On Mon, 24 Jun 2013 11:04:01 -0700, (Dave Platt)
wrote:

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).

Umm... if that were true, then 5/8 wave base station antennas, which
all have 1/4 wave ground radials, shouldn't work or require extra long
ground radials. Here's a stacked dual 5/8 wave 6 meter ground plane
antenna:
http://802.11junk.com/jeffl/antennas/gnd-plane-05/index.html
(Note that the characteristic impedance is about 125 ohms and that the
necessary matching xformer is not shown). Plenty more 5/8 wave base
stations antennas with 1/4 wave radials found with Google images:
https://www.google.com/search?q=5/8+wave+base+station+antenna&tbm=isch


However, I will confess that the aluminum duct tape ground plane was
easy. The fiberglass roof had stiffener ridges, projecting lamps,
other antennas, and metal stiffeners. Wiggling the tape around these
was not easy. I finally gave up, dropped the entire headline, removed
all the other hardware, and covered most of the roof with the foil
tape. It wasn't an electrical issue, but rather that we didn't have
time to do much experimenting. There was no reason to minimize the
use of the aluminum duct tape, so I just plastered it onto the
underside wherever possible. I therefore did NOT determine if 1/4
wave radials were adequate. Also, no swept response with an antenna
analyzer... just a VSWR check.

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.

I would think it would be the other way around. Large ground planes
are more reflective causing more of the RF to go towards the sky. I'll
need to run an NEC2 model to be sure. However, at VHF, I don't think
it's a problem. The vertical radiation angle of a 5/8 wave antenna is
sufficiently wide that a small change in takeoff angle isn't going to
make much of a difference in coverage.

End fed collinear antennas, with or without a ground plane, usually
have a non-zero takeoff angle. If you want the major lobe to point to
the horizon (i.e. zero takeoff angle), the antenna should be center
fed, which is not going to happen on a mobile antenna. However,
that's not necessarily a good thing, as such a wide vertical radiation
angle antenna, that is so close to the ground, is going to send much
of the RF into the absorbent ground. Better to have some uptilt and
hope that some of it goes in the right direction.

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.

That doesn't sound like it would be easy to test on a vehicle. I
think a computer model might be easier and probably more interesting.
(No, I'm not volunteering to do one).

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.

Yep. I have quite a bit of experience with 1/2 wave antennas on
fiberglass vessels. They work just fine. However, there's an
additional problem on marine applications which limits antennas to
fairly low gains. If the gain is too high, and the vessel rocks and
rolls with the waves, the narrow radiation angle could easily send the
signal into the sky or into the water, instead of towards the horizon.
There's a similar problem in vehicles going up and down hills, but is
less serious. Fortunately, for VHF, it's not too horrible.

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.

Yep.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558