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Re: [TowerTalk] Fwd: Bandwidth of a 75/80 meter dipole

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fwd: Bandwidth of a 75/80 meter dipole
From: K4SAV <RadioIR@charter.net>
Date: Tue, 17 Mar 2009 09:54:35 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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