[600MRG] RF Ammeter

[Scott Armstrong]

Thanks for all the  information received on and off the list.  I've got a
couple usable designs to build something. The challenge now is to find what
is available in the junk box!

@John

I place a VOM across the terminals of the Simpson meter and resistance is
equal to or slightly higher than what the lead resistance of the  VOM is.
Need to check with a more reliable VOM but looks promising thus far.  The
Simpson meter  will be of more use if/when I get something up in the
200-300W range.

Seeing the information that Niel provided and other info found on the web
about testing with a AC /60Hz source....

Would a clamp-on AC ammeter work for measuring RF currents at 2200M and
630M or does the frequency response of clamp-on/inductive coupling roll off
and is no longer useable?

-Scott AA5AM

On Sat, Nov 21, 2020 at 7:45 AM Neil Klagge <w0yse at msn.com> wrote:

> Scott, here is a link to W5JGV’s projects page  Showing how he made a
> homemade  meter. I have built several of these.
>
> http://w5jgv.com/rfa-2/rfa-2.htm
>
> You can calibrate it with AC through 100 W bulb if necessary. Of course be
> careful with the clip leads when you do that LOL.
>
> Neil Klagge
> w0yse
> Northern Utah
>
>
> Sent from Neil's iPhone
>
> On Nov 21, 2020, at 05:51, John Langridge <kb5njd at gmail.com> wrote:
>
> Hi Scott, the way to tell if its a thermo is to put a VOM across the
> terminals.  If the R is very low or zero, its likely a thermocouple.
> If you see winding resistance, like 20 ohms or 100 ohms, for example,
> its just a volt meter with a current scale that will need to be tied
> to a therm or a current transformer...  I often find that the meters
> with deeper cases tend to be more likely to be a therm... Also, my
> experience with buying working thermocouple at hamfess is about 50%..
> often times they show up with a burned-up junction or coil...
>
> W5JGV had a nice transformer-based current meter with remote sampling.
> I dont have the link handy right now but it may be on my links page...
>
> I use both here and monitor antenna current in the ham shack
> religiously.  I calibrated a volt meter with a current scale to a
> transformer in the ATU to  a thermocouple ammeter.  Its linear where I
> use it.  but I find the information far better at determining system
> health than SWR...
>
> as you suggest, your meter may have a hard time at lower current
> levels.  but determine what it actually is first.  A current
> transformer and a volt meter can better resolve those low levels if
> you have an accurate meter to calibrate with.
>
> 73!
>
> John..
>
>
>
>
>
>
> On 11/20/20, Scott Armstrong <aa5am at vntx.net> wrote:
>
> Hi Guys,
>
>
>
> A couple of questions....
>
>
>
> Wondering if anyone has any information on this meter.
>
>
> I have a Simpson Model 135 RF Ammeter.  Full scale is 2.5A
>
> Does this meter have a thermocouple built in or did it use an external
>
> thermocouple?
>
>
>
>
> Can anyone point me to a schematic for a RF Ammeter that will work at 2200m
>
> and 630m and has a top end of say 1-1.5 A.
>
>
> The Simpson meter, if it does have a TC built in, will not work very well
>
> for low power ~25Wapplication. The meter scale being non-linear  does not
>
> have much resolution on the bottom end of the scale around the .7A range.
>
>
> Thanks,
>
> Scott AA5AM
>
>
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