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Re: [TowerTalk] 1/4 wave coax "Balun"?

Subject: Re: [TowerTalk] 1/4 wave coax "Balun"?
Date: Wed, 29 Dec 2010 09:12:17 EST
List-post: <">>
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 
choke  type?
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 
Gerald K5GW
In a message dated 12/29/2010 4:32:23 A.M. Central Standard Time, 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 
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 
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 
antenna and 
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 
are in  
your comment regarding W8JI.  Without a more complete description of  the 
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 
value is.  I recommend looking up  "Series Coaxial Transformer" (ARRL 
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 
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 
impedance  transformation.  

An ODD quarter wave of 75ohm cable is also  used in coupling two identical 
50 ohm 
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 
arrangement is 
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.  

Norm   W4QN

Date:  Tue, 28 Dec 2010 07:04:13 -0800
From: "Jim Thomson"     <>
Subject: [TowerTalk]  Baluns...again.
To:     <>
Message-ID:     <52EE1FDD307B4F319F0EB430BFCF95D2@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    
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 [ 
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 
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 
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 
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 
'correct height'.... the concept    would not work.     Does anybody 
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 
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 
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  
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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