[TowerTalk] 1/4 wave coax "Balun"?
TexasRF at aol.com
TexasRF at aol.com
Wed Dec 29 06:12:17 PST 2010
Quarter wave baluns are used extensively in VHF/UHF applications.
There two variations, one using a coax of an impedance needed for desired
impedance matching and surrounded by a sleeve connected to the quarter wave
point on the coax shield. The open end of the sleeve is not connected to
Two, using parallel conductors shorted at the quarter wave point and one of
them with a coax inside. This type balun provides a 4:1 impedance step up.
Both parallel conductors are connected to the feed point.
Perhaps the coax grounded to the tower at a quarter wave from the feed
point is a crude attempt to emulate the sleeve type balun. If so, it probably
is better than no balun, but how can it be as effective as well designed
Half wave coaxial baluns are typically used with a Tee Match and give a 4:1
impedance step up. They don't HAVE to connect to a Tee Match, For example,
a folded dipole would have an impedance of 200 to 300 ohms and can be
stepped down 4:1 for a better match to 50 or 75 ohm coax.
All of these methods will work at HF as well, but some get kind of bulky in
In a message dated 12/29/2010 4:32:23 A.M. Central Standard Time,
npalex at bellsouth.net writes:
The use of an ODD electrical quarter-wave of coaxial cable as a BAL-UN is
correct. The correct description would be "series coaxial transformer".
there are certain other conditions to be satisfied when constructing this
impedance matching network. An odd quarter-wave of 75ohm coax is a
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
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
the shields are together, the feed coax center to terminal 1 of the
the 1/2w piece center from T1 to the other terminal of the antenna - this
is a 4:1 impedance transformer as well.
Your description of connecting the "braid" to the tower or mast top
apply in the applications I am familiar with. One of the most common
arrangements is in a multi-band quad. where several of the quad driven
loops would be connected to a remote antenna switch via 1/4 or 3/4 wave
of 75ohm cable for a 2:1 impedance match.
It is not clear what the actual arrangement of cables, tower and antennas
your comment regarding W8JI. Without a more complete description of the
arrangement and feed points it is not possible to determine what the
the 1/4w cable sections are. You speak of Hi Z, again not clear what
value is. I recommend looking up "Series Coaxial Transformer" (ARRL
Book)as this will discuss a more general solution for impedance matching
length of coax equal to the source Z, and a larger value of Z for the
section. The lengths and value of cable Z is determined by the frequency
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
A 1/4w of 75 ohm coax is a special case series transformer and provides a
An ODD quarter wave of 75ohm cable is also used in coupling two identical
antennas into a phase array.
A half-wave of same characteristic impedance coax is required to create a
Balance feed and results in a 4:1 impedance transformation. This
frequently use in large mono-band yagi's where the driving Z may be 10 or
ohms and the 4:1 provides a balanced match to 50 ohms.
Most amateur applications of the odd quarter-wave transformer use 75 ohm
which results in a 2:1 transformation.
Date: Tue, 28 Dec 2010 07:04:13 -0800
From: "Jim Thomson" <jim.thom at telus.net>
Subject: [TowerTalk] Baluns...again.
To: <towertalk at contesting.com>
Message-ID: <52EE1FDD307B4F319F0EB430BFCF95D2 at JimboPC>
Content-Type: text/plain; charset="iso-8859-1"
Looking through some old notes plus the info on Jim Brown's site....
was a question about using
an electrical 1/4 wave or electrical 3/4 wave of coax. The 1/4
wave of coax was run between the
DE of the yagi and the tower... with the braid of the coax bonded to
top of the tower.
The theory is... if a 1/4 [3/4] wave of coax has it's braid
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 [
3/4 wave] pieces of 50 ohm coax
were brought to a remote switch box at the top of the tower..where of
all the braids are bonded to the tower top. The author claimed that
baluns were needed on the various loops used on each band. I have
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
top...and bonding the braids to tower top. Seems to me that W8JI
that if a balun was used in conjunction with the 1/4 [3/4] wave coax,
the balun would makes things worse. There was some discussion on one
other refelctor's years ago about the 1/4 and 3/4 wave concept.
something do do with the height of the tower. If the tower was not
'correct height'.... the concept would not work. Does anybody
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
again.. just past a 3/4 wave. OK, what gives? Is an electrical
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
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
does work... and a high Z is presented to the DE... how high is this
Is is mainly reactive... mainly resistive..or a bit of both ? Why
a bead balun or torroid balun, when used in conjuntion with a odd qtr
of coax makes things worse ?
later........ Jim VE7RF
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