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From: hajo.brandt.dj1zb@t-online.de (Hans-Joachim Brandt)
To:  <rsgb_lf_group@blacksheep.org>
Date: 13 Nov 2006 21:49 GMT
Message-ID: <1GjjhM-0giu7m0@fwd29.sul.t-online.de>
Subject: Re: LF: RE: LPF coils  and VY1JA to 1W ERP
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Status: O

Dear all,

a former collegue of me had to design an amplifier in the frequency range 1=
0 to 20 kHz delivering a few watts. For the output filter he had used tunab=
le pot cores. He was wondering that he could tune the filter just for a spe=
cific output level only. Changing this level would change the performance o=
f the filter and required a retune of the output filter.  =20

I could just help him by explaning that, as far as I know, all iron powder =
or ferrite coils show a certain dependance of AL value versus excitation of=
 the coil. This can even be shown in a simple VFO employing either air coil=
s or iron powder or ferrite coils. When the supply voltage of the oscillato=
r is changed, changing oscillator power too, the influence on frequency is =
much higher when using iron powder or ferrite coils.

Therefore in the case of the low pass filter I guess the rf power will chan=
ge the desired inductivity values slightly - I confess I do not know in wha=
t direction. But there should be an influence.

Assuming that the inductivity values of the output filter have been determi=
ned correctly, I would propose to prove this thesis at VY1JA if the annoyin=
g phase effect would become less when reducing power.

If this should be true, the filter coils should be retuned slightly, not th=
e antenna.=20

HW?

73 Ha-Jo, DJ1ZB


"Talbot Andrew" <ACTALBOT@mail.dstl.gov.uk> schrieb:
> 1)=09
> Slight Saturation in the core causing a loss in inductance at high power
> perhaps?
> Assuming the air wound and cored inductors were adjusted to exactly the s=
ame
> values initially.=20
>=20
> Or
>=20
> 2)
> Direct coupling in the near field from your original air wound coils
> influencing the coupling into the scope of your V and I pickoff, that is =
not
> present in the closed torroidal cores.
>=20
> Can you wind the power down and measure the phase difference which would
> show up 1) but not 2)?
>=20
>=20
> Andy  G4JNT
>=20
>=20
>=20
> -----Original Message-----
> From: owner-rsgb_lf_group@blacksheep.org
> [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of J. Allen
> Sent: 13 November 2006 12:57
> To: rsgb_lf_group@blacksheep.org; LowFER at LW Cafe
> Subject: LF: LPF coils and VY1JA to 1W ERP
>=20
> Gentlemen,
>=20
> During the amp repairs at VY1JA, the coils in the LPF were changed from a=
ir
> wound to torroid core at the suggestion of one from our group as a possib=
le
> way of removing ringing from the transmitter.  There was no difference in
> the operation of the two, except that now there is a very slight phase sh=
ift
> with the current waveform lagging the voltage waveform.  The bottom line =
is
> that the air wound coils produced a better LPF only because of this very
> slight lag which is not apparent in the LPF using them.  The inductances =
of
> each coil was measured using an MFJ 257 analyser and was wound to the sam=
e
> value.  The current and voltage traces were picked up for the scope using
> the same scope and scopematch coils, and measurements were made at the sa=
me
> power level with the same transmitter, coax and dummy load connection.
>=20
> Have any others seen this very slight lag on LPFs with torroid cores?  Do=
 I
> now begin tuning the antenna slightly to correct for the phase shift whic=
h
> comes from the LPF, so that the transmitter sees a purely resistive load?
>=20
> Repairs to the interim transmitter at VY1JA are complete and the station =
is
> transmitting now at license limit 1W ERP on ~137778.8 QRSS60.  Again,
> reports and captures are appreciated.
>=20
> Thanks,
>=20
> J.
>=20
> =20
>=20
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>=20
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>=20
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