Fw: [TowerTalk] Wideband 80 & 40 dipoles

[Joe Reisert]

Tom,

Dave's technique (after AI1H article) for a broadband dipole works great. I 
use it here on two inverted V shaped half wave dipoles on 80 Meters. Due to 
available feedline lengths and an remote antenna switch at the tower, I 
could only use a 1/2 wavelength of RG231 between the feedpoint (about 88 
feet as I recall rather than the full 1 wavelength he recommended). Then I 
used a 1/4 wavelength of RG59 (about 45 feet) in series with the feedline 
to the remote antenna switch. I now have very low VSWR from 3.5-3.8 MHz.

Easiest and best technique for broadbanding I've ever seen for 80 Meters.

73,

Joe, W1JR

At 01:59 AM 9/11/00 -0600, n4kg@juno.com wrote:

> >From my archives, here is the "double dip"
>solution to broadbanding dipole SWR from
>W6NL ( ex W6QHS ) written in 1997.
>
>I had a similar experience with an 80M wire
>'fan' vertical (wire loop element 2 ft across at
>the top, 1 ft across 2ft above feedpoint) fed
>with 1/2 WL of RG58 50 ohm coax followed
>by a long run of 75 ohm CATV hardline.
>
>de  Tom  N4KG
>
>--------- Forwarded message ----------
>From: Leeson <leeson@ix.netcom.com>
>To: towertalk@contesting.com
>Date: Wed, 26 Nov 1997
>Subject: [TowerTalk] Wideband 80 & 40 dps
>
>There's an interesting method of broadbanding antennas mentioned in
>various ARRL publications by Frank Witt, AI1H.  It uses the transmission
>line length you are going to need anyway to match at two frequencies in
>the band (say 3.52 and 3.80 MHz).  It's also mentioned in a text I use
>teaching a graduate microwave course at Stanford, and I have been
>wanting to try it.
>
>Here is a summary of an 80m inverted vee I put up that matches 1:1 at
>the cw and ssb frequency.  The inverted vee is designed using AO to
>resonate at 3.67 MHz (geometric mean of two frequencies of interest),
>and to have enough height and included angel to have a resistance at
>resonance of 70-75 ohms.  The apex is at 120 ft, and the angle is about
>120 degrees.  I use the AO optimizer, then knock off 1% length from
>experience.
>
>Now for the interesting part.  I measured a full wavelength of 50-ohm
>coax using the MFJ-259 antenna scope.  Using Belden 8214, it's about 205
>feet long, connects to the antenna 1:1 balun and comes down to the
>ground.  Next is conected a 1/4 wavelength of 75-ohm coax.
>
>At the transmitter end of the 75-ohm quarter wave, the match is perfect
>(less than 1.1:1) at 3520 and 3800!  This should work for a horizontal
>dipole, or any antenna with impedance in the 75-ohm range.
>
>To see if this was a fluke, I put up a 40m dipole at 50 feet for
>Sweepstakes.  It uses a half-wave of 50-ohm coax followed by the
>quarter-wave section of 75-ohm coax, and it matched fine at 7.0 and 7.25
>MHz.
>
>To see how this works, the best tool is Wes Hayward's Microsmith Smith
>chart program (ARRL), but you need a reasonable RLC model for the
>antenna impedance.  I do this by getting the impedance of the antenna
>from AO, using a spreadsheet to figure out what the equivalent series L
>and C should be, then using that in Microsmith.  A better dipole
>impedance model has the resistance across the inductor, so the variation
>of R with frequency is better modeled.
>
>Once you have the antenna impedance model, you can see why this scheme
>works.  The electrical line length (in wavelengths or degrees) is less
>at the low end of the frequency range and more at the high end, and it
>tends to wrap the impedance plot into a much smaller range.  Once you
>have gone enough half-waves, you can convert the impedance to 50 ohms
>with a transformer or quarter-wave section.  AO, Microsmith and the
>MFJ-259 working together make it easy, and my curiosity is satisfied.
>
>You can use this on any antenna, but if you want to do it with a Yagi,
>you need to work harder on the model (try RLC plus a short negative line
>length), and you will find it takes an awful lot of line length if the
>resitive part is less than 70 ohms or so.  But I plan to use a W2FMI
>transformer to get the impedance up where I can use this on a 3-el 40.
>
>Hope this is of interest, give it a try.
>
>73 de Dave, W6NL
>
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