Dear Petr,
at the
input of my VLF-LF converter I have just a simple low pass filter with a
cut-off
frequency of about 80 kHz. This is sufficient even for a low-power
mixer IC
like an NE602 or similars. For the higher frequencies I have built a
preselector-amplifier combination with four tuning ranges, 110-190 kHz,
310-1590
kHz, 1,6-9,1 MHz, and 6,7-40 MHz, in the form of a simple parallel LC
circuit,
tuned by a double variable capacitor with a maximum
capacity of about 800 pF. The toroids for the low bands tuned
circuits
are ferrite toroids with an AL value around 700. Their Q ist not as high
as with
pot cores but sufficient. The amplifier is an LT1252 op amp driven
directly from
the high impedance parallel
circuit. This preselector-amplifier
combination ist mostly employed behind the loop antenna, because
from my
location in eastern bavaria most LF and MF activities come
from northern directions.
OK?
73 Ha-Jo,
DJ1ZB
"Petr Malý" <[email protected]> schrieb:
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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