specific range of frequencies.
C2 is used to reduce the maximum capacitance of
the tuning capacitor when a smaller maximum value is
desired; the minimum capacitance is only slightly
affected relative to the maximum. This is desirable at the
higher frequency bands in order to allow a higher
Using the spreadsheet, FLC Calculations.xls (available
at the article link), you can see that if the full 360 pf
range of the capacitor is used on the highest band, the
required inductance is about 1/6th of the value listed in
the table. I chose to reduce the effective ΔC of the
tuning capacitor in order to use larger values of
inductance, which are easier to implement.
The spreadsheet has several sections that I
developed to help me with the tuned circuits of the
oscillator. The sheet is password protected (k3pto) to
ensure that I don’t make any unintended changes to the
formulas. The protected cells are highlighted with a
The first section calculates frequency, inductance, or
capacitance, knowing any two of the three. The next
section calculates frequency, inductance/capacitance,
and reactance, also knowing any two of the three. Next
is the formula to calculate inductance based on coil
diameter, number of turns, etc. This is followed by a
listing of the turns per inch for several wire gauges.
The next section derives the table show in Schematic
5 using formula ( 2) for inductance as shown above. The
last table is the coil winding information for each of the
I wanted to wind the coils on forms that would be
relatively easy to duplicate. Five of them are wound on
the cases I saved from some pills; they are 0.7” diameter.
Note that one of these five indicates 0.8”. That is
because I used heavier gauge wire which did not stay
tight against the form.
The first coil in the table was wound on an empty
thread spool which my wife graciously gave me. I tried
several variations of coils on a few bands. They all
worked equally well, so if you don’t have the forms I
used, anything relatively close will work. Simply plug in
the diameter of the forms you are going to use and the
spreadsheet will calculate the number of turns.
The last line of the spreadsheet shows the
inductance for a “stretched” coil. That is, one that is not
tightly wound. This is helpful for making a coil with a
very low inductance.
The #14 wire I used for the two highest frequency
bands was retrieved from a short length of Romex I had
left over from some house wiring. The #24 wire is from a
spool I’ve had for over 20 years. I would be happy to
mail the required length of #24 wire (as long as it lasts)
to anyone who wants to build this project and sends me
an SASE. Again, please email me first to ensure I have
some left and to get contact info. Photo 3 shows the six
coil assemblies I made for this project.
You should note that with any capacitor tuned
circuit, using a standard variable less than the first 25% of
the band is covered by 50% of the capacitor rotation.
Page 2 of the spreadsheet contains a graph which shows
this. The reason I mention this is that you may want to
customize one or more bands for the frequencies that
most interest you.
The one that first comes to mind is the band which
covers six meters. It might be worthwhile to design the
oscillator to cover from 49 MHz to at least 65 MHz in
order to get good band spread on six meters.
You may wonder why I used #14 wire for the higher
two bands. Basically, for two reasons: (1) Fewer turns per
inch requires more turns for a given inductance, making
it easier to get the desired value; and ( 2) Making a
“stretched” coil is much easier with heavier gauge wire.
I mounted the tuning capacitor to the back wall of
the box — the same wall that has the coil socket. I
threaded two of the holes in the capacitor body using a
6-32 tap for mounting and 6-32x1/4” bolts. You need to
make sure that the bolts do not touch the rotor plate as it
If it does, you will need to shorten the bolts. I
extended the capacitor shaft using a piece of 1/4-20 all-thread with a homemade shaft coupler to hold them
together. You might want to use the shaft coupler
mentioned in the Parts List.
The oscillator Det_Out signal is a DC voltage which
increases when the coil of the oscillator is brought into
30 July 2016
■ PHOTO 3. Coils.