Topband: measuring transformer core inductance

[Robert Kavanagh]

At 11:22 AM 11/17/03, Richard Karlquist wrote:

>Also, the RF chokes used to feed the DC into
>the coax cannot be wound on non-gapped ferrite
>cores, like toroids.  I see this all the time
>in commercial equipment supposed designed by
>engineers.  My favorite is RF amplifiers that have
>beads on a DC feed with 20 amps going through it.

To make RF chokes I use iron powder toroids. Then the permeability changes 
only a little when you pass d.c. through the winding.

I have made RF chokes for low frequencies (400 kHz) using T-130-26 cores. 
About 9 metres of #30 wire wound on these cores gives an inductance of 
about 2.5 mH. But it takes a lot of winding !

For feeding a relay control current along the coax to remotely switch my 
160 metre antenna I also use powdered iron cores to make the chokes. I 
don't know what material I used, unfortunately, because I just used a 
couple of unlabelled cores which I found in my junk box ! But I know that 
they were powdered iron and probably they were material 26 or something 
similar.

Bob, VE3OSZ