Dick, I may be wrong about this but the "power" linearity of the meter in
the SWR circuit depends upon the diodes bring operated in their square law
region. This requires the right range meter, because this supplies the load
to the diodes....they are usually fairly easily obtainable these days from
flea-markets though they may not always be scaled as you might expect. You
may put an amplifier in the circuit but you must be sure the meter amplifier
is linear and provides the correct load to the diodes. I would suggest you
really should use an op-amp but I appreciate there is then a supply problem.
I think it unlikely you will be able to retain the linear power calibation
with just a single FET or BJT.....but is it important ?? you really only
need an indication, and a indication of the tuning.
Cheers de Alan G3NYK
----- Original Message -----
From: Dick <[email protected]>
To: <[email protected]>
Sent: 14 May 2006 20:58
Subject: LF: 'meter problem'
> Hello all,
> I almost finished my new 136kHz transmitter, and after several hours of
> fault-finding it seems to work upto
> 600-700Watt DC input.
> However, I do have a 'problem' ..........
> Instead of using a 200uA meter for the forward and returned power, I'm
> a 10mA meter.
> This 10mA meter needs 53mV for full scale, internal resistance somewhere
> around 6 Ohm.
> When I put this meter into the G0MRF design it is a to heavy load for the
> measurement circuit.
> Open voltage drops from 13V to almost 0 V due to the low internal
> This also effects the SWR protection circuit........
> So, I did some experiments by using a FET or transistor as a driver for
> The measurment circuit voltage is almost not effected by the meter circuit
> now, however
> it is very difficult to find a good setpoint for the meter.......
> i.e when doubling the power, the meter must read also twice the power.
> Somehow I'm not able to get the right linearity or what ever.
> Anyone out there with a circuit diagram that does work? or suggestions.
> It is difficult to exchange the meter because I cannot easily reach the
> mountingscrews anymore.....
> So I have to use this 10mA meter........
> Dick, PA4VHF