[TowerTalk] [Bulk] 160m Shunt Feed

[Bill Coleman]

On Dec 11, 2010, at 6:57 PM, Kelley wrote:

> Multitudes of web pages show, well, multitudes of setups for shunt feeding a tower. Many appear to use a couple of variable capacitors in order to adjust for lowest SWR. Having a limited junk box, what kind of parts should I be looking for to build a generic matching network? 

It really depends on the shunt-feeding arrangement. Some can get by with a single shunt capacitor, others use an omega match, still others use L or T networks. 

It really all hinges on the complex impedance that shows up at the feedpoint.

I have a short tower that I have shunt-fed for 160 and 80m. It works pretty well. Just about any installation is unique though -- how tall is the tower, how big are the antennas at the top, how high and how far away is the shunt wire(s), etc. K9AY suggested I run a shunt wire as high as I could, and then measure the impedance at the base and build a matching network to suit. And that's what I did, basically.

You can read the entire article on my blog here:
http://boringhamradiopart.blogspot.com/2009/04/shunt-feeding-short-tower-on-80-and.html

I didn't have any sophisticated equipment to measure the impedance, so I designed an L-network through trial and error.

If you want to run more than 100 watts or so, you are likely to need pretty beefy components in the matching network. Capacitors with 1-4 kV ratings recommended. Coils (if you need them) would be very large. 

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Web: http://boringhamradiopart.blogspot.com
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901