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feed line length question

May 6th 2017, 18:16

KK4BHS

Joined: Apr 15th 2011, 16:46
Total Topics: 0
Total Posts: 0
Hi new here and don't post or look here much but figured this was best place to ask a ham related question.
I have two towers I would like to put up and location is giving me fits as to exactly where.
I would like both of them to be where I could hang wire antennas from one to the other and have the feed lines come down between them. I would also place a rotator with antennas on tops of them as well.
I have several locations I am considering for the vhf uhf tower. One is approximately 40 feet away the other is 125 feet away.
My question is several. 1. how far is too far for vhf uhf ? What types of feed line would you recommend at those distances? Obviously I know the closer the better but I'm hoping someone can give me more specific answers. Also the tower is 72 foot tall and vhf uhf would be at the top of said tower so I realize that would add to the distance of feed line.
Any tips or help is much appreciated as I am new to tower stuff and different kinds of feed line. Currently using lmr 400 thru most of the shack.
May 6th 2017, 19:09

WA0CBW

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The amount of coax loss (attenuation) increases with length and higher frequencies. You can lookup each type of coax and determine the amount of loss (measured in db) for the length and frequency you will be using. You can then balance the least amount of loss against the amount of money you want to spend. Typically coax will have more loss than "hard line". Also‚Äč connectors for hard line are more expensive than for coax.
Bill
May 19th 2017, 17:43

KA8WBF

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Hello

I notice older Handbooks suggest a 1/4 wavelength feeder (or multiple) to a 1/2 wavelength zepp, while later ones list a length that appears to be a multiple of 1/3 wavelength (42 feet). Parallel currents or feedline radiation are mentioned, but not a lot of explanation of why they exist or where they come from.. Can anyone add more details?

Cordially
KA8WBF
May 19th 2017, 19:32

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Quote by KA8WBF
Hello

I notice older Handbooks suggest a 1/4 wavelength feeder (or multiple) to a 1/2 wavelength zepp, while later ones list a length that appears to be a multiple of 1/3 wavelength (42 feet). Parallel currents or feedline radiation are mentioned, but not a lot of explanation of why they exist or where they come from.. Can anyone add more details?

Cordially
KA8WBF

The older Handbook advice still works well for a single band antenna, but I tried for years to sell that idea to beginners without much success. Modern hams want multiband antennas. The older Handbook advice doesn't work for multiband antennas and modern tuners. A multiple of a 1/2 wave antenna is high impedance, and the feeder presents a high impedance to the tuner. Tuners like the AH-4 and others specifically tell you not to do that. Actually, you can if you run very low power, like 5 watts, but that idea is even harder to sell than starting off with a simple single band antenna.

The modern Handbook advice is a compromise based on the idea that a modern ham wants to run as much power as they can on as many bands as possible.

One of a kind high voltage vacuum variable capacitors are still available on Ebay if you wish to homebrew a tuner that will handle very high voltages.

Zack W1VT
ARRL Senior Lab Engineer.

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