Petr,
you can use a Pi filter, but I am afraid that the values might become
rather inconvenient:
A short (relative to the wavelength) wire antenna will act as a R-C
series circuit where C is the antenna capacitance and R mainly the loss
resistance.
In most cases R is in the range of 10 .. 200 Ohm while Xc is in the range
of 500 .. 50000 Ohm (depending on antenna length and frequency).
A long wire goes with a relative small Xc but also relative small R
(footprint theory), a short wire with a large Xc and large R.
So in almost any case Xc >>> R. This will result in a large
value for the L of the filter an relative small values for the C's.
If R = 50 Ohm you would end up with Pi filter with an large (XL =
Xc) and both C's = 0 (so you can omit them).
73, Rik ON7YD - OR7T
At 15:15 2/04/2009, you wrote:
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 -----
- From:
[email protected]
- To:
[email protected]
- 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
-
-
-
- Disclaimer:
http://www.kuleuven.be/cwis/email_disclaimer.htm for more
information.
-
Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm for more information.
|
