Please scroll down to see test result graphs.
Ground Plane Sizes Tested:
25 mm dia mag mount only
73mm dia can
100 mm dia can
165mm dia can
Antenna Used:
1/4 wavelength mag mount whip
Image 1 of 5: Test Setup
Images 2 & 3 - First Round of Trial Run
Images 4 & 5 - Second Round of Trial Run
.
I found the best results using a 9x13 baking pan upside down of all things (don’t tell the wife). Am actually in the middle of plotting results/tests from two identical setups using different antenna (stock, chopped 68mm, 52mm, coiled, ebay 5/8, 2 different ebay coco’s) all against the FA antenna outdoors. Results are surprising so far and gathering the last few days of data with as many variables removed as possible and will post in a separate thread when complete. More to follow, but point being, see if you can raid the kitchen to find yourself a baking pan and slap that on for a test with no other changes. Ideally need to test on separate rigs since air traffic is never constant.
Just to suggest more work, set up 4 identically, run & tweak till you get consistent results. Then start testing different configs, leaving one as a control. That should reduce the differences from traffic variability. (Easy to say, I’m not the one doing the work 
)
A question about the ground planes, are these attached to the ground on the antenna or just sitting under the base of the antenna. Mine had a plastic base on them covering any metal so there wouldn’t be a direct connection.
@IAUCKLAN63
If black plastic disc shaped tape is there, then the connection is established through the capacitor formed between the base plate of can and metallic base plate of antenna.
The antenna’s metallic base plate disc is hidden behind the disc shaped plastic tape. The disc is about 1 inch (25mm) in diameter. With a thin plastic tape (0.5mm thick, dielectric constant 2.2) between the can & base plate of stock whip, the capacitor formed is of the order of 20 pF. At 1090 mhz 20 pF capacitor’s reactance is low enough (7.3 ohms) to provide a reasonably good connection.
If the disc shaped tape is removed, then there will be a direct contact of can and antenna base’s metallic disc.
Have seen some very mixed results as it pertains to different ground plane setups. In the shots below, I simply ran a noise generator through a ZFDC-20-50-S+ directional coupler to bounce on the antenna and swept 88Mhz ~ 1400Mhz. Dongle used was an NESDRSmart .5ppm, not that it matters. I used Spektrum for these tests as it’s much quicker than SDR_Panarama and running an Airspy-mini using SpectrumSpy looks much the same.
Below is a stock antenna trimmed to 51mm (measured from the base to tip of the screw-off thing’a’majig) sitting on a cardboard box:
http://puush.hopto.me/puA0
Same antenna placed on an 85mm diameter can of Bush’s Baked beans (Brown Sugar Hickory, :mrgreen: ):
http://puush.hopto.me/EjKJ
Ate the beans and replaced can with a 65mm diameter can of Campbell’s Chicken Noodle soup:
http://puush.hopto.me/ljye
Appears to me that placing anything under the stock antenna base royally screws the antenna’s VSWR at our intended 1090Mhz frequency. Obviously this is far from professional equipment, but still has to make one wonder… I’m too lazy to sit and perform side by side tests using separate (but identical) rigs and I only have this chopped stock antenna and an untouched antenna to work with so far as “stock” antenna’s are concerned.
By the way, for those wondering - Here is a shot of the antenna in it’s unmolested form (out of the box):
http://puush.hopto.me/ig15
Maybe someone with some real equipment and some extra time on their hands can shoot off a few sweeps to share.
For those wondering what this is:: simply a poor-man’s way to graph what frequency(s) the antenna in question is most resonant, or has the lowest VSWR. (most radiation potential) for lack of smart man’s terms.
PS: I never bothered to sweep the antenna sitting on top of the baking pan I spoke about in my prior post as my wife took it back and hid it. 
Great, thanks for the clarification
@Nitr0
Yes, we lack the proper test equipment. Cost of these equipment (Spectrum Analyser, VNA, Signal Generator, etc) runs in several thousand dollars at minimum, so out of reach of a normal hobbyist.
Due to lack of test equipment, I resorted to trial runs and performance graphs. You used Noise Generator & Spektrum software.
Now I have run a simulation software 4nec2. The results of these software carries an eror of 5% to 10%, and is not suitable for building a high gain antenna. However these are good aproximation to general behaviour & trends.
Below are screenshot of results of frequency sweep for 4 different sizes of ground plane disc. The whip in each case is 1/4 wavelength (69mm).
The X-axis represents frequency in Mhz. Frequency Sweep from 500 Mhz to 1500 Mhz
Screenshot 1 - 25 mm dia ground plane disc - Resonant at 1300 Mhz
The 25mm (1 inch) dia disc is actually the built-in base plate of stock antenna.
Screenshot 2 - 65 mm dia ground plane disc - Resonant at 1120 Mhz
Screenshot 3 - 80 mm dia ground plane disc - resonant at 1090 Mhz
Screenshot 4 - 100 mm dia ground plane disc - resonant at 1070 Mhz
Interesting shots abcd567! Wish I could figure that software out, I’m a little slow in that department (or in general).
It appears the simulation somewhat mirrors the sweeps I performed above where the SWR tends to increase with the ground plane size. Resonance aside, which can be somewhat corrected by whip length. Oddly enough the message counts of your original tests don’t follow that same logic. (Your original tests are what I would have expected without having swept anything). I did notice that moving the whip to a different location (within a few feet even) changed the results - but I suspect that could be caused by nearby objects, etc. as the sweeps were performed indoors and not at a reasonable mounting location for long-term. Interesting subject as it concerns the stock whips for sure - be nice to get to the bottom of it with some clarity so others who don’t wish to spend their retirements on specialized antenna can get the most out of what they do have.
