RadioBanter - View Single Post

John Popelish · July 11th 06, 02:02 AM posted to rec.radio.amateur.antenna

wrote:
Roy Lewallen wrote:

John - KD5YI wrote:

Actually, on elevated antennas (as in the usual VHF setup), just two
quarter-wave radials 180 degrees apart is almost indistinguishable from
4 or more radials. EZNEC shows very little change in terminal impedance
and pattern by removing two radials from a 4 radial ground plane.

I once used copper tape on a window to make a ground plane vertical like
that for 70cm. It worked very well.

George Brown, the inventor of the ground plane antenna, found that only
two radials were necessary. But when his company went to sell it, the
marketing department decided that no one would buy a two-radial ground
plane antenna in the belief that it would be omnidirectional. So they
added two more to make it "look" more omnidirectional. The four-radial
ground plane persists to this day.

The real reason to use 4 radials or more is decoupling the feedline
shield.

Decoupling is very bad with two radials unless you get lucky with
feedline and/or mast length or use a decoupling aid like a common mode
choke.

Awe, you ruined the suspense.

On a commercial 47 Mhz GP I designed that had 4 radials, the radials
had to be isolated from the mounting and a ferrite decoupling sleeve
placed over the coax. I can't imagine how bad that problem would be
with only two radials.