[Antennas] Re: Link coupled tuners and other comments

Sun Mar 19 07:46:25 EST 2006

All of my tuners are link coupled with a series tuned link, as i use my 160 
metre dipoles with open wire feeders on all bands, from 160 to 10 mtrs.
The size of the series capacitor depends on the number of turns and the 
diameter of the link coil and some other minor factors.
On 160, one of my tuners uses a 3 turn link, so the series capacitor is 
about 2500pf whereas the other tuner has about 5 turns and the series 
capacitor is about 1000pf (both are about 3.5 inch diameter).
The adjustment of this series capacitor is mildly sensitive and the number 
of turns is also not critical.
The other issue with link coupled tuners is for a particular frequency and 
for a particular feeder length, what is the open wire feeder impedance, it 
may be very low, indicating a current maximum point, in which case a 
parallel tuned coil in the tuner may not have enough tap points to enable 
such a low impedance matching.
This then requires the introduction of series capacitor tuning of the 
antenna coil, that requires removing the parallel capacitor and replace it 
with two separate capacitors, each in series with the feeder wires.
Changing to a series mode, requires extra switching or complications and so 
this may not be suitable in your tuner box, so the other solution is to 
change the feeder length. Add (or remove) an extra 10 or 20 feet. This will 
then enable the parallel tuner to cope with a higher impedance.
Another issue is that the feed point impedance may be very high, indicating 
a very high impedance point requiring a very wide spaced parallel tuning 
capacitor. This also can be solved by lengthening or removing some feeder 
length and so reducing the feed point impedance.
Lastly when working with a parallel mode antenna side coil is to provide 
plenty of tap points for the feeder connection and ensure they are 
symmetrically spaced, the same number of turns for each tap point for each 
half of the coil, this will ensure a balanced feed, provided your antenna 
and feeder are symmetrical (balanced).
Good Luck, from Ron vk3io vk3io at iinet.net.au 