Dear LF Group,
I'll try sending this again...
-----Original Message-----
From: james moritz [mailto:[email protected]]
Sent: 13 November 2006 17:03
To: '[email protected]'
Subject: RE: LPF coils and VY1JA to 1W ERP
Dear J, LF Group,
If the low-pass filter is introducing a small phase shift, it could be
because:
1)The component values are off a bit. The various reactances in the filter
should cancel out at the operating frequency to give a resistive load
impedance at the input of the filter. But any error in any component value
will result in a net reactive impedance, and so a phase shift. It is quite
difficult to produce these components with high accuracy; in particular,
air-cored coils will tend to have their inductance reduced slightly by
nearby metal objects (or increased if close to ferrite cores). Mutual
coupling between the coils would also alter the impedance. I don't know what
the capabilities of an MFJ-257 are, but most of these analysers are
specified for 1.8MHz and above - if measuring at 1.8MHz, the apparent coil
inductance will be different to that measured at 137kHz due to distributed
capacitance, varying permeability etc. If actually measuring at 137kHz, the
accuracy of the instrument may be degraded.
2)The filter design does not actually give you a resistive input impedance.
The way LC low-pass filters actually "reject" unwanted frequencies is by
producing a huge impedance mismatch at the unwanted high frequencies (where
the input impedance is almost all reactance and no resistance),
transitioning around the cut-off frequency to a small, but usually not zero,
mismatch within the wanted frequency range (mostly resistance with a small
reactance component). Even well below the cut-off frequency, most low-pass
filter designs result in some small mismatch at most frequencies. It is
possible to design filters with a perfect match at one or more spot
frequencies, but this depends on the design method used.
Either way, you could get back to a resistive load at the filter input by
re-tuning the antenna to produce a slightly reactive load at the output of
the filter, which will cancel the reactance introduced by the filter. Or,
you could try adding or subtracting 1 or 2 turns from the toroidal coils. In
any case, if the phase shift is only a few degrees, it won't make much
difference to the TX.
Cheers, Jim Moritz
73 de M0BMU
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