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,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
-----
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