That's very interesting. I can't see how the mag mount could change the
pattern shape any, so your experience indicates a severe loss in the
coupling capacitor, maybe as high as 90% or more. That's hard to
imagine, so I wonder if anyone else has had a similar experience -- or,
much better yet, actual measurements.

Roy Lewallen, W7EL

wrote:
By how much? Why?


I would assume the direct connection with the roof
metal. I tried this one time, after trashing a larson
magmount with about 84 knocks off the car...
When I converted it over to a hole mount, with the
usual NMO, there was a good bit of difference. I didn't
measure, as the mag mount was toast, but it was quite
large actually. Like maybe a change for say S9 on a
machine, to maybe 15-20 over S9. Very noticable in my
case using the larson 5/8 antenna on top of a car roof.
Was like a different antenna..
MK

I can't see how the mag mount could change the
pattern shape any, so your experience indicates a severe loss in the
coupling capacitor, maybe as high as 90% or more.....

A pretty large amount...I guess it is possible maybe my
mag mount performance was down a bit due to it being
knocked off the car so many times...Not sure why that
would effect it much though..Maybe the connections were
getting a bit corroded also...Hard to say really...
Probably would be a better test to compare against a
brand new one...The one I had was the standard rectangular
looking larson mag mount. Looks sorta like a small box with
magnets underneath..I do have to say though...When I switched
to the hole mount, the increase in performance was instantly
noticable, as I knew what all the local machines did as far
as strength before the switch.
BTW, my comparison was mainly just receive on
various repeaters...Being things are reciprical, I never
really worried about transmit tests...MK

The mag mount depends on capacitve coupling to ground while the "drill a
whole in the roof" mount is directly connected to the ground plane.

Also, when using a hole in the roof antenna there is no problem withcoaxing
being crimped in a door or window, etc. I have used the NMO (Motorola
style) in the roof mounts for 2 meters and 450 MHz since 1978 with various
vehicles. No problems. IMHO, it is the ONLY way to go.

John

All you say is true. I'm just having trouble envisioning what it is
about the mag mount's effective capacitor that could cause it to
dissipate 90% of the power being fed to the antenna. That's one
outrageous ESR, even more so considering the size of the effective
capacitor.

I personally find a mag mount to be convenient, since I use my two meter
radio only a few times a year at most, and in two different cars. I'm
perfectly willing to take a little hit in performance for the
convenience, but not to the tune of 10 dB or more like MK reported. And
if there really is any significant performance sacrifice at all, it
means that there's something I don't understand and would like to. If
there's a 10+ dB difference, there's something I *really* don't
understand. The fact that I get a nearly 1:1 SWR with the mag mount
makes it even more puzzling -- if the capacitor had an ESR of over 300
ohms (which it would have to in order to drop the signal 10 dB), the SWR
would be terrible.

But "Korbin Dallas" said: "Yes a properly installed antenna will out
perform a MagMount, always has alway will." That's a very positive
statement, with no room for exceptions. But so far I haven't seen any
reason why it should be true except for MK's single experience that
doesn't make sense unless there was some other problem, like damaged
coax, with his mag mount. (I can see the possibility for poor
performance in an HF mag mount -- but like the OP, I'm talking only
about VHF/UHF ones.)

I've used a VHF mag mount for over 20 years and have had to replace the
coax only once. But then again, I use it very infrequently.

Roy Lewallen, W7EL

SBC News Server wrote:
The mag mount depends on capacitve coupling to ground while the "drill a
whole in the roof" mount is directly connected to the ground plane.

Also, when using a hole in the roof antenna there is no problem withcoaxing
being crimped in a door or window, etc. I have used the NMO (Motorola
style) in the roof mounts for 2 meters and 450 MHz since 1978 with various
vehicles. No problems. IMHO, it is the ONLY way to go.

John