Current transformer with RF ammeter?

[WB3JOK]

I have a brand-new 3.5" Simpson "Wide-Vue" RF ammeter (0-2.5 amps) gathering dust in the junkbox, and had the idea to measure the current into my dummy load. The scale is rather compressed below 1 amp, so at the minimum useful indication that's 50 watts (into 50 ohms), and 2.5 amps full-scale is obviously 312.5 W. In order to extend the range, if I could use a current transformer with a 2:1 ratio, then full scale would actually be 5 amps = 1250 W, enough for anything I would run.

Can someone help me design such a transformer? I figure 1 or 2 turns primary in-line between the SO239 and the resistor, therefore 2 or 4 turns secondary to the meter... but what mix and how physically large a toroid is needed (for the HF bands)? I don't know the burden of this RF ammeter.

(Yes, I know I can also use a voltmeter to calculate power - just wondering how to use the ammeter instead) 

[W4NEQ]

This link is about the best RF CT discussion I've seen ...

http://www.g3ynh.info/zdocs/bridges/Xformers/part_1.html

Chris

[WB3JOK]

Thanks... that is certainly detailed (and beyond my atrophied EE skills but that's another problem) 

However, I can't find anything about my particular application (driving an ammeter). The conventional use of current transformers, as exhaustively detailed in that article, is to drive a resistor and measure the voltage across it.

It occurs to me that it may not be possible to do what I'm trying to do - an ideal ammeter acts like a short circuit to the transformer secondary...

[W4NEQ]

And that short is reflected into the primary as a near short too.  The better to pass your RF current with ...

I have found that very flat frequency response 160-40 meters occurs with 22 turns on a 140-61 torroid core, when terminated in 50 ohms, using a pass-through primary.  I used a 1n5711 diode detector to obtain a dc voltage ( minus about .38v) which can then be coupled to a 0-1 ma meter with a calibration resistor to read accurate current.

Your application would require some experimentation ...

Chris

[WB3JOK]

I had tried a small (3/4" diameter) yellow core that I thought was suitable, but it hardly registered anything on the ammeter and the core itself got quite warm. Apparently it's not the yellow -6 mix.

Anyhow, I found a (marked) T140-61 core in the toroid drawer and it works great.. except that to get a 2:1 current ratio, I ended up with a 2 turn primary and a 3 turn secondary... each pass through the center of the core is counted as one turn. I would have thought it would require 4 turns.  

I rigged up a toggle switch with short leads for x1 (transformer not in circuit) or x2. So 100W is 1.4 amps on x1 and 1KW is 4.5 amps, 2.2 on the x2 range... covers most of my 50 ohm power measurement needs  

The dummy load itself is ten 470 ohm 10 watt wirewound resistors, fan cooled, with an air variable in parallel to tune out the inductance on the higher bands. DCR is 48.7 ohms. It's nearly a perfect match up to 10m when the variable is set for minimum SWR (barely moves the needle on my HM-102 bridge). Who needs noninductive Globars anyway

Edit: even better match than I remembered