[Antennas] Mosley/Balun

[James]

From: "Don Boyer" <don.boyer at gte.net>

 

 

I recently obtained a Mosley Classic 33 antenna that appears to be in
excellent shape.  It has an SO-239 coax connector to the antenna's "Classic
Match".  The Mosley website indicates that for The Classic System:  "To
isolate the outer coax conductor from ground, the coax line is coiled for a
few turns near the antenna end.  This is designed to prevent the very
unlikely effect of Feed Line Radiation".  I have also heard that with the
Classic 33, you can connect your coax directly to the SO-239.

 

Is this coiled coax actually needed and if so, how many turns, what diameter
is the coil and where is the coil located? (I typically use RG-8 coax)  Any
comments regarding this interface would be greatly appreciated.  The coiled
coax may not even be required. This is unclear to me.

 

Also, any repair / maintenance pointers would be helpful before I put the
antenna on my tower.  Thank you.

 

Don - WB0AIN

 

>From KV4FE  JIM 

 

>From the ARRL Antenna Book 1994

Chapter 26 page 12 

-      - Construction of a choke (current) balun is described here.  The
objective is to obtain a high impedance for currents that tend to flow on
the line. - - These baluns work best when used with antennas having a
feed-point impedances less than 100 ohms or so - - 

-      The simplest construction method for a 1:1 balun for coaxial line is
simply to wind a portion of the line into a coil - - A flat coil shows a
broad resonance that easily covers three bands. 

 

Table 1.  effective choke Balun 

 

Multiple bands   3.5 to 30  7 ft. of RG8 coiled into 7 turns. 

 

Single band       20 meters    10 ft of RG8 coiled into 4 turns. 

End of Handbook quotes. 

I have learned from reading the information on Balanced/Balanced

Antenna tuners that the coil should be in the form of a tube.  If you tape
each turn as you are forming it there will be no need for a form.  When
cutting the coax cut longer that the need so you can form a lead in and a
lead out.  If you plan ahead you can make the form on the end of the coax
and run it into the shack with out signal reducers inserted. (connectors)

Also there is a minimum radius that coax should be bent and I think a 4 inch
form (pvc pipe) exceeds that minimum. 

10 ft. of RG8 into 4 turns is 2 1/2 ft. per turn and  7 ft. into 7 turns is
1 ft. per turn is not to small a radius for the coax.  

 KV4FE 

JIM 

 

Joan & Jim 

On The River 

 


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