Antenna and Coax Length

This brings to mind a clever circumvention of the FCC regulations on Family
Radio Service (FRS)tranceivers. The rules say that a FRS transmitter and
antenna must be integrated - i.e. handhelds are OK but not roof antennas.
Radio Shack brought out a transceiver/antenna unit with a very long mike
cord and all transceiver controls in the mike which meant that the
antenna/transceiver with a magnetic base could be on the roof of a car. It
met the rules and worked really well but the FCC frowned on the effort and
it was withdrawn from the market.

So, why not extend the idea to built-in units? Put a small tranceiver unit
in the antenna base and a separate control head in the panel. This
eliminates all the RF coax problems.

Not without problems of course. I'm sure it't more important in the GHz
bands than in the 2-meter bands. Mounting a tranceiver in the fin with the
antenna creates access and W&B issues but it looks solvable.

Bill Daniels


"Ralph Jones" wrote in message
...
On 31 Jan 2007 08:00:31 -0800, "ContestID67"
wrote:

I have a question for this august body.

I found a link on the 1-26 associations web site talking about the
proper length of an antenna for operation at 123.3Mhz. This turns out
to be;

1/4 wave Length: 0.61 m or 23.95 in
3/4 wave Length: 1.82 m or 71.84 in

I was curious about the length of the coax. I was under the
impression that to get the maximum power out of the antenna, that the
combined total length of the coax *AND* the antenna needs to be taken
into consideration and needs to be an even number of wavelengths.
That prevents power from reflecting at the tip of the antenna and then
back into the trasceiver. This can not only rob radiated power but
might also damage the transceiver. A VSWR meter is used to tune this
for maximum radiated power typically by adjusting the length of the
antenna.

I may be all wet on this subject as I am an electronics engineer (a
bit pusher) and not an RF electrical engineer. Any comments?

If the transmitter output circuit, the coax and the antenna all have
the same characteristic impedance, no reflections occur; the VSWR is
1; radiated power is maximized; and the length of the coax is
essentially immaterial.

If there is an impedance mismatch anywhere in the system, there will
be reflections and the VSWR will be more than 1. This will affect the
signal at least two ways:

(1) The impedance the coax presents to the transmitter will be
something other than its characteristic impedance, creating an
impedance mismatch at the output circuit. That will reduce the amount
of power transferred into the system to begin with, as well as
possibly overloading the output circuit.

The amount of this impedance mismatch will depend on the length of the
coax. The math gets interesting he as an extreme example, a 1/4
wavelength line shorted at one end looks like an open circuit from the
other, and if open at one end it looks like a short from the other!

(2) Every time a wave hits one end of the coax, some part of it gets
transferred and some gets reflected back to the other end, where the
same happens again. Some of the energy goes into the antenna the first
time, and some of it makes multiple trips.

If the coax were made of perfect conductors with perfect insulators,
this back-and-forth travel would be immaterial and all the power that
gets into the line (after being reduced by the impedance mismatch at
the transmitter) would eventually get radiated by the antenna. But
they don't sell perfect conductors and insulators at Radio Shack, so
every foot of coax represents measurable resistive losses.

rj