It's a long story, here's the short version.

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

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

Our local Motorola sales rep has his head stuck firmly up his ass and
keeps trying to sell us some basic 1/4-wave verticals.

The current antenna is a vertical whip with a loading coil wound
along the length of the antenna.

The dimensions a

-- 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

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

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. 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.


I have Googled every term I can think of to find this antenna,
Motorola sales rep tells me he thinks its a "cellular antenna" . .
..which it clearly is not. My MFJ antenna analyzer shows a resonance
at 154 MHz.

Anyone help me identify this antenna?


- - - - -

Fat, Dumb, and Ugly
is no way to go through life.

But, if you're a
Republican,
you have no choice.

"Kickin' Ass and Takin' Names" wrote in message
...


It's a long story, here's the short version.

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

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

Our local Motorola sales rep has his head stuck firmly up his ass and
keeps trying to sell us some basic 1/4-wave verticals.

The current antenna is a vertical whip with a loading coil wound
along the length of the antenna.

The dimensions a

-- 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

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

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. 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.


I have Googled every term I can think of to find this antenna,
Motorola sales rep tells me he thinks its a "cellular antenna" . .
.which it clearly is not. My MFJ antenna analyzer shows a resonance
at 154 MHz.

Anyone help me identify this antenna?

I don't think the Motorola man has his head anywhere, but maybe your head
is.

To be a 5/8 wave or gain antenna at 154 mhz the antenna will be about 35 to
45 inches long. If the antena you have is only 14.25 inches long, it is
less than 1/4 and will have even less gain than a 1/4 wave whip. A 1/4
whip will be about 18 inches at 154 mhz.

You do not shorten a 5/8 antenna by winding coils. The coils are either
matching or phasing coils.

I hate to break it to you, but what you have is a
less-than-quarter-wave vertical with a loading coil
to bring the terminal impedance up to 50 ohms. Only
the straight parts of the antenna radiate and that
thing doesn't have very much. Your Moto guy was
right in recommending a quarter-wave whip; it would
out-perform this loaded version you have now.

To counteract hitting trees and other stuff, you
could substitute thin, strong 0.03"dia stainless
spring stock for the radiator. It has a lot of
"give" and could recover from close encounters.
I use that for my 1/4-WL whip that daily bangs
into my low-hanging garage door.

Jim, K7JEB

On Monday, June 24, 2013 5:57:05 AM UTC-7,
Kickin' Ass and Takin' Names wrote:

The current antenna is a vertical whip with a loading coil wound
along the length of the antenna.

The dimensions a
-- 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

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

El 24-06-13 14:57, Kickin' Ass and Takin' Names escribió:


It's a long story, here's the short version.

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

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

Our local Motorola sales rep has his head stuck firmly up his ass and
keeps trying to sell us some basic 1/4-wave verticals.

The current antenna is a vertical whip with a loading coil wound
along the length of the antenna.

The dimensions a

-- 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

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

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. 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.


I have Googled every term I can think of to find this antenna,
Motorola sales rep tells me he thinks its a "cellular antenna" . .
.which it clearly is not. My MFJ antenna analyzer shows a resonance
at 154 MHz.

Anyone help me identify this antenna?


- - - - -

Fat, Dumb, and Ugly
is no way to go through life.

But, if you're a
Republican,
you have no choice.


I agree with others. The full size quarter wave with correct size
radials will perform better then the current 14.25' stick, no matter
how you wind it.

Make sure you have some VSWR indication to tune it to your frequency
range, or just to check the complete installation.


--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM

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

El 24-06-13 18:14, Wimpie escribió:
El 24-06-13 14:57, Kickin' Ass and Takin' Names escribió:


It's a long story, here's the short version.

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

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

Our local Motorola sales rep has his head stuck firmly up his ass and
keeps trying to sell us some basic 1/4-wave verticals.

The current antenna is a vertical whip with a loading coil wound
along the length of the antenna.

The dimensions a

-- 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

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

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. 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.


I have Googled every term I can think of to find this antenna,
Motorola sales rep tells me he thinks its a "cellular antenna" . .
.which it clearly is not. My MFJ antenna analyzer shows a resonance
at 154 MHz.

Anyone help me identify this antenna?


- - - - -

Fat, Dumb, and Ugly
is no way to go through life.

But, if you're a
Republican,
you have no choice.


I agree with others. The full size quarter wave with correct size
radials will perform better then the current 14.25' stick, no matter
how you wind it.

Make sure you have some VSWR indication to tune it to your frequency
range, or just to check the complete installation.


I overlooked the mobile operation from a car. Of course, when the
antenna is mounted on a metal surface, you don't need radials..

--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM

On Mon, 24 Jun 2013 18:46:46 +0200, Wimpie
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.
http://www.homedepot.com/p/Nashua-Tape-322-1-57-64-in-x-50-yds-Aluminum-Foil-Tape-3220020500/100030120#.Uch4_Ng9pjZ

The fiberglass roof problem is also common in marine VHF (156-163Mhz)
installations. Those tend to use 1/2 wave antennas, which do not
require a ground plane. The automobile version of the 1/2 wave:
http://www.theantennafarm.com/catalog/laird-tech-bb1322w-4470.html?zenid=6bc9236b727ed1e483c9037fb2ac52db




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

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
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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