The Coaxial Dipole

Coaxial Dipole Driver for Yagi-Uda Antennas

Antenna geeks rejoice… you have another great antenna web site to visit during your next armchair browsing session.

I came across the website of Derek Hilleard, G4CQM, containing a wonderful assortment of details to help make your next Yagi-Uda project antenna a success.

I’m always on the prowl for innovative, and functional, ways of feeding Yagi-Uda antennas with 50 ohm cable. Derek explains the Coaxial Dipole as a way to obtain 50 ohm, and perhaps a self-balancing, feed for Yagi-Uda antenna designs using two hollow tubes containing a wire within.

It’s worth noting this topology doesn’t miraculously turn any parasitic antenna into a 50 ohm system without due diligence and design of the entire antenna system. In fact it’s not quite clear what the natural impedance of this thing is. The more important claim is the full wave path that provides, at resonance, a self-balancing feature.

Also note the axiom “Coaxial Dipole” describes more than this concept such as the vertical coaxial dipole shown in this J-Pole vs. Coaxial Dipole article.

Here’s the site…

Details include:

• It’s a loop partly contained within a tube.
• The folded branch (inside the tube) does not radiate resulting in increasing the bandwidth, like other folded dipoles, but without affecting the input impedance.[1]
• Allegedly performs the same self-balancing behavior like any full wave loop as discussed in the Loop Fed Array (LFA) article. NOTE: Simulations (non NEC) suggest this is not the case… see below.
• Free to use for anyone as a 1963 paper describes the technique.[1]

Preliminary Findings

Without a doubt, this innovative technique begs for some simulation and lab verification. The details on his web site are food for thought for now, but stimulus for experimentation. I’ve added this to my list and will share results soonest.

A quick run of modeling the various configurations in an FDTD simulation provides the following data.

Test #	Description	1/4 Wave Feedline	Feedline Current	Result
1	Cylindrical Dipole	None	n/a	Classic Dipole Pattern
2	Cylindrical Dipole	Yes	High Current	Tilted Pattern
3	Folded Cylindrical Dipole	Yes	Low Current	Almost the same as test #1
4	Coaxial Dipole	Yes	High Current	Tilted Pattern

This simulation suggests:

• The Folded Dipole exhibits the self-balancing behavior. The feed impedance is several times higher than the cylindrical dipole’s.
• The Coaxial Dipole lacks the self-balancing behavior. The feed impedance is similar to the cylindrical dipole.

Both being full wave loops, this difference in behavior of the Folded vs. Coaxial dipole is a surprise.

Lab testing is the logical next step before we proclaim a verdict.

References

• Page, H. (1963). An Introduction to the Theory of Aerials. (p 69 – pdfpage 77). British Broadcasting Corporation Engineering Division.