[ARC5] 400784741321

[Ian Wilson]

I would be surprised if the Q of a solenoid in the HF region increases
with frequency - for Cu, the skin depth between 1MHz and 10MHz
decreases by a factor of more than 3; so, as a first-order approximation,
the Q at 28MHz would be close to the Q at 8MHz (the increase in wL
is more or less cancelled by the increase in r).

In practice I suspect that the Q drops off faster than this because of
the effects of distributed capacitance (which in effect requires more
turns to achieve the desired inductance at higher frequencies, hence
yet more loss).

The combination of tube gain falling off above about 10MHz together
with losses in coil switching is presumably responsible for the "deaf
as a post" performance for some classic boatanchor receivers above
20m....

73, ian K3IMW

On Thu, Oct 16, 2014 at 12:04 AM, AKLDGUY . <neilb0627 at gmail.com> wrote:

> > I completely rewound all three inductors in the receiver following the
> article
> > that Gordon White wrote some time ago entitled, "Command Receivers for
> > All Frequencies", in which article Gordon described and listed the turns
> on
> > the coils that ARC came up with for THEIR early HF receivers up to 27
> MHz.
> >
> > I used ARC's 27 MHz data, in fact, as the starting point for my coils.
>
> OK, you rewound the coils on the basis of A.R.C.'s data.
>
> Q is the reactance of either the inductor or capacitance at resonance,
> divided
> by the series resistance, which in this case is in the coil if we ignore
> other very
> small losses.
>
> In my suggested modification (no modification of the coil) the Q is more
> than
> tripled by going from 7-9 to 28-30 MHz because the coil reactance more than
> triples. The higher Q at the higher frequency means the selectivity curve
> is the
> same as at the lower frequency. The curve will be the same number of dB
> down
> at the same frequency offset at both 9 MHz and 30 MHz.
>
> There would therefore seem to be no good reason for taking turns off the
> coils.
> Why did A.R.C. use reduced turns? Probably because they were faced with a
> too large value of tuning gang (no smaller gang available), so they used a
> large
> amount of **parallel** capacitance across the gang to reduce its tuning
> range
> and reduced the number of turns to compensate.
>
> > Now I WILL say that your method would most certainly eliminate the
> > necessity for removing any plates from the capacitors, but I STILL say,
> that
> > the inductances would have to be "adjusted" also in order to maintain
> circuit
> > "Q".
>
> Yes, my method does eliminate the need to remove plates, but the
> selectivity
> self-adjusts by virtue of the rise in Q as described above. A Q of 100 at 9
> MHz
> (easily possible) becomes a Q of more than 300 at 30 MHz, and the
> selectivity
> (percentage change) at both bands is the same.
>
> I'd suggest that anyone contemplating modifying a 6-9.1 Mc/s receiver for
> 10m
> try my suggestion first. You have nothing to lose. It's simple and
> reversible. Use
> very good quality capacitors and let them cool before evaluating.
>
> 73 de Neil ZL1ANM
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