[Boatanchors] ge 8dw44 rf amp meter,, ideas please

[Gary Peterson]

I have used RF ammeters for many decades, both in AM broadcast service and amateur radio.

First, determine if the meter works.  A quick, simple test is to rock the meter from side to side.  If the pointer shows little movement, this is a good sign.  If the needle freely flops, the internal thermocouple may be burned out and it might be toast.  You may still have a good, sensitive meter movement but you don’t have a usable RF ammeter.  Another test is to measure the meter’s terminals with an ohmmeter.  If the resistance is low, this is good.  If it measures a very high resistance or open, the thermocouple has burned out.  

These RF ammeters have a heating element in series with the meter terminals.  When RF passes through, the heating element heats a thermocouple, which produces a small DC current that drives the meter movement.  The heating elements are subject to burnout, as are the filaments in incandescent light bulbs.

Older (1950s) ARRL handbooks describe mounting RF ammeters in enclosures with input and output SO-239 connectors.  The RF ammeter is almost always in between the center conductors of the two RF connectors.  A metal enclosure completes the circuit for the coax shields.  If using a non-conductive enclosure, it is necessary to connect the shells of the RF connectors together with a heavy conductor, such as copper strap.

Power is determined by multiplying the square of the reading, in amperes, by the resistive part of the impedance in the circuit.  If the incoming and outgoing connections to the meter have a resistive part that is 50 ohms and the meter is reading 4.0 RF amperes, the power is 4.0, squared times 50, which is 800 watts.

Unless you know the resistive part of the impedance, where you insert the meter, it only gives relative readings.  It is a good way to adjust a matching network for maximum RF current at the output.  If you do know the resistive part of the impedance, you may use the I squared R method to determine the power at that point in the circuit.  This works well for CW (continuous wave or continuous power).  It is not a useful method for determining peak output power, as the response of the thermocouple is sluggish.  On AM it’s a good method for determining carrier power, without modulation.  On CW it’s a good method for determining key-down power.  It is not useful for determining power on SSB signals, other than relative power.  

Hope that helps.  Have fun & 73,

Gary, KzeroCX

    



“hello BA group.? I came home from a hamfest with a meter I know little 
about.? It is for measuring RF current and I do not know how to use it 
or even test it.? Considering that I do not have a true RF power meter 
(e.g. Bird, etc.) I am wondering if this might prove useful for this 
purpose or any other helpful shack information.? I am currently using a 
restored Ranger-1 for transmit (no RF amp).
.
If you simply search on the name you will find some pictures. Mine is 
the eight amp version. Beyond this, I found little how-to details. thanks.
==> ge 8dw44 rf amp meter
”