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The DDRR antenna |
The DDRR (Direct driven ring radiator) was invented by Dr. Boyer from Northrop, for military applications in the 50s. It stayed classified until Dr Boyer published an article titled "Hula hoops antennas" in Electronics (Jan 11, 1963). "73 Magazine" published also a two part article from Dr. Boyer titled "Surprising Miniature low band antenna". The article is quite technical and contains a very precise mathematical formulation of the DDRR based on transmission line theory.
In layman's terms a DDRR is just a short vertical monopole (vertical post) attached to a transmission line tuned by a reactance (ring plus vacuum capacitor). The reactance of the capacitor is transformed by the transmission line and will under specific conditions (length, value of the capacitor, etc.) make the vertical post resonant. Slight variations of the capacitor will lengthen or make the transmission line "shorter" and allow tuning of the antenna on a certain range.
First lets make a difference between the DDRRs patented by Dr Boyer and the ones that we can find here and there in amateur publications. Boyer designed two basic models: the one ring DDRR and the two rings DDRR.
The
first one is made of an opened ring made of aluminum tubing (4 in.) over
the perfect ground of a metallic structure like a warship, and with two
aluminium posts connected at the extremities of the ring. A set of fiberglass
post supports the ring. One of the vertical post is attacked at the base
by the coax, the other one contains a variable vacuum capacitor to tune
the antenna. This faded picture from the 50s from Northrop show a set of
concentric DDRR for 2 to 30Mhz. The engineer is standing below the 75m
loop (6 feet height) and is looking at a 50Kv vacuum capacitor. This type
of DDRR was installed on some special communication and ELINT warships,
and was apparently used in the early stages of the Apollo project.
Contrary to most ham publications, Dr Boyer in all the articles he wrote, and in his discussions with VE2DLJ and VE2AMT stressed the importance of using big fat aluminium or copper tubes and to avoid at all cost chicken wire as ground plane. In a one ring DDRR he insisted on the fact that you needed a solid metallic ground plane.
The advantages of DDRR compared to a vertical are obvious, especially on a ship. Small size and height, remotely tunable, low noise due to loop structure, extremely low impedance and therefore no or little influence on connected electronic equipment from voltage transients and lightning. Low angle for long range communication and possibility of a secondary high angle for NVIS, indispensable in fleet communications, can be achieved by using a configuration using a central post and two posts with vacuum capacitors.
The second
type is a double ring system isolated from the ground with the same two
vertical posts connecting the opened extremities of the rings. One is solid
aluminium, the other one contains a Vacuum. A third aluminium post cut
in the middle is used to feed the antenna and can be moved to adjust the
SWR. The coax enter in the middle of the right post, then up then down
to the "feed point" where the shield is connected to the top part and the
center of the coax to the bottom part of the post. This configuration creates
a coaxial balun for a symmetrical attack. Matching is achieved by moving
the "feed point post".
The antenna is standing and supported by very high quality isolating posts made of PVC or fiber glass, and Dr Boyer even recommended beehive isolators at the extremities of the posts to limit losses. The whole structure is placed some 3 feet above the ground.
Contrary to the one ring model, there is no need here for a perfect ground and Dr Boyer was confident that this antenna could perform well even on very poor ground. He referred in his articles to the fact that DDRR are essentially "magnetic" antennas in the near-field and that losses in the ground my magnetic field are negligible. This is not entirely true and computer simulations and experiments by VE2DLJ showed that even if losses in the ground are negligible there is an improvement when radials are inserted.
Tests made by Boyer and his team for Northrop and the US Navy showed that the antenna was very low noise and very low angle. This was done in side to side comparisons made with a collapsible vertical on 160m in the Arizona desert, using a half square mile ground plane made of solid copper sheets soldered together (visible in the picture).
VE2DLJ and VE2AMT found some ten years ago an old copy of the 73 Magazine
article and decided to build a prototype. They contacted Dr. Boyer and
have numerous discussions with him. He warned them and told them "not to
cut corners". His recommendations were:
- Use big tubing: 4 inches or more for 75m.
- Do not use automotive exhaust pipe. It will rust and contacts losses will transform the antenna into a dummy load.
- All contacts and connections must be A1. Solder corners if you make it square, use Penetrox everywhere.
- Use very high voltage vacuum capacitors, and high quality isolators to support the rings.
- No chicken wire. Dr Boyer was horrified by some description of DDRRs using chicken wire. He explained that measurements made by Northrop engineers showed that the near field is in concentric rings on a one ring DDRR and that chicken wire could add losses.
VE2DLJ followed suit a few years later and his DDRRs are visible in
the Picture page. He is very often in the 75m
DX window working VK, ZLs and other pacific stations.
Models
| NEC4WIN Model |
| CM ********** 80m DDRR
**********
CM CM Designed by JM Boyer for Northrop CM Supposed to be Low Noise, DX antenna CM VE2DLJ, Tony Cicchetti, is using a full size CM version for 80m and can be heard in the DX Window CM CM US Patent #RE026196, RE 3,151,328 Northrop Corporation CM "Hula-hoop antennas", Electronics, Jan 11 1963, Boyer. CE
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| NEC4WIN95 Beta v1.00.5 Log File
DDRR.N4W
Impedance = 0.46 - j 0.08 Ohms at Source
1
DDRR.N4W Zenith
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| AO Model |
| DDRR for 80m
Ground 13 5 1 zone 50 radials #12
3.795 MHz
10 0 0 3
14 0 3 0.3
1 source
1 load
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| ANALYSIS
Impedance 0.678 - j 18.7 Ohms
Wire Losses 0.33 dB
At 18.0ø Elevation:
Forward Gain 3.41 dBi
Maximum Gain 3.81 dBi at 132ø
Azimuth
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| DDRR with radials vs a Dipole at 130 feet DDRR is better at low angle ( <18 deg) |
Comments and remarks
The DDRR impedance is very very low. At 0.5 ohms, any wire or connection losses will transform the antenna into a dummy load. This is why one needs big fat tubes. Losses in the tuning capacitor must be minimized too and a vacuum of very high voltage is indispensable.Contrary to Dr Boyer's predictions, the addition of radials increased gain and lowered radiation angle. This was confirmed by VE2DLJ experiment's. We see from the zenith patterns that the TOA is below 20 degrees and that the antenna is a low angle radiator. This could be OK for long range DX like VK and ZLs but it is not very good for medium range DX. This was confirmed by long term testing over a year and a half period. Tony, VE2DLJ, has a horizontal loop "reference antenna" and the loop will always beat the DDRR toward Europe in transmission even in Winter. The horizontal loop will also give better reports (in average) than DDRR on long range DX outside of Winter DX period but starting around beginning of October the DDRR will start beating on average, the horizontal loop by one to two S units in transmission on VK, ZLs and JAs.
In reception, the DDRR (according to Tony, and most hams who listened on them) will always beat the loop for QRN and man made QRM. Signals will be sometimes one to two S units lower but always perfectly Q5. The bandwidth is some 20 to 30Khz and the Q is in the 300 to 400 range, so the antenna is very selective which is a big advantage.
We can see from the comparison of a dipole at 130 feet vs the 3 feet high DDRR that the DDRR is better on low angle signals. The difference is even more important for a lower dipole.
What are the advantages of the DDRR? Low noise antenna, low angle. "Small size" requiring little real estate. Could be easily installed in a backyard at ground level or on a flat roof. No tower needed, no climbing. Immune to noise, static electricity and ligthning. Tunable, very high Q. Boy that seems great!
Does that mean that I will "recommend" a DDRR? Hell No! To build a good system you need some 100 feet of 4 in. tubing, then solder the corners (or use special machined brackets) to reduce losses. If you want to transmit full legal power you need 50 KV vacuums, and they are not cheap. Add the cost of good isolators to support the rings etc and you end up with a two to three thousand dollars antenna. Even if ground does not influence much the DDRR in the near field, ground is important for the first reflection and you need good ground around the antenna. If you have neighbours the low angle may create TVI. Another major point is directivity. The DDRR doughnut is omnidirectional and to work DX you need directional antennas. That's the only way to eliminate QRM and concentrate the energy in one direction. This means that one DDRR is not enough and you will need at least two.
To build low band DDRRs, there are cheaper alternatives using salvaged Heliax. Three or four VE2 designed DDRRs using plumbing copper tube, wire and Heliax with variable success. The only area where price/performance is interesting, is in VHF/UHF. At these frequencies, DDRRs are easy to build and very small. Otherwise, stay away from them unless you know what you are doing.
Madjid
VE2GMI
Patents: Thanks to Brooke Clarke, N6GCE (http://www.PRC68.com) who found that some numbers I had were wrong and found 2 other patents here are all the patents we could find.
US Patent #3,151,328; #3,247,515; RE026196 & 3680135 by J.M. Boyer all assigned to Northrop Corporation. for more antennas.
If you have questions, comments, critics, suggestions click here: ve2gmi@orionmicro.com
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