Coil of coax used as a J-Pole antenna choke balun
Most of the instructions and plans you see online for J-Pole antennas often include the admonition to use a choke balun with the antenna. The purpose of the balun is to limit the amount of RF radiation coming back down the feedline and into the radio. Even though limiting the return of RF energy is a good thing, I still get the question ‘Is the choke balun necessary?’
To answer this question, you need to look at the antenna itself. The J-Pole is a half wave antenna by design. As a half wave antenna, it doesn’t require any special grounding or a groundplane to operate. The purpose of the groundplane is to ‘balance’ the antenna, that is to give the antenna something to push against as it releases the RF energy. For example, with a quarter wave antenna, not all the RF energy will be released up the antenna itself unless it has an effective groundplane to direct that stray energy.
Half wave antennas don’t need the groundplane, but they also have the tendancy to send excess RF energy back down the feedline, increasing your SWR and limiting sensitivity. The choke balun, which can be as simple as six turns of coax or ferrite chokes near the feedpoint of the antenna will stop the excess RF energy from returning down the feedline.
If the antenna is perfectly tuned and is mounted in a location where it isn’t couple with any nearby structures, then the choke isn’t necessary. I’ve used J-Poles with and without the choke and a well tuned antenna mounted in a good location will work fine without out it. But as I usually say, the choke will never hurt and always help. Therefore I recommend the choke balun for any new J-Pole antenna installation.
IF MY COAX GOES INTO THE MAST WOULD THAT DECOUPLE FEEDLINE…OR DO I NEED TO KEEP BALUN
Feeding your coax cable inside your mast will not cause it to couple with the mast or antenna. The way coax is constructed (inner wire with an outer braid) is designed so that it doesn’t radiate RF energy. The choke balun is designed to keep stray RF energy from the antenna traveling back down the feedline. So I still recommend the choke, even though your feedline is encased inside the mast.
Michael
Does the Slim Jim antenna require the choke balun too?
I think the balun is also a good idea for the Slim Jim antenna. You’ll receive the same benefits in using it on the Slim Jim as you do on the J-Pole: it will keep any stray RF energy from returning back down the antenna.
Michael
First thanks for the fast shipment and professional job on the J-Pole I recently ordered. I’m a newbie.
So what type, and how many, ferrite chokes (1,2,3) would i need on lmr-400? seems like the lmr-400 is too stiff to get a tight balun coil with the feedline, so I’m thinking i need the ferrite snap on chokes, however i’m unsure which kind and how many.
Also, should the ferrite choke be placed 12″ from the attachment to the antennae?
Thanks!
KC3BUY
Bryan,
LMR-400 cable is pretty stiff, so using 2-3 ferrite chokes near the feedpoint (about 12 inches) of the antenna is a good substitute. Radio Shack carries a couple of ferrites (#273-1005) but I’m not sure if the diameter of the core is big enough to accommodate LMR-400. LMR has a little bigger diameter than RG-8, so you may need a bigger choke. DX Engineering is a great source for chokes if the Radio Shack store doesn’t have anything.
Michael
10mm (or .4 inch) ferrites fit LMR-400 perfectly.
Just to be more specific, that is inside diameter. These fit perfectly:
Jeff,
Thanks for the recommendation. LMR-400’s outer jacket is a hair thicker than RG-8U or RG-213, so it can be difficult to find that perfect fit without crushing the cable. These look like they are the perfect size.
Michael