Re: LF: LF SWR Bridge

[Gary - G4WGT <g4wgt@tiscali.co.uk>]

Hello Rob & Stefan,

That is the one I used & I can highly recommend it. I have now changed to using the "ScopeMatch" tuning aid which is in "The Low Frequency Experimenters Handbook" by the same author. Of course you will require an oscilloscope for the display but I find it more accurate & very easy to use. Construction is very simple.

If you wish to see the article (with other tuning aids) I will place it on my web site for down load, size is 2.6Mb.

73

Gary - G4WGT.

2011/5/15 Stefan Schäfer <schaefer@iup.uni-heidelberg.de>

Hello Rob,

Instead of a SWR bridge i would recommend a voltage/current and a phase meter. The voltage/current ratio should be 50:1, so you can easily check for 50 Ohm. As i started with LF in 2003 i found that pdf from Jim/M0BMU with a good description. The advantage of that tuning meter is that you can see if the mismatch is caused by a reactive part (= not in resonance) or due to an impedance that is not equal to 50 Ohm.
I'm not sure if the link is still working but i found the pdf in my old LF folder and put it to http://dl.dropbox.com/u/19882028/LF/LFtunemeter.pdf ...if you want to take a look there...

73, Stefan/DK7FC



Am 15.05.2011 12:09, schrieb Rob, M0DTS:

Hi All..

I'm trying to put together my 137KHz transmitter after it has gathered dust in the loft for a few years and will hopefully generate some signals again soon..
I'm not so good with this LF so need a bit of help with an swr bridge..  This will stop me blowing FET's this time i hope!

I've seen a couple of designs for an swr bridge and have tried out G0MRF's one here which appears so simple who could go wrong?!

I'm using a 3c90 core,  a very small one (13mm) but i should be able to get the same effect I'm sure,  i just can't get any difference between forward/reflect at all with 50R on the load port..  the only difference i have in my build is the different core size.
I've tried increasing the bifiliar turns but just gave more coupling as you would expect..

I guess I'm just very low on inductance by using the different core so the q is too low and i don't 'see' the dip in reflected signal when tuned?

Any ideas and a simple explanation of how it works would be useful..

Thanks

Rob
M0DTS