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Is there an antenna design somewhat like a coaxial dipole, where the coax ends in a quarter-wave radiator, but instead of an explicit coaxial radiator running back over the coax body, the outer surface of the coax is the radiator, much like common-mode currents arising with some coax-to-dipole antennas? The coax could have a choke at the quarter-wave point along the coax to prevent the remaining coax from radiating. How well does / would this work?

(My internet searches have not identified anything similar to this idea, just current baluns and coax dipoles.)

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  • $\begingroup$ coaxila colinear antenna and carolina windom both do this in different ways. $\endgroup$
    – user10489
    Commented Jul 17, 2023 at 11:24

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Yes, for example see "End-Feeding a Center-Fed Vertical Dipole" and iiuc sometimes the concept is called a "bazooka dipole" as well.

While not a fatal flaw, I think the "gotcha" lies with this part of the plan:

The coax could have a choke at the quarter-wave point along the coax to prevent the remaining coax from radiating.

If you look in that first presentation link (by Jim Brown, K9YC) you can see it takes some significant engineering to implement a choke that can handle the requirements placed upon it in this application, and handle them efficiently.

I've mostly only done "armchair" research on this so far; personally I keep circling back to it as an attractive idea. But each specific time, a more traditional approach (e.g. a monopole, or perpendicular feeding, or an end-fed, or … etc. etc. …) ends up seeming like a more practical way to accomplish my actual real-world goals in the end.

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Yes. You can google for a "flower pot antenna" to find this design for VHF and UHF bands, so named for the ability to stealthily have an antenna disguised on one's apartment balcony in a flower pot.

This VHF example uses a simple coax-coil as a choke (as you posit in your question), though I suspect it's a good idea to augment this.

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As noted above, the K9YC vertical dipole is a good simple example of using the coax shield as part of the antenna.

There are more examples I can share:

End Connected Off Center Fed Dipole - An alternate OCFD design.

Gain Master Antenna Model - Center fed 5/8 wavelength vertical dipole makes use of the coax shield for the lower leg. I created an EZNEC model of it in order to understand how it worked, and how it was matched to 50 ohms which is explained.

The End Fed Half Wave (EFHW) design using the 49:1 transformer makes use of the coax shield as a counterpoise for the return currents, most designs don't show it, but a common mode choke should be placed about 0.05 wavelength (on the lowest band) down the coax.

Getting off topic, but relevant:

Anywhere someone is using an unun (unbalanced to unbalanced) transformer - the ground lug is common to the coax shield, often without realizing it the coax shield is part of the antenna system. Others realize this and place a common mode choke at the appropriate point depending on the design - often to get multi band operation with an ATU (and accepting SWR related coax losses). Take a look at Balun Designs ununs where they show pictures of the inside.

The private life of coax will further show how any imbalance results in current "spilling over" and flowing on the coax shield if its not properly choked.

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