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!

Rad Com - Slipping Again

The latest edition of Rad Com, the RSGB's glossy magazine, is out. 

Earlier in the year, I was sufficiently impressed with the improvements in writing style and content that I sent a note to the editor of Rad Com. 

A few months later, we're back to square one.

This month's cover says it all.  An elderly establishment figure standing-in for Prince Phillip, who's on his last legs, mulling over the displays at the National Radio Centre, or whatever it's called.  Flick through the pages, and ne'er a man under 60 can be seen.  Women merely appears as wives.  Sepia images of the centenary dinner, as though anyone actually gave a toss.  And I really, really am fed-up of seeing the NRC plastered everywhere in the RSGB's desperate and continuing attempts to justify the ludicrous sums of members' money spent on it (now devalued to about half of that paid.)

Yes, Rad Com is back to the good old days of terrible, often obscure writing, content and a focus that just can't get away from old-man-in-shack-builds-RF-meter type stuff.  There's an article about a reliable power meter that one could build, but is written in such a crap style, it ends up being the usual put-off for anyone who hasn't spent 90% of their lives sweating over a soldering iron.

Read this, Rad Com: Not being a qualified radio engineer isn't a crime.  Amateur radio is not professional radio.   The days of 'better than thou' due to background, money or simply callsign, are over.  No, really, they are.  It's just you haven't woken up to it yet.

Why on earth can't Rad Com, when it comes to circuits, publish both the circuit diagram and a diagram for noddy folk like me, with the actual components required - plus where you can get them from - so that beginners and those who operate but don't normally build can get on with it?  That way, us simpletons would learn.  We're good at that.  That's how we passed our licence exam(s.)

It seems the answer is that the old-timers are often dismissive of people not as earnest about the hobby as they.  This is a pity, because like all exclusivity-driven agendas, it is killing the hobby and killing it fast.  I have moaned about this before and, no doubt, will do so again as amateur radio rides the hill down towards oblivion borne of its failure to appeal to and engage with newcomers and those not retiring from the comms industry.

It really is time the RSGB pulled its head out of its 100 year-old backside and start dealing with the very real problems of an ageing, dwindling membership.  It has very little time left to do so. 

When the ship is taking on water, you spend all your effort plugging the hole.  You don't ignore it and start buying new upholstery for the saloon.  Take note, all ye RSGB 'worthies'.

Antenna Bases - Clearing-Up Amateur Misinformation

I recently came across some interesting discussion threads online about the strength of concrete, especially in relation to erecting antenna towers.

The stressful - and expensive - part of antenna tower preparation.

There was an awful lot of nonsense and misunderstanding about.  An exchange in one thread, stated with the characteristic certainty of someone who doesn't have a clue, believed concrete was "as strong after three days as it will ever be."  The overall consensus in that debate was that after 28 days, concrete had reached full strength.

The above views are complete nonsense.

Concrete does not harden, as some think, because of drying out or 'setting' like some evaporative glue.  It hardens as a result of a series of chemical reactions that form an interlocking microscopic mesh of minerals, all based on calcium, that provide the mechanical strength.  If you aren't convinced, mix a small amount yourself and let it harden on a warm, sunny day.  A few days later, you will be able to crush it with your hands, it will be that weak - all because the essential water for crystal growth dried out.

Microscope image of mineral crystals developing in concrete.  The crystals need a considerable time - many years - to fully develop and reach maximum mechanical strength.

After three days, concrete will certainly have 'set', but the crystals that make up the strength have only just begun to grow.  After 28 days, which is a period chosen only for its real-world convenience and not any inherent feature of concrete, it will have reached about 70% of its ultimate strength.  Note that, in the case of very hot and very cold weather, the ultimate strength is greatly reduced.  That's why you should pour structural concrete during reasonable weather.

Reasonable warmth and a good dollop of time are prerequisites for strong concrete.

The full mechanical strength of concrete is never reached for many years.  Five years is about as early as you would want to make the "as strong as it will ever be" claim, though even then, it will still be adding strength for some years again.

So, whilst you may want to put up your tower as soon as that expensive concrete has 'set', don't!  Wait for a month, keep the concrete moist and covered for as long as you can, and then feel sure that the strength is up to scratch.