Bill,
I don't think it matters much, but just to be safe you can
make the 50 ohm pieces an integer mutiple of 1/2 wavelength
at the operating frequency. This will ensure that the there is
no impedance change due to the length of 50 ohm coax.
The impedance seen looking into length of coax that
is an integer multiple of 1/2 wavelength coax will be equal
to the load impedance that terminates the coax (in this
case the antennas feedpoint impedance. Using the N/2
wavelength pieces of 50 ohm coax, will thus be, in effect,
the same as going to the elements directly with 75 ohm
lines.
BTW, I hope the judge to doesn't go too hard on the
horse :)
73 de Mike, W4EF.........................
----- Original Message -----
From: "BILL STRAW" <wb0o@yahoo.com>
To: <towertalk@contesting.com>
Sent: Wednesday, October 31, 2001 10:38 AM
Subject: [TowerTalk] 4 Square question
>
> A question to the experts:
> I have a 40M and an 80M 4 square, each has been up
> 6-7 years. They are both made of Rohn 25, and the
> base of each vertical is elevated 15 feet. Thus, a
> simple 1/4 wavelength 75 ohm phasing line won't
> reach from the phasebox on the ground to the "base"
> of each vertical.
> When I built the 40M 4 square, the then owner of
> Comtek told me to use a multiple of 1/4 wavelength
> 75 ohm coax (I think I have 3/4) so it would reach
> between the box and base. A lot of coax, but it works
> great.
> When I built the 80M 4 square, the owner of DX
> Engineering told me to just use equal lengths of 50
> ohm coax to drop down from the elevated bases to the
> ground, then hook up the 1/4 wavelength 75 ohm coax.
> This also works great.
> Now, I find one of the 40M 3/4 wave 75 ohm phasing
> coaxes is destroyed. (A horse was implicated and
> arrested). It is just 3 days before CWSS and I have no
> way to get any 75 ohm coax.
> Questions: Are these two feed methods equivilant?
> I plan to feed the 40M like the 80M, all I would
> have to do is cut the good 75 ohm into 1/4 wave
> pieces, cut up some 50 ohm coax and solder a few
> plugs. I guess I could elevate the 40M phasebox on
> a pole and string 1/4 wave phasing lines to the
> elevated bases, but the 50 ohm drop down line
> method sure would be quicker.
>
> Any ideas?
> Thanks, Bill WB0O in northern North Dakota, 10 miles
> south of VE4 land.
>
>
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