LF: Re: LF SWR Bridge

["James Moritz" <james.moritz@btopenworld.com>]

Dear Rob, LF Group,

The inductance of the transformer is probably a bit on the low side (the 
reactance of the total 24turn winding should be much greater than the 68R 
load resistance), but should be enough to get at least reasonably close to 
balance - you should be able to see at least some reduction on the "reflect" 
side with 50R load connected. Check that the phasing of the secondary 
winding is correct - the centre tap should be the connection between the 
"start" of one winding and the "finish" of the other.
What should happen is the RF voltage on one half of the current transformer 
secondary is equal in magnitude, but opposite in phase to the load voltage 
sensed by the capacitive voltage divider when the load is 50R. The component 
values shown look about right to do this. One way to check this would be to:
-Disconnect the "hot" end of the capacitive potential divider from the TX 
input terminal. Note the meter readings with the TX running into 50R load.
-Disable the current transformer by shorting the 24 turn secondary and 
re-connect the capacitors .Check the meter deflections with 50R load - it 
should be possible to get these the same as in the first test by adjusting 
the trimmer capacitor.
You can find some other ideas about LF antenna tuning aids in the file 
LF_Tuning_aids.pdf at 
https://sites.google.com/site/uk500khz/members-files/files
Cheers, Jim Moritz
73 de M0BMU