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Thermocouple RF Ammeter

hevans1944

Hop - AC8NS
Jun 21, 2012
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even with their age, they are still one of the best units out there
Yeah, I just might score one at the Dayton Hamvention this year if the price is right. I have this strange attraction for D'Arsonval meter movements, with or without a mirrored scale for parallax error prevention.
 

dorke

Jun 20, 2015
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RF ammeters like the ones @Y2KEDDIE has, or the simple vacuum thermocouple attached to a heater wire as in the image I posted, are functionally obsolete, replaced by better technology. However, the first principle of measuring RF power by measuring the amount of heat it produces is always valid. Only the methods have changed, not the fundamental principle. Thermocouples are still very much a part of RF power measurements, as are square-law diodes and thermistor sensors. Here is a very thorough discussion of the state-of-the-art in RF power measurement, published by Agilent (formerly Hewlett-Packard), a world-class leader in RF power measurement instrumentation.

The Bird Model 43 wattmeter is a tried-and-true in-line RF power meter that, using a directional-coupler, measures forward and reflected power when inserted in a 50 Ω transmission line. But it is limited to CW measurements. The Bird has been the "gold standard" in both commercial and radio amateur practice since practically forever, but it too is rather primitive technology. At the this year's upcoming Hamvention® in Dayton, Ohio, there will be dozens of vendors offering Bird wattmeters and "slugs" for same... along with upcoming clone products.

The Bird is popular, not because it is particularly accurate and inexpensive (it is not!), but because it simply works. Many hams shun the SWR bridge, which is constructed using similar principles, and simply tune their rigs for minimum reflected power using a Bird wattmeter. Among hams, the forward power measurement is mainly for bragging rights, IMHO, because the type of ham who uses a Bird is usually aiming for maximum power output as compared to the ham who pursues QRP operation, or the "minimum power necessary for communication" as specified in the FCC rules for amateur radio.

The Bird 43 is an outstanding engineering item .
So well build and excellent for the job,
practically sells the same product from the beginning ,about 65 years back.
That is truly an amazing thing for a test instrument.

The directional-couplers and SWR bridges are not only used by hams.
Many RF transmitters have them inside for the purpose of automatic antenna tuning or PA protection.
 

Y2KEDDIE

Sep 23, 2012
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I've seen them mounted at the transmitter output , permanently, at the transmitter end on FM Commercial broadcast installations. I alway's thought they work best connected at the base of the antennae , but not many techs, including myself ,
The Bird 43 is an outstanding engineering item .
So well build and excellent for the job,
practically sells the same product from the beginning ,about 65 years back.
That is truly an amazing thing for a test instrument.

The directional-couplers and SWR bridges are not only used by hams.
Many RF transmitters have them inside for the purpose of automatic antenna tuning or PA protection.

like climbing 100' towers. Two units in series, back to back lets you read both forward and reflected power simultaneously, then you can calculate SWR.
 

dorke

Jun 20, 2015
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I've seen them mounted at the transmitter output , permanently, at the transmitter end on FM Commercial broadcast installations. I alway's thought they work best connected at the base of the antennae , but not many techs, including myself ,


like climbing 100' towers. Two units in series, back to back lets you read both forward and reflected power simultaneously, then you can calculate SWR.

For SWR monitoring BIRD has the model 4342 Dual meter.
With scales for foreword/reverse and SWR scales
Very nice dual needle movement;).
bird dual SWR.JPG

 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Yeah! That's the meter movement most hams would use! Who invented this? How long has it been around? The meter movements are quite popular items on the Web because any idiot can build a directional coupler... no electronics experience required if you can follow directions.

Well everybody's heard about the Bird.
Bird, bird, bird. Bird is the word.
Don't you know about the bird?
Everyone's talking about the Bird!
Bird, bird, bird. Bird is the word...

(Lyrics stolen from Surfin' Bird performed/sung by The Trashmen)
 

Bluegrass

Dec 20, 2019
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I just found this forum discussing RF Thermocouple meters.
While older threads, I wanted to add some update in using RF Thermocouple meters.
There is the notion that these meters cannot measure in a Coax system very accurately and then only at relatively low frequency.
I have one GE meter described in the thread a 0 to 4 amp unit.
Have always wanted to see if packaged a certain way, the meter frequency range could be extended in coax application.
Some basics of the meter movement.
A resistive element, thermocouple placed in a closed container, leads internally brought out to a sensitive readout meter.
Build:
Meter mounted to a 4 inch electrical plate with round opening.
Meter face and outer rim on the outside.
4 inch electrical box with UHF connectors through the back directly connect to the meter terminals as short as possible.
4" case is the coax shield continuation.
Test results:
This assembly caused no impedance upset when a non inductive flat 50 ohm RF load is connected to one terminal on test frequencies as high a 6 meter amateur radio band (to 60 MHz). Testing done initially with an Antenna Analyzer then under transmitter power.
A bird 43 placed in series also verifies this parameters as well as accurate power indication using I x I x R.
Needless to say I was dumb founded at the performance of this project.
.
Does this 'now" instrument have a use?
Not much these days in the presence of modern power measuring methods unless one has a specific application.
Bottom line is, if the working transmission line impedance can be maintained, this type meter appears as good as any other.
Also this meter responds the same way any other meter does in the presence of a mismatch from the load.
Said another way, reflected power can result in greater or lesser heating of the series resistance the Thermocouple measures and adds to or subtracts from the meter indication.
Lastly, some will say the meter has to be mount on non metallic panel.
Can't be in this application and be used for coax current measurements. There is no hand or proximity effect on the out side housing 'unless' there would be a mismatch causing common mode current.
Hand effects in a metal case is only sensitive to placing fingers 'directly' on the meter face glass that one would normally not do for any reason I can think of during normal measurement use.
Most all other meter movements can be sensitive to glass charge ,as well.
Further testing in a in a real transmitter system, antenna tuner adjustments to see effects shows the results of miss matches both j+ and j- situations. Since 4 amps in 50 ohm systems is 800 watts I chose to test well below this power so as not to make a mistake and ruin a nice project so limited it to about 300 watts. Of course the accuracy is no better than any other instrument at +/- 5% or so and depends on the part of the meter face the reading is taken.
.
I built a second meter in the exact same way but using a different make.
The results are similar except the highest usable frequency is the10 meter band.
This leads me to believe the thermocouple is built differently than the first one, never the less still usable.
.
To close, be a where a meter that does not say Thermocouple but only RF amps is likely not a Thermocouple as self contained, but regular meter with an RF scale used with a known matching external thermocouple device at a much different impedance level.
 
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