J-pole with a gamma match ???what's that all about

[davev8]

i see a photo of a 27mhz j-pole sold by a French company and in the photos it seems to have a gamma match....so i assume its no longer a J-pole i believe a J-pole is a end fed 1/2 wave with a 1/4 wave matching stub and behaves much like a 1/2 wave
so if it fed with a gamermatch will it work like a 3/4 wave and have a 3/4 pattern so less gain than a 1/2 wave???
photos below

 

[Shadetree Mechanic]

The "dog bone" at the top of the gamma looks like plastic.

 

[Captain Kilowatt]

The "dog bone" at the top of the gamma looks like plastic.

Yup. Just a spacer to keep the short stub from breaking off at the base and to maintain spacing in the wind. Without it the SWR would be all over the place in a high wind.

 

[Marconi]

I agree with the others. It looks just like a support near the top of the stub...not a gamma tap.

Can you post the link to this antenna?

 

[bob85]

The gamma is tapped up the monopole like a 1 radial sigma4,
mechanically simple to make vs a bottom fed j-pole,

I would prefer that setup to a j-pole with the coax tapped up the stub where voltage is higher & potential for cmc even worse.

 

[Marconi]

Looking closer at image #1 it does look to have a gamma match close to the radiator on the bottom left, with the stub tube, that is taller...to the right.

The author talks about the black strap at the top of the stub, IMO that is a support strap for the top of the stub just like the other operators spoke about.

Bob, could be right about the gamma location on the left connected near the base of this antenna. I've never seen a J-Pole setup with a gamma, but I would not doubt for a minute that Bob is right about this...upon second look.

 

[Captain Kilowatt]

The image shows what looks like an aluminum connection at the bottom but a black piece of "something" at the top. If it was a gamma match it should be a conductor like aluminum. I still think it is black plastic and therefore NOT a gamma match. I have made J-poles for 2m before consisting of a 3/4 wave main radiator and a 1/4 wave stub joined at the bottom with a coaxial TEE connector for the main radiator and a 90 degree adapter for the stub connection.There was NO sliding tap for the coax cable. SWR was tuned by adjusting the lengths of both the radiator and stub sections. I made them using two telescopic rod antennas. More than one way to skin a cat as they say.

 

[bob85]

Look closer CK

there is a gamma capacitor & rod & gamma strap at the base of the monopole tapped up for 50ohm like a sigma4 with 1 parallel leg in place of 3 angled legs,

the black thing is a none conductive brace to keep the stub spacing stable.

 

[Marconi]

Bob, I did not see the gamma at 1st.

I was looking at what Dave8 was pointing out at the top of the stub. I just made a mistake in looking at what Dave8 pointed out. I see the gamma now.

I think Captain and ShadetreeM both saw the same thing I did...that is why I agreed with them at first.

 

[Shadetree Mechanic]

Oh, I see it now. So can we assume that it is fed through the gamma?

 

[Marconi]

Oh, I see it now. So can we assume that it is fed through the gamma?

I see it a clear as a bell now SM, but I have never seen a J-pole that was gamma fed...only stubs and maybe a collinear or two with various forms of collinear phasing near the middle of a longer radiator.

On the other hand there are lots of things I haven't seen too.

 

[Marconi]

I haven't seen a J-pole with a gamma match yet, but this demo by Cebik was sorta revealing when he modeled a J-Pole overlaid over a Starduster type Ground Plane antenna. He writes...where is the difference with both antenna at 36' feet.

https://www.hamradio.me/nec-shootouts/monopole-vs-jpol-eznec-shootout.html

I continue to look for a J-Pole fed by a gamma.

 

[Captain Kilowatt]

Even after blowing the image up I can't see the capacitor. I do see a small stub that could be the gamma arm so I give you that. Be nice to know the actual model of antenna it is to get better info. I cannot understand the need for a gamma feed along with a stub.

 

[Marconi]

Here is what Cebik said specifically about these two antennas compared at 30' feet each.

Quote:
"The plot above shows the monopole as the primary trace in red compared with a j-pole in blue. Both have their feed points up 30 feet. The j-pole and the monopole have, for all practical purposes, identical gain in the primary low-angle lobe just above the horizon. Hmm, where is the benefit?"

 

[bob85]

You have seen one before Eddie, a sigma4 is a 3 leg variant,

heres what one ex avanti employee said about the gamma fed j-pole,

"The gamma fed J pole is probably the only correct way to feed the J and
reduce (not eliminate) feed and support radiation. Avanti Antennas
patented this idea back in the 70's

Dale W4OP"