FlightAware Discussions

Antenna on mast performs worse?

I’m pretty sure this is the answer.
I tried a lot of changes yesterday between the rain, and couldn’t find anything definitive.

I ran the Spektrum scan with both mounts and there was not much difference. Neither had any stand-out interference that only showed up on one of the locations but not the other. I plan to re-test next weekend when the weather is better and can try more changes.

I changed to separate 978MHz and 1090MHz LNAs (previously used one that boosted both).
I moved those LNAs to the roof.
I changed the antenna feed to 6’ of LMR400 into a simple splitter to the 2 amplifiers, then 25’ of LMR240 to the SDRs.

After that change, the only difference between the mast and no mast was the dB of the received signal. On the AC unit itself gave me a slightly hotter signal, while on the mast seemed to be down maybe couple of dB (looking at the graphs and too-strong signal %).
Aluminum tape coated PVC mast didn’t change anything, for the record.

So with the LNAs moved, and better cable, the difference seems to be rather diminished.

I’ll keep playing around with things though, and plan to replace the 1090MHz RTL-SDR blog V3 with an air-spy mini. I plan to get separate 1090MHz & 978MHz antennas eventually.

On a possibly related note (but probably not), the RTL-SDR Blog V3 dongles I have (both of them) seem to be completely broken with a gain of 48.0. If I set it to 49.6 it works fine (obvious way too much gain though). If I set it to 44.5 or 43.9 or anything else it also works (showing planes and messages). However it seems that both dongles, if set to a gain of 48.0, don’t show any messages at all. Very odd.

I raised my antenna by about half a meter last weekend and my range is slightly less rather than slightly more as I was expecting as there are several nearby 2 and 3 floor houses not to mention my own roof north of the antenna, all of which use galvanized iron sheeting as roofing material. Raising the antenna slightly didn’t affect signal level or messages received though. Mine was right next to my aluminum gutter while now it’s slightly above it. It makes me wonder if close proximity to metal near the base of our antennas is actually beneficial somehow. Maybe the gutter was acting as a reflector, and the aircon in your case? Weird.

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By mounting on the grounded air conditioning unit, you made a good ground plane connection. Vertical antennas like that. When you attached a non conducting mast to the antenna, you removed the ground plane effect. Either use a metal pole or run some wire at the base of the antenna (silver part) to the air conditioning unit. That should help.
Regards, Don WA1DF

In theory, these antennas should be ground independent.
If they do behave as you suggest, there are a lot of stations that could be greatly improved.

Perhaps someone with an easily accessible antenna might like to run some tests?

The OP has a ADSB Exchange colinear antenna which isn’t the same as a vertical monopole. JamesMun posted a picture of its innards upthread on Oct. 4. As geckoVN said, if what you say is true about them benefiting from a ground plane that would be very good to know.

Does your coverage graph show a change when the mounting position/location changed? I would think if the air handler is acting as a ground plane the antenna reception would be more biased toward the ground plane…

Seems like all the discussions keep coming back to ground planes. I was thinking about a classic dipole antenna. The pattern looks like to “almost circles” around the line that the dipole forms (google “radiation Pattern of a Half Wave Dipole”, or Wikipedia Dipole Antenna). This is why you point the broad side of the antenna toward the area where you want to listen.
Now imagine that your vertical antenna is 1/2 of the dipole, and the feed line coming off the bottom is the other half. It would seem like the patter would be good for a long ways away because the max signal is at the horizon 90° off of the vertical antenna. But in that same pattern, half the pattern is below the horizon. The pattern is terrible directly above the antenna, but, the aircraft is so close to the site it all works out.
So thinking out loud, this poses these question for me:

  1. What happens to the pattern when the antenna is on a non metallic mast making the feed line the ground plane? or when the coax comes off at an angle. Does this affect the pattern?
  2. The pattern of a 1/4 wavelength antenna changes depending on its distance to the ground plane. At 1090 MHz a wavelength is pretty small, around 8.5"/.2144 Meters. So small changes in height relative to a large ground plane should affect the pattern ?
  3. Especially with the gain antennas, is the coax a good ground/counterpoise or would they work better with 1/4 wave radials that attache at the top of the metal of the antenna? All the high gain amateur radio antennas I’ve played with have ground radials. Has anyone played with radials on the gain antennas? Wikipedia “Monopole Antenna” has a great graph showing the change in the pattern, and a good picture of a ground plane antenna with radials.

Some radiation pattern molding software could help understand this. Might be time to break out my ARRL Antenna Handbook from the early 1970’s… I have to figure these sorts of things out for some work I’m doing at 915 MHz, and hope to get access to a test chamber. Great thread here, it has me thinking of things to try…

The Coaxial Dipole

Die Koaxialantenne

Modelling is very much a case of GIGO (garbage in, garbage out).
If you can’t accurately describe the input parameters, the model will predict ‘what you asked for’ not ‘what you want’.

I’m not suggesting modelling doesn’t have value, only that you need to be skilled to use it, and experienced to recognised if the result is plausible.


I very much “get” GIGO ! I often prefer to build a small circuit and don’t use various SPICE-like software because the details (and parasitics) matter. I built a 900 MHz Return Loss Bridge (RTB) on a small PCB, and while it worked, the dynamic range was terrible- about 10 dB. Too much leakage at those frequencies. I suspected I’d see this since most RTB designs are more of an adventure in milling aluminum that electronics, But it let me see that, and play with shielding a bit. I ate a big slice of humble pie and learned a bit about high frequency parasitics in the 2D circuit board world.
I ended up buying the RTL made for my spectrum analyzer. Perfect fit to the device. Holding antennas and playing with clip leads for grounds got me thinking more about ground planes. What a great learning path!
What I was thinking here is modeling a theoretical quarter wave at various heights above a perfect theoretical ground plane. This might provide a good “feel” for that happened to the OP when his antenna went from having a big piece of metal at the base of the antenna to being up on a non conductive pole.
I don’t know if it’s possible, but model a ideal vertical dipole, then change the ground out for radials and see if the angle of the radials matter.
Finally, all predictive tools (SPICE, models, spreadsheets, Ghant charts, 3-D CAD for stress analysis) are prone to manipulation to get the result you want. Part of the skill of using models is to remain humble.

Great image! What I’m pondering is what happens with this (or some form of this) when you then install the antenna such that there is a huge ground plane at the base, which in this case would be the bottom of the copper pipe ? That’s the antenna on the air conditioning unit scenario.
One of the fun things about playing with HF frequencies, and to some extent up to the 146 MHz ham bands is seeing the interaction of an antenna with ground, be that the earth or the roof of a vehicle. How does this translate into what happens around a GHz ?
For the image you’ve provided, what impact does the feed line have on the antenna ?
While ducting happens at 1 GHz, it’s rare. We’re trying to receive signals from aircraft that are 5-6 miles high but 100 miles away. This is a very low angle. Fortunately, in many regards it’s easier to build gain antennas at these high frequencies and get the patterns we want. But it seems unlikely that the proximity and shape of nearby grounds stops coming into play.