In a message dated 98-07-07 18:56:43 EDT, you write:
<< I may kibbitz ...
I intend to run 75 ohm catv cable from the foundation of the house to the
tower base and up to near tower-top. The cable will terminate at an
Ameritron remote switch at the tower. Then I will use short runs of
flexible cable (probably 50 ohm) to connect to the two beam and two wire
antennas (I also intend to do the same at the shack at the other end).
What matching and/or misreading issues might I face, if any? Should I run
75 ohm flexible cable at both extremes?<<
The worse thing that can be done is to mix in random alternating sections of
75 and 50 ohm cables! (Properly done that would be a good idea, you could
build a matching transformer by mixing 50 and 75 ohm cables properly).
You can wind up with a horrid SWR doing that! (or a good one with lottery
winning luck)
Try to use the same Zo line from the antenna terminals to the rig, unless you
add a 50 ohm line at a "sweet spot".
>> I have been told that most beams are designed closer to 65 ohms than 50
ohms since the 50 ohm standard is not an international standard and that
other countries use 60, 65, 70, and 75 ohms. This would make 75 ohm cable
no greater issue for matching than 50.<<
I've been told Aliens are real . Both seem just as plausable.
I'd check with the antenna manufacturers. I'd bet most yagi antennas are
designed as 50 ohm systems (or lower).
>>Also, which device might best handle this situation as I make adjustments,
the new MFJ-259B or the new AEA-CIA (I am leaning towards the new MFJ-259B
since it is 1/2 the price of the AEA-CIA -- will a lighter duty user like
me really miss the features of the AEA-CIA?).<<
The MFJ is a 50 ohm unit, but is quite usable on lines of differnt Zo (except
for SWR readings, they will be referenced to 50 ohms). I have no idea about
the AEA, but would guess it is the same.
*******Remember the SWR we measure is the ratio of the METER design impedance
to the system impedance at the point where the meter is connected. A 75 ohm
line with a 1:1 SWR will measure as a 1.5 : 1 SWR on any instrument designed
or normalized as a 50 ohm meter!!!!!!!!*******
Meter SWR might not be the line SWR at all when design impedance of the
transmission line and meter are different.
73 Tom
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