Tuesday evening, I checked into the Swap Net real quick mainly just to get a signal report. I just got done throwing together and throwing up an antenna I've had for a while and had put away. It's a 300 ohm, twin lead type that uses one of the versions of the old Radio Shack, black(or brown), with white foam filled, twin lead. I'm sure everyone's seen various versions of these, made from various different types and colors of twin lead, if you've ever done any kind of search for homemade or home brew "ham" antennas.
For those who live under a rock somewhere, basically it is an off-center fed, J-pole type of antenna with one side at 3/4 wave(1/2W on top of 1/4W) and a matching stub that is 1/4 wave. This eliminates the need for a ground plane or counterpoise. Depending on specs such as velocity factor, wire size, measurements.......some are designed to be as is and hung from a tree and some are designed to be placed inside some sort of enclosure like PVC pipe. It is a very efficient antenna and is very, VERY cheap and easy to make!!! Everyone and their cousin's uncle's Grandma has different plans on the web for this type of antenna with different material and different measurements, all with (obviously) different results.
As with most 2 meter, vertical antennas that resonate at 70cm(because it ends up being around 5/8 to 3/4 wave on 440), it is only working off of a harmonic and the take off angle is pretty high. Unless you're talking to martians or shootin' for the moon, it will work but isn't the best application.
A fellow "ham" named Edison Fong, WB6IQN, has designs for a better way of building this type of antenna which involves using a "trap" made from a piece of RG-174 coax that is open on one end and closed(shorted) on the other end and placed in the radiating element. This makes the J-pole 1/2 wave on 2 meters AND 440!!! It is then inserted into a 5 foot piece of class 200, PVC pipe.
He also has design plans for the same antenna with slightly different measurements for the same good SWR with-OUT the PVC pipe so it can be rolled up and thrown into a backpack or GO-Bag! Ed's designs have been spotlighted in QST magazine!!! Don't remember date or issue, sorry.
So, with that.....I will post the design plans here and maybe throw up a couple links if I can find some and of course, some pictures. Sorry, I didn't even think about taking any pics of the antenna itself as it comes from Ed Fong. But those can be found online. Besides, the end result is what you're interested in, anyway. Right?? RIGHT!!
There's other types of designs out there on the web for "true dual band" twin lead, J-poles, including a lot of 450 ohm ladder line designs. This post is for Ed Fong's "DBJ-1" and "DBJ-2" designs.
Also, a big thanks to Jim, KI7AY, for giving me the antenna. He was actually the one who bought it and had it laying around and then gave it to me to see if I could copy it with MY own twin lead material I had laying around, because Jim knows how much I LOVE building my own antennas. Then later he told me to just go ahead and keep it, very generously!! Thank you very much, Jim!!!!
Anyway, I finally got it put together and temporarily mounted with a new 50 foot run of RG-8/U to test SWR and realized the Swap Net was just starting so I checked in to see what they thought.
So far I've had nothing but great signal reports and I might even buy or make one or two for "roll-up" duty just to throw in the glove box!!
Sorry this is so long! Here's a couple (online).PDF files of the articles that were in QST Magazine back in '03 or '07.(Can't remember actual date......)
(.PDF files, even online, require Adobe Reader.)
DBJ-1 "TRUE" Dual band, twin lead J-pole....
DBJ-2 "Roll-up" version(no PVC pipe).....
Here's a few pics of mine.....
***Sorry I didn't take any pics of the twin lead itself! Didn't even think about it because I didn't think about pics at all until someone on the swap net was asking about the antenna and I got the idea of posting this here on the forum. Later on, I'll look around on the net for a pic or two of the antenna the way it comes before you put it in the PVC, or maybe I'll find a pic of the "roll-up" version.