Behold, Greg_False signalled from keyed 4-1000A filament:


"Avery Fineman" wrote in message
...
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


Hi,

Thanks for the prompt and very helpful response.

I do not know enough about radio circuits to fiddle with installing
additional preamps directly into the radio it' self.

I want to improve my reception as I normally travel with a group of
other Land Rover owners who also have 29MH radios but they are not as
fussy as I am about their installations and in many cases they can hear
me but I cannot hear them.

I was thinking of using a double pole double throw relay.

I could fit a micro switch to the mic which activates before the press
to talk switch activates the transmitter?

Thanks again
Greg

Hey Greg ;-)

A DPDT relay is what you need, but to simplify things, drive the relay
with an RF sensitive circuit. Usually a 1N914 into a 2N2222 will work,
with some bias resistors and a delay capacitor of course.

That way, you let the circuit do the worrying for you. You key mike, the
transmitter outputs RF, the relay switches immediately to "transmit",
bypassing the preamp. You unkey, the relay drops out. If you use SSB, add
a 0.5 second delay before the relay drops out.

Google is your friend here. Their are literally thousands of 10M preamp
designs and almost as many RF operated TX/RX circuits.

I like vacuum tubes myself. I can post a Nuvistor-based design if you
wish, to ABSE.

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca