[UK-CONTEST] OT but Likely I'll get answer....

[Adrian Rees (MW1LCR)]

A question that hopefully some one may know the answer to....

I need to measure the impedance of long lengths of Open Wire feeder,
when
attached to monoband aerial systems. How do I do that ?

Here's the scenario:

An aerial (for discussion, a 14MHz Yagi) is some 350 metres from the
shack.
Only efficient way to feed it is with Open Wire Feeder.
At each end of the Open Wire Feeder is a Balun, changing the impedance
to 50 Ohms.
(One end to coax + radio the other to coax + 1:1 Balun  and to Yagi Feed
Point).

Because of the distance involved (350m) of Open Wire Feeder, the actual
impedance of the Open Wire Feeder may not be 450 Ohms (or whatever the
manufacturers state).

(For example, if the open wire feeder is terminated by a half wave
dipole
with an impedanceof 50 Ohms, then a 9:1 Balun would see  an impedance of
50
Ohms when the Open Wire Feeder is a half wavelength long.......or 4050
Ohms
when its a quarter wavelength long.....high impedances presented to
Baluns
in this manner would most likely cause harmful flux in the core and
excessive
Heating).

How do I measure or calculate accurately, the impedance of the Open Wire
Feeder at these distances....
and therefore build the correct Balun ? 
(Building Baluns that can handle anything more than say 10:1, and be
efficient etc is not easy, so if I can get the impedance of the Open
Wire Feeder run down to, or close, to 300 or 450 Ohms at the design
frequency will be good).
Is it as simple as calculating odd numbers of Quarter Wavelengths in the
Open Wire Feeder length ? Or have I missed something ?

Each Open Wire Feeder run will serve one monoband aerial, so that should
help.

Glad evryone had a good time at the HF Convention, might make it next
year....

TIA

Adrian MW1LCR