J-Pole antennas


Don Wilhelm <w3fpr@...>
 

Dale,

The best impedance for the 1/4 wave transmission line section should be 300 ohms.� If you can find the definitive work of Gary O'Neil N3GO related to the J-pole, you can see the reason for that choice.� Sorry, but I cannot locate that reference at the moment.
There are charts and formulas in the ARRL Antenna Book and Handbook for determining the spacing of the conductors in a transmission line given a particular diameter.

I don't know about perturbations using a 5/8 wavelength radiator, but I suspect they will buy you little and because the impedance at the base of the 5/8 wavelength will not be the highest impedance possible from the radiator, the transmission line section would have to be changed from a 1/4 wavelength to something else and another element (capacitor) would likely have to be used to cancel out the reactance.� I just do not think it worth the effort for that extra 1 or 2 dB.

I would stick with a 1/2 wavelength radiator and 1/4 wavelength transmission line section to keep the reactive element in check.
If you want to increase the gain, put an insulator at the 1/2 wavelength point of the radiator and insert a folded 1/2 wavelength of wire, then stack another 1/2 wavelength on top of that.� You then have a co-linear J-pole.� Practical for 2 meters, but may become physically too large on 6 meters.
I do have a co-linear J-pole on 2 meters constructed of 1/2 inch copper pipe. A co-linear should give you 3 dB of gain and bring your radiation pattern down to a stronger (but more flat) pattern in the horizontal direction.

When tuning a J-Pole, the method I use is to first adjust the 1/2 wavelength section to resonance (reactance equals zero on the antenna analyzer), and then adjust the tap to produce a 50 ohm match to the coax.� If you do not have an antenna analyzer, borrow one.� Trying to adjust both the radiator and the coax tap position using only lowest SWR can lead to confusion between the two and a poorly performing J-Pole.

Gary, N3GO is a local friend (as well as a superb engineer), and I will ask him where his paper on the J-pole can be found.� If it is still available on the web, I will post the link here.

73,
Don W3FPR

On 8/11/2015 7:16 PM, Dale Putnam daleputnam@... [4sqrp] wrote:
�

Interesting that you mention the J-pole Don, as I am building one for 6m from 2 in aluminum mast..
can't find any details as to how to adjust for the much larger diameter.. .or what to expect...
NOR can I find a really good general explanation of how the J-pole works... the short 1/4 wave section is the ground for...
the 5/8 wave length??? fed section? �That pretty well matches some of the commercial antenna designs... but there are a whole boatload of diagrams that simply don't fit that. Some of them have well over a wavelength being fed, with a 1/4 grounded to the shield.�
� I expect to find the larger diameter J-pole to be wider in response, and acceptable across a wider portion of the band.
I think I just missed a great 6m opening, tho... ��


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