[Antennas] MORE " G5RV " ANTENNA QUESTIONS !!!!

Mon, 22 Jul 2002 13:57:16 -0700

John,
Feed lines when they are 1/2 and 1/4 wave lengths on a given frequency
(electrical length) become impedance transformers. 1/2 wave is 180 DEG
out of phase and therefore not reactive in a harmful way. 1/4 wave
length of 50 ohm will have a resistive component changing the impedance.
Random length feed line might have a frequency that you can't operate by
hitting a 1/4 wave length increment 5/4 or another. I deal is to measure
what you need and cut to the nearest half wave length still a guessing
game due to bandwidths and multiple bands.
When you change from 450 ohm to 50 ohm balanced to unbalanced a (BALUN)
is needed. Then the vertical part of the 450 ohm feed line can have RF
on it in fact this can be done on intentionally say for a 10M vertical
unless you have a choke BALUN at the feed point.
Having two different types of feed line both need to be in 1/2 wave
lengths if possible.
Personally I don't do all this math and figuring. I just do it randomly
and if I have a problem on a frequency I want, I just add or subtract
some of the feed line to make it work.

Randy
AC7NJ

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of j.hilton1
Sent: Monday, July 22, 2002 1:23 PM
To: [email protected]
Subject: [Antennas] MORE " G5RV " ANTENNA QUESTIONS !!!!

HI again,
              And first off all many thanks to all who helped me with my
last questions I asked on here regarding the full size 80 - 40 mtr G5RV
antenna I had installed recently, a few more questions regarding the
above, if anyone can be so kind as to help me out again ?? have the G5RV
installed with the centre point at around the 25 feet level just below
the rotator holding the 6 mtr beam with both 51 feet legs coming sloping
down there full length to the ends which are supported at 10 feet each
side. the problem that I have is that the feeder which is 30 feet 6
inches off 450 ohm ladder line is more than enough to feed from the
centre off the 5RV to the actual radio, according to the instructions
supplied with the antenna any length off 50 ohm coax cable can be
attatched to this ladder line, so whats best to do ?? bring the ladder
line into the shack and attatch a small length off coax cable to it, say
2 feet in length ?? or loose some length off the ladder line on the
outside by having it going further down the tower first ?? to  then
attach the coax cable say 10 - 15 feet in length ?? from the radio going
way out to the end off the ladder line ?? also many thanks to the
amateur who    advised using a length off platic tubing to insulate the
ladder line from the actual tower, great idea !! using the inbuilt tuner
with the yaesu ft920, am I likely to be able to get a sattisfactory tune
on all bands 80 -40 mtrs using the above arrangement ??

As always thanks to everyone who helps me out with any tips / advice it
is always very much appreciatted.

kind regards,

john hilton

" gm1zvj / mm3zvj "

scotland

- uk -         

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