DDRR Antenna - A Forgotten Legend

Back in the days when military spending was simply a matter of writing cheques, the floor of an US desert was covered with welded copper plate, and strange ring antennas placed above it, suspended a few feet by insulating props.

Military and commercial DDRR experiments of the 1950s.

This was part of the development of the DDRR antenna.  At the time, the DDRR received a relatively good coverage in the radio media.  Roll the clock on to the 21st century, and we seem to have all but forgotten about this mythical antenna with some surprising performance characteristics.  Key amongst these is relatively low, vertically polarised radiation.

The original DDRR was a quarter wavelength ring, fed against ground and tuned with a high-voltage capacitor.  Italian researchers later experimented - to good effect - with a ring of half wavelength circumference, which apparently didn't need a capacitor.  Both versions were claimed to be directly feedable with 50 Ohm coax, though the half lambda version kept roughly that impedance across a very much wider bandwidth, and was largely insensitive to the feedpoint position.

First wire iteration of a 20m DDRR.  You can also hang your clothes out to dry on it...

How do you make one, I hear you ask?  It seems that, to avoid losses, it is necessary to use at least some form of tubing to make the radiators.  Old texts show ops using exhaust tubes suspended above chicken wire ground screens, though more considered experimenters quickly pointed out that the nature of steel was such as to negate any benefits arising from using such an arrangement.

Although this is nominally a 'ring' antenna, there is no need to spend a fortune buying two lengths of copper tube and having it bent into a perfect ring by some machine shop or other (of course, you will find US ops who have done exactly this!)  Anything from a square to an octagon and beyond will work without much or any consequence.

So, it's best to start with some copper tube, maybe fork out for 3/4" tubing and some compression fittings.  You could also try conductive metal foil tape, or aluminium cooking foil.  Insulating supports can be anything from fence posts arranged in a ring to, well, whatever you can think of!   You need two rings of half a wavelength each for the band of interest (let's assume it's a monobander for now.)

The lower ring can be as high as you want, but the aim here is for an antenna at roughly chest-height, and that might evade any sort of neighbour attention.

Next, you just need to connect the two rings, which must be spaced one above the other about 0.05 lambda apart (about 53cm for a 20m DDRR.)  You feed the upper ring.  A simple 'tee' connector and a short piece of copper tube does this.  Then, just work yourself anything from 60 to 120 degrees round the ring and attach your feed.  I used 300 Ohm twin but you can also try coax.  An ATU is likely to be required, at least at first, before you trim and change.  Rig ATUs may not achieve a match, but you can try.  Rain and moisture tend to affect precise matching, but a slight twiddle of the ATU soon sorts that out.

In fact, I started off with a wire DDRR, which in theory ought to mean significant losses.  That said, WSPR tests, and couple of PSK contacts across the EU and Russia showed the antenna to be doing pretty well, considering it took 20 minutes to make and had no optimisation whatsoever. Oh, and that it is only 1m off the ground and so immune from planning control, assuming anyone ever recognised it to be an antenna at all!

So, I am just at the start of a DDRR journey.  There is clearly plenty of scope for good performance from an improved construction, and the interest for those living in ham-hostile environments is obvious.  Certainly worth spending some time on in the coming months.  A site as clear of obstructions as possible is also handy!