At 08:17 PM 10/10/01 -0400, Tom Rauch wrote:
>
>> Some years ago, Dave Leeson, now W6NL posted the
>> explanation here on Tower Talk. The length of line creates a
>> double tuned transformer which per theory does, in fact,
>> increase the "system" bandwidth. It is NOT added loss, it is
>> a form of matching transformer.
>
>Perhaps you misunderstood what was said, because that is clearly
>not correct.
>
>The only mechanism that increases bandwidth is loss in the line.
>Nothing else.
Let's clear this up. Here's the original e-mail:
>From: Leeson <leeson@ix.netcom.com>
>To: towertalk@contesting.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
>
73, Pete N4ZR
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