K5TR has another way. He sent me how to do it but I can't find it. I
recall a piece of 75 ohm coax that was switched into the line.
Maybe Geo will post it.
Mike W0MU
CC Packet Cluster W0MU-1
W0MU.NET or 67.40.148.194
"A slip of the foot you may soon recover, but a slip of the tongue you may
never get over." Ben Franklin
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of K4SAV
Sent: Tuesday, March 17, 2009 8:55 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fwd: Bandwidth of a 75/80 meter dipole
There are several ways to make a dipole cover all of 75/80 meters, and also
many ways that don't work.
A double Bazooka doesn't cut it. It only provides about 14% increase in
bandwidth at the expense of extra loss at the band ends. Even the crossed
double bazooka that provides about 55% increase in bandwidth at the expense
of even more loss, doesn't do it.
A cage dipole doesn't do it either unless you make the cage an extremely
large diameter.
A folded dipole will provide a little more bandwidth but it won't cover the
whole band either.
A fan dipole with one dipole cut for CW and one cut for the phone end will
work if you place the dipoles at right angles to each other.
This open-sleeve version of a dipole will cover all of 75/80 with less than
2 to 1 SWR and have good efficiency.
http://rudys.typepad.com/ant/files/antenna_broadband_dipole.pdf
Be careful if you try to analyze this antenna with NEC. It is possible but
extremely tricky to analyze.
The method of applying a resonant LC network at the feedpoint of a dipole,
as shown in Chapter 9 of the ARRL Antenna Book will work, however the
version with the whole LC network connected directly across the antenna will
be limited to low power unless you use some honker size caps to handle the
current. The method used in the "DXers Delight"
(same chapter) can be used at high power. It provides a step-up impedance
for the caps so they don't have so much current. Neither of these work well
at high power using a toroid for the coil because of the huge flux values.
This technique produces three resonant points in the 75/80 meter band
although resulting in an increase in feedpoint impedance which then has to
be matched. It is possible to implement all the whole network and the
matching with a single tapped coil.
There are many other ways to do this that also work with good efficiency.
Some of them are shown in Chapter 9 of the ARRL Antenna Book. Beware of
networks using coax in the matching network. Many of those (but not all)
are very inefficient.
Jerry, K4SAV
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