Here are a couple messages I posted on eHam net. Any comments would be
appreciated.
Thanks,
Doug VE3XDB
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MESSAGE #1:
Hi everyone,
I am thinking of building a center loaded fan dipole. One wire dipole
to cover 20 and 40 metres, and a second wire diple, connected at the
same feedpoint, to cover 30 and 17 metres. Coils would be placed at the
resonant point of the higher frequency, acting as both an RF choke and
a loading coil. My question is this. I would like to have the wire
beyond the coil as long as possible, but would still like the coil to
have enough inductance to act as an effective RF choke for the higher
band.
Other than through experimentation, how would one calculate or
determine this mix of inductance/length?
Best regards,
Doug VE3XDB
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MESSAGE #2:
Thanks for your replies. I know the technique does work. The Alpha
Delta DX-CC puts a loading coil at the end of a 40 meter dipole, and
adds about 9 feet of wire past the coil. This creates a dipole that
resonates on 40 and 80 meters. Here is a link:
http://www.alphadeltacom.com/pg1.htm
As well, coil loaded HF antennas are described in "The Radio Amateur
Antenna Handbook" by Bill Orr W6SAI and Stuart Cowan W2LX (Published in
1993 by Radio Amateur Callbook, a Division of BPI Communications Inc.,
pages 147 to 151).
Typical dual band antennas, of the type used for 2 meter/70 cm
operation mobile are coil-loaded whips. Reg G4FGQ wrote a program
called LOADCOIL.EXE that calculates the characteristics of a loading
coil at any point on an antenna radiator of a given length.
I know the length of wire necessary to resonant the antenna at the
higher frequency, and can figure out the coil required based on the
length of wire past the coil. What I don't know is what is the maximum
wire length, and therefore minimum inductance, necessary to have the
antenna resonate on 2 bands.
Here is what I calculated, using Reg's program.
20-40 meter combination (resonant at 14.150 and 7.150 MHz)
Feedpoint to coil 16.54'
Coil 17.73 uH
Coil to end of antenna 7.46'
30-17 meter combination (resonant at 10.120 and 18.100)
Feedpoint to coil 12.93'
Coil 11.21 uH
Coil to end of antenna 5.07'
This creates an antenna that is about 75% full size on 30 and 40
meters, full size on 17 and 20 meters.
What I'm trying to determine is the maximum length of the radiator (and
minimum inducatance) necessary to achieve dual band capability.
Any insights?
Best regards,
Doug VE3XDB