Can't Drill any holes - but thanks for the suggestion.

wrote in message
oups.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.

Thanks Stefan - I will keep that in mind.


"Stefan Wolfe" wrote in message
...

"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!

The thickness of the balloon will result in a much larger capacitance to
the 'ground' plane counterpoise so it should be a big improvement over the
felt.

"Cecil Moore" wrote in message
...
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

Thanks Cecil - I think that this might be the best possibility of all.

73's

"Dloyd Lavies" wrote in message
ups.com...
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

Dloyd - The radiating element will be too short if it is cut, and there is a
problem with it resonating.

On Nov 3, 1:54 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!

Just use a very thin piece of plastic. Some companies that make mag-
mounts sell circular pieces with adhesive to stick to the antenna
base. But you can just cut out a disk from a zip-lock bag and put
underneath the magnet. It won't slip around.
The thicker the material, the more it affects the performance because
it reduces the capacitance to the metal that the antenna is placed on.
Thinner is better here.
By the way, someone told me to try felt years ago, and I did. The
antenna had more tendency to slip around, and it DID mar the paint on
my car, more than if I hadn't used anything.
Last comment: a balloon would probably get water in it, and keep the
antenna base wet for a long time after it rains (at least here in
Portland where I live!) Not a great idea. And balloons aren't known
for their tougness.
Gary

On Nov 4, 10:48 am, "John Doe" wrote:
"Dloyd Lavies" wrote in message

ups.com...



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

Dloyd - The radiating element will be too short if it is cut, and there is a
problem with it resonating.- Hide quoted text -

- Show quoted text -

I wasn't implying that you should cut the element. What I was trying
to understand/suggest, is whether the whip is too short already. Have
you checked the antenna with an analyzer? Is it resonate anywhere on
10?

Dloyd

" wrote in message
...

"Stefan Wolfe" wrote in message
...
sign your own mr crow not mine

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

"Stefan Wolfe" wrote in message
...

"Dave" wrote in message
...

"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. Dave

Dave (and John Doe), the idea is to incease capacitance. Large capacitance
is a solution to the problem not a cause of problems, agree?


I don't know, but if that's the case, than a thinner medium between the mag
mount and the roof would have a higher capacitance than a thicker one, like
felt. Or it least I think it would... If so, than Saran Wrap might be the
answer.

Dave