Re: LF: Tune and match the ant for 50kHz...550kHz

[Rik Strobbe <rik.strobbe@fys.kuleuven.be>]

Hello Petr,

50-550kHz seems a "big shot".
If you want to tune (bring to resonance) the antenna using a variometer
it would need a ratio of (550/50)^2 = 121.
The variometer I use on 500kHz has a range of 40-450uH, thus a ratio of
11. The variometer comes from an 500kHz marine transmitter and it would
be rather complex to build a copy (mechanical).
Running low power you could use a parallel LC circuit. One end to the
antenna and the other end to ground. The TX is connected to a tap at the
coil, close to ground. You can tune the antenna to resonance by changing
C (variable capacitor) and match to 50 Ohm by changing taps on the coil.
I did that with success in the early days on 136kHz, but I could run only
30W power before the capacitor (plate distance 2mm) started arcing. 
Now you will need a capacitor with a ratio of 121, but that is not so
hard: most variable capacitors have a range of 20 or better and you can
put some fixed capacitors in parallel (via switches).

73, Rik  ON7YD

At 09:26 2/04/2009, you wrote:
> Hi all,
>  
> I want to make a small transmatch (RX, TX up to 10 Watts, or
> so) to tune LW 41 m (or smaller T-ant) in the range 50 ... 550 kHz.
> I am not too good in theory... but I believe that the most efficient
> system is the popular loading coil (home made variometer, and taps to
> find 50 Ohms match).
> The simple variometer (cylindrical coil in another cylindrical coil) is
> easy to make and works fine on 136kHz. However, it is possible to change
> inductance in the range about 1:2 or 1:3 only, not much better. Therefore
> the redudant inductance is too high to fetch the ant to resonance on 550
> kHz.
> Solution would be to make a more sophisticated variometer (best: sphere
> in sphere) to reach the ratio 1:10 or so.
> Or, to use a different kind of network. Pi network, L network or T
> network. BTW I would also prefer to use a rotary switch and solder twenty
> condensers rather than to make twenty taps on a coil wound with litz
> wire...
>  
> How did you solve this yourself?
>  
> Thanks, 73, Petr, OK1FIG
>  
>  
>  


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