Hi J, yes you may still have some effect there. My simple thoughts on
tranformers are as follows.
If you are only aiming at 136kHz wide-banding the transformer is not so
important so bi-filar winding is not so important, though it will keep down
the losses. Overwinding the two winding should be adequate.
The turns should be calculated so that the reactive inductance is about 3 to
5 times the expected / required load impedance. So the 50 ohm side should
have enough turns to have a reactance of 150 to 250 ohms at 136kHz.
My guess is you do not want a 1:1 transformer anyway as it looks as though
the resonant impedance of your antenna system is about 12 ohms, so you
actually need a 2:1 step-up from the antenna to the coax. Roughly this means
you need to double the number of turns on the coax side of the transformer,
the antenna side is probably about right.
This being the case just overwind the two windings on the core with a few
taps on the 50ohm side to achieve the matching, after the antenna has been
resonated (made purely resistive.) The best way to find the right tap maybe
to look at the scopemeter with a dummy load on the end of the coax, and
adjust the taps after resonating to get the same picture.
To Mike G3XDV, J's problem has been that he had several millihenries in
the coil following our literature but he has a much bigger antenna system
that is common in the UK....he has a capacity of 1200 to 1300 pF !! so he
only need a coil of several hundred micro-henries....maybe 1mH at the very
most. He was not geting up to resonance, and at the end of 250feet feet of
coax was getting strange results.
Well done on getting some fragments to Scott, you should be able to bridge
1000miles easily on QRSS with 100W and that antenna. I have achieved 500
miles (just !!) with 2W to a much smaller system (30foot high 800pF
Marconi).
Best Wishes de Alan G3NYK
----- Original Message -----
From: J. Allen <[email protected]>
To: LF (RSGB) <[email protected]>; <[email protected]>
Sent: 09 July 2006 05:00
Subject: LF: transformer question
> Alan and All,
>
> Alan, you were concerned about the transformer that was added between the
> coax and the loading coil at the base of the 137 kHz. antenna. Here
> follow-up to your question.
>
> If you remember it is a bifilar winding of ten turns which takes up on
inch
> length on a 10-3/4 inch form.
>
> I used Solenoid3 which may or may not be the right tool for the job, and
> calculate the self-impedance of the windings to be just about 50 Ohms at
> 137.8 kHz. I do not know if 50 Ohms is good, bad or what. Can anyone tell
> me?
>
> I am concerned that the 50 Ohms of self impedance might be giving me a
false
> OK on my system and that I actually still have antenna problems even
though
> the system radiates and looks matched.
>
> Even in the bad conditions yesterday, some bits of the transmission have
> appeared on Scott's screen 1000 Miles distant, and we are trying it again
> tonight.
>
> The transformer was wound by guess, since I cannot remember if I have seen
> any data on properly wining a solenoid type transformer with bifilar
> winding.
>
> Anyone know the answer or have a link to the information on winding these?
>
> J.
>
>
>
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