[Boatanchors] RF current sensor

[Dave Brown]

Roger
It certainly would be OK.
Put as many turns as you can easily- number of turns not critical in 
your case as calibration is not an issue- on the toroid and load the 
winding with a shunt resistor. A series diode from one end of that 
winding to the meter and a cap across the meter (say 0.1 uF) with the 
other end of the winding going to the other meter terminal. Add series 
resistance between diode/cap junction and meter terminal if needed - 
maybe a pot- and/or juggle the shunt resistor on the toroid winding to 
get whatever reading you want on the meter.  The toroid is threaded 
onto the conductor you want the current indication from, of course!
 The resistor values to use will depend on the sensitivity of the 
meter movement and the rf power in use- all values non critical in 
your (uncalibrated) case.  Try 4k7 across the winding for starters 
(assuming say 100 watts rf) and maybe a similar value pot to set the 
meter reading as required.  Be aware that the reading will be 
frequency dependant-you can reduce this effect by keeping the shunt 
resistor across the winding as low as possible.
 73
 Dave, ZL3FJ


----- Original Message ----- 
From: "Roger Basford" <roger at new-gate.co.uk>
To: <boatanchors at mailman.qth.net>
Sent: Thursday, May 03, 2007 6:40 AM
Subject: [Boatanchors] RF current sensor


> Hi All,
>
> I've been looking without success for an 15 amp RF meter for my 
> BC-939 ATU. As I'm not really bothered about a calibrated scale for 
> the meter, just tuning for maximum output, I think that a current 
> sensor driving a milliampere meter would probably be OK. I used to 
> have a simple design used on a VLF TX using a toroid with a link 
> passing through the centre carrying the RF current. Unfortunately, I 
> can't find the circuit again. Anyone tried a similar method of 
> replacing an RF ammeter?
>
> Regards,
>
> Roger Basford/G3VKM
>
>
>
>
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