Hi Marconi,
It was many years back but let me give you some background info.
Scroll down the images to the one titled Woburn 1997:
https://www.qsl.net/g3tso/Top Band.html
You can see me far left (ham call G4ZOW) holding my mono-band 160m (1.8-2MHz) Texas Bugcatcher antenna (lower section as it's a centre loaded design antenna). I had a much smaller coil which I used on 11m but it was still a 12ft+ antenna in total.
In my testing a local park was used so that there would be no obstructions. We used a portable spectrum analyzer on a table several wavelengths away from the vehicles tuned to spot frequencies on each band, in this test, 160m & 11m (CB). So, measurements were carried out in the far-field. You can read about near field & far-field here (but you need a degree in high mathematics
https://en.wikipedia.org/wiki/Near_and_far_field
Spectrum analyzer was used simple to measure any increase or decrease in voltage or field strength and not the purity of the signal.
Two vehicles were used in the testing, a regular compact family car (I think you call them) and a commercial VW LT truck roughly double the size of the car. For the testing the same antenna was used on both vehicles and in the same location (centre of the roof), Both vehicles used the same amp: The SGC Power Cube which ran around 500W pep output. We had the export models which covered up to 30Mhz and was opened up in the factory so covered 27Mhz as it did not have to comply with FCC reg's.
A Bird 4410 meter was used to set the transmitter power at 100W and also at 500W CW.
I found a good image on the Italian Gain-Master antenna site showing similar to what we were looking at on our spectrum analyzer all those years ago. In this image you see two antenna field strength, one stronger that the other. A spectrum analyzer used in a line of sight scenario takes no prisoners and there will always be a winner, and a loser. I do not like 2nd place
http://gain-master.it/vsconventional.php Scroll down to the image entitled Fair Field Signal Comparison.
Btw, this was of course a line of sight 'groundwave' test as scientific as a pair of regular hams could make it. Personally speaking you would see the same or similar results at a CB shootout or on CH6 it's just that rules go out the window there LOL.
The truck beat the compact with a very noticeable increase in field strength. I no longer have the written results as it was too long ago and I have move home, and country since. I'm now based in Cyprus, where the sun always shines in the Eastern Mediterranean.
Unit 148