Experiments in shunt feeding

kc5uta

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For a little background, my old tower is coming down soon due to deterioration. (one leg at about 35 feet has rusted through..yikes! :eek:) So in the process of setting up some new antennas NOT on the tower, I did an experiment. I shunt fed a grounded pole. My rationale, was since my 65ft tower had been shunt fed for 160 and 80 with good results, I could do the same for a shorter version and maybe get 40 or 20, and maybe 15. I used a 20 foot chain link top rail stuck in the ground, with a (approx 2 foot) arm at the top perpendicular to the pole, attached a copper wire to it, dropped it to the bottom, with a fiberglass spreader about halfway to keep the spacing. The feed point was an air variable in line on the center conductor of the coax, and shield to ground. Surprisingly it tuned to 160? (huh? WTH?) wouldn't touch 80, or 40, seemed to like 10mhz. Decided to add more ground radials (old grounding was sketchy) which made tune 80 ok but not 40, or any other band. As a radiator on 160, 80...not so good. Tried to make a Texas traffic net on a hundred watts, they could hear only an unintelligible weak signal., lots of RF in the shack. :sneaky:I guess 20 feet is not a good length to cheat with on 160, 80. Live and learn I guess. Any thoughts?
 

prcguy

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You should be able to tap the tower maybe 1/4 to 1/3 the way up with a wire that goes down to an auto tuner that is well grounded at the base to some ground radials. Then lots of ferrite on the feedline and/or power cables to the tuner. This should tune all the lower bands ok.

For a little background, my old tower is coming down soon due to deterioration. (one leg at about 35 feet has rusted through..yikes! :eek:) So in the process of setting up some new antennas NOT on the tower, I did an experiment. I shunt fed a grounded pole. My rationale, was since my 65ft tower had been shunt fed for 160 and 80 with good results, I could do the same for a shorter version and maybe get 40 or 20, and maybe 15. I used a 20 foot chain link top rail stuck in the ground, with a (approx 2 foot) arm at the top perpendicular to the pole, attached a copper wire to it, dropped it to the bottom, with a fiberglass spreader about halfway to keep the spacing. The feed point was an air variable in line on the center conductor of the coax, and shield to ground. Surprisingly it tuned to 160? (huh? WTH?) wouldn't touch 80, or 40, seemed to like 10mhz. Decided to add more ground radials (old grounding was sketchy) which made tune 80 ok but not 40, or any other band. As a radiator on 160, 80...not so good. Tried to make a Texas traffic net on a hundred watts, they could hear only an unintelligible weak signal., lots of RF in the shack. :sneaky:I guess 20 feet is not a good length to cheat with on 160, 80. Live and learn I guess. Any thoughts?
 

kc5uta

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LOL....yup a whole LOT of ferrite:).
The 65 foot tower version rocked, but the 20 footer ... meh. BUT!!! Now that you mention it, I may drop the feed point halfway and try again before dismantling ..Gracias! Ill update in a few days
 

RFI-EMI-GUY

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Are you sure that your experimental 20 foot loop isn't magnetically coupling with your existing 65 foot tower and reflecting its resonance?
 

kc5uta

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not sure.. ill give it a "plausible but not probable maybe". I separated the ground system, and the loop is roughly 15 feet away from the tower, with a corner of my new shack (with tin sides) in between. But RF can be weird, who knows?

My guess (and a poor one at that) is that it's trying to behave sort of like a magnetic loop/shunt fed hybrid but not with any efficiency to speak of. it's roughly a 40 ft long ( squashed) loop grounded at one end fed with an air variable cap at the other. But I'm sure brighter minds than me would be able to explain what is really going on. :p
 

ko6jw_2

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Check out this article on shunt feeding:


I was once chief engineer of an AM station that had a shunt fed antenna on 1490KHz. Of course it was a full quarter wave with an excellent ground system. It still had a matching network with an Rf ammeter. The feed was not as high up the tower as you might expect but It worked very well. Ground conductivity was extremely good as it was very near to the ocean.

The link I posted is more relevant to your situation because it discusses the shunt feeding of a short vertical. Note that the beam at the top apparently functions as a capacity hat.

I doubt if there is much in the way of practical guidance for this type of antenna. As you say it is an experiment.
 

kc5uta

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I do believe the article you supplied was the one that got me thinking of feeding my old tower to begin with :) I had a great signal on 160, 80 with the 65 footer. AND as you also mentioned grounding, I found out that it is VERY critical on these feeds. Appreciate the input!
 
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