On Fri, 05 May 2006 23:53:07 +0200, switcher
wrote:
Strange thing is that with var cap + 10nF the loop only tunes between
say 207 and 225 khz, while on AM (without the 10nF), it tunes 1 MHz wide
..

explanation ??

Your variable capacitor (probably in the 20 to 400pF range) is a small
percentage of the total capacitance when the 10nF is added to it.
Alone, and at a higher frequency, it is enough to cover that much
range.

I saw that some people make their own var cap ... (with very large
plates...)

Now, I intend to buy some nF caps from 1-50nF or so, to see what's
happening ...

Buy some 1-50nF? You may discover why others build them.

Today I picked up two old grundigs, and after some tweaking (knobs) they
work !
melody boy 500 (excellent condition, the AM-FM- etc knobs aren't touched
...)
and a Yacht boy with lamp, batt indication, AFC ... tunes very well on
SW...
Great sound on both radios ...

yes indeed.

---and now, for a great song: ---
http://www.asklyrics.com/display/Chu...rics/12119.htm

Chacun à son Goût!

73's
Richard Clark, KB7QHC

Well, if he does cover 207-225 w/ a fixed 10nF in parallel with
whatever tuning cap he has, that implies Cmax/Cmin = (225/207)^2 =
1.18/1. Since Cmin is at least 10nF, the variable to accomplish that
must be 1800pF, at least. That in itself is a pretty good size
variable cap.

Then the implication would be that L = 50uH, and 1800pF would tune that
to 530kHz. The minimum capacitance including stray and effecitve coil
shunt would be 175pF to tune to 1700kHz.

I think he means to buy some fixed caps in the range 1-50nF, not a
1-50nF variable. (Though in this forum, maybe he does mean the
variable!) The fixed caps could make some sense. Maybe put them on
dual stackable banana plugs, and plug together whatever value is
desired.

Switcher, you can do the calcs yourself: 2*pi*freq = 1/sqrt(L*C) --
or use something like the RF calculator that's in RFSim99. And you may
find it's better to not try to cover too wide a range with one loop by
adjusting only the capacitance, for performance reasons in addition to
the fact that the tuning range will get very small as you go to larger
fixed capacitances.

Cheers,
Tom

K7ITM wrote:

I think he means to buy some fixed caps in the range 1-50nF, not a
1-50nF variable. (Though in this forum, maybe he does mean the
variable!) The fixed caps could make some sense. Maybe put them on
dual stackable banana plugs, and plug together whatever value is
desired.


You could use a solid state analog mux IC to switch capacitors at
those frequencies. Maxim, Analog Devices and others make a wide
selection of these chips.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida