An Off-Center Fed Sleeve Dipole

At Ham Radio Outside the Box the urge to experiment is always front and center. Isn’t that what radio amateurs are supposed to do – continuously improve our knowledge and hone our expertise? In the minds of the government departments that give us our spectrum allocations we are a reserve resource of communications expertise to be called into public service when needed.

Even if public service were not part of the picture some of us are sufficiently intrigued by the mysteries of the universe to be self-motivated when it comes to experimentation. Ever wonder exactly how a radio signal gets from one place to another – even through the vacuum of space?

In that spirit I was suddenly smacked in the head by yet another brilliant idea recently. I had been corresponding with a Ham Radio Outside the Box reader about the T2LT antenna I built some time ago. A T2LT (Tuned Transmission Line Trap) is also, and perhaps more commonly, called a Sleeve Dipole.

Do you believe in magic?

A Sleeve Dipole is a cunning idea that employs the Skin Effect to achieve a physically end-fed, but electrically center-fed dipole antenna. It comprises a quarter wavelength of coax with the center conductor connected to a quarter wavelength of wire. The braid of the coax is unconnected at its far end. Now here’s the magic: the coax braid is actually two conductors in one. Current flows along the inner surface of the braid toward the feedpoint and back along the outer surface of the braid. The outer braid surface becomes one half of the dipole, while the inner braid surface and center conductor form the transmission line. The quarter wavelength of wire is, of course, the other half of the dipole.

The transmission line is terminated at the electrical feedpoint at the center of the antenna where it is radiated. [How? My theory, as a long-in-the-tooth physics graduate, is that the signal is conducted along the plane of the space-time continuum; but back to the subject of antennas].

Current flowing in the reverse direction along the outer braid surface also needs a termination, which is arranged by means of a choke at the physical (end) feedpoint. The choke has no effect on the current flowing along the inner braid surface, but significantly attenuates the current flowing back towards the radio along the outer braid surface.

A dipole is a dipole is a dipole

Now here’s where my brain’s neurons began firing on all cylinders. A dipole is a dipole is a dipole. Ignore the magic bit for a minute. A dipole can be fed anywhere along its length nicht wahr? It is typically fed in the middle, but End-Fed Half Wave (EFHW) antennas are currently very popular among those of us who like to operate out in the Big Blue Sky Shack. An EFHW is like a dipole that is fed at one end.

Another variant is the Off-Center Fed Dipole (OCFD) that is typically fed at around one-third of its length. But the reason end-fed antennas are so popular with portable outdoor operators, such as POTA and SOTA etc, is that end-fed antennas are easier to erect in the field. Now what if we applied the skin effect magic to an OCFD, would it work?

An EFHW antenna is resonant on its fundamental frequency and may also be resonant on its odd harmonics (see the post “Is an End-Fed Half-Wave Antenna Truly Multiband“). An OCFD is resonant on its fundamental frequency plus its even harmonics. An OCFD is typically used with a “tuner” (technically an “Antenna Matching Unit”) to make it usable on multiple bands.

So, if this works, we can build an Off-Center-Fed Sleeve Dipole antenna (OCFSD) that is physically fed at one end, for ease of temporary erection in the field, but electrically fed at some other point along its length. Adding a “tuner” should allow us to operate on multiple bands.

But wait a minute … why not just use an End-Fed Random Wire (EFRW) antenna? It can also be tuned on multiple bands? Yes, but the EFRW requires a counterpoise, the OCFSD does not!

It is mid-winter here in the Great White North and Mother Nature has chosen 2025 to dump an extraordinary amount of snow on us. It ain’t easy to pop outside and throw a wire in a tree when there is an accumulation of a couple of feet of compressed icy snow on the ground and the tree branches are bowing down with the weight of the fresh snow that has fallen in the last few days. So, I built a test version to see if this idea is crazy or not. It is a short OCFSD for 20m and up. I can erect this indoors for testing purposes using my RigExpert antenna analyzer.

Scavenging bits and pieces from my junque collection I found 10ft of RG-174 coax, 24ft of wire, and a ham made 1:1 choke (tested using a nanoVNA: >25dB Common Mode Current attenuation) The wire was shortened to 22ft when tuning for best SWR. The SWR will probably further improve when I can get outside to erect the OCFSD properly.

“Common knowledge” holds that an Off-Center Fed Dipole has a feedpoint impedance of around 200 ohms and should be used with a 4:1 impedance transformer to provide a good match to 50 ohms. But Ham Radio Outside the Box lives in downtown Contrarian Thinkerville, so I measured the feedpoint impedance for myself. Impedance transformer? Humbug!

Band	Untuned SWR	Untuned R ohms	Untuned X ohms
20m	2.4	21.2	5.39
17m	10	9.82	48.7
15m	10	46.4	140.1
12m	8	78.3	-152.3
10m	4.3	57.4	-84.5

If you look at the table above you can see that the measured impedances can be easily tuned with the Ham Radio Outside the Box “Old Barebones” Z-match tuner (and any other tuner in all likelihood).

Band	Tuned SWR	Tuned R ohms	Tuned X ohms
20m	1.03	50.4	1.61
17m	1.10	52.4	4.48
15m	1.07	47.6	-2.41
12m	1.01	49.6	-0.17
10m	1.10	52.0	4.24

Both the SWR and the complex impedance values are very acceptable when measured after tuning. NB: the above measurements were taken at the physical feedpoint of the OCFSD.

What about the radiation pattern?

A regular dipole has a nice donut-shaped radiation pattern with a low-angle maximum all around the circle. Great for DX and no wasted signal going straight up to warm the angels. Could it be possible that this bizarre variant of a dipole would have the same radiation pattern? EZNEC says yes, it does have a nice a donut radiation on every band although on 12m the pattern starts to develop some baby lobes in the vertical direction. These lobes become a little more prominent on 10m.

That is the state of the science on the Off-Center-Fed Sleeve Dipole at this stage. When the snow has melted and the temperature has returned from its wild swing into double digits with a minus sign in front, we’ll get this hunk of wire up in the air and see if we can make some QSOs with it.

NB: This post has been edited to comply with feedback received.

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