIK, all RGC cables are 82% VF, and all RG cables are 67%. So my datasheet is right about it being 82%.

What this means? Nothing, made another 6-elements coco, carefully measured (113mm) and guess what? Half the range of a Slim Jim

DC continuity carefully tested on each segment and after full assembly

From your Coco, remove top two elements, leaving 4 full (4 x 11.3 cm) bottom elements only, and try it.

Same poor result


Looks to me like you have a great build so I don’t understand why you are getting bad results.

Can you build 2 more CoCos with 103mm and 93mm segment lenghts and try them. I was sure the coax I had was VF = 0.85 but doing the above I found that it was actually VF = 0.66.

When you say you check continuity you should get continuity alternating Braid, Core, Braid, Core etc. Is this what you are seeing?


I’m really tired of making CoCos with those cables… maybe I’ll source some RG213 cables to try with a .66 VF (didn’t find it in my city).

In the meanwhile I’m trying some J-Pole/Collinear J-Pole to try something different. The the Slim Jim gave me pretty good results (for it’s size/complexity), going to try some more complex ones now.

flightaware.com/adsb/stats/user/ … tats-33310

Fairly good converage up to 100nm


Since you are the NEC’s guru, would you mind simulating the antenna below on it? If possible with 3 radiating elements as well?

It’s a Collinear J-Pole with a copper VF of .96.

I made the matching coil with 16 turns of AWG 16 copper wire. Inner diammeter is 5mm and height is 53mm.

Spacing between elements is 6mm and the feed point is also 6mm from the bottom (center wire to longer element)

Good Luck with “Wire Collinears”.

After making about one dozen FAILED “Coaxial Collinear” (Coco) antennas, I decided to make “Wire Collinear” (vertical wire sections with interposing stubs and/or coils). I tried over a dozen designs, and all performed much less than a 1/4 wavelength ground plane (Spider/Cantenna).

Here is a photo of some of these Wire Collinears, which I found in my scrap parts drawers. There were many more, but have been digested by trash bin. The top 7 are only whip part, which were soldered to a SO239 connector with radials. The next two are variants of J-Pole, and last 2 at bottom are Zeppelin, the original antenna used in German Zeppelin air ship (1895), and Slim Jim was derived from it.

Zeppelin Antenna Hanging from a Balloon
German Patent Number: 225204
Year: 1909
Inventor: Hans Beggerow
Original Document: German Patent Number 225204
Claim (translated from the original German): Airborne wiring diagram, marked by two dangling wires of unequal length, which in the vicinity of the airship form a Lecher system.

J-Pole and Variants

Make a Spider or Cantenna, and use it as Benchmark/Reference antenna. Comparison of performance with the Benchmark antenna will give you real picture of how your other antennas perform.

Ok, I will do the simulation of your Collinear J-Pole design and post results. By the way I am not NEC guru, just a hobbyist.

Thanks very much.

My current benchmarks are a ground plane (aka Spider) and a Slim Jim. The Slim Jim from this post200914.html has a much better range and was really easy to build and match. Actually I didnt match, just soldered the feed to the calculated point and it worked fine. Thus my adventures with the J-Pole Collinear

Thanks for input. I never built & tried Slim Jim, only J-Pole and Super J-Pole, both performed less than Cantenna. I will now make a Slim Jim and compare it with Spider & Cantenna.

Actually I didnt match, just soldered the feed to the calculated point and it worked fine.

Can you please give the link to the J-Pole calculator you used?


m0ukd.com/calculators/slim-jim- … alculator/

m0ukd.com/calculators/slim-jim- … n-version/

I had a lot of success with a 6 element Franklin antenna


I eventually built a CoCo that was better but it took me a long time to get right 8)

Yes, I agree with triggers about Franklin. It was my second antenna after 1/4 dipole. It had 4 vertical elements, 2 hairpins (stubs) for phasing. Its photo is here:


Shortly afterwards, I added a stub in the middle for impedance matching, and cable tap-off, similar to j-pole stub. It performed good and I used it for a long time, before switching to Cantenna. Later I added one more Pi with FA 26" antenna. The Franklin antenna’s diagram is here:


My experience is that Franklin is the least tricky of all wire collinears, and is relatively easy to get right. Worst are those with coils: easy to make, hard to get right without antenna tuning instruments & test equipment. Coco is also hard to get right without necessary test equipment.

Some interesting findings from the latest tests.

Made a Collinear J-Pole (3 elements), tested on bench and it performed fairly better than a Slim Jim (my reference now). Both handheld at my window, while the Slim Jim tracked 5 aircrafts, the Co J-Pole tracked 12. Nice. Time to put it on the roof. Result: 2 to 3 aircrafts tracked (the Slim Jim on the same place/cable was tracking 17~20).

The difference between the setups are

  1. Bench: RTL Dongle + RG6 75Ohms + 20dB Sat Amplifier
  2. Roof: Pro Stick + RGC58

Replaced the dongle + cable on the roof by my testing ones and the result was great, good range, great number of messages, no shadow areas. By miles the best antenna I built ever.

Looks like the Pro Stick is very strict on the impedance and matching a homemade antenna is very difficult. Now I’ll leave the J-Pole running for a couple of days and then will bring my RGC213 back to life for comparison.

\o/ finally some progress!

CONGRATULATIONS for the progress!

Now in responce to your request, here is NEC Simulation of your Collinear J-Pole


Thanks for the simulation.

Could you please try different feed points to try to improve the SWR?

Looks like I’ll have to learn this software :smiley:

Ok, I will run the simulation again to find best feed point, but dont depend too much on software simulations. The results have errors around 5% to 15%, enough to knock down a collinear. These can at best be taken as crude guide. The trial & error method to adjust feed point gives better results. Try 6mm +/- in steps of 2 mm.

OK, here is sweep of feed (tap point) from 2 mm to 22 mm in steps of 2 mm.
Minimum SWR is at 6 mm tap from bottom, and its value is 4.65 :frowning:
But dont get disappointed, the simulation results cannot be blindly relied upon. Try to adjust feed point manually in steps of 2 or 3 mm.

Would you mind sharing the .NEC? I’m going to spend some more time on this antenna.

The .nec file is on my Desktop, and I am not at home now, replying from my phone. When at home, I will upload it to dropbox and post a link.

I have uploaded following .nec file to dropbox
"J-Pole Collinear halfwave132mm coil turns16 dia5mm length53mm.nec"

You can download it from here:
dropbox.com/s/qtkinnn6twaea … m.nec?dl=0