Looking through some old notes plus the info on Jim Brown's site.... there
was a question about using
an electrical 1/4 wave or electrical 3/4 wave of coax. The 1/4 [3/4]
wave of coax was run between the
DE of the yagi and the tower... with the braid of the coax bonded to the top
of the tower.
The theory is... if a 1/4 [3/4] wave of coax has it's braid bonded to the
top of the tower... the Z at the DE
should be sky high..and there should be no requirement for a balun.
Apparently there was an article out
about doing just that. The ant was a quad... and individual 1/4 wave [ and 3/4
wave] pieces of 50 ohm coax
were brought to a remote switch box at the top of the tower..where of course
all the braids are bonded to the tower
top. The author claimed that no baluns were needed on the various loops used
on each band. I have not seen the article,
but it was brought to my attention by a local ham.
W8JI also mentioned the use of 1/4 and 3/4 wave lines from DE to tower
top...and bonding the braids to tower top. Seems
to me that W8JI mentioned that if a balun was used in conjunction with the
1/4 [3/4] wave coax, that the balun would makes things
worse. There was some discussion on one of the other refelctor's years ago
about the 1/4 and 3/4 wave concept. It had something do do
with the height of the tower. If the tower was not the 'correct height'....
the concept would not work. Does anybody remember this ?
In one of Jim browns write ups.... there is a depiction showing feedlines of
various lengths... from 0 up to... but not quite 1/4 wave. Then
the chart starts up again... just a bit longer than a 1/4 wave..... and up
to... but not quite a 3/4 wave. Then it starts up again.. just past a 3/4
wave. OK, what gives? Is an electrical 1/4 wave and odd quarter waves
the real answer to a high Z on one band ? In the case of
something like monoband yagis.... this may prove to be a huge benefit... if
the theory actually works. If I remember correctly, the main coax
braid was bonded to the base of the tower as well. IF the theory does work...
and a high Z is presented to the DE... how high is this Z? Is is
mainly reactive... mainly resistive..or a bit of both ? Why would a bead
balun or torroid balun, when used in conjuntion with a odd qtr wave
of coax makes things worse ?
later........ Jim VE7RF
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|