My antenna for 160/80/40/30 is as follows:

160m loaded dipole shortened to 200' with 30uHy inductors so it will fit in
the lot.
80/40 parallel fan full size dipoles at right angles to 160m dipole
30m the loading coils on the 160m dipole act as RFC's or traps on 30m to
isolate the center section of the dipole as a full size dipole on 30m.
All with a common feed point through a home brew 1:1 current balun.

I was modeling the whole array before moving the loading coils in EZNEC and
noticed that I had a near perfect resonate point at about 6MHz defined by
the loading coils on the shortened 160m dipoles center loading coils. As
this is in a shortwave broadcast band it was hitting the front end of my
radio with very strong signals. As I'm a DXer I didn't like the idea that
strong out of band signals may have been degrading my in band receive
performance. The idea of moving the coils in to make a 30m dipole came to
me as a good way to fix the problem and give me a antenna on a band I had no
antenna for. To keep the length of the 160 dipole at 200' when moving the
loading coils in to make 30m dipole required increasing the inductance to
30uHy's. The SWR is a reasonable 1.2:1 and seems to work quite well. As 160
and 30m are the only bands I don't have DXCC on, I'm making good progress on
30m to getting it, not so good on 160m as I don't have an amplifier on
160m.


--
John Passaneau W3JXP
State College Pa.


wrote in message
ups.com...
Here are a couple messages I posted on eHam net. Any comments would be
appreciated.

Thanks,

Doug VE3XDB

--
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

--
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