I've never heard of using one as a receiving antenna. I thought they were designed as a transmitting antenna only. Not that you couldn't use it for receiving, of course. Made of metal and all...
Very interesting regarding FM. I've got about a 12' ceiling up in the attic. This could be interesting!
Antennas that work good for transmitting work exceptionally well for receiving on the same band it was cut for.
At the end of the day antennas are nothing more than RF tuning forks. When you design one to pick up a band or specific frequency for transmission they are equally good for reception.
J-Pole antennas are nothing more than glorified dipole antennas. They are technically end-fed dipoles.
Regular balanced dipole antennas have to be fed where the voltage is highest. Problem with this is that center fed antennas being balanced require balanced feed lines like twin lead which is what most FM receiver antennas are made of. They are often folded 300 ohm twin lead types that come with your stereos when new, you know.. the old two leg types you hang up horizontally where the lead comes down the centre.
J-poles are absolutely no different, except they are fed at the END. That means they need a means to transform the current at the end into voltage at the centre.
This requires an extra 1/4 wave section, this is the J part of a J-Pole, or the end section at the bottom that acts as an electrical short transformer (impedance match).
I hope I got my info all right here.
My point is that a J-Pole is a dipole is a J-pole is a dipole regardless if it is made from metal tubes or wire. Metal is metal (please no one talk about velocity factor, no one wants that)
They are identical antennas in how they operate. One is end fed (j-pole), one is center fed (dipole).
Yes Dipoles are electrically balanced antennas by nature which means either you need twin lead wire to your receiver input.
J-Poles look like they are not balanced, after all we connect coax to one side of the legs, usually to the ground lead to the short end.
However THEY ARE STILL BALANCED antennas. The ONLY way to remove the artefacts of coaxial interaction is either by making a current balun, or using twin lead to it as if it was a dipole in the first place.
TL;DR :
Everyone thinking a j-pole is somehow better than your wire or fixed install dipole are fooling yourselves which is cool because I love to study RF and help.
They both have the same gain and electrical characteristics when it comes to the radiating element. Only difference is how you feed it.
In fact j-poles often have a gain loss because of the balun used in most setups.
It gets far weirder though. There are charts using software showing j-poles can benefit because of feedline radiation, there are unpredictable circumstances that makes a j-pole either work miracles or less worthy than your dual lead 300 ohm tacked up on the wall dipole.
Just remember this one fact if everything I said was too long, an antenna made for transmitting will usually work better for receive than a receive antenna that could work as a transmit antenna.
Transmitters require accuracy in element length because of SWR and so much more. You can tie a random wire to a receiver and hear something.
Hope that makes sense.. the comparison.
/me lays down the beer can
