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[TowerTalk] Re: Force12

To: <towertalk@contesting.com>
Subject: [TowerTalk] Re: Force12
From: jreisert@jlc.net (Joe Reisert)
Date: Thu, 11 Oct 2001 09:41:29 -0400
Mike,

This discussion is mixing apples and oranges so let's try to get the record 
straight and sort it out.

The Force 12 feedline approach using 100 feet (the length is not critical 
or magical) of transmission line to feed an antenna is simply adding a loss 
in the line that improves the VSWR seen by the transceiver. The longer the 
line, the better the VSWR at the transmitter, but the greater the loss of RF!

What has been mistakenly thrown in to this discussion is the "Transmission 
Line Resonator" concept. It was first written up by Frank Witt, AI1H, in 
the ARRL Compendium, Volume 4, pp 30-37. In Frank's article, he showed how 
to broadband an 80-meter dipole.

It was quite an involved article and uses resonant (half and one 
wavelength) sections of transmission lines augmented with other 
transmission lines of different impedances. What it does is create a double 
humped VSWR response such that the lowest VSWR is obtained not at the 
center frequency where the dipole is resonant but instead separated out 
like a Chebyshev response. In other words, a dipole resonant at say 3.65 
MHz fed with his transmission line transformer has it's lowest VSWR at 3.5 
and 3.8 MHz with a slightly higher VSWR at 3.65 MHz.

Dave Leeson, W6NL, later popularized this technique on 80-meter dipoles to 
broadband them with a somewhat simpler approach using the same concept 
proposed by AI1H. In Dave's basic form, you feed a dipole with a half 
wavelength of 50 Ohm coax FOLLOWED by a quarter wavelength of 70 Ohm coax. 
Note that the 70 Ohm coax is AFTER the resonant transformer of 50 Ohm 
transmission line.The double frequency low VSWR match is attained again at 
3.5 and 3.8 MHz.

I have used this technique on my two 80 meter dipoles for a  few years now 
and it works great since I mainly only operate 80/75 meters near 3.5 and 
3.8 MHz. At 3.5 and 3.8 MHz, my VSWR is approximately 1.2:1 but at 3.65 MHz 
my VSWR is higher (perhaps 1.5:1). Above 3.8 MHz, the VSWR sky rockets! 
This is considerably broader than a straight dipole which would be closer 
to 2 or 3:1 at 3.5 and 3.8 MHz when tuned to 3.65 MHz.

Note that this transmission line transformer is a ONE BAND approach. Hence, 
it is not the kind of matching system to use on a tri-band Yagi.

I hope this finally clarifies all the mis-information that has been on 
Tower-Talk for the last few weeks.

73,

Joe, W1JR


At 06:11 PM 10/10/01 -0700, Mike wrote:

>Hey Tom,
>
>I don't think it is absolutely true that line loss is the only mechanism that
>can increase system bandwidth. If the reactance change of the mismatched
>line happens to work in opposition the reactance change of the load, then
>then a lossless cable could in principle increase system bandwidth. This
>certainly works when a series resonant circuit of proper loaded Q is placed
>in front of a load with with an opposing reactance change versus frequency.
>Unfortuntately, the direction and or rate of the reactance change is usually
>wrong, so this technique only works in specific cases.
>
>73 de Mike, W4EF.......
>
>----- Original Message -----
>From: "Tom Rauch" <w8ji@akorn.net>
>To: <towertalk@contesting.com>
>Sent: Wednesday, October 10, 2001 5:17 PM
>Subject: Re: [TowerTalk] Re: Force12
>
>
> >
> > >               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.
> > 73, Tom W8JI
> > W8JI@contesting.com
> >
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> > -----
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>
>
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>-----
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List Sponsored by AN Wireless:  AN Wireless handles Rohn tower systems,
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