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

To: <[email protected]>
Subject: Re: LF: Tune and match the ant for 50kHz...550kHz
From: [email protected]
Date: 02 Apr 2009 18:02 GMT
In-reply-to: <[email protected]>
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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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