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.

--
If it doesn't contravene the Criminal Justice Act (1988) it has no place
in a kitchen. -- Peter Corlett in the scary devil monastery.

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.

In article .com,
"terryS" wrote:

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.


Ah, right. In that case:

The ex-RAF meters (marked A (crown) M for Air Ministry) have the resistance
wire soldered directly to the back of the terminals on the case. The
thermocouple is then spotwelded to the middle of the resistance wire,
and one lead goes to one side of the movement directly, the other goes
via a "coil spring" of nichrome resistance wire (length cut to get the
thermocouple calibration correct) to the other side of the movement.

If it's open circuit across the terminals, then the resistance element
has blown. The thermocouple and movement should still be perfectly OK.

(I've considered trying to replace the resistance wire but calibrating
it will be the difficult part.)

Somewhere I have a box of dead meters....

I may be some time.

Chris.

--
Most uses of "spider" have noninsect meanings. -- Christine Malcom-Dept.

On Mar 5, 3:26 pm, (Chris Suslowicz) wrote:
In article .com,

"terryS" wrote:
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.

Ah, right. In that case:

The ex-RAF meters (marked A (crown) M for Air Ministry) have the resistance
wire soldered directly to the back of the terminals on the case. The
thermocouple is then spotwelded to the middle of the resistance wire,
and one lead goes to one side of the movement directly, the other goes
via a "coil spring" of nichrome resistance wire (length cut to get the
thermocouple calibration correct) to the other side of the movement.

If it's open circuit across the terminals, then the resistance element
has blown. The thermocouple and movement should still be perfectly OK.

(I've considered trying to replace the resistance wire but calibrating
it will be the difficult part.)

Somewhere I have a box of dead meters....

I may be some time.

Chris.

--
Most uses of "spider" have noninsect meanings. -- Christine Malcom-Dept.

Back in the old marine electronics days we had those 410 to 512 Kcs MF
transmitters. They were all required to have built in RF Ammeters. The
transmitters had to meet minimum SOLAS radiation requirements by
multiplying the RF current by the antenna height above the deepest
water line. Usually they were 15 amps or 5 amps fsd (15 for the main
tx and 5 for the emergency tx). These meters were pretty accurate on
mf but some of the transmitters used them on hf also. On hf they were
only accurate depending on the length of antenna/frequency, where the
current node is. We are talking a single wire type feeder. At mf the
current is pretty much the same anywhere on the antenna due to the
long wavelength. I still see some of these meters being sold at
hamfests. Virtually every RF Ammeter used had a built-in shunt. Randy,
wb5kcm

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.

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.

Here's the article:
http://206.124.141.169/albums/acammeter/index.htm

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."