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LF: Re: TX-ouput transformer problems

To: [email protected]
Subject: LF: Re: TX-ouput transformer problems
From: "urban ekholm" <[email protected]>
Date: Mon, 10 May 2004 08:29:41 +0200
References: <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
Hi all !

I made some tests on a TV line output core during the last winter, just to
see if is
suitable for an antenna matching transformer. Maybe this can be of some
help:

I used a core from an old coulour TV (22"-screen). The core was first taken
apart and
the thin pvc pieces giving a small air gap were removed and the cores
were glued together with epoxy. On each leg (the ones whitch are circular) I
wound 25 turns of 1mm wire (with electricians tape between the core and the
first turns)
which gave an inductance of about 330uH.
This was connected between the tx and a Heathkit "Cantenna" dummy load with
an SWR-meter between the tx and the transformer.
The output was changed by changing the voltage on the PA. The SWR was very
high, obviously
too loose coupling between the windings (or wrong ferrite !)
I then removed one winding and instead rewound it on top of the other (the
same number of turns).
Making the same test again and now the SWR was 1:1 ! Running 200W output for
a couple of minutes
gave no heating at all of neither the core nor the winding.
The power output was increased to 350W and after about 10min a slight
heating of the core was noticeable.
With 470W output the temperature increases quicker and maybe using QRSS with
longer transmissions will
give problems.
The last test was with 750W output. Still SWR 1:1 but the core temperature
rises more quickly now and I did not test this
fore a long time. The winding also now started to get warm
Probably it would be possible to use 750W by using two cores "side by side"
and thicker wire. However, I did
not have enough thicker wire to perform this test.

Good luck !

73 SM5EUF / Urban

----- Original Message ----- From: "Dick" <[email protected]>
To: <[email protected]>
Sent: Sunday, May 09, 2004 10:02 PM
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.

73

Dick, pa4vhf






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