Slimmer Slim-Jim

GA INQUIRY ABOUT THE VALIDITY OF THIS DESIGN

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

Hi Joe--

Note: The antenna can be hung from almost anything usingsome string through a hole

SEE INQUIRY & RESPONSE BELOW

on YouTube

The 2-Meter TV Twin Lead Roll-up J-Pole Antenna

Are you ready to use your amateur radio communicationsskills in a real emergency?

Would you like to be better prepared?

Over the next few issues of this newsletter, this columnis going to present some information that you probably already know, but,like most of us, have not gotten around to doing anything about it. Whenthis series is completed, you will have created a communications "GO-KIT".A bag full of goodies that you would not normally be using, but, has beenprepared, tested, and packaged to GO with you anywhere, quickly. With simpletools and basic skills, you will be able to create the contents of theGO-KIT to be more prepared for effective emergency communications.

The 2-Meter 300 Ohm TV Twin-lead Roll-up J-pole.

As we all know, the antenna is the most important partof any transmitter system. It must be placed as high as possible, properlymatched to the transmitter, and designed to effectively radiate the powerapplied to it. The antenna that we are going to construct will be madefrom ordinary 300-Ohm TV Twin lead wire. This wire is available at almostevery hardware, drug store, and other stores that carry such items as telephoneextension cords.

In the diagram below you will see the construction detailsof the antenna.

Now, on to the step by step.

Tools: Ruler, Wire strippers, Wire Cutters, SolderingIron.

Measure 54" of Twin lead and mark this spot with some maskingtape. Cut the wire about 1 ½ inches longer than this measurement.If this is your first attempt at building something, you may want to leave4 " of extra wire on the piece you are working with. This will allow anadditional attempt of the critical portion of construction.
Remove the center insulation from the bottom 1-inch of twinlead. Strip the insulation off of the bottom section of twin lead (onlythe 1 "). Connect the wire together at this point and solder.
From the splice you just made at the bottom, measure up about1 " and remove ½ " - ¾ " of insulation from each conductor.This is where we will be attaching the coax.
From the splice measure up one side 16 ½" . CHECKTHE MEASUREMENT TWICE. Cut a gap on this side about ¼" wide. Besure to remove the conductive material from the notch.
From the splice, re-measure the total length to 54 " andtrim the top.
Attach some coax to the opened area of the wire about 1 ¼"from the splice. The shield of the coax MUST connect to the notched side.
Attach a suitable connector on the end of the coax for yourtransmitter and GO!

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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