In article .com,
"terryS" in rec.radio.amateur.boatanchors
writ:

On Mar 4, 8:57 am, (Chris Suslowicz) wrote:
In article .com,

"terryS" wrote:
Came across a couple of those RF antenna current meters. Typical they
have a scale up to three of five amps. One is US and other is UK war
surplus.
AIUI the meter movement is driven by a small DC current produced from
the output of the thermocouple.
The thermocouple being heated by the RF current. Looks like the meter
itself is a one or five milliamp movement. Can anyone comment/advise?
Thanks.

The British ones (black bakelite cases) almost always have the thermocouple
built into the meter. They will usually be marked "Thermocouple" or "For
Radio Frequency Only" on the dial, and the resistance is usually shown
as well.

Be careful: the thermocouple is *VERY* delicate and will not stand up
to overload - the nichrome wire melts and it's useless thereafter.
(I've blown up an ex-Admiralty plug-in meter that was scaled 0-500mA
but had "requires x20 shunt" pencilled very faintly on the back.)

Chris.

Thanks Chris and others. Part of what I was wondering is a seemingly
'dud' RF ammeter. The movement might be OK and could be, say, 1 or 5
milliamp etc. fsd.

When I was a kid there were thermocouple ammeters ex T1154 etc on the
surplus market, usually at about 25% of the price of a normal surplus
meter. So, snip out the thermocouple and get a cheap voltmeter. Not
so, the meter relied on the low Z of the thermocouple for damping.
10 to 20 seconds of oscillation before a stable reading if used as a
voltmeter, about passable if used with a 20A shunt.

jww.

Richard Hall wrote:
Somebody recently said they might have a suitable thermocouple in their
junk box and asked which type the meter used. The article, in the May
1969 issue of Popular Electronics, doesn't say.

No, but the manual for the command set may. Or you might be able to figure
it out by looking at the color of the metal used.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Richard Hall writes:

The May 1969 issue of Popular Electronics has plans to make a 5 Amp. AC
Ammeter out of a BC-442 meter. It uses 300 ohm twin lead as a 0.1375 ohm
shunt. They show 31" plus 1/2" for connections at each end. I have the
proper meter but never acquired the thermocouple.

K3HVG wrote:
Well, they could be either a self-contained meter; one that one hooks up
directly, inline/series, with the RF current and the other is as you
have described. If the meters are not of the sealed type, you can
remove the movement from the shell and have a look. Try and see if
there's a thermocouple mounted somewhere as an integral part of the
unit. Meters, like on the TCS, BC-191/375, and the ART-13 for example,
are self-contained. They generally all work the same, its just where
the thermocouple is located. That's my observation on the subject.....

terryS wrote:
Came across a couple of those RF antenna current meters. Typical they
have a scale up to three of five amps. One is US and other is UK war
surplus.
AIUI the meter movement is driven by a small DC current produced from
the output of the thermocouple.
The thermocouple being heated by the RF current. Looks like the meter
itself is a one or five milliamp movement. Can anyone comment/advise?
Thanks.


It is the BC-442 antenna current meter. I built the meter from the article.
Still use it.

Fair Radio had the thermocouple units-- now out of stock, but it might
be worth an email.

Steve.
--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA

"COLIN LAMB" writes:

You can easily determine if the meter is a remote reading meter or one with
a built in thermocouple. Put a digital ohmmeter on the meter leads. if the
resistance is very low, then the thermocuple is built in. If the resistance
reads tens of ohms or more, it is a simple dc meter that can be used for any
purpose.

I have found that many of the remote reading meters are not calibrated in
actual amps, but are relative reading.

Often the rf ammeters that are self contained contain a bakelite or formica
back to give clearance to the rf.

Colin K7FM


http://www.fairradio.com/catalog.php...witem&item=551

--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA