No engineer here, hoping to draw on some of your experience.
I’m reading and trying to understand the differences between RF ground vs counterpoise vs the constructed ground plane elements- doing the same thing but in different ways.
I understand the radials of the design you posted serve as the “ground plane” for the vertical element, my father had several back in the 60’s. My question probably leaned more towards other designs where the antenna is more dependent on the RF ground being provided by “earth” in some form…dirt, water, metal roof, etc. ( It comes from somewhere other than the monopole )
A number of people here have been messing about with CoCo antennas or other collinear designs, all of which are said to not require a constructed ground plane at the base of the antenna, the assumption being that the RF ground is formed by other stuff, so “don’t worry about it.”
Well, I am starting to worry about it.
I observed that a location A) right on a large body of salt water B) that also has a section of tin roofing a few feet below the testing platform - noticeably improves nearly every antenna I have. Apparently great RF grounds make happy antennas regardless of design. ( The Spider ground plane naturally improved the least, but still worked better as well )
But it did have enough metal to make an element of around 67mm, which is a figure for the length of spider radials often quoted around here (and visible in the opening photos in this thread)?
Indeed, it’s not great but enough to get going very cheaply. It is what it is so I treated it as a constant across different groundplane setups, taking the coax out of the equation to a degree. I no longer use that antenna or cable.
The performance dropped when I tried the lid before I got the preamp. The magmount without a lid performed way better than the magmount on the large lid. I since tested the Kenco tin versus both without the preamp and the Kenco tin outperformed everything, followed by the magmount without anything, and the worst was the magmount on a large metal lid which saw messages, positions and aircraft numbers take a big dive.
Isn’t it just right? Again ~67mm which is quoted here all the time as an optimum diameter for a groundplane for 1090Mhz?
This ought to help shed some light on the effect: https://www.w8ji.com/ground_systems.htm
(about 2/3rds of the way down the page in the section titled Induced Ground Currents)
A quarter wavelength at 1090 MHz is 68.8 mm, so your element was a tad short.
Hmmm. That’s interesting. My results were the opposite. The el cheapo mag mount performed poorly when it didn’t have any couterpoise. (groundplane) I tried it on a can 100 mm x 115 mm (D x H) and while it worked, I got much better results when I changed to a one gallon paint can lid ~155 mm in diameter.
The best results were obtained by planting the mag mount in the center of my car’s roof and driving the car to a location in an essentially open area. I say essentially as there was one house nearby. Other than that I was on a road that runs throught farm acreage. (the fields were fallow at the time)
Just a tiny bit short, i.e. it should be 68.8 mm.
That “spec” should read “69 mm or longer.”
Which is why a mag mount antenna works so well on a vehicle roof.
(provided there’s enough sheet metal underneath it to form the image element)
