Dear Petr,
I guess the easiest
solution would be to seperate receiving and transmitting.
For
receive it
is not necessary at all to tune the antenna. I am using my LF T-antenna
for
receive from 10 kHz to 30 MHz (in addition I also have an untuned wire
loop
covering 10 kHz to at least 500 kHz, pointing to the north). Tuning the
antenna
would just increase the antenna noise too, as SM6BHZ has also told me.
You may
just need somewhat more amplification before the rx, and possibly an rx
preselection before this preamplifier.
Concentrating on just
tuning
the antenna for transmitting on either 136 kHz or 500 kHz would be much
easier.
And, by the way, the "sphere in sphere"
variometer has
a big drawback: Its Q is quite OK at maximum inductivity but
very poor at
its minumum. Explanation is simple. Just assuming the resistance of
the
wire being constant, at low inducitivity the relation XL to R
becomes very
poor.
OK?
73 Ha-Jo, DJ1ZB
"Rik Strobbe" <[email protected]> schrieb:
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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