[Antennas] Re: Link coupled tuners and other comments

Sun Mar 19 12:41:13 EST 2006

Ron, thanks for that very useful information. Although I'm only using 
resistive loads on the bench right now, I want to feed an 80M EDZ (344' 
doublet) using balanced feedline for 160M. I am using a 3" coil with a 
five turn section cut out of the center for the link. The SWR is 
actually quite responsive to the setting of the parallel output 
capacitor, but there is hardly any effect from the link capacitor. I 
have tried series capacitances from about 500 to 2500 pF, although I had 
to use fixed parallel capacitors to get above 500.
In a discussion with another correspondent, it occurred to me that the 
series capacitor was originally added to link tuner designs as a 
refinement. It may be that I simply don't need the link capacitor with 
my combination of component values. I am just trying to put together 
something simple to use on 160M. I use an old Viking MB on 80M.
Right now I am using two sections of a three section broadcast capacitor 
for the output capacitor. The largest split stator capacitors in my 
junk box with suitable voltage ratings for a KW are only around 100 pF 
per section. So I'm currently looking for a split stator cap in the 
250-300 pF range, or I will wind up paralleling some fixed capacitance 
there too.
On another front, I have a good friend in Melbourne, VK3JD. You may 
know John. I hope to get back Down Under some time soon. When I'm 
down that way, I use the call VK8XX.
73 and tnx agn,
Joe
K2XX
Ron vk3io wrote:
> 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 
> ______________________________________________________________
> Antennas mailing list
> Home: http://mailman.qth.net/mailman/listinfo/antennas
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:Antennas at mailman.qth.net
>>