In article ,
Jeff Liebermann wrote:
I overlooked the mobile operation from a car. Of course, when the
antenna is mounted on a metal surface, you don't need radials..
If it's a metal car roof, you don't need radials. Unfortunately, I've
had to deal with verhicles that have a fiberglass roof. Aluminum duct
tape ground plane (on the inside) to the rescue.
That will help but not entirely resolve the situation.
What I have heard, is that the theoretical gain advantage of a
5/8-wavelength monopole over a 1/4-wave monopole, is dependent on the
antenna being operated over a fairly large groundplane (one which
reaches out several wavelengths from the feedpoint). A simple set of
ground-radial "tapes" won't be big or extensive enough... and,
actually, neither will be the typical vehicle roof (at VHF wavelengths
at least).
According to these sources, in the absence of a good groundplane, the
5/8-wave monopole tends to "squint" - its highest-gain lobes are not
towards the horizon but aim upwards somewhat. Gain towards the
horizon may be *less* than a quarter-wave monopole on the same vehicle
mount.
So, the theoretical gain advantage of a 5/8-wave vehicle antenna may
not work out in practice. Testing would be required to see if there's
actually an advantage, or whether a "high gain" antenna of this sort
is actually a loss in practice because the gain is aimed in the wrong
directions.
And, I agree that for many vehicle mounting situations, a "ground
independent" antenna such as an end-fed half-wave may be the best bet.
I believe you can get these in a shortened form (with distributed or
lumped inductive loading in the center of the radiator) to keep the
height within reason... but going for a full-length end-fed radiator
would give you somewhat better gain and efficiency, if it's safe to
install on the vehicle.
--
Dave Platt AE6EO
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