[AMRadio] Re: Thermocouple rf ammeter


Donald Sanders w4bws at comcast.net
Sat Feb 10 13:34:40 EST 2007


With a DPDT knife switch and the feedline to the center, connect the amp
meter in one line to the antenna matcher/transmitter, connect the outside
contacts in a crossed fashion and then switching from one side to the other
puts the amp meter in each line.

Healthfully yours,
                          Don W4BWS
----- Original Message ----- 
From: "D. Chester" <k4kyv at charter.net>
To: <amradio at mailman.qth.net>
Sent: Saturday, February 10, 2007 12:04 PM
Subject: [AMRadio] Re: Thermocouple rf ammeter


> Usually it will be stamped on the meter to indicate whether or not an
> external thermocouple is required.  If the back of the meter is brown
> bakelite instead of black, that usually (but not always) indicates a
> built-in thermocouple.
>
> Otherwise, try measuring the resistance using an ohmmeter or DVM.  The
> thermocouple should read no more than a fraction of an ohm.  The DC meter
> movement will  read in the tens of ohms.  Be sure to use the highest
> resistance range available, then work down to the next lowest, etc, since
> some ohmmeters run enough current in the lower resistance ranges to
destroy
> a sensitive movement.  If the meter deflects positive with the ohmmeter
> connected, then deflects backwards when the leads are reversed, that is a
> sure sign that the meter requires an external thermocouple.
>
> For open wire balanced feeders, ideally you would use a separate meter in
> each feeder.  If a matched pair of meters is not available, one meter in
one
> leg would be ok.  With a 4PDT knife switch, you could rig up a switching
> circuit to switch the meter  from one side to the other.  Be sure to
> disconnect the meter when not in use, since it doesn't take much of a
> lightning jolt to destroy an rf ammeter.  I  lost my very best one a
couple
> of years ago when lightning turned thermocouple, meter movement and all,
> into a block of charcoal.
>
> Don k4kyv
>
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