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Hello Ha-Jo
Thanks for your suggestion. My transmatch woud be
used mainly for RX, however, I still believe that a tuned antenna is better than
detuned one. At least it is a good preselection for RX. Most of the RX's
have just a LPF at these fq's and therefore they are "broadband" in some way.
The transmatch is then the only tuned circuit in front of the RX.
I am still wondering whether a standard Pi network
could be used? There would be two large banks of condensers on both sides (high
voltage ones on the side of ant) switched by rotary switches. Fine tuning would
happen by the variometer in the middle. The only disadvantage would be that on
the lower end of range (say 50 or 100 kHz) the vario inductivity would be
too low and tuning would be difficult (woolly), while on the higher end (550
kHz), it would be too high, tuning would be rather sharp and losses in the
vario also rather high.
Could this work?
73, Petr
----- Original Message -----
Sent: Thursday, April 02, 2009 1:15
PM
Subject: Re: LF: Tune and match the ant
for 50kHz...550kHz
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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