Cra Antenna From Comrad Industries
Your first impression will be "What is it?" The second may be [fHow does it work?1' or "Hey, that's sure different—í wonder what it's for?" My neighbors thought I had left a fancy dish on the top of the car, and my wife remarked that it looked like a flying saucer (whatever they look like).
To me, it looks like a directional discontinuity ring radiator, or DDRR., .which is exactly what it is, Okay, I guess I do owe you an explanation about the mystery, so let's take first things first.
A few months ago Jim Waldron of Com-Rad Industries (which means Compact Radiating) called me on the telephone and asked me a few questions about advertising. It seems that Jim has been playing around with antennas foryears and has been particularly interested in space-saving antennas, aerials that will permit hams to operate from apartments, condominiums, mobile homes, offices, and the like.
This is all well and good when we are talking two meters and higher frequencies, but what about the HF bands? Aye, that's the rub...what about them? How can you operate from an apartment or condo when the landlord or the management (or your neighbors for that matter) objects to any outside antennas, What do you do if you're a DXer and an avid rag-chewer, let's say on 20 and 80 meters, respectively.
Weii, Jim had thought a lot about these problems and, being a man who has done extensive travelling and his share of
operating from the car, from motet rooms, and from other sites, knew about the problems. The one thing he didn't do was give up; he researched various shapes and arrangements of radiators in a variety of combinations and finally developed a combination of radiator, inductor, and capacitance that would allow him to operate on any band from ten through 80 meters with good results. Checking into the several nets he usesT he asked for reports on his signal. He asked DX stations how they copied him, and he rag-chewed with many hams all over the country. ..ail on the space-saving designs he developed. They were successful, and he knew that others would like them or more likely need them.
The question he asked me was 'How do I get the message across?'' and my answer was advertise first in a New Products release, and second in a display ad. So we introduced the first and smallest of the Com-Rad antennas in our May, 1982, New Products section., .the ODRR model CR2A,
In his travels, Jim had discovered that two meters is almost indispensable for mobile communications. However, there are problems associated with whip antennas \n general, and we have all faced these from time to time in varying degrees of severity. Example: I nearly had the entire whip, mount, and coax removed from my car when I drove into the parking garage at Logan international Airport in Boston. . .a high-profile antenna in a tow-profile environment. Another case; Using a 5/8-wave mag-
mount antenna for two-meter FM, I lost the whole shebang when it blew off the car at speed (yes, it was a bit over 55). Once again, f had become used to the flutter, the QSB, the pruning to length, the removal of the whip before driving into my own garage. . ,all nuisances for those of us who operate mobile.
The answer to all this came to Jim in a flash—use a rugged, low-profile antenna of solid construction. Enter the DDRR.
The CR2A antenna has a considerable height reduction over that of full-sized verticals. This lowers wind resistance and completely eliminates mobile flutter (picket fencing). The extremely low angle of radiation enables the directional discontinuity ring radiator to compare, most favorably, with most full-size units. The feed system connects the antenna directly to ground, which provides an automatic static drain from charges induced by fog, dust, and precipitation, affording an improved signal-to-noise ratio.
The DDRR has been around a long time. It was developed for use in hostile environments and in instances where lots of abuse could be expected but had to be withstood. One of the first articles about the DDRR, or <NHula Hoop11 antenna as it has been called, was by J. M. Boyer in Electronics, January 11, 1963. He suggested that an antenna only two feet high could perform nearly as well as a 60-foot-high vertical antenna.
The problem had always been that when the vertical antenna height is reduced physically and resonance is achieved by loading with lumped reactance elements, efficiency drops drasti cally. In the new system, however. which Boyer termed a "teaky waveguide radiator," the circumferential aperture replaces the vertical height The small height (only 2.5 electrical degrees) and the small diameter (about 28 electrical degrees; ¡.e.T slightly more than a quarter wavelength) together with its ability to be tuned over a frequency range of about 2:1 and matched to transmission lines of between 36 and 500 Ohms make the DDRR an ideat anten* na. Jtsonfy drawback ts the need for it to be placed over the best possible continuous ground plane with very low losses. At two meters, of course, the steel automobile body serves admirably, but copper or aluminum would be even better.
Early models oi the DDRR performed within a few dB of thefr full-size quarter-wave vertical counterparts at frequencies of between 2 and 4 MHz. However, as the frequency increased, so did the apparent efficiency (possibly because of the increased conductor diameter as a ratio to wavelength), and at 30 MHz, the DDRR was shown to be supenor to a quarter-wave ver tical antenna mounted to the same ground planel
Two hams who have done extensive work with these antennas are W4M1P and W6UYH ., . and now Jim Waldron W1HGZ,
The Com-Rad antenna is formed of SrtT-diameter stainless-steel tubing and is bolted to a chrome-plated roof-top magnetic mount. Stainless-steel hardware connects the single insulated wire from the mount to the ring, and a wide copper strap (actually phosphor-bronze strap about 3/8" wide and .010r thick) is connected between one end of the ring and a point on the circumference, The wire and the strap are adjusted to provide the lowest possible reflected power. A coaxial UHF type chassis con^ nector on the chromed magnetic mount is used to attach your 50-0hm coax to the antenna, permitting it to lie flat against the car roof.
The tuning ring enables the band-center to be placed anywhere within the two-meter band and beyond, making the device usefui outside of the amateur band. The tuning ring replaces the variable capacitor used with conventional DDRR designs and broadbands the system by making a large loop at the voltage end of the antenna The high-Q oMhe device provides increased selectivity during reception, minimizing adjacent channel interference image response and crosstalk. The low profits eliminates entangle ments with trees, and it doesn t need to be removed from your car when you park in a garage. The low profile and magnetic mount make it easy to operate the antenna virtually anywhere a square foot of ground plane exists.
How does it work? Wei), let me give you an example As soon as I received the antenna, I mounted it on the car roof and connected it to my NDIHC 1400 two-meier synthesized transceiver by means of a piece of RG-8/U. First, I should explain that the terrain here in New Hampshire is extremely hilly. Repeaters are on hiiltops here, as everywhere, but there is an awful lot of rugged terrain in between, and many shadow areas. Consequently, many of us use beams to hit the repeaters from our homes, and (at the very least) 5/8-wave whips on our cars. With a good 5/8-wave whip, and the car parked in the driveway, I can usually bring up about four or five New England repeaters, including the Derry machine, the Mt Greylock machine, and several others. With my quarter-wave whip, things are a bit tougher, I usually have to go to the top of the mountain to bring up the machines. . .all except for our local repeater in town,
Thus it was with considerable interest that II put the siMy-looking (but tough and rugged and low-profiie) Com-Rad DORR on the car. What the heckr 1 coutdn t do worse than the whip...or could I? So, I scanned the band, . . and wait a minute! I'm hearing many repeaters. One after another, I listened, called, and "made the machine/' I got good reports (ful[ quieting) and scratchy reports, but J got out—eight times... Eight different repeaters!
The crowning achievement was making the Topsfield, Massachusetts., repeater — about 70 miles distant and on the other side of the mountain behind which my QTH lies. Okay, lHm not going to tell you that the Com-Rad DDRR antenna has to be for you. All I can say is that it's going to be my antenna for two-meter FM mobile from now on. It s strong, it's easy to tune icovers a range from 143-150 MHz and matches 50-0hm :oa:< at near unity swr), it has a low profile, it's small (not much larger than a saucer), it works like a bandit. can be removed or replaced in seconds, and it has no mobile flutter, Besides all that, it's, well- .er, ah.. . gotta say it,., kinda cute, and a sure conversation starter wherever you go.
The price of the CR2A is $39.95 (plus $2 postage} delivered to your door, mag mount included, pretuned to about 146 MHz and matched to 52-Ohm coax. For more information about this and other space^sav-ing antennas, write Jim Waldon at Com-Rad industries. 1635 West Rtver Parkway. Grand Island NY 14072 Reader Service number 477,
Jim Gray W1XU 73 Magazine Staff
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