On Nov 3, 5:54 pm, "John Doe" wrote:
If you can't contribute anything positive to this question, then stay out of
it, with your trash!

I have a 10m base loaded mag mount antenna, that I had to put a piece of
felt over the magnet as not to scratch a new vehicle.

You are wasting your time trying to protect the paint when using a
magnetic mount. Grit will find its way under your magnet, and within
a year of normal driving, you will have scratched paint. Take a deep
breath, get out the hole saw or metal punch, and punch a hole in your
new car. Your radio will be happier and so will you as the paint
around your antenna will not be subjected to the inevitable damage
caused by magnet mounts. BTW, before anyone says anything about
lowering the value of the vehicle, I've sold and traded-in multiple
vehicles with holes punched in them, and not once has anyone ever said
a word about the holes. When I trade it in, I just put in a rubber
plug and no one notices.


When I did this I apparently changed the capacitance between the mag mount
and the roof of the vehicle. Someone suggested that I might want to replace
the felt with either a large balloon or a large prophylactic as it is
thinner and the capacitance would then return to almost where it should be.

Positive comments only!

However, if you have committed yourself to using a magnetic mount, I
would suggest using ultra thin vinyl sheeting, this should give you
enough grip to keep it on the vehicle. Nevertheless, anything that
you use is going to require that you retune the antenna, a longer whip
may be also be required because of the decreased capacitance.

73, Dloyd

On Nov 3, 9:27 pm, Dloyd Lavies wrote:
On Nov 3, 5:54 pm, "John Doe" wrote: If you can't contribute anything positive to this question, then stay out of
it, with your trash!

I have a 10m base loaded mag mount antenna, that I had to put a piece of
felt over the magnet as not to scratch a new vehicle.

You are wasting your time trying to protect the paint when using a
magnetic mount. Grit will find its way under your magnet, and within
a year of normal driving, you will have scratched paint. Take a deep
breath, get out the hole saw or metal punch, and punch a hole in your
new car. Your radio will be happier and so will you as the paint
around your antenna will not be subjected to the inevitable damage
caused by magnet mounts. BTW, before anyone says anything about
lowering the value of the vehicle, I've sold and traded-in multiple
vehicles with holes punched in them, and not once has anyone ever said
a word about the holes. When I trade it in, I just put in a rubber
plug and no one notices.

When I did this I apparently changed the capacitance between the mag mount
and the roof of the vehicle. Someone suggested that I might want to replace
the felt with either a large balloon or a large prophylactic as it is
thinner and the capacitance would then return to almost where it should be.

Positive comments only!

However, if you have committed yourself to using a magnetic mount, I
would suggest using ultra thin vinyl sheeting, this should give you
enough grip to keep it on the vehicle. Nevertheless, anything that
you use is going to require that you retune the antenna, a longer whip
may be also be required because of the decreased capacitance.

73, Dloyd

Try some 3mil Teflon, Teflon has a very high capacitive coefficent.

Jimmie

JIMMIE wrote:

Try some 3mil Teflon, Teflon has a very high capacitive coefficent.

I'm afraid you're misinformed. The dielectric constant of PTFE Teflon is
2.1, or just about twice that of air. Quite a few plastics are higher
(e.g. Mylar at 3.2 and PVC around 3.5), and many materials, such as
those used for capacitors, have dielectric constants that are a lot
higher (e.g., barium titanate at 1500 - 2000). Of course, a lot of the
latter aren't physically suited for this application. A long time ago, I
had trouble with microstrip line dispersion in a high speed delay line
compensation network design. So I chose Teflon for the substrate
material because of its *low* dielectric constant. The previous design
was on an alumina substrate having a dielectric constant of about 10.

The capacitance of two parallel plates is directly proportional to the
dielectric constant and the plate surface area, and inversely
proportional to the plate spacing. So putting 0.1 inch of Teflon between
the plates gives you the same capacitance as putting the plates 0.05
inch apart with air between.

This isn't to say that Teflon might not be a good choice. It's a very
low loss material, and chemically very inert. It's soft so won't
scratch, but it's slippery which might be a disadvantage. It's also
subject to cold flow, but there probably won't be enough pressure for
that to be a problem.

Roy Lewallen, W7EL

Roy Lewallen wrote:
JIMMIE wrote:

Try some 3mil Teflon, Teflon has a very high capacitive coefficent.

I'm afraid you're misinformed. The dielectric constant of PTFE Teflon is
2.1, or just about twice that of air. Quite a few plastics are higher
(e.g. Mylar at 3.2 and PVC around 3.5), and many materials, such as
those used for capacitors, have dielectric constants that are a lot
higher (e.g., barium titanate at 1500 - 2000). Of course, a lot of the
latter aren't physically suited for this application. A long time ago, I
had trouble with microstrip line dispersion in a high speed delay line
compensation network design. So I chose Teflon for the substrate
material because of its *low* dielectric constant. The previous design
was on an alumina substrate having a dielectric constant of about 10.

The capacitance of two parallel plates is directly proportional to the
dielectric constant and the plate surface area, and inversely
proportional to the plate spacing. So putting 0.1 inch of Teflon between
the plates gives you the same capacitance as putting the plates 0.05
inch apart with air between.

This isn't to say that Teflon might not be a good choice. It's a very
low loss material, and chemically very inert. It's soft so won't
scratch, but it's slippery which might be a disadvantage. It's also
subject to cold flow, but there probably won't be enough pressure for
that to be a problem.

Roy Lewallen, W7EL

It was a joke. I actually tried it on my wifes car, she wouldnt let me
drill a hole. The antenna stayed on about 3 minutes, just long enough
for her to get out of the neighborhood and get up to about 45MPH, then
it slid off and scratched the side of her car.Slipperiness is a BIG
disadvantage.

Jimmie

In message . com,
JIMMIE writes

Roy Lewallen wrote:
JIMMIE wrote:

Try some 3mil Teflon, Teflon has a very high capacitive coefficent.

I'm afraid you're misinformed. The dielectric constant of PTFE Teflon is
2.1, or just about twice that of air. Quite a few plastics are higher
(e.g. Mylar at 3.2 and PVC around 3.5), and many materials, such as
those used for capacitors, have dielectric constants that are a lot
higher (e.g., barium titanate at 1500 - 2000). Of course, a lot of the
latter aren't physically suited for this application. A long time ago, I
had trouble with microstrip line dispersion in a high speed delay line
compensation network design. So I chose Teflon for the substrate
material because of its *low* dielectric constant. The previous design
was on an alumina substrate having a dielectric constant of about 10.

The capacitance of two parallel plates is directly proportional to the
dielectric constant and the plate surface area, and inversely
proportional to the plate spacing. So putting 0.1 inch of Teflon between
the plates gives you the same capacitance as putting the plates 0.05
inch apart with air between.

This isn't to say that Teflon might not be a good choice. It's a very
low loss material, and chemically very inert. It's soft so won't
scratch, but it's slippery which might be a disadvantage. It's also
subject to cold flow, but there probably won't be enough pressure for
that to be a problem.

Roy Lewallen, W7EL

It was a joke. I actually tried it on my wifes car, she wouldnt let me
drill a hole. The antenna stayed on about 3 minutes, just long enough
for her to get out of the neighborhood and get up to about 45MPH, then
it slid off and scratched the side of her car.Slipperiness is a BIG
disadvantage.


Use RUBBER.
--
Ian

Try some 3mil Teflon, Teflon has a very high capacitive coefficent.


But seems like teflon - being so slippery - would contribute to the antenna
slipping off to roof.??

"Dloyd Lavies" wrote in message
ps.com...
On Nov 3, 5:54 pm, "John Doe" wrote:
If you can't contribute anything positive to this question, then stay out
of
it, with your trash!

I have a 10m base loaded mag mount antenna, that I had to put a piece of
felt over the magnet as not to scratch a new vehicle.

You are wasting your time trying to protect the paint when using a
magnetic mount. Grit will find its way under your magnet, and within
a year of normal driving, you will have scratched paint. Take a deep
breath, get out the hole saw or metal punch, and punch a hole in your
new car. Your radio will be happier and so will you as the paint
around your antenna will not be subjected to the inevitable damage
caused by magnet mounts. BTW, before anyone says anything about
lowering the value of the vehicle, I've sold and traded-in multiple
vehicles with holes punched in them, and not once has anyone ever said
a word about the holes. When I trade it in, I just put in a rubber
plug and no one notices.


When I did this I apparently changed the capacitance between the mag
mount
and the roof of the vehicle. Someone suggested that I might want to
replace
the felt with either a large balloon or a large prophylactic as it is
thinner and the capacitance would then return to almost where it should
be.

Positive comments only!

However, if you have committed yourself to using a magnetic mount, I
would suggest using ultra thin vinyl sheeting, this should give you
enough grip to keep it on the vehicle. Nevertheless, anything that
you use is going to require that you retune the antenna, a longer whip
may be also be required because of the decreased capacitance.

73, Dloyd

I've been down that road, too. The mag mount is OK for short term use, say
for a day or so but it is not a good idea to leave one on the vehicle for
any length of time. As Dloyd pointed out, grit WILL find a way to get under
the mount and it WILL damage the paint. I learned the hard way that moisture
will also linger under the mount and the paint will be further damaged.
Then, too, is the possibility of the antenna striking branches, door tops
and other low-hanging obstacles and when the magnet slides across the roof,
guess what? You got it! More damage to the paint.
At VHF/UHF frequencies you will not see enough of a change in the standing
wave to be concerned with if you use a buffer between the magnet and the
body. It will be miniscule.
Dloyd is correct again when he advises you to do it the right way. Drill a
hole. Mount the antenna and have fun.
Oh, as an addendum? I once had a magnetic mount antenna stolen from my car.
It was a 2 meter quarter wave whip but that didn't phase the CBer who cut
the coax and stole it. And before you jump on me and ask how I know it was a
CBer, read on.
I had engraved part of my soc number on the underside of the antenna. I just
happened to visit a local CB shop one afternoon and saw my antenna on a
shelf and the owner of the shop told me it was for sale for five dollars.
Without going into further detail, I arranged for a local detective to visit
the shop and buy the antenna, whereupon the owner of the shop was promptly
arrested for selling stolen property. The icing on the cake, so to speak,
was when the detective discovered that the owner also had several stolen CB
radios for sale as well. The CBer who stole the antenna was also caught and
charged.

73!

name and call withheld

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Not Dloyd wrote:

I've been down that road, too. The mag mount is OK for short term use, say
for a day or so but it is not a good idea to leave one on the vehicle for
any length of time.

I don't see what the big deal is. I have a mag mount on my car and
haven't noticed any scratches. It doesn't move at all. It is a 2 meter
antenna, and the magnet is very strong. Do other mag mounts typically
have week magnets?


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Dloyd Lavies wrote:
On Nov 3, 5:54 pm, "John Doe" wrote:

lowering the value of the vehicle, I've sold and traded-in multiple
vehicles with holes punched in them, and not once has anyone ever said
a word about the holes. When I trade it in, I just put in a rubber
plug and no one notices.



don't mean to go off topic, but I bought a new hyundai in 2000, and it
came from the manufacturer with a hole in the roof! Roof antennas seem
to be popular for cell phones and GSP. It had one of those plastic
plugs, and it looked perfectly normal.

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In message om, Dloyd
Lavies writes
On Nov 3, 5:54 pm, "John Doe" wrote:
If you can't contribute anything positive to this question, then stay out of
it, with your trash!

I have a 10m base loaded mag mount antenna, that I had to put a piece of
felt over the magnet as not to scratch a new vehicle.

You are wasting your time trying to protect the paint when using a
magnetic mount. Grit will find its way under your magnet, and within
a year of normal driving, you will have scratched paint.

I used a 7" magmount for a 5/8 WL on 2m.
It had a rubber boot.
I used it most days to and from work, which involved putting it on and
taking it off twice each day. And then there was other use.
Each time, before I put it on, I wiped the mounting spot (in the centre
of the roof) with a soft rag (with maybe a bit of spit if nobody was
looking).
Four years later, when I got rid of the car, the mounting spot was
extremely highly polished, which (being a company car) is more than you
could say for the rest it.




--
Ian