Reg Edwards wrote:
Let us know when your rule-of-thumb is available from your website.
I'm looking forward to seeing the details.
The rule of thumb is pretty simple and is for 2L pop bottles.
Wrapping at 2 turns per inch (RG-213) around a 4 inch diameter
pop bottle, the optimum number of turns for a particular band
is equal to the numbers of meters in a wavelength. In other
words, use 40 turns on 40m, 20 turns on 20m, and 10 turns on
10m. It's not actually linear - just a rule of thumb. It is
most accurate around 20m-17m.
I guess there will be the usual collection of over-meticulous
nit-pickers.
Well, here's some data points that will probably surprise you.
I don't have a pop bottle so I am using a Quaker's Oats box
for a coil form. It is 5 inches in diameter which makes the
diameter of the coil using RG-400 about 5.4 inches. I wrapped
8 turns at ~4 turns per inch. My EXCEL spread sheet indicates
that coil is 1/4WL self-resonant at about 22.5 MHz making it
a good choke on 15m. Doubling that 1/4WL self-resonant frequency
to estimate the 1/2WL self-resonant frequency gives 45 MHz.
Now brace yourself.
The series impedance of that choke falls below 650 ohms at
about 27.2 MHz. 650 ohms is the maximum impedance that my
MFJ-259B will display. The choking impedance is never again
higher than 650 ohms as frequency is increased. It goes to
a minimum of 49 ohms at the 1/2WL self-resonant frequency of
45.6 MHz. Note that is reasonably close to double the 1/4WL
self-resonant frequency of 22.5 MHz *calculated* by my EXCEL
spreadsheet and by the BASIC program that I sent to you.
There is a one-wavelength self-resonant point at ~88 MHz and
a 1.5WL self-resonant point at ~122 MHz.
These measurements seem to prove that the coiled coax choke
acts more like a transmission line than like a lumped
inductance. Conclusion: A coiled coax choke designed for 20m
doesn't function very well on 10m or at any higher frequency.
(A coiled coax choke designed for 10m also doesn't function
optimally on 80m.)
--
73, Cecil
http://www.qsl.net/w5dxp