"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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Stefan Wolfe wrote:

I think that when considering plastic and mica type materials for high
capacitance values, component engineers do not necessarily look at
dielectric constant as the most important property. Mylar, mica and
polyethylene are common capacitor materials and have rather low dielctic
constants just a little higher than teflon. What makes them most attractive
is their ability to be manufactured in very thin sheets (increased C) and
their high dielectric strengths (increased V ratings). Teflon is higher cost
but is very good for high voltage ratings due to superior dielectric
strength.

Aluminum and tantalum oxides tend to have very high dielectric constants due
to the capability of these metal oxide molecules to store electrons. What is
more desirable however is their property of being very thin. What makes them
a problem is their relatively low dielectric strengths thus their low
voltage ratings.

Actually, there are a lot more considerations yet. Some plastics are
self-healing, so a momentary arc won't permanently destroy the
capacitor. And some have exceptionally low leakage current. Various
materials also have widely different temperature coefficients. In
general, the very high k (dielectric constant) ceramics have higher
temperature coefficients than lower k materials. Some ceramics, like the
very high k ceramics used for Z5U and similar capacitors, are also
hygroscopic, microphonic, piezoelectric, and their k varies with
frequency and voltage. Many capacitors, depending largely on the
dielectric, also have nonlinear properties such as "soak" (dielectric
absorption) and "hook". Loss, expressed as loss tangent, ESR, or power
factor, is also often an important consideration, and it can be very
different for different dielectrics. Choosing the right capacitor for a
particular job can be pretty demanding. This one, fortunately, is easier
than some.

Roy Lewallen, W7EL

"Cecil Moore" wrote in message
et...
John Doe wrote:
Positive comments only!

I use aluminum foil.
--
73, Cecil http://www.w5dxp.com

At least one dualband magmount, no longer offered by MFJ, came with an
aluminum foil bottom. I have used several of them and seen no adverse
effect on the paint -- and I whiz along at highway speeds a lot.

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

John

Nobody seems to have mentioned these...

- Tune the antenna length to allow for the change in capacitance?
- Increase the diameter of the magnet to increase the capacitance...

Cheers Bob

John Doe wrote:

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.

"John Doe" wrote in message
...
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.

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!



Maybe black, electrical tape on the bottom of the mag mount (applied to that
mag mount, of course.) Would be thinner, but I suspect it might still cause
some problems with increased capacitance. Count paint that mag mount base
with liquid electrical tape, which I get at my local ACE Hardware store.
But then, you could still get some grit underneath the mount. Saran Wrap?
Very thin. Might be your best bet. Tough and flexible. Maybe that would
help...

Good luck.

Dave

On Nov 4, 4:00 am, Ian Jackson
wrote:
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

Interesting Ian,

I suppose the fact that you removed the antenna twice a day and wiped
the spot made a substantial difference. In my experience, I did not
remove the antenna daily, but as a lot of amateurs, I used the magnet
mount in lieu of punching a hole. After a year, the paint had
terrible scratches where the magnet had been placed. Nevertheless,
this still does not solve John's problem with the detuning he is
experincing when using a rubber boot or other protective device. What
John does not say, and I would like to know, is why he is unable to
retune the antenna to compensate for the change in capacitance. Will
the antenna not resonate? or is there some physical limitation, such
as the radiating element being too short, ect.

73 Dloyd

Ian Jackson wrote:
I used a 7" magmount for a 5/8 WL on 2m.
It had a rubber boot.

My Larsen mag mount came with a thin sheet of
aluminum across the bottom. After much use, the
aluminum started to tear. I replaced the factory
aluminum with ordinary aluminum foil taped to the
base. It's not as sturdy as the original but it
costs virtually nothing to replace.
--
73, Cecil http://www.w5dxp.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.

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!

Consider drilling an NMO mount and get the Larsen whip for 27-30 MHz.