FlightAware Discussions

Mag Mount Antenna's Whip Replaced By V-Stub Wire Collinear

It’s a matter of convenience mostly - 50 and 75Ω connectors tend not to be interchangeable. Avoiding adapters is best if practical.
Generic dongles tend to have 75Ω connectors, while brand name dongles tent to have 50Ω. If you are using a purchased 1090MHz antenna, it’ll probably have 50Ω.
Of course if you are building from scratch, use whatever is convenient!

The Wheeler version of COCO found its way into commercial production in the 460-MHz range.

The antenna used a dielectric housing for support of the somewhat floppy coaxial cable and for weather protection. To place the antenna at DC ground relative to its mounting location, the top section used a short between the inner and outer conductors at the ¼-λ mark, although the section continues for the full section length.

Along the base section, also a full electrical half-wavelength, the makers installed three ¼-λ rods connected to the coaxial cable braid 1/4-λ from the bottom of that section. However, section continues below the rods for its full length.

The feedpoint — close to 50 Ω in the antenna described in the article — is at the base of the lowest section.

Source:
Is COCO Your Cup of Tea?
By L.B. Cebik, W4RNL

http://on5au.be/Cebik-2/IsCocoYourCupOfTea.pdf

 

From ARRL Handbook

Lots of conflicting information above (no offence to anyone). I’d love to see top to bottom (bottom to top?) instructions from those who have had success with the design. VF’s are one thing, actual logical construction is another. I’m glad we have a dedicated thread now to unleash the experience from those who have success.

Not only above, but every where on the web, and much more conflicting.

Next to VF, this is the 2nd key hurdle in successful Coco making

Yes, that will be great. As far as I know @theresjam and @vkirienko have built successful cocos. It wil be great if both of them (and also any one else with a successful coco) can post full details of their built in the newly created dedicated thread.

Probably best not to try this if you have bias-T turned on and no DC isolation.

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In order to bring the off-topic discussions back to the original topic, here are some sketches & photos of the V-Stub wire collinear antenna. Back from coaxial collinear to wire collinear :wink:

Vspider-built-3

 

 

That and some illustrations use a balancing resistor with zero mention of blowing things up if you have power coming up the line. Hence the conflicting and rather shoddy information. I’m on the learning side of this one - my attempts all sucked and I knew the VF and am rather OCD with construction otherwise, so I blame the directions to keep my ego intact.

ACK - wrong thread - this isn’t the new one. Sorry.

I have posted the two configurations to highlight the issue of conflicting and incomplete information available on web in heap. This is very confusing for almost any one, particularly for a beginner.

No problem, it is OK.

 

Well, I got VLad up to the roof today and found that he does NOT like heights. At deck level where I was testing him, he did very well tracking about 50 aircraft (good for the time of day). As I raised the platform from deck-level to roof-level he slowly tracked fewer and fewer aircraft until it was only 4 at the roof line. I thought maybe the connector had loosened or maybe even the cable pulled out of the crimp (even though it was well supported) so I started lowering the platform. Sure enough, as I lowered it, more aircraft popped back in until it was back to around 50 again. Hmmm. So I wiggled all the connectors, raised it again and played with adding another filter and adjusting the gain but to no avail. On a hunch, I brought it down and replaced VLad with the Mutant Spider and hauled the platform up again and by the time it got to the top, it picked up 25 more aircraft for a total of 75.

I’m not quite sure what the deal is. I’ll post some pics of the platform and antenna mounts in another thread so you can see that changing out the two antennas involved only replacing the antenna itself. No cables were even reconnected. I suspect it has something to do with the strong RF field from the TV and FM transmitters but I’ll have to test more.

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What does it look like when you sweep it from the bottom of the longer cable as opposed to when you sweep it at the antenna itself?

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Sounds like it is also a good antenna for [random interfering transmitter here]. Wideband SDRs don’t deal with strong out of band interference too well.

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Can you please run RF scan of your V-Lad while it is up on the platform?

Scan method given in the thread Do I Need A Filter?

Yes, ¼𝛌 ground plane antennas are much better in rejecting out of band frequencies than the wire collinear antennas.

I faced similar problem when I purchased FA antenna (also a wire collinear) and replaced my ¼𝛌 ground plane with it and I was shocked to see that the performance substantially dropped instead of increasing. I investigated connectors & cable, but nothing was wrong, till I purchased and added an FA filter, and performance jumped up many folds. :slightly_smiling_face:

I find the exact opposite. Pretty much all the collinears I’ve swept have quite a narrow and steep VSWR curve than most other designs and have a much higher VSWR on out of band frequencies making their usable range much more narrow (and tougher to tune for this very reason, if off by a few MHZ, performance goes to $hit). Higher gain designs, usually carry a narrower frequency. Wider sweep bands usually tell the story a bit better. A correctly tuned collinear can also negate the need for an external filter for this very reason, I just wish I was smart enough to build a decent one…LOL

EDIT/ADD: Also I guess regardless of underlying design, most every antenna will be resonant at several frequencies - hopefully just not at one where you have out of band interference. I thinks that’s what OBJ was getting at, I could be wrong.

Did you compare their RF scans also by the method given in the “Do you need a Filter” thread?

I have no clue what you are asking, not trying to be a smarta$$. The “do you need a filter thread” has to do with XYZ environment and plotting interference/out of band frequencies. I personally dislike plots of that nature and would rather watch my entire spectrum with real-time sweeps marking the peaks, but that’s personal preference… Unfortunately, I haven’t seen anything in that thread that explains that different antenna can/will change one’s need for a filter - to my knowledge it hasn’t even been addressed, but I could have missed it.

I was just making a simple point that I find the complete opposite effect between collinear and 1/4 wave ground plane designs - 1/4 wave (usually) has a much flatter SWR curve and in those cases will NOT reject out of band frequencies as well as a design with narrower working frequencies such as collinears. More or less a blanket statement from my own experiences (All of the coco’s I’ve swept are much narrower band)- nothing to do with environment otherwise. Sometimes tough to convey on a forum, apologies.

I mentioned my own experience that my Flightaware antenna picked interference much severely than my ¼ 𝛌 ground plane antenna picked, in exactly same location and same rf envirnoment. How can that be explained?

~2dBi vs ~5dBi, effective gain aperture differences, angle, radiation pattern, maybe you have a lot of interference in the ~600Mhz range (26inch FA antenna is relatively resonant around there as well) - could be a number of things… I’m not smart enough to answer that, especially since you’re not testing in large amounts of free-space (another thing that really messes with collinears and many designs)…

My findings are based on s11/SWR antenna sweeps. Logically if a unit has a very sharp and narrow s11 and SWR slope and frequency range (picture a sharp V), it has high SWR and s11 on out of target frequencies and will reject them better as a rule of thumb all else being equal (which it never ends up being). Problem is, there is a lot more to it. A low SWR over a wide® frequency range is more susceptible to out of target frequencies and as a rule of thumb, your lower dBi designs like 1/4𝛌 will have flatter curves. There are always exceptions to everything in the RF world. I’m far from an antenna expert, so take with a grain of salt. There are some here that are guru’s and may chime in when they grow tired of my babble.