Dave Heil wrote:
Cecil Moore wrote:
How about a rule of thumb? Using a 2 liter pop bottle, make the number
of turns equal to the wavelength in meters. That will optimize for a
specific wavelength but will render it not very useful for any shorter
wavelengths, e.g. 20 turns is too many for 10m.

So it is your "rule of thumb" that 160 turns would be used for 160m?
I don't think so, Cecil. Six or eight turns on such a form should work
fine for 20-10m.

For maximum impedance, i.e. optimized for a single frequency,
the ugly balun would be 1/4WL self-resonant. I think you will
find that, for HF frequencies, the number of turns of RG-213
on a 4 inch coil form is in the ballpark of the wavelength in
meters. Thus, close to ten turns would be optimum for 10m.
Such a balun, although it would work, would not be optimized
for 20m as the impedance would be maximum around 10m.
--
73, Cecil http://www.w5dxp.com

Buck, N4PGW wrote:
"Does anyone know where I might find that formula?"

Look at the ARRL Antenna Book. The 19th edition (old) has a choke balun
for low-frequency antennas on page 6-9. The text says:
"In general, an isolation choke inductance of 50 to 100 microhenries
will be needed for 3.5 and 1.8 MHz, ground-plane antennas. One of the
easiest ways to make the required isolation choke is to wind a length of
coaxial cable into coil as shown in Hig 18. For 1.8 Mhz, 30 turns of
RG-213 wound on a 14-inch length of of 8-inch diameter PVC pipe, will
make a very good isolation choke that can handle full legal power
continuously."

On page 11 of Terman`s 1955 opus is found formula (2-2):
Inductance in microhenries = F n(sqared) d

F is a constant that depends on length/diameter. There`s a chart in the
book for F.
n is the number of turns.
d is the diameter of the coil measured between the centers of the wire
in the coil.

The purpose of the coil is to present a high impedance to currents on
the outsifde of the coax so quick and dirty is OK so long as there is
enough impedance. You may get away without a coil form at all. You might
lose some of the isolation that coil length provides so I don`t
recommend just coiling coax like garden hose.

Best regards, Richard Harrison, KB5WZI

On Wed, 20 Jun 2007 15:15:03 -0500, (Richard
Harrison) wrote:

Buck, N4PGW wrote:
"Does anyone know where I might find that formula?"

Look at the ARRL Antenna Book. The 19th edition (old) has a choke balun
for low-frequency antennas on page 6-9. The text says:
"In general, an isolation choke inductance of 50 to 100 microhenries
will be needed for 3.5 and 1.8 MHz, ground-plane antennas. One of the
easiest ways to make the required isolation choke is to wind a length of
coaxial cable into coil as shown in Hig 18. For 1.8 Mhz, 30 turns of
RG-213 wound on a 14-inch length of of 8-inch diameter PVC pipe, will
make a very good isolation choke that can handle full legal power
continuously."

Page 26-21 of the same 19th edition ARRL Antenna Book has a whole
table of choke baluns sized for each of the HF bands. I believe Mr.
Lewallen worked out the specs.

bob
k5qwg



On page 11 of Terman`s 1955 opus is found formula (2-2):
Inductance in microhenries = F n(sqared) d

F is a constant that depends on length/diameter. There`s a chart in the
book for F.
n is the number of turns.
d is the diameter of the coil measured between the centers of the wire
in the coil.

The purpose of the coil is to present a high impedance to currents on
the outsifde of the coax so quick and dirty is OK so long as there is
enough impedance. You may get away without a coil form at all. You might
lose some of the isolation that coil length provides so I don`t
recommend just coiling coax like garden hose.

Best regards, Richard Harrison, KB5WZI