Using RF Ammeters

Discussion in 'Amateur Radio' started by W5HRO, Jul 15, 2015.

  1. W5HRO

    W5HRO Administrator

    I just bought two new in the box 0A to 5A Simpson RF Ammeters on ebay. I asked the seller what the backsides looked like and he said they were metal. I then went to Simpson's website and fired off a few emails trying to find out if they had the thermocouples built in or not. The head engineer said we cannot help you because the meter was at least 15 years old. Yeah right, more like 50 years old ::) I then finally received another reply from an old Sr. Tech. Plant Services & Product Repair guy at Simpson who remembers working on the meters years ago and he said they had internal welded cross thermocouples and were self-contained. The metal housings were probably for RF shielding.

    Anyway, I bought the last two the guy had, because you just cannot find these meters still new and unused anymore and with the 5A scales and I needed one for my HB antenna coupler. I have an old 3A Triplett in there now, but I don't yet have the thermocouple for it and 3A is really too small anyway. 500W carrier out into 50-ohms is 3.16 amps so the 3A meter would often get pegged. The 5A meter is perfect.

    I did find an old 10A Westinghouse meter a few years ago, but 10A is too big. It does have the brown back so it has the built-in thermocouple. That was the indicator on many of the old RF ammeters. If the backs were black then you needed an external thermocouple. If they were brown they were built-in. I guess Simpson started making them with metal housings for RF shielding with the built-in thermocouples so not all brands were the same.

  2. W5HRO

    W5HRO Administrator

    I received the two meters yesterday and below are pics of the back and inside of the box.


    I knew they were the smaller 2.5" size and I've been trying to find one of my 3.5" to 2.5" panel adapters. I use to have one or two, but I don't know where they are anymore. They were for adapting the smaller size meters to the bigger size hole in 19" panels. The hole in my antenna coupler's panel is for the 3.5" meter. I'll either have to make one or just change the panel.
  3. K4TQF

    K4TQF Member

    I think those are the same as in my BC-939s.
  4. Opcom

    Opcom Member

    Have you tested it yet? The ones used with the external TC usually have some "+" or other marking for polarity.

    I was told that the older meters with external thermocouples are not always good up to 30MHz let alone 50.
    I don't know about being no good - I might expect some inaccuracies, but if an indication is all that's wanted then it may not be so important.

    Has anyone experience with this sort of thing?
  5. W5HRO

    W5HRO Administrator

    Hi Patrick,

    No, I have not used them yet, but see the red paint mark on the rubber above the right terminal. That should be +