Antenna Ammeter

[Ed/KB1HYS]

So how is this coupled to the transmission line?  If it's placed in series with one of the feeders won't the meter and associatted parts have to handle the full RF?  Or is this capacitively or Inductively coupled? 
 

[k4kyv]

All the thermocouple rf ammeters I have seen have the thermocouple mounted either on the inside of the back case of the meter, or the thermocouple unit is separate, with a couple of terminals to  go to the DC meter movement.  The whole thing is insulated, so the meter can be mounted on a metal panel.  I have never used one at a high voltage rf point, so I don't know how much voltage they could stand before arcing over.

I would assume that the W4JGV diode version would work the same way.  The internal resistance of the meter should be very low compared to the impedance of the circuit where the meter is inserted.  You can insert the meter in line, just as you would a hammy hambone wattmeter, or you can put it in series with one of the feeders of an open wire line to measure the current in the  feeders.  Ideally, two identical meters would be used, one in each feeder to monitor  line balance.  I have seen circuits use a double 4PDT knife switch to switch a single meter from one feeder to the other.  The extra poles are used to close the gap on the opposite feeder from the one being metered. It would be advisable to momentarily kill the transmitter while the meter is being switched over.

$139 seems like a lot to pay for an rf ammeter, since they have been available at hamfests for as little as a dollar or even less at hamfests ever since the War, but if you look up the price of a quality thermocouple rf ammeter in catalogues from the 20's and 30's, and adjust for to-day's devalued dollarette, you will find the MoFo Junque meter cheap by comparison.  But I would much prefer to use a real thermocouple meter than a rectifier type.

[Opcom]

Interesting discussion here with lots of food for thought.  However, I seem to learn more by doing than reading so....

I found this article on the web.... http://w5jgv.com/rfa-2/rfa-2.htm

I found a bag of 25, 3 amp Schottky diodes on Ebay for $5.

So, MFJ is going to have to wait for my $139 for that ready to go meter.

I have lots of meters to choose from but most of them do not specify the resistance and I am curious as to how one would go about measuring this.

My digital says 3.5 ohms and my needle multimeter pegs the meter.

It probably does not make much difference anyway. As I understand it, the absolute value is pretty meaningless and relative current is all we really need to know.

js

May I suggest silicon carbide rectifiers for when you run out of Schottkys? Zero recovery and 93 cents each.
http://www.cree.com/products/pdf/CSD01060.M.pdf

[WU2D]

Probably safer and more legal to use low power and an RF Milliammeter when tuning at the antenna site with remote TX activation! And do it when the band is dead!

Mike WU2D

[Mike/W8BAC]

Ed/KB1HYS Wrote

So how is this coupled to the transmission line?  If it's placed in series with one of the feeders won't the meter and associatted parts have to handle the full RF?  Or is this capacitively or Inductively coupled?

My understanding is you have two copper plates and a DC path between them made of a conductor with a slight resistance like nichrome NiCr wire. Attached to the nichrome wire you have two wires of dissimilar metals going to the meter.

I can understand the nichrome. That will heat slightly with RF current flow like the electric toaster. The dissimilar wires to the meter isn't as easy for me to understand but I'm going to check into it.

I work with welders all the time. High chrome content carbon steel alloy pipe requires preheat and post welding heat treat. Service companies like Cooperheat and others use two wire thermocouples attached (spot welded) to that pipe to measure and track the heat treatment accurately. I'll see what I can learn from them.

Remember ED your measuring a voltage only on one side of the AC line and that voltage is a result of heating effect on dissimilar metals. The RF is not terminated in the meter. If the RF load is missing you have Don's situation, POOF.

[N3DRB The Derb]

all other things being equal, the RF ammeter is a "tune for max blowsmoke" instrument on any given antenna and load. It's worth noting that AM broadcast stations almost always use a RF ammeter ( Made by Delta) full time in their transmission line.

Considering you can pick them up all day long at hamfests for a buck each, go buy some and see. Just make sure they say "internal thermocouple" somewhere on the meters face, or come with the matching shunt block with matching type and serial numbers.