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help appreciated with unusual j pole feed
hi there, i'm looking for some help with a j pole antenna, it's actually a vhf marine antenna so i hope it will be ok to ask here. my problem is that this antenna used be atop the mast of my yacht untill it blew off breaking the end fitting and pulling free of the coax, because these things are quite expensive and don't seem to last that long, I decided to try repair it as a spare. It fits inside a fibreglass tube, so I carefully cut through the tube and pulled out the antenna parts, which are all intact. my plan was to solder some coax to it and put it back in the tube, then glass the tube back together, but this is where i ran into trouble, i'm unsure where to afix the coax. the antenna i believe is a j antenna, with a 3/4 wave leg and a somewhat less than a 1/4 wave leg returning from the base, however there is no connections on this stub part where the coax has been connected, i had expected there would be. There is, however, 2 pins below the 2 upright legs, these are not electically connected to the upright stub elements, (the 2 upright elements are joined across the bottom). These 2 pins have a length of insulated wire attached to them which stretches over and parallel to the uprights of the stub. The 2 pins are staggered in height which means 1 side of this insulated wire run is approx 75 mm and the other side is approx 70 mm, it runs like an inverted v up the stub from these pins at the base. I'm assuming that the coax should also be soldered to these 2 pins and interacts with the antenna through induction or something. If this is correct does it matter which side of the coax goes to which pin, neither of them are conneced to anything other than each other through the insulated wire, which as i've said, runs above what i beleive to be the antenna stub section. or should the coax be connected at the top of the stub, ( a halfwave from the top), i can't see any sign that the coax was previously fixed there, but then there is no sign that it was previously connected anywhere for that matter hope this makes sense and someone is able advise, thanks for looking and if you can help thanks again. if this all sounds like nonesense, i may be able to put a picture as an attachment but i'm unsure whether it is acceptable in this group. kevin |
help appreciated with unusual j pole feed
No, posting a picture here wouln't be acceptable. You could send it to
those who ask, though. From your description, it sounds like the loop of insulated wire between the two pins couples to the stub--the 1/4-wave bottom of the antenna. I guess that stub is over 400mm long, so your wire only goes up a fraction of that distance. Someone may have engineered it, or they may have determined it empiracally, but it sounds to me like quite an acceptable way to couple to the antenna. And that seems like the most likely reason it's there, so that's where I'd connect the coax. I'd guess that there isn't much difference between the two possible polarities of connecting the coax to those pins. Cheers, Tom |
help appreciated with unusual j pole feed
thanks for that tom, i have tried it experimentally coupled to the pins but
it was dissapointing, infact i needed to disconnect the outer coax to get any signal at all. think i'll try build something, perhaps j pole or maybe skirted dipole type. thanks for your answers tom. cheers kevin "K7ITM" wrote in message oups.com... No, posting a picture here wouln't be acceptable. You could send it to those who ask, though. From your description, it sounds like the loop of insulated wire between the two pins couples to the stub--the 1/4-wave bottom of the antenna. I guess that stub is over 400mm long, so your wire only goes up a fraction of that distance. Someone may have engineered it, or they may have determined it empiracally, but it sounds to me like quite an acceptable way to couple to the antenna. And that seems like the most likely reason it's there, so that's where I'd connect the coax. I'd guess that there isn't much difference between the two possible polarities of connecting the coax to those pins. Cheers, Tom |
help appreciated with unusual j pole feed
hi fred, thanks for your input, if i read you correctly this is cut for 2mtr
and the one i'm working on is actually a marine vhf, (usually I think cut for 156.8mHz), i realise this is a ham group but hoped it would be ok to ask here. i have found some online calculators, maybe i can use the information from them to recut this design. thanks fred kevin "Fred McKenzie" wrote in message ... In article , "kevin" wrote: hi there, i'm looking for some help with a j pole antenna, it's actually a vhf marine antenna so i hope it will be ok to ask here. Kevin- One version of the J-Pole antenna has the cable connected an inch or two above the connection between the two vertical rods. The following article from my archives, shows how it could be connected, except it is based on Television Twin Lead instead of the two rods yours has. If you use a mono-spaced font like Courier, it should be clear. The conductor above the 1/4" gap is not essential, it just comes with the twinlead! Fred --------------- 2M Jpole with 300 ohm twin lead From: (Andrew A. Kinsman) Newsgroups: rec.ham-radio Organization: Eastman Kodak, Dept. 47, Rochester NY The following is a description of a J-Pole antenna made from 300 ohm TV twin-lead. Quite a few of the hams in the area have successfully built one or more of these antennas. They have quite a few advantages which include improved performance for 2-meter HTs, portability, and last, but never least, they are inexpensive. | | do not short this end. | | | | | | | | | | 54" | | | | | | | | | 1/4" gap | | | | | | 16 3/4" coax ctr conductor=* *= coax shield | | 1 1/4" | | -*- solder the twin leads together at bottom 1. Start with 54 1/2" of TV twin lead (foam core does not work as well) 2. Strip 1/2" of insulation at bottom and solder wires together. 3. Measure 1 1/4" from soldered wires and strip insulation on both sides. This is the solder point for a coax feedline. 4. Measure 16 3/4" from bottom and cut out 1/4" notch on one side. 5. Feed with a length of RG58U coax. Tape coax at feedpoint to the twin lead for strength and weather protection. I have also attached an alligator clip to the plastic on the top of the antenna in order to conveniently hang it. I am successfully using one in my attic to avoid any CCNR problems. I have another that I can take with me camping, sailing, etc. This design has appeared on many BBSs and in club newsletters; the earliest reference that I know of is a Jan. 1984 D.A.R.C. antenna article by James Burks, KA5QYV. Enjoy, N6UBM ------------------ |
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