Let me start with the story so maybe this all makes more sense.
I am fairly new to plane tracking – only started when all the noise about ADSBExchange happened a couple of weeks ago. I bought a $15 indoor antenna from Amazon and an RTL-SDR which I put on a Raspberry Pi4b I already had, using the scripts from @wiedehopf to set things up. Within a day I was hooked and wanted better reception. I placed the antenna outside (I have a metal roof, the antenna has a magnet foot - perfect) and reception got better. But - still only 40-50nm max. So I ordered the FlightAware antenna and some high quality cable.
I mounted the whole thing at the top of my roof with great visibility - heywhatsthis says I should have about 170miles to the west and 60 miles to the east @ 30’000 ft. Instead I get maybe 15 miles and only see maybe 1/4 of the planes that I can see on FlightAware should be even in that small radius.
I played with different gain settings, checked that all the connectors are tight, rebooted, all the usual stuff.
Finally I connected that initial indoor antenna again - also outside, but one floor down from the top of the roof (odd multi-level house) - actually one floor of my own house is blocking the view from that antenna to the west. And with that I get easily 3x the range that I get with the recommended antenna and the recommended cable with the unblocked view.
So - how do I figure out what’s wrong with my setup?
Do you have a filter? It’s not uncommon for someone to get a better antenna and immediately overload their SDR with the improved reception. Especially if there are other “noise” sources in the area - cell phone towers, for example. There have also been reports of issues when the pi, SDR, cable, or antenna is too close to a poorly shielded HDMI cable.
Here is the one I have: FlightAware Band Pass Signal Filter, Dual 978-1090 MHz | ADS-B Flight Tracking
I get almost 300 miles to the south, about 250 to the north, 100 miles west and about 50-60 east (mountains on that side). My setup is an FA antenna on the roof (30 feet above ground) → 50 feet of LMR-400 → the light blue FA filter → a cheap Chinese “low noise” amplifier → the FA SDR → and a Pi 3B+.
If I take out the amp and the filter, I max out at 50-60 miles in every direction. If I swap the amp and filter I can get to maybe 100 miles. I might replace the amp with something more reputable someday, but if I’m honest, there are about 25 other feeders within 10 miles of mine, and my station is routinely first or second among them. I’m not sure how much improvement there is to be had - at least without adding some significant altitude to my antenna - It just might not be worth the cost and effort for me.
I do not have a filter at this point. I did turn down the gain in the software and can see reception dropping below about 18, so I figured it wasn’t interference (no cell towers near by, the only other cables on the roof are the solar system)…
Definitely something to try. Thanks
So I replaced my antenna cable with a different one - on the assumption that that might be damaged.
No improvement.
So the remaining two options appear to be either that the antenna itself is damaged, or that something about being situated above the metal roof completely messes with reception…
(if I attach the tiny little indoor antenna to the SDR I get WAY WAY better reception)
Maybe the coax isn’t actually making contact with the antenna. Does the coax have an N-type connector, or do you have an adapter to go from N to what ever you have on the coax.
Yep, coax is SMA-male to N male.
I now have made some progress in understanding the issue… well, no, to narrowing down the root cause.
As mentioned I have a standing seam metal roof and mounted the antenna on an aluminum plate attached to the two adjacent seams with clamps - and mounted on that plate is a J-shaped satellite dish mount to which in return the antenna is attached. So all of this is metal. And with that I get terrible reception. If I sit right next to this contraption and hold the antenna in my hand, I get great reception.
So it’s something about the way I mount this to the roof that is causing my issues 
Can you confirm the the end of N-SMA coax is actually “SMA” vs. “SMA-RP”.
Many others have been caught by this subtle, but critical difference.
Yes… I found those posts and checked… The cable is the right one. It’s the way the antenna is mounted…
I was going to suggest you check for an RP-SMA, but @dongerrard204 got there first - plenty have been caught by those.
Is your roof painted or exposed gal-steel?
I’m thinking you’ve set up a cell that is putting a DC voltage down the coax.
Can you mount your antenna exactly as you did before, but insulate the antenna from the mount?
An Al-Zn cell could be putting 0.9V on the feed.
That sounds… sub-optimal
How well would I need to insulate this? I tried with just some hard foam around the metal J-mast (to insulate the mounting brackets), but that definitely was insufficient…
I’m trying to figure out a reliable way to decouple this - because mechanically this is the mounting option that I want…
Depends what (if any) scrap you have on hand (it’s just a test at this stage).
Maybe cut up a PET drink bottle (brand of your choosing!) or similar.
A short length of 25mm PVC pipe would be good for a longer term solution - cut a length say 100mm, then slit along the length to allow it to expand over the antenna base.
I was thinking of picking this up tomorrow: https://www.homedepot.com/p/Everbilt-1-3-8-in-O-D-x-1-in-I-D-x-24-in-Clear-PVC-Braided-Vinyl-Tube-HKP002-PVC006
Should fit perfectly over the J-pole that I am mounting the FlightAware antenna to… and should insulate it very well from the mast and roof…
This is the sort of thing I use for antenna building
At this stage, just try something from your rubbish bin to see if there is any improvement - you can sex it up later if it works.
I think that is very likely to be an incorrect assumption.
You may not have a cell site nearby but high power broadcast and PMR sites can also cause intermod problems.
Old radio guy here. You mentioned holding the antenna gave good reception. Attaching it onto the roof satellite mount trashed the signal. That combination kinda rules out interference.
geckoVN’s intuitive suggestion above about the galvanic voltage from dissimilar metals may hold the key.
I like your idea of the clear plastic sleeve. Gotta admit, the trip to the store and only one roof adventure instead of several would be my choice. At 73, roof time adventures hold little appeal. Have fun, and figuring out what is going on gives us satisfaction with this hobby time.
Filter should arrive today 
I mean, what’s the point of asking if I don’t listen to the suggestions 
I’m a bit younger than you are, and the roof is easily accessed and very shallow (~12°). So it’s not too bad…
So it turns out that @geckoVN had the best suggestion. The true issue was having the metal J-pole co-linear with the antenna. 
Once I replaced that with simple PVC pipe, my reception (as in messages/sec) went up about 10x.
I still don’t get anywhere near the range that heywhatsthat would make me hope for - but the range has also doubled (and I’m still waiting for the filter to be delivered).
Definitely a huge step forward, though.
Thanks, everyone, for the extremely useful input!
Yes, good old PVC pipe. It’s been nearly 5x more expensive in 2022 than it was in 2019, but it was down about 20% from the peak last autumn at Home Depot.
Some lessons I learned the hard way about outdoor antenna supports with PVC pipe (with larger antennas at much longer wavelengths than 1090 mhz):
Schedule 80, while more expensive, is worth every extra penny in a windy climate.
Make sure the inside diameter will pass the coaxial cable connector after assembly on the cable. Check this on the ground, not on a roof or tower. Yes, I made that mistake once. Google for PVC schedule 80 pipe inside diameter vs trade size, and measure your coax and connector to avoid surprises.
All PVC will eventually be damaged by UV in both direct sunlight and also from Rayleigh scattering in deeply shaded locations. It is wise to paint it if you plan to leave it up for years, even in temperate or cold climates.
In sunny hot desert climates, summer heat (often 45C, 113F or higher air temperature in the shade) plus direct solar heating can soften the pipe enough it bends and droops, especially if extreme heat is accompanied by wind.
If desert heat and sun is a problem, use the most reflective white paint you can find that is formulated specifically to stick to plastic, to help keep the pipe from going above ambient air temperature. In extremely hot climates that can reach 50C (122F), or where you can get extreme winds on a regular basis at any temperature, about 3 feet should be the unsupported height limit.
Be careful of white paint - the common pigment is Titanium Dioxide which will almost definitely attenuate RF signals.