The first element is inverted, ie, the center of the cable is connected to the shield of the bnc, and the grass to center pin, is this correct or the first element shouldn’t be inverted
I use F connectors so the centre core of my downlead is connected to the braid (grass) of the first element. At the dongle end, the core is connected to the centre pin of the SMA connector.
What does the inner insulation look like? If it is translucent (translúcido) then it is probably foamed polyethylene (VF about 0.85). If it is opaque (opaco) then it is probably polyethylene or more rarely PTFE (VF=0.66)
Make up two 4 element CoCos one with element length 91mm and the other 117mm. Put them on test for a couple of days and see which gives the best results.
According to this site timesmicrowave.com/calculator/ every cable I heard about being used to make CoCos (RG 58/59/6/213) are 66% VF. Why there is so many people assuming they are 82~85%?
If it’s correct, my 113mm is tuned in 875Mhz, which would explain its poor performance.
Testing in SDR# my noise floor in 875Mhz is -25dB, where in 1090Mhz it’s -50dB. Not sure if there is extra noise in 875 or if the antenna is really working on that frequency.
@aldoir
Coaxial collinear (Coco) is easy to make, but hard to get right. Most DIY coco makers (including me) have passed through the same frustrating experience you are having now.
Try to make another one using a coaxial cable for which manufacturer’s specifications are available, and use the Velocity Factor from those specifications. Note that each make and model has different VF depending on the insulation material used by the manufacturer.
The one main reason for failure of DIY Coco is using a “general VF” (for example using 0.82 for RG6 FPE Insulated, 0.66 for RG213 PE insulated). Be specific and use the VF given by that particular manufacturer for that particular model of coax you use. Do NOT use a coax of make/model for which manufacturer’s specifications are not available.
Definitely I need to source some quality cables. Tried to shorten the antenna to 91mm and the same results (< 20nm).
Then decided to give the old Slim Jim a try. 160nm easily, using the same setup, place, cable connectors. Just took the CoCo of and put the Slim Jim on the very same connector.
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
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.
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.
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.
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?
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.
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
Bench: RTL Dongle + RG6 75Ohms + 20dB Sat Amplifier
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.
