RF Ammeter Help

[Mike Langner]

Here's a trick I used to use when looking at/for RF ammeters.

Typically, but not always, if there is one pair of posts for connection on
the back of the meter, it is a DC meter.  This visual test works on older
Weston, Simpson, Marion, and others, but does NOT work on many of the more
modern "sleek look" RF ammeters, particularly the square thin-faced
Simpsons.

If you see a second set of bolt heads or nuts (which typically do not
protrude much if at all from the back of the meter housing) the thermocouple
is typically inside.

Second test:  all the RF ammeters I've purchased over 50 years that do NOT
have an internal thermocouple are low-current DC meters.  To check without
test instruments (the military surplus organization where I used to get them
did not allow test equipment to be brought into the premises) place a short
across the (usually single) set of large terminals. I used to simply use one
of my car keys when testing surplus meters. Now rotate the meter back and
forth quickly on the axis of the armature. The meter hand should barely move
as you have shorted the generator (d'Arsonval movement) and put a load on
the generator.  Now rotate the meter back and forth quickly with no
connection to the terminals.  If it's a DC unshunted, un-thermocoupled
meter, the meter hand should swing back and forth much farther.

Note:  this trick does not work on moving vane meters, or AC meters with
internal rectifiers.

I've purchased many an RF ammeter in 50+ years of hamming, and almost as
many years in broadcast engineering where we use RF ammeters in AM antenna
systems.  This trick has never failed me!

Mike
K5MGR
________________


-----Original Message-----
From: Boat Anchor Owners and Collectors List
[mailto:BOATANCHORS at LISTSERV.TEMPE.GOV]On Behalf Of J Forster
Sent: Sunday, February 24, 2008 1:46 PM
To: BOATANCHORS at LISTSERV.TEMPE.GOV
Subject: Re: RF Ammeter Help


Some RF ammeters need an external thermocouple assembly. Look at the writing
on the meter face. Also, try a VOM's ohmmeter. If the meter kicks, you need
a TC.

If an external TC is not needed, you can usually apply DC current to see if
it's working. Start very low, in case the TC is not indicated on the face.
See above.

If you in fact have a ot wire RF ammeter, you may well need an RF current
transformer,  otherwise the meter guts will be at antenna voltage.

Is this from a BC-442 type surplus unit?

Best,

-John




You can see if you need a shunt

Chris wrote:

> Hello To the Group
>
>  I have a GE RF ammeter which reads 0-5 amps...I am wanting to use this
inline in front of my Tuner so I an monitor the RF output to my antenna, My
question is do I need to use any type of Shunt resistor on the meter or can
I just put a pair of SO-239 connectors on the meter and start using
it...Please let me know what I need to do to use the meter as I am wanting
to...Thanks so much for your help...Chris

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