The Open Stub J-pole

[The DB]

A friend is making and selling these Open Stub J-Poles that you can find directions for on-line. The question is does this description accurately reflect how this antenna actually works? I personally think that, despite the design's looks, that the name given to this antenna is in error.

I'm curious to others thoughts are on this antenna design, do you think it is a J-Pole or not, and why?

The DB

 

[Marconi]

A friend is making and selling these Open Stub J-Poles that you can find directions for on-line. The question is does this description accurately reflect how this antenna actually works? I personally think that, despite the design's looks, that the name given to this antenna is in error.

I'm curious to others thoughts are on this antenna design, do you think it is a J-Pole or not, and why?

The DB

DB, I've seen this style before...is it a multi-element J-Pole antenna or is the short element part of the tuning mechanism?

 

[bob85]

id call that an asymetrical fed j-pole which creates common mode currents.

 

[Marconi]

id call that an asymetrical fed j-pole which creates common mode currents.

How about this Bob.

 

[bob85]

same thing eddie, not shorted at the base so its open stub and not fed symetrically.

 

[Marconi]

touche

 

[The DB]

To call it an open stub I think is way off as it isn't actually a stub, which would have a tap point at some point other than the end of the 1/4 wavelength. Also, a 1/4 wavelength open stub would convert a high impedance resonant source to a low impedance resonant antenna, in fact just the opposite of a shorted stub. For it to be an actual open stub with the same impedance at the antenna end, the stub would have to be 1/2 wavelength long with a tap near the middle, which would be the low impedance feedpoint.

This design also has the feedpoint at the end of the so called "stub" which makes it not act like a stub but a feedline. Think 1/4 transformer here as that is what it appears to be to me. The two elements essentially become a high impedance ladder line, and over 1/4 electrical wavelength transform the low 50 ohm source to a very high impedance, which can then directy feed the end fed half wavelength antenna above.

Yes there will be some imbalance, but I have yet to encounter a significant amount of common mode currents from one of these antennas yet...


The DB

 

[camaro1]

would like to build one,, what are the dimensions between the elements??

 

[bob85]

DB
w8ji and others call an antenna fed at the lower end of the stub without the shorting bar a bottom fed j-pole,
it looks like a zepp to me, i have seen the same thing described as either a zepp or j-pole,

i agree the 1/4wave section of transmission-line transforms the high impedance at the top of the stub to a low impedance at the bottom of the 1/4wave transmission-line,

the antenna pictured looks like a modified version of this antenna using an asymetric feedpoint, i don't know if there is a correct name for it.

 

[Marconi]

I wonder if when you TX on the multi-band antenna if the signal can be detected on both frequencies or does the TX signal only excite the proper stub?

 

[Captain Kilowatt]

I wonder if when you TX on the multi-band antenna if the signal can be detected on both frequencies or does the TX signal only excite the proper stub?

Same thing as a fan dipole. The vast majority of the RF goes to the antenna that presents the best impedance match due to the law of energy transfer that dictates that maximum energy transfer will occur when source and load impedances are equal.

 

[Mongoose]

would like to build one,, what are the dimensions between the elements??