Ground Plane Verticals
Ground Plane antennas and common mode
We often hear a groundplane only needs two or four radials to prevent feed line radiation. That isn't true. Years ago when I was designing a commercial antenna for lower VHF, I discovered significant SWR changes in a four-radial groundplane as I altered the feed line length. There was also significant pattern distortion.
Here is the freespace pattern of a 1/4 wl groundplane antenna with four radials without proper feed line decoupling:
Significant common mode current exists on the feed line in the groundplane antenna. Common mode current is nearly 1 ampere with 2.5 amperes at the antenna element (500 watts applied). To behave properly, a 1/4-wave groundplane with four radials actually needs a "current balun" or some other form of feed line and mast decoupling.
The pattern distortion is caused by feed line common-mode current. If you think this is bad, imagine what happens with an end-fed antenna (even a 1/2 wave antenna) and NO radials! In that case all of the antenna current flows over the feed line shield! Many antenna designs actually use the feed line and mast radiation that others dismiss as "insignificant" to increase antenna gain. In some cases, the antenna designers really don't even understand what they did to create a "magic" antenna.
Here is the same 1/4 wl groundplane, with radials and radial ground connection insulated from the mast, with a feedpoint "feed line choke" of moderately high impedance (3000 ohms):
In this case feed line currents are under .4 amperes, and antenna current increases to 3 amperes.
The pattern is much cleaner and more like what we expect from a groundplane far above earth. The antenna, even with a good isolator, still needs more work. The system needs a ground on the "unbalanced" side of the isolator (the station side) to be 100% effective. (This is why I also ground the unbalanced side of Yagi baluns to the antenna's boom. A coil of coax hanging in the air is probably not good enough without a shield ground on the station side of the balun.)
With a perfect balun and optimum de-coupling (a few extra radials below the balun or an extra-ordinary balun), the pattern looks like this:
This is the ideal pattern of a 1/4 wl groundplane. Despite what we might think, it takes extraordinary care to obtain an ideal pattern. We often assume we have this pattern, because we model antennas without the feed line or mast included in the model!
To obtain this pattern, I had to:
Use a good balun just below the radial tip level
Ground the feed line shortly below the balun to a small groundplane
Slope the radials downwards at 75 degree angle
It is possible to accomplish this in the real world. All we need to do is insulate the radials from the mast or tower, use a feed line isolator, slope the radials downwards, and ground the shield to a reasonable RF ground on the station side of the isolation device.
We often assume we don't have a problem with common mode current upsetting antenna pattern because we have four radials. We can find numerous articles that ignore the feed line, and "pretend there isn't common mode current on the feed line. Some articles even claim the groundplane antenna only needs one or two radials!! This happens because the person who modeled the antenna never included the mast or feed line. This eliminates the real-world problem through an omission in the model, there can't be common mode current and pattern distortion from the mast and feed line if the antenna "hangs in space" without a feed line or mast.
If an antenna model does not include the feed line, losses in the feed line, or imperfections in the source it much more likely than not is a flawed model. The real-world antenna often will be much different if the model ignores the feed line and mast.