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Re: [TowerTalk] Comments by K9YC

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Comments by K9YC
From: Ward Silver <hwardsil@gmail.com>
Date: Mon, 23 Jun 2014 08:28:23 -0500
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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

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