On Mon, 24 Jun 2013 08:57:05 -0400, Kickin' Ass and Takin' Names
wrote:

Our volunteer rescue squad dispatch operates in the 152 - 154 MHz
range -- transmit on 154.XXX, receive 152.XXX.

Antennas are normally tuned to the transmit frequency.

Our main antenna barely survived contact with a tree limb and needs to
be replaced.

The tree limb will replace itself by growing back. All it takes is
time.

-- Overall height: 14.25 inches
-- 4 inches from the base the antenna is wound into a coil, about 3/8
inch diameter, 5 turns
-- the coil is 1.75 inches long
-- above the coil is 8.5 inches of antenna
-- NMO base

Like this perhaps?
http://www.11junk.com/jeffl/antennas/Misc/slides/Motorola-850mhz-NMO.html
That's an 850 Mhz 5/8 wave antenna, commonly sold by Motorola for
trunking systems. There's another model, with the same dimensions,
but with thicker wire. It does work on VHF frequencies as a center
loaded monopole, but is not designed for the purpose. I have about a
dozen if you need one or two. Also, please check your frequency of
operation just to be sure you're on 152 and not 850 MHz.

I suspect this antenna is an old model 5/8-wave VHF antenna, shortened
by winding a coil in the antenna.

Nope. A 5/8 wave antenna is ummm... 5/8 wavelength at 152 MHz long or
about 1 meter long. A 1/4 wave antenna will be ummm... 1/4 wavelength
long or about 50 cm long.

If it is a 5.8-wave, it should be giving us a few dB gain. The
1/4-wave whip he wants me to install would give unity or less gain. In
our rural area, we need all the antenna help we can get.

Some reading on the topic of 1/4 wave versus 5/8 wave mobile antennas:
http://www.k0bg.com/images/pdf/mobile_vhf_ant.pdf
Gain is a good thing, but not always beneficial. The mounting
position has a big effect on the pattern. Any monopole, mounted on a
flat ground plane, will have a tendency to "uptilt" the pattern,
providing more RF to the sky than to the horizon. Too much gain and
when the vehicle is at an angle, such as going uphill or downhill,
there will be some pattern tilt. Fortunately, these type of problems
are not much of a consideration, although they are important an UHF
and up.

Also, there are some mobile antennas, with open loading coils in the
middle. Tree branches like to become entangled with the coil and try
to rip the antenna out of the roof mount. I have one of those do
exactly that to a previous vehicle.

If you're going to be garaging the vehicle, there are 5/8 wave
antennas that will tilt over with a hinge just above the base loading
coil.

I an
thinking about installing a full-length 5/8-wave whip, but, we go into
a lot of driveways with low tree limbs and I doubt a full-length
antenna would survive very long.

If there's a spring near the base, it will survive, as long as the
tree limb doesn't hit the spring and get stuck. I prefer a magnet
mount antenna, which simply falls over. The rectangular bases fall
over better than the round bases. Do try to remember to put the
magnet mount back when leaving the garage.

Anyone help me identify this antenna?

Sure. A photo would be helpful. If it's not the one in my photo,
start he
https://www.google.com/search?tbm=isch&q=vhf+mobile+antenna
and see if any of the photos match your existing antenna.


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

On Mon, 24 Jun 2013 09:15:46 -0700, Jeff Liebermann
wrote:

-- Overall height: 14.25 inches
-- 4 inches from the base the antenna is wound into a coil, about 3/8
inch diameter, 5 turns
-- the coil is 1.75 inches long
-- above the coil is 8.5 inches of antenna
-- NMO base

Like this perhaps?
http://www.11junk.com/jeffl/antennas/Misc/slides/Motorola-850mhz-NMO.html
That's an 850 Mhz 5/8 wave antenna, commonly sold by Motorola for
trunking systems.

The antenna in the photo was made by Maxrad (PcTel). I have others
that appear identical from Antennex (Laird). I couldn't find a
similar antenna in either online catalog. The one's supplied by
Motorola use a heavier gauge wire which I guess(tm) were Antenna
Specialists (PcTel) like this:
http://www.ebay.com/itm/321096706872

This is the closest equivalent that I could find:
http://www.antenna.com/artifacts/2012511BASELOADEDNOGROUNDPLANEANTENNAS.pdf

I would advise against using this antenna for 152 Mhz and suggest you
get a real 1/4 wave or 5/8 wave antenna.




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