I think it might be the shielding effect, but could be wrong.
If you want to see what you could ultimately achieve in terms of aircraft count per day, take a look at the best nearby site.
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I use round steel pizza trays. They are strong and wonāt deform. Cost is $2 at Walmart. I drill a hole in the center so that they mount to the bottom of the antenna. Then they go into my attic and are hung from the top by a line.
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Thanks for all the replies.
@astrodeveloper thanks for the advice on the permanent plate. Will see what I can do. Would a card board disc, cut and folded as you suggest, covered with aluminium foil on both sides be the type of thing? That way I can get the shape and support.
Gain is currently 49.6, havenāt touched any settings. As you can see from. The picture was a very rushed job. Currently strong messages are around 1%
Will move it up to 60, see what that does.
Cooling will keep this in mind. Currently in the cold north of the uk Iām at 36Ā°, pi2 is good to 85Ā°
It would need āsummerā to happen here and thatās been in short supply for a number of years. Will keep an eye on it.
Time permitting will try to drop it from the metal plates.
Thanks again.
@tech0 thanks. Will have a look. Would need to remove a section to allow it past the roof support. But could look to fill the gap with a section of foilā¦
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The cardboard disk only needs to be covered on one side, and making contact with the antenna metal base. Essentially, you are making a solid set of radials. Hope it works as well as your 30x15 plate. You have a much cooler attic than mine, living in Florida with lots of sun. Dropping the antenna away from the metal plate should help too, but take your time, stay safe and have fun.
I like @tech0ās idea on the pizza tray also. Great idea!
I have ordered a pizza tray. Might be a while until I can get in the atticā¦
Weather: hottest day of the year has an average temperature 21Ā° and an average low of 12Ā°
Send the weather we can use itā¦
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I found an 8" x 12" plastic flat, cut a 5/8 inch hole (N connector size) near the center, covered it with Aluminum foil on the top and put it at the bottom of a test system FA 66cm antenna & FA Blue receiver. Not fancy, but operational for test purposes.
Range is up a bit, plane counts are up too. Looks like l now have to build a better ground plane and then get into my attic to add it to the antenna up there. Never a dull time in this hobby as we improve our systems.
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Where on the system should these be placed?
In the base if the antenna?
Top or base of the filter?
Would grounding help?
My pizza try required cutting, and would be a mess, so Iām back to the cardboard option like yourself.
Or maybe a other thinner foil try.
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Quite honestly, your current plate is working amazingly well, and changing it to a different plate may not work as well. Definitely move the antenna down at least 3 inches below the metal plate a the top. That will help the antenna receive better. Maybe use zip ties to hold the antenna instead of bolts.
My antenna had a nut and washer at the top of the N connector. Diameter is 5/8 inch, measured with a caliper. I think the cardboard covered by wide foil is a good way to do this. I used a 5/8 inch drill and hand twisted it through the thin plastic, or just cut a hole with whatever knife you have. I left the foil hanging over the edge a bit, and if your cardboard is wider than the foil, use two pieces offset so you have overhang on all sides. Punch a hole through the foil and let the foil drop into the hole, then disconnect your N Connector adapter, with the foil on top, push it up the antenna N connection, thread on your washer and nut to press the foil to the base of the metal mount of the antenna.
Yes, you will need to cut out a space for the antenna mount. I suggest you cut a slot from the outside and make a notch where ever you need to. The idea is to have an electrical connection to the metal base of the antenna from the foil. It does not have to be perfect and the slot will not affect the performance.
Damned near home-brewed. Signal up!
Seriously though, wouldnāt a circular groundplane/counterpoise be more efficient?
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If it werenāt for the thousands who are interested in ADSB signals (aviators or not) there would be no FlightAware!
Not exactly! The FAA requires ADS-B equipment in various air spaces.
FlightAware provides several services to the commercial aviation community and its infrastructure providers.
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So, reading these posts on adding a ground plane ā what does the ground plane do, does it change (ie improve) the antenna pattern? What about antennas mounted on a poll outside? They donāt seem to have a ground plane.
Whether ground plane is needed / beneficial depends on antenna design. Some designs require ground-plane, while some other do not. For example Flightaware antenna does not require ground-plane. Another example is Coaxial Collinear (CoCo) antenna which does not require a ground-plane.
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@ DaveSaunders added an old cooking pan inverted, with holes and his stats improved significantly. He took the step to see what would improve his reception.It worked for him. Kudos!
I made a simple little ground plane using a plastic face shield repurposed to support thin Aluminum foil and my stats for that testing site also improved. Photo somewhere above. I also learned during the tests, the blue FA receiver works better on a Pi4 when connected via USB2 than USB3. Who knew!
As an old ham, having used many kinds of antennas, end fed antennas like the FA 66cm antenna seemed to give better results when there was a ground plane more than a 1/4 wavelength at the base of the antenna. So, Dave tried an idea to put some kind of metal flat area horizontally around the base of a 66cm FA or similar antenna. My guess, it changes the reception profile of the antenna, flattening it out and seems to improve the gain horizontally. This worked well in the lower frequencies I enjoyed as a ham, and seems to work here too. If it seems to be crazy but it works, It isnāt crazy at all. Maybe FA will offer a ground plane upgrade sometime in the future?
I will be adding a ground plane for a 110 cm ADSB antenna in my attic. I will wrap a large flat cardboard rectangle with AL foil, probably using 2 sheets or more, connect it to the metal mounting cylinder of the antenna, wiggle the whole thing through the studs in the attic and just see what happens. It may be nothing, worse, or better.
This is where we experiment to see what happens if we make a change. I will get this done soon, likely tomorrow. Others should do a similar experiment and see if it helps their own systems. This is my kind of fun.
@adsb567, you have design software for antennas as I recall. What would it take to add a ground plane to the CoCo design and run it through a test or two? Should it be flat or sloped down perhaps?
It works very well 
(Antenna in the attic, jetvision cavity filter, uputronics preamp)
More planes and a higher message rate.
And some extra distance (about 40 km)
Thank you for the (simple but very valuable) tip 
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@Ruudjuhhh
Which antenna you have?
@abcd567 the āstandardā flightaware antenna
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Thatās what I thought. So adding a ground plane to any antenna in the hopes of improving performance is kind of like cargo cult programming.
My best guess is that itās not really acting as a ground plane, itās possibly just blocking noise / interference from all the devices below and in the house.
The placement of a metal shield there should pretty well prevent noise from the house below to couple into the antenna.
Or itās possibly even interference traveling along the coax shield being somehow reduced by the capacitance such a construction will have.
I like the shielding idea from above better though, iām not sure if there is actually any capacitance on such a ground plane and if high frequency content can even travel on the coax shield to somehow couple into the antenna.
All these setups described so far had the pi pretty close to the antenna, thatās one of the main points why iām going with the āshieldingā idea.
Once you have even 1m of coax directly down and only have your setup there, the angles make it much harder for interference to couple into the antenna.
Yes, and contrary to the general thinking on here, having a long USB cable from the SDR dongle to the Pi is actually a good thing. I have an SDR dongle near the antenna in the attic with a 5 meter USB cable to a Pi in an upstairs closet. It works great, and Iāve never had a problem with MLAT due to the long USB cable. Course, the USB cable should be good quality.