With a regular 1/4 wave (or 5/8) mobile whip (VHF and/or UHF) the coax
normally is grounded to the sheetmetal to form the ground plane -- but a mag
mount lacks the ground at the antenna end of the shield, though hopefully
the metal of the body is ultimately connected to the transmitter chassis and
coax braid (assumes ground braids for trunk lids etc. for rear deck lid
mounts), so is there a significant performance penalty for the mag mount vs
the same whip on a grounded base at the same location?

Well, it depends. The antenna does not need a DC ground path to work well.
Inductive/capacitive coupling to an adjacent metal surface is generally
adequate. But there are so many installation specific issues that it is not a
simple answer. However, most people are unaware that one of the best antennas
for 2m is a 1/4 whip in the center of the roof. At highway speeds, the 5/8
whip bends over and distorts the pattern and causes a null in the forward
hemisphere. For stationary instances, the gain antennas may indeed offer gain
over the 1/4 wave. Just some thoughts...

73s,

Evan

"K9SQG" wrote in message
...
Well, it depends. The antenna does not need a DC ground path to work
well.
Inductive/capacitive coupling to an adjacent metal surface is generally
adequate. But there are so many installation specific issues that it is
not a
simple answer. However, most people are unaware that one of the best
antennas
for 2m is a 1/4 whip in the center of the roof. At highway speeds, the
5/8
whip bends over and distorts the pattern and causes a null in the forward
hemisphere. For stationary instances, the gain antennas may indeed offer
gain
over the 1/4 wave. Just some thoughts...
73s,
Evan

There is a significant amount of capacitance between the base of the
magnet mount and the roof. Some mag mounts have special arrangements of
conductive material, sometimes a foil which is on the bottom and is
(usually) insulated by a thin plastic membrane. This provides a low
impedance or Capacitive reactance to the roof at the base of the antenna and
therefore is effectively an AC ground as far as the antenna is concerned. I
have often wanted to measure it, but haven't yet. The ground through the
coax, rig and negative power lead is completely irrelevant for the RF. This
is because the distance along this path presents a significant impedance
(without going into too much detail) and should be considered, for all
intents, an open circuit at RF.
My 5/8 MFJ has surprisingly little wind induced bend at the kind of high
speeds I travel (60 mph). Thicker whip sections are better in this regard.

--
Steve N, K,9;d, c. i My email has no u's.

"The Masked Marvel" wrote in message news:1g%hc.5154$0u6.1124791@attbi_s03...
With a regular 1/4 wave (or 5/8) mobile whip (VHF and/or UHF) the coax
normally is grounded to the sheetmetal to form the ground plane -- but a mag
mount lacks the ground at the antenna end of the shield, though hopefully
the metal of the body is ultimately connected to the transmitter chassis and
coax braid (assumes ground braids for trunk lids etc. for rear deck lid
mounts), so is there a significant performance penalty for the mag mount vs
the same whip on a grounded base at the same location?

There was some difference in one case where I had the chance to
compare. I once had a larson 5/8 mag mount on the center of the roof.
You know the common model with the black box looking mag mount... It
always seemed to work pretty well as a mag mount, but eventually I
tore it up by knocking it off the car once too many times...So I had
to ditch the mag mount, and drill a hole and do a hole mount. Did it
make a difference? Yes. Quite a bit actually. It was quite noticable
anyway...I found the hole mount was superior. But I still like mag
mounts...It's nice being able to slap it on any car you are in. But on
my truck, I have a hole mount right in the center of the roof of the
cab. At the moment, I only have a homebrew 2m 1/4 wave whip on it...I
don't work a heck of a lot of 2m mobile, but I do want the antenna
there ready if needed.. MK