In article , "Greg_False"
writes:

I want to fit a receiving preamp to my 29MH mobile radio which runs a 50watt
linear amplifier. My plan is to fit the preamp at the base of the 102" whip
and to switch it in and out of the co-ax circuit (when transmitting) by
means of relays activated by the mic press to talk button.

Can we presume the frequency as about 29 MHz?

What you propose is quite proper to do but that raises the question
of WHY the need to do it?

It would seem that it would be easier all around to add some
amplification in the radio's receiver, behind its built-in switching
or T/R circuitry. The amount of loss experienced in the 10m
band coax isn't going to affect much unless the mobile is a
double-trailered semi with antenna at the aft trailer and radio is
in the tractor's cab. That coax loss will be there as much on
transmit as it will be on receive...which is to say, very little.

Can I use any
12v relays, well shielded by copper boxes around them and what will happen
to my 50ohms impedance?

Very little will happen to the 50 Ohm impedance if you use most
any common small relays that can handle 50 W of anything (with
enough fudge factors to withstand about 200 W of anything).

Use Ohm's Law of Resistance to find the RMS voltage and current
at 50 Ohms for 50 Watts. Multiply those by 1.414 to find the peaks.
Since the mobile antenna isn't a perfect 50 Ohms, multiply the
peaks by 4 as a safety margin. Those are the breakdown voltages
and current-carrying requirements of the relay contacts. Try to keep
the relay contact wiring to/from antenna and radio's coax as short as
possible.

It would be wise to use a "break-before-make" contact set on the
relays. That avoids having a "one-time-use" preamp circuit where
the transmit RF can wipe out the preamp input. Contact action
can be checked at DC and with a scope to be sure.

First of all, the typical (8 foot) whip isn't going to be at a common
impedance over the whole 10 m band. It's characteristics will
change between different vehicle shapes/conductive surfaces even
if at the same frequency in comparing vehicles. No matter, those
things work okay as they are...work better with some kind of
antenna matching network. Antenna pattern will vary depending
on the vehicle and its conductive surface shape and topography.
That is normal, nothing to be overly concerned about.

Only nit-pickers will get into hissy-fits on antenna impedance and
antenna patterns and so forth. :-)

As you can see I need all the guidance I can get:)

We can all help there although this thread might get into a different
direction with nit-picking that is sure to evolve...:-)

Relays are okay to use even if they have (what seems to be) "large"
contact arrangements (of around an inch or two). Some will insist
that such things will mess up the 50 Ohm impedance (of the coax)
but then they probably haven't measured actual mobile antenna
installations in detail, such as the connection between coax and
antenna base. No matter, any relay that can handle 200 W of
anything has contact structures that are about 0.01 wavelength
at 10 m and won't affect the characteristic impedance very much.

What I see as a potential problem is the relay contact making
the transmitter output directly into the preamp input, even for a
few milliseconds. That's blow-out time for the preamp with 50 W
potential output...50 W in a 50 Ohm system equals 50 V RMS
or 71 V peak, current is 1 A RMS or 1.4 A peak. [P = E^2 / R
and = I^2 x R] Relay contacts can operate in about 5mS (reed)
to 10mS (rotary spider-armature) to 100 mS (larger leaf-type
contact stacks). It may be that the transmitter output may not
reach full output in a short time (most probable) but that is an
unknown here.

A protective box for the relay is very good for environmental
protection (think salty water in northern climes in winter) but it
won't help otherwise for RF in this case. A totally-enclosing
ABS plastic box should be fine.

Putting on the skeptic's hat, I'd have to ask why the need of a
preamp? If more receive gain is desired, the safer place to do
that could be in the transceiver itself. The T/R switching is already
there and presumably safe to use as it is.


retired (from regular hours) electronic engineer person