In message , Richard Clark
writes
On 27 May 2004 19:06:26 -0700, (Bruno) wrote:

I would like to operate a 5 watt 151. MHz mobile transceiver into a
ball and spring low band whip antenna similar to the old bumper mount
CB antennas except that the antenna would be mounted directly to the
top of the rear fender of a large 1960's American car and needs to be
around 5-7 feet high. I need to use this type of antenna to give it
the appearance of an old police car.

Is it possible to do this effectively without damaging the
transmitter. I realize that this is an amateur radio board but I
thought that amateurs would be the best group to ask. Thank you.

Bruno

Hi Bruno,

MURS allows only 2W effective radiated power, not 5W going to an
antenna. Further, this is into a simple antenna. Yeah, I know,
yours seems to be a simple one, however as long as it is (5-7 feet
high), it is both a poor match (almost no power out), and would have
gain (a power multiplying factor) if you did match it (making it more
powerful, hence the term "effective" radiated power). Even more
unfortunate is that this gain (and most sensitivity for receiving)
would be up into the sky! This is like drawing three cards to an
inside straight: antenna too long, poor match, deaf too.

Now, this sounds like a lot of contradictions I am sure. The simplest
antenna would be roughly 18 inches tall. Use the AM radio antenna for
that, use a legal transmitter, and put up your buggy whip to impress
the buddies. As an alternative, the antenna length you are describing
is quite close to those used in the CB bands which would work just as
well, if not better (as long as who you talk to use the same bands).

By the way, are you located near Norwalk Connecticut?

73's
Richard Clark, KB7QHC


Actually, there is no problem.

As it stands, at the impedance looking into the coax (TX end) won't be
50 ohms. To keep the TX happy, all you need to do is to insert a simple
matching circuit (at the TX end) to transform whatever the impedance is
to 50 ohms.

This won't change the impedance looking into the coax, and the SWR on
the coax will be the same. As a result, the loss on the coax will be
higher than if the SWR was 1:1. However, as the length of the coax won't
be that great (even in those large American cars!), I doubt if you'll
notice it.

For a suitable matcher, I suggest you get one of those very cheap small
CB type (the kind in with the two compression trimmers, often found in a
junk box), and rebuild it into a C-L-C pi configuration (if it isn't
already). The coil will probably be about 4 turns 1/4" dia (some
experimentation will be necessary). To adjust the matcher, insert an SWR
meter between the TX and the matcher, and connect the matcher to the
coax.

Cheers,
Ian.
--