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LF: Re: Re: TX-ouput transformer problems

To: [email protected]
Subject: LF: Re: Re: TX-ouput transformer problems
From: "Vernall" <[email protected]>
Date: Mon, 10 May 2004 09:31:27 +1200
References: <[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
Dick and Alan,

I agree with comments from Alan, it does seem your core is either lower
permability material, or there is still some unintended "air gap" (I have
heard of gaps being set by glass beads in glue).

I suggest that what is needed is a test of inductance of the primary side,
at or near working frequency.  A method I use is to series resonate the
inductor under test with a known capacitor (I use a switchbox with
polystyrene caps) and look for the resonant peak.  Inject from a signal
generator and a stepdown ratio (say 20:1 but it is not critical) toroid (to
give lo Z source) and put one leg of the series circuit through the toroid.
Monitor voltage across either the inductor or capacitor with a 10:1 scope
probe (minimal loading at 10 megohms).  Pick the capacitor that gives
resonance nearest to the band of interest.  This can also act as a
reasonable Q meter, using the -3 dB response method.

73, Bob ZL2CA

----- Original Message -----
From: "Alan Melia" <[email protected]>
To: <[email protected]>
Sent: Monday, May 10, 2004 8:56 AM
Subject: LF: Re: TX-ouput transformer problems

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
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
your LOPT cores do not have as high a permiability as Dave's cores, or
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
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.
> 73
> Dick, pa4vhf

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