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Author Topic: Antenna feedline coupling.  (Read 869 times)
KG4OLW
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Posts: 165




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« on: June 27, 2013, 07:06:46 AM »

I have a question. I have a 50' run of coax, that terminates on my patio to a uhf bulkhead. The other side of the bulkhead has radials soldered on to the bulkhead and the center pin is soldered to a 30' run of 18 cage cable, which is screwed into a 25' downspout. Based on the low strength of received signals and the fact that when I transmit I don't seem to be radiating very much, I am thinking that what I am actually tuning is the coax, and I am using the coax like a random wire except laying on the ground.

My question is, how do I ensure the downspout is acutally being used as the main radiating element in the antenna system instead of the coax, What is a good way to check that the downspout is actually electrically coupled to the feed line?
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WB3BEL
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Posts: 78




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« Reply #1 on: June 27, 2013, 07:34:26 AM »

An antenna radiates by having RF current flowing on a conductor over a length that is useful at the frequency of operation.  This usually means that the length of the conductor is a substantial fraction of a wavelength.

You could measure the RF current flowing on the various conductors of your antenna system.  You could search for RF current meter plans.  I think that there was one within the last year or two in QST.  Its a simple thing to make or you could buy one from MFJ.  One made with clip on pickup coils can be valuable to determine what is actually radiating in an antenna system, the wires, the coax, etc.

Alternatively you could make or buy a common mode choke and attach it to your coaxial cable near the UHF bulkhead.  This will prevent RF currents from flowing on the outside of the coaxial cable braid.  If you see a significant difference in your antenna SWR this is a good clue that the current path has been altered with the inclusion of the common mode choke and that previously the feedline was radiating.
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KG4OLW
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« Reply #2 on: June 27, 2013, 08:14:53 AM »

I have a feeling the coax is fine, I think the actual 18 gauge wire coming out of the bulkhead center conductor is radiating along the ground (30' length) and very little reaches the downspout. 

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WX7G
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« Reply #3 on: June 27, 2013, 08:53:25 AM »

You are right, the wire along the ground is very lossy.

An improvement is to run coax from the UHF connector to the downspout and place radials at the downspout. A further improvement is to place an autotuner at the downspout.
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KE3WD
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Posts: 5694




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« Reply #4 on: June 27, 2013, 09:16:21 AM »

I have a feeling the coax is fine, I think the actual 18 gauge wire coming out of the bulkhead center conductor is radiating along the ground (30' length) and very little reaches the downspout. 



Bingo. 

The OP might do better to extend the coax to the point where the downspout is. 

And by all means, use a wide range Matchbox as well. 


73
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