Hmm.. I think the center conductor of the coax is suppost to be connected to the long element of the J-pole and the ground connected to the short part. I don't know if it makes a difference, maybe someone can fill us in on this.
BTW I found this online PDF tutorial on J-Pole antennas with some interesting design ideas you all may want to look at... www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=3482 The dimensions in the guide is for ham bands but with recalculating the sizes all the info is good for FM broadcast also.
One more thing, you have to use a balun at the feed point of the jpole antenna if you mount of your antenna to a mast that is earth grounded. From what I have read online you can simply loop 4 to 6 turns of coax to work like a balun.
Post by Reverend Bow Voodoo on Mar 18, 2009 6:24:37 GMT -6
The changes look a LOT better.
As much as you mess with radio, you should know the range you "Should Get" and the range you "Do Get" are never even close...
Theory looks great, on paper...
When I built my 2 Meter J-Pole back in the day, I soldered the SO-239 to the Long radiator, and used a piece of solid cooper house wire straight across to the match, then soldered it.
Those hose clamps are just asking for corrosion and failure in the the lovely Houston humidity...
Post by HighMountainRadio on May 19, 2018 8:42:12 GMT -6
Greetings From High Mountain Radio !
I thought it was very important to mention an often overlooked issue that can very easily cause a lot of grief when installing a FM broadcast antenna and tuning it for the first time !! ALWAYS, ALWAYS, be certain to utilize a 'RF Choke' in your coaxial cable feedline right where the coax is fed to the antenna feed point. The RF Choke is nothing more than 6 turns of your coaxial cable transmission line wound into a 6 inch circle, making sure to wind each subsequent loop of coax ADJACENT to the previous turn or loop. You DON'T want the choke to be wound in a 'scrambled' fashion, it MUST be coiled very neatly and tightly placing one loop exactly next to the prior loop, etc., until it is complete. I used small wooden spacers that I cut from an old ferring strip of wood. I placed them strategically around the coil of coax and secured them into place with electrical tape to assist in holding the choke together securely, assuring that it holds the RF choke together properly. I have included a link to another post that I placed on this forum that shows how to fabricate this type of 'coaxial choke' assembly. You can either incorporate this type of choke right into the end of your coaxial cable transmission line or you can 'retrofit' it into an existing setup using RF adapters where and if necessary to insert it into the transmission line at the antenna.
An alternate method and form of RF choke is through the use of 'ferrite sleeves' of the proper MIX TYPE 43. These are 'sleeves' made from ferrite and that have a hole going through the center along the length of the ferrite sleeve and can be simply slipped over your coax and heat shrink tubing of the proper size can be used to secure them onto the coax. This method is also 100% effective in suppressing RF energy from being dissipated and radiated along the shield of your coaxial cable transmission line which very frequently occurrs and will cause an abnormally high VSWR which can drive you crazy when trying to tune any antenna where this becomes an issue !!
While it is not mandatory to use a RF choke in the transmission line on all transmitting antennas as some antennas, depending on whether they are of the balanced or unbalanced type do not require a RF choke simply by their unique technical nature, it is a good rule of thumb to use one anyway and be SURE !!
Here is the link to fabricating a coaxial choke, I will post another thread soon showing how to fabricate an equivalent RF choke using the ferrite sleeves in order to assist others who may be unfamiliar with them.
Hope that this helps you understand why, in all likelyhood you were having such difficulty tuning the antenna you may have already, or are about to give up on !
73... Spooky...
"Knowledge Is Always Made More Valuable By Sharing It With Others !" Always Remember To Be Kind ! True Greatness Often Has Very Humble Beginnings ! Help A Kid Out Today ! *** High Mountain Radio *** "Broadcasting From Somewhere High In The Remote Appalachian Mountains"