It's even better than that - a piece of 75-ohm coax permanently in the line
at the right place gives a frequency characteristic that has a
double-dip. I think the idea was first thought of (or at least publicized
in amateur circles) by W6NL. Here's a reference, from
N3BB
<www.ctdxcc.org/n3bb/broadband_antenna/A_Broadband_80_Meter_Antenna_System.ppt
>
73, Pete N4ZR
At 04:13 PM 3/17/2009, W0MU Mike Fatchett wrote:
>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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