Antenna=SlimJim

GA INQUIRY ABOUT THE VALIDITY OF THE 300ohm DESIGN

from KF7IRF Gaaren to N9TAX Joe Krajacic joe@N9TAX.com

Hi Joe--

I have bought several of your slim jims & they work great. I have a question for you. Are you familiar with this guy http://www.youtube.com/watch?v=nu_dY4VABic ? He says he built a slim jim out of the narrower 300 ohm ladder line antenna wire, & that it works phenomenally. The guy is kind of a clown, but maybe he has something. I was wondering if you had tried to build one that way, & how it compares to yours. You have the really good SWR meter & like to do measurements---just thought you might know. Joe

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RESPONSE TO INQUIRY from Joe K N9TAX

Gaaren,

The spacing of the conductors does play into the math when figuring out the antennas. Higher frequencies like narrower wire and lower frequencies wider. The 450 ohm ladder is more suited to 2m than the 300. I use 300 ohm ladder for my 440 only antennas it works much better for that. I wish there was a wider spaced wire I might be able to make them work better on 6 meters. That is my take on the different ladder lines. I have never seen that guys videos but I will check them out.

Thanks, Joe N9TAX

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Hi Joe,

Thanks for your response. A few clarifications:

When you say, "Higher frequencies like narrower wire and lower frequencies wider. The 450 ohm ladder is more suited to 2m than the 300.",

--it sounds like you are referring to the wire diameter(?).

--Is there a mathematical ideal wire size + resistance rating for 2m?

--& for 70cm?

Is there a theoretical ideal for distance between the wires for 2m & for 70cm?

For your 6m antenna, shouldn't you be able to just get the right diameter wire, & run one down each side of wood or fiberglass that is the right width apart, or get 600ohm ladder line? Info Here is a place that sells it.

Thanks,

Gaaren

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Gaaren,

Ok first I bought some ladder snaps at Dayton. I was able to brew up a 6m antenna that is < 1.5:1 from 50-54 this confirms my thinking that conductor spacing is at the heart of the problem. There are however many engineering hurdles with the concept of using a true open wire ladder line. Like what to do at the notch where there is nothing to keep that side of the antenna together. My temporary solution would make Rupe Goldberg proud. I also need to come up with a more professional method of attaching the cable. I am working on this slowly as I have time. Maybe more progress on the long weekend. I will keep you posted as I am sure you would like to test one of the prototypes ;) Final thing the wire diameter has some effect on circuit Q and velocity factor but not bandwidth. I was talking of conductor spacing when I said wider and narrower.

Thanks, Joe N9TAX

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