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LF: Re: TX-ouput transformer problems

To: [email protected]
Subject: LF: Re: TX-ouput transformer problems
From: "Alan Melia" <[email protected]>
Date: Sun, 9 May 2004 21:56:42 +0100
References: <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
Hi Dick, I dont know whether the calculation is valid in this case but I
dont see why not. I have always measured the inductance of the the primary
winding and adjusted the turns until the reactance (secondary open of
course) is at least 3 times the driving impedance. The driving impedance is
probably not quoted by Dave but you can work back from the turns ratio to
the 50 ohm secondary. In the case of a narrow band transformer like this
situation, it might be better (more efficient) for the reactance to be 5
times the driving impedance impedance. You should also be able to work out
what the reactance of Dave's transformer would be because the turns per uH
should be quoted for 3C89 cores in the references. It would sound as though
your LOPT cores do not have as high a permiability as Dave's cores, or maybe
there is a slight effect from the air gap. I have certainly wound working
transformers on old LOPT cores.
I hope that helps ...if I've screwed it up no doubt someone will correct me.

Cheers de Alan G3NYK
[email protected]

----- Original Message -----
From: "Dick" <[email protected]>
To: <[email protected]>
Sent: 09 May 2004 21:02
Subject: LF: TX-ouput transformer problems

Hello all,

I completed my G0MRF-transmitter, and today I've done the
first tests on it.
All is working fine, except the output circuit.

As far as I can see now, the outputt-transformer on the FET's is
not good.

I'm using an old core from the HV-circuit of a computer monitor.
I got this tip from Jim, M0BMU a while ago.
Core size is about 63x39x14mm (2 "C"shaped halves, held together
with a metal spring clip)
I payed a lot off atention to have no air gap.

Used 10 turns on the primary side, centre tapped.
(That means 2 times 5 turns.......)
Secundairy coil max. 21 turns with taps, all as described in the
original design.
Well, at 15V drain voltage I already had 9Amp........
With 40V over 50 Ohm.......
FET's got hot........
I changed primairy to 2 times 7 turns instead of 2 times 5 turns,
and that gave me 15V, 5Amp with also around 40V over 50 Ohm.
So, higher efficiency......

I wonder if anyone has experience with these core's and have hints about
number of turns to be used, before I start to get into endless rewinding
coils etc.


Dick, pa4vhf

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