Greg_False wrote:
"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
Even at 10 meters a modern radio should be fine, unless you have an
older rig or one that's got a reputation for being crappy I'd suspect a
fault with the radio or the antenna installation. I'd want to do a bit
of troubleshooting before I went off building modifications to something
that may just need repair.
At lower HF frequencies you can test your receiver noise level pretty
quickly by attaching an antenna -- if the atmospheric noise is
discernible then you have no trouble. I honestly don't know if this is
still a valid test at 10M.
If you can, see if you can swap rigs with somebody temporarily and go
driving. If the problem follows the rig then you have rig problems, if
it follows the car you have car problems. If you have no problem with
your car shut down but the noise level goes up with it running
(particularly if you hear the ignition) then you have ignition noise
problems. You may have an antenna that's more of a dummy load than a
radiator (and receiver).
If you still think you must have a preamp you should have a better time
installing the preamp inside the radio than outside -- there will be a
spot in the radio that comes after the T/R switch but before the mixer.
If the radio already has a preamp then you probably have one of the
aforementioned faults, and should be looking at "fix" rather than "modify".
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com