[TowerTalk] mfj259 and 75 ohm system

[Lowell, Mark]

>This weekend I built a 10m dipole which is fed by 75 ohm coax (uses 75
>ohm catv cable already in the building) .

>Since the 259 is designed for a 50 ohm system I am curious to know if
>it is useful when the coax is all 75 ohm.  If so what would be the
>procedure for adjusting for resonance?

>Thanks, Pete ws4g

Hi Pete,

You can use any impedance feedline if its *electrical* length is any
multiple of a half wavelength. When you do this, the feedline will exactly
reflect the feedpoint impedance of the antenna itself.

Find out what the velocity coefficient of your particular feedline is.
Solid dielectric stuff is usually around 0.66. Foam is around 0.88.

Now, just a little math to calculate electrical length:

one full wave, in feet:
(984 x velocity coefficient)/frequency

for 28.4 Mhz and solid dielectric:

one wave = (984 x 0.66)/28.4 = 22.86 feet (that's 2 half waves)

one half wave = 11.43 feet

for solid dielectric, then make your feedline a multiple of 11.43 feet and
you're all set.

For foam:

(984 x 0.88)/28.4 = 30.49 feet (one full wave)  half wave = 15.24 feet

Once you have set your feedline length, you can prune your dipole normally.
 Dont's forget to wind a choke balun with 6-8 turns at 6" diameter at the
dipole feedpoint and be sure to include this length of coax in your overall
feedline length measurement.

--...MARK_N1LO...--




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