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

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.

What you said is logically & theoretically right, and I had the same opinion initially.
However my opinion got changed by practical experience. Lot of users of FA antenna complain of performance drop when they replace their DIY antenna with FA antenna. They are almost always advised to add a filter between Antenna and the Dongle, and when they do that, the performance becomes much better than their DIY antenna. I have myself passed through this phase.

Bigger omni-directional ears can hear more noises? :slight_smile:
The FA antenna isn’t that sharp, but it works great for most. Not even sure what to call that design…

ADD: A couple crappy sweeps I took a couple years ago with the N1201SA and a random newer sweep to show a sharp s11/SWR

Flightaware 26inch SWR

Believe it or not, a PCB antenna:

And remember how poorly the old FA 48inch Antenna performed for many?

Talk about the predisposition to pickup a lot of out of band frequencies…But in all fairness, FA sold a limited amount in hopes of having a good 978/1090 8dBi antenna all-in-one - worked fine with really sharp cavity filters inline.

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No doubt FA antenna works great, but it has one drawback that it picks out-of-band RF noise easily. Therefore in RF noisy environments, it performs poor, but returns to its great performance once a filter is added between antenna and Dongle.

The Flightaware Antenna is a Wire Collinear, approximate dimentions:
𝛌/2 - phasing coil - 𝛌/4 - decoupling sleeve

The V-Stub is also a Wire collinear, approximate dimensions:
𝛌/2 - phasing stub (V) - 𝛌/4 - radials

FA Antenna Inside_349x500px

@Nitr0

Couple of years ago I have done SWR vs frequency sweeps of two antennas through simulation software. The sweep was done from 100 MHz to 2500 MHz.

I used models of following two antennas for this sweep:

(1) A ¼ 𝛌 ground plane antenna (Cantenna)
(2) A Wire Collinear antenna similar to Flightaware antenna

The frequency sweep results of a Cantenna (𝛌/4 ground plane antenna) show that it gives low SWR at and near its designed frequency only, and at out-of-band frequencies its SWR is very high. Hence the ¼ 𝛌 ground plane antenna picks much less out-of-band signals.

The frequency sweep results of the Wire Collinear antenna show that its SWR is good at multiple off-band frequencies. This necessitates a tuner circuit or a filter at front end (i.e. between antenna and receiver).

(1) CANTENNA (1/4 Wavelength Ground plane Antenna)
Click on the thumbnails to see full size images

Thumb%20-%20Cantenna%20(QuarterWave)%20Freq%20Sweep%20-1 .

Thumb%20-%20Cantenna%20(QuarterWave)%20Freq%20Sweep%20-2 . Thumb%20-%20Cantenna%20(QuarterWave)%20Freq%20Sweep%20-4

 

(2) WIRE COLLINEAR ANTENNA (𝛌/2 - phasing coil - 𝛌/4 - sleeve)
Click on the thumbnails to see full size images

Thumb%20-%20Wire%20Collinear%20Freq%20Sweep%20-1 .

Thumb%20-%20Wire%20Collinear%20Freq%20Sweep%20-2 . Thumb%20-%20Wire%20Collinear%20Freq%20Sweep%20-4
.
.
.

Using simulation, I have just now finished frequency sweep from 100 Mhz to 2500 MHz for following antennas
(1) ¼ 𝛌 ground plane (Spider) and
(2) Wire Collinear (V-Stub)

The sweep results clearly show that @obj’s conclusion is right.

 

SPIDER

Spider-Geometry

 

Spider-Freq-vs-SWR

 

Spider-Gain-vs-Freq

 

V-Stub

V-Stub-Geometry

 

V-Stub-Freq-vs-SWR

 

V-Stub-Freq-vs-Gain

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To answer a few questions…

When I was testing the v-stub a few weeks ago, I had it mounted to a 1m PVC “hard coax” and connected to the SDR via 10m of RG142 and 1m RG316. When I moved it to the new platform, I simply moved the whole assembly without disconnecting anything. Same mount, same cable, etc.

The platform had 4 antennas on it…

  • The v-stub (now replaced with the 1090 mutant spider)
  • The 1090 FA
  • The 978 Mutant Spider
  • The 978 FA

The 1090 FA performed better at roof-level than it did at deck-level

Wow! That explains a lot.

@gtj0
@Nitr0
@obj

POSSIBLE REMEDY FOR V-STUB:
Add an external Filter between Dongle and V-Stub antenna.

With my V-Stub, I am using a Radarbox24 FlightStick (Green), which has a SAW filter chip between antenna SMA connector and front-end LNA / tuner chip R820T2. This is equivalent to adding an external filter to ProStick/ProStickPlus.