[TowerTalk] Comments by K9YC

[Ward Silver]

A clarification - there are two basic mechanisms in play regarding the 
effect of the feed line's outer surface.  Each influences the antenna 
pattern and SWR but in different ways, sometimes creating the impression 
of one complex problem.  The fix for each is somewhat different - divide 
and conquer.

The first mechanism is the current path from the inside of the shield 
that splits between the antenna element and the outside surface of the 
shield.  See Roy Lewellan W7EL's "Baluns: What They Do and How They Do 
It" for a complete explanation. 
(www.eznec.com/Amateur/Articles/*Baluns*.pdf) To minimize the effect of 
the split current path, install a current balun (aka - choke balun) at 
the feed point.  This increases the impedance of the path to the outer 
surface of the shield and restricts current flow from the inside of the 
shield to the antenna and vice versa.  A current balun at the feed point 
is very useful in balancing current in the driven element, creating the 
symmetrical pattern we expect.

The second mechanism is the effect of having the outer surface of the 
feed line in the antenna's near field and interacting with the radiated 
energy.  As we all know, having a nearby conducting surface coupling to 
the antenna can greatly affect the antenna pattern and also affect the 
feed point impedance and SWR.  The orientation and electrical length of 
the feed line determine the effect's magnitude which can range from 
minimal to dramatic.

For example, you might have a current choke at the feed point of a 
coax-fed dipole, but if the outside surface of the shield is resonant or 
the coax is asymmetrically placed with respect to the dipole, 
significant current is likely to be induced on the shield's outer 
surface, despite the choke's impedance located at the feed point.  This 
can greatly upset both the pattern and feed point impedance.

You can tell if this second effect is occurring if moving the feed line 
around changes the SWR significantly or you observe a beam's pattern to 
be very asymmetric.  (You can also sniff for RF current with a current 
probe.)  Another way to check might be to coil up some of the feed line 
partway along its length (one-quarter wavelength from the feed point is 
a good place to start) into a choke and see if that changes SWR.  At VHF 
and UHF, just placing your hands on the feed line is enough to affect 
the impedance and that can be observed as changes in the SWR.  Don't 
obsess over small changes in SWR or pattern - get on the air and work 
people :-)

To minimize the coupling effect of the shield's outer surface, run the 
feed line symmetrically with respect to the antenna for as far as you 
can or tape it to the boom of a Yagi and then run it down the leg of the 
tower.  If you have significant current flow on the outside of the 
shield, additional current chokes placed a quarter-wavelength apart can 
help.  If you are using open-wire feed line, you'll have to rely on 
symmetrical orientation to keep the system balanced and minimize 
common-mode currents on the open-wire line.

Where have we seen this second mechanism before?  Guy wires! Insulators 
are used in steel guy wires to "break them up" into non-resonance 
lengths that minimize interaction with antennas atop the tower.  Exactly 
the same effect occurs with the feed lines that are suspended below 
antennas.

73, Ward N0AX