On Wed, 26 Jun 2013 13:16:39 +0100, Channel Jumper
wrote:

A mobile vertical antenna has no gain - gain is only achieved when you
have gain in one direction and rejection in one or more directions.

Wrong. Gain on a mobile antenna can be achieved without
directionality. Just reduce the vertical radiation angle, which puts
more RF towards the horizon, and less RF towards the sky and the
ground.

The only measurable gain would be gain as compared to a dipole or gain
over isotropic.

Yep. That's the way gain is normally measured. dBd or dBi.

1/4 wave antenna's can sometimes produce a better signal locally,
because the radiation pattern is spread out over a larger area.

I've seen that. However, it's usually the result of misusing a "gain"
type antenna, such as a dual band 5/8 ham antenna being used on marine
or commercial frequencies, or a 5/8 commercial antenna, being used on
ham frequencies. Lots of ways to do it wrong. Where a 1/4 wave
antenna really shines is when one needs to cover a wide range of VHF
frequencies, from aircraft to marine.

It will give no distance gain - just local reception.]

Ummm... the range depends more on the terrain than on the antenna.

Because it is all one radio - the reception length does not matter, just
that it is resonant at X mhz - transmit.

VSWR is highly over-rated. The only real reason to keep VSWR low is
that high VSWR will cause the transmitter to protect itself and partly
shut down.

Try this experiment. Take a piece of sheet metal (or aluminum foil
covered cardboard) to act as a ground plane. Insert and SO-239
connector in the middle. Add a length of moderately stiff electrical
wire to the SO-239 that is longer than 1/4 wave at the weather
frequency (162.xxxx). Find a receiver that will measure the actual
receive signal strength. An all mode or AM (not FM) receiver will
work nicely. Extra credit for using a service monitor. Make a
measurement and start cutting the length of the antenna in roughly
1/2" intervals. Measure the receive signal strength.

What I've found when I've done this, is that the antenna gain, which
is what the receive signal level indicates, doesn't change very much
until you get down to about 1/8th wavelength. I modeled this test
using 4NEC2 and found the same thing.

Now, if you believe that the tx and rx performance of an antenna are
identical, this would suggest that you could make the antenna almost
any length, and still have adequate gain and function if you could fix
the VSWR.

Public service here all uses Larsen antenna's - especially the PA State
Police, and they have very deep pockets.

Obviously, the more expensive the antenna, the better it works.

Shakespeare also makes a decent mobile antenna.

They mostly make marine and military antennas. Their commercial
antennas are overpriced versions of the antennas that they sell to the
military. They're very well built, rugged, but not cheap.
http://shakespeare-military.com

You need a antenna analyzer or a Dip Meter to set to resonance.

Have you ever tried to resonate a 1/4 wave antenna with either of
those? You'll find that it's affected by the position and location of
just about everything within about a 20 ft radius. I run a sweep
generator, directional coupler, detector, and scope combination to
test antennas, but no way would I ever use that to tune the antenna.
Just getting near the antenna ruins the display. Incidentally, for
complex antennas, such as a dual band J-pole, minimum VSWR isn't
always at resonance.

The only thing the SWR meter can do is tell you what is happening in the
feed line.

Wrong. A VSWR meter reading is affected by the xmitter output
impedance, feed line impedance to the VSWR meter, characteristic
impedance of the coax cables(s), feed line impedance after the VSWR
meter, and of course, the antenna impedance. That's actually a
problem because a VSWR meter is affected by literally everything.

Drivel: I run mostly 75 ohm systems (because the coax is cheap and
easy and has less loss). I had to build my own 75 ohm directional
coupler in order to get accurate VSWR measurements. (Yes, Bird makes
a 75 ohm wattmeter 4307, but I don't want to spend the money).

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