[CQ-Contest] Adding 80m Shunt feed w/160

[David A. Pruett]

Scott,

Find somebody who has a set of old QSTs (or the CD-ROMs) and find the 
September 1975 issue (I think - it might be October).  There was an article 
by Earl, W5RTQ (who later became K6SE, still a well-known call on top band) 
that was pretty much the holy grail for people trying to shunt feed 
towers.  Earl's tower was approximately 70' with a TH6 on top which is not 
all that much different from your setup.  He had it successfully shunt fed 
on both 160 and 80.

In my experience, I've tried shunt feeding towers such as yours and agree 
that it is difficult to make work.  The reason I believe is that the height 
of the tower, along with the top loading of the yagi, results in an 
electrical length approaching a full quarter wavelength on 160M.  While 
this results in a very effective 160M antenna, the same structure is almost 
one half wave long on 80M which results in a very high radiation resistance 
when fed at one end (the opposite of the 160M situation).  This is why it's 
so hard to match.

I *have* been able to make this work, but it takes a LOT of cut and try.  I 
would start by moving the feed point *lower* on the tower rather 
higher.  Again, the K6SE/W5RTQ article would be a great starting point.

73,

Dave/K8CC



At 12:51 AM 11/15/04 -0500, Scott Bauer wrote:
>Has anyone out there combined an 80m shunt feed on a tower that already 
>has a shunt feed for 160m ?
>
>I have an 76 foot tower, with a 160m shunt feed, with the shunt wire 
>connected at about 60 feet.  It works very well for me on 160m, or at 
>least it seems to.  I only run 100W on 160m, since my amplifier does not 
>have 160.
>
>I tried adding an 80m shunt feed, first with the shunt wire connected at 
>about 50 feet, which didn't work then moved it to 60 feet, which also 
>didn't work.  I am using an L network, with a shunt roller inductor and a 
>series variable capacitor to the shunt wire.  I have more than enough 
>inductance and capacitance to work with, both value and tuning range.
>
>The 160m shunt feed took all of 30 seconds to tune.  On 80m I can't even 
>get it close.
>
>The tower has a PRO-67 on top at 77 ft (40m thru 10m), with three other 
>yagis at 64, 50 and 37 feet, and an 80m dipole.
>
>Since the dipole I have on 80m doesn't seem to do well, I want to shunt 
>feed the tower on 80m and take advantage of the radials already installed 
>for 160.
>
>The only other option is to buy tubing and make an 80m vertical and put 
>down another set of radials.  Anyone have experience with the tubing that 
>Texas Towers sells ?  I don't really want to use irrigation tubing, looks 
>like it is harder to support.
>
>Thanks in advance for the help.
>
>73 Scott W2LC
>
>
>
>
>
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