Jim,
The use of an ODD electrical quarterwave of coaxial cable as a BALUN is not
correct. The correct description would be "series coaxial transformer". And
there are certain other conditions to be satisfied when constructing this
impedance matching network. An odd quarterwave of 75ohm coax is a special
case
of the Series Transformer (which uses two different pieces of coax) and is
nominally a 2:1 impedance transformer. Its application for Cubical Quad
antennas was popular as often the loop impedance of a quad was around 100 ohms,
and required a 1:2 impedance change to match a 50 ohm source. It does not
perform a BALUN function (Balance to Unbalanced). Coaxial baluns can be
constructed using electrical 1/2w piece of coax (Same Z)and connected so that
the shields are together, the feed coax center to terminal 1 of the antenna and
the 1/2w piece center from T1 to the other terminal of the antenna  this also
is a 4:1 impedance transformer as well.
Your description of connecting the "braid" to the tower or mast top doesn't
apply in the applications I am familiar with. One of the most common
arrangements is in a multiband quad. where several of the quad driven element
loops would be connected to a remote antenna switch via 1/4 or 3/4 wave
sections
of 75ohm cable for a 2:1 impedance match.
It is not clear what the actual arrangement of cables, tower and antennas are
in
your comment regarding W8JI. Without a more complete description of the
antenna
arrangement and feed points it is not possible to determine what the function
of
the 1/4w cable sections are. You speak of Hi Z, again not clear what that
value is. I recommend looking up "Series Coaxial Transformer" (ARRL Antenna
Book)as this will discuss a more general solution for impedance matching using
a
length of coax equal to the source Z, and a larger value of Z for the other
section. The lengths and value of cable Z is determined by the frequency and
impedance differential being matched. These are single band solutions.
Summarizing  1/4w transformers do not provide a balanced feed. The
characteristic impedance of a series transformer is different then the source
impedance.
A 1/4w of 75 ohm coax is a special case series transformer and provides a 2:1
impedance transformation.
An ODD quarter wave of 75ohm cable is also used in coupling two identical 50
ohm
antennas into a phase array.
A halfwave of same characteristic impedance coax is required to create a
Balance feed and results in a 4:1 impedance transformation. This arrangement
is
frequently use in large monoband yagi's where the driving Z may be 10 or 12
ohms and the 4:1 provides a balanced match to 50 ohms.
Most amateur applications of the odd quarterwave transformer use 75 ohm cable
which results in a 2:1 transformation.
Norm W4QN
==============================================================
Date: Tue, 28 Dec 2010 07:04:13 0800
From: "Jim Thomson" <jim.thom@telus.net>
Subject: [TowerTalk] Baluns...again.
To: <towertalk@contesting.com>
MessageID: <52EE1FDD307B4F319F0EB430BFCF95D2@JimboPC>
ContentType: text/plain; charset="iso88591"
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
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