Low Band Antenna

Joseph ¿J,, Boyer Ï 7302 Yukon, Suite 63 Torrance CA 90504

Ddrr Antenna

Fig, L Close-up view of military-type Directly Driven Ring Radiator (DDRR) antenna set up for long range communications tests in the 2.0 to 30 MHz frequency spectrum. Dark vertical posts supporting the ring elements are fiberglass tubing topped with "beehive" insulators. The hands of the engineer at the left rest on the 50 k V variable vacuum tuning condenser. The height of the ring in the foreground is six feet; the innermost 17,2 to 30 MHz ring element is 1,5 feet in height {Northrop Corporation photograph).

small height. All of them offer the ham with a small size QTH certain advantages over conventional antennas; some even afford a real advancement in overall ham band c o m m u n i ca t io ns practice. Therefore, we will discuss the new antennas from the ham's point of view.

The Directly Driven Ring Radiator (DDRR)

The DDRR antennaE

151, 329; #3, 247, 515; #RE26 196; all assigned to The Northrop Corporation, Hawthorne, California.

shown in Fig. ] might well be called the Little Wonder AU-bander. It tunes continuously from 2,0 to 30 MHz, and will accept simultaneous input from up to four 10,000 Watt auto-tune transmitters (Texas kW men and DX contest types take note!). The close-up view in Fig. I shows the DDRR set up for long range

1 IF communications tests prior to being installed on the naval ship ¿755 Wheeling. Fig.

2 is an aerial view of the same DDRR aboard the Wheeling during early sea trials. Although t.he rugged antenna operated well even when taking "green water" during storms at sea, it was later covered with a piilbox type fiberglass radome. Its location is on the roof of a helicopter hangar aft, the metal roof and surrounding sea serving as a highly conducting ground plane.

The maximum diameter of the Wheeling DDRR is thirty-five feet, with its outermost ring element at a height of only six feet above the ground plane. Although such dimensions may not immediately convey the idea of "small size,11 the antenna is seen to be f'small" in terms of the four hundred ninety-two foot wavelength at 2.0 MHz. The Wheeling served in the Apollo space exploration program as an HF worldwide network control center, and is now off on similar scientific missions. There are a total of five concentric ring radiator elements. The outer one tunes 2,0 to 3.3 MHz, and the inner four rings tune the bands 3.3 to 5.7 MHz, 5.7 to 10 MHz, 10.0 to 17.2 MHz and 17.2 to 30 MHz, respec^ lively, "The ends of each of the ring elements are "grounded" to the metal image plane through metal posts, Half way around the circumference of each ring conductor, connection is made to ground through individual 50 kV rated, variable vacuum condensers. Lach variable condenser is remotely controlled from the ship's radio room by means of two-phase servo drive motors of variable speed. Each DDRR ring element is directly fed with Individual fifty Ohm coaxial transmission lines, no auxiliary impedance matching networks being required to obtain low vswr. Originally, provision was made to install reflectometers at the input terminals of each ring element to permit fully automatic servo tuning and frequency tracking with associated transmitters. To my knowledge, however, these units were never installed,

In the radio room there is a visual display readout console which gives constant information for (a) the identity of the transmitter currently in use with a given ring element, (b) the frequency to which each DDRR element is tuned, and (c) the vswr in cach of the feedlines. In spite of all this automation, however, an experienced operator in total darkness can hand ''slew1' the tuning of the DDRR until a background noise peak is heard in a receiver tuned to the desired frequency. When 1 he noise peak is observed, the input vswr in the feed line to the antenna is less than 2:1 and the antenna is ready to accepi full transmitter power, Ofr this tuned frequency, the receiver connected to the DDRR sounds "dead," its S-rneter resting on the zero peg-

In a DDRR, all antenna elements arc at dc ground potential through extremely low impedance, high current capacity shunts. As a consequence, associated electronic equipment is quite well protected against damaging effects from voltage tran sients induced by lightning strikes on the ship's structure. Such dc shunts also serve as "static drains11 during weather conditions when Impact with charged snow or rain particles can build up very high magnitude voltage potentials on conventional antennas. Under such weather conditions, noise level during reception on the DDRR is a minimum of twenty decibels less than that attained on non-drained antenna systems.

Up to here we have been discussing a military antenna. Fortunately, however, I am able to give an account of how such a multiband DDRR operates on the ham bands. During preliminary land-based tests of the antenna in southern California, it was only natural that licensed amateurs serving as engineers and technicians on the project literally itched to know how the thing would work on ham frequencies. A notice of portable operation under the call W6UYH was sent off to Uncle, and one evening the gang adjourned to the large communications van nearby. To get a fee! for band conditions using a standard radiator, a one hundred ten foot tall vertical quarter wave tower antenna was used to put out the first call on 160 meters, I his antenna, of variable height, was available as a reference ¡V/4 monopole during military tests, and could be raised or lowered to ground within three minutes,

A number of contacts were quickly made with rete-tively local stations; naturally, excellent reports were secured using the big vertical skyhook. The QRN level was substantial and considerable Loran "buckshot" was noticed. The vertical was then dropped, and a touch of a control button sent the motor-tuned DDRR down Into the high end of 160 meters (we had made sure it would "inch" a bit below 2,0 MHz, hi!). Almost immediately we heard a number of stations calling us from the Hawaiian Islands. The KH6s said they had been calling repeatedly since our first CQ. We had not heard their relatively weak signals, however, due to the fact that they were

buried down under the QRN; now they stood out loud and clear against a much lower background noise level. We also observed that Loran buckshot was way down in magnitude and QRM from very near-channel strong locals was almost absent. To the island stations, there was very little, if any, difference detected in FS between the DDRR and \/4 monopole in subsequent comparison transmission under conditions of slow fading.

As engineers, the hams present were a little surprised. We had all been somewhat concerned about the narrow frequency bandwidth of the electrically small height DDRR when operating efficiently with hard fought-for, low Ohmic environmental loss resistance Yet here, with this narrow frequency width antenna acting as a sharply tuned "bandpass" filter ahead of the first receiver stages, it was preventing loss of sensitivity due to random white noise loading, greatly reducing QRM and delivering a considerably superior signal-to-noise ratio advantage over the big antenna in a real world, two way HF radio communications mode. As one old-timer in the shack sagely observed: "You got to hear 'em, boys, before you can work 'em!" Shifting to 75 meters, it was the same story. Reception, using the relatively wide-banded tower X/4 vertical, was a noise pain to the ear; on the DDRR we worked VKs, ZLs and Js, slicing them out from under the noise and QRM as if using a hot knife on butter.

Again, due to the unavailability of test stations on military assigned frequencies at intermediate distance range from our land-based test site, we were able to first observe on the ham bands another deliberately provided performance feature of the two post model DDRR antenna: the ability to work stations during daylight hours at distances greater than that of the ground wave fade out zone produced when using quarter wave vertical antennas. In the two post design DDRR, provision had been made to generate an auxiliary, very high angle radiation pattern lobe in addition to the DDRR's normal,

vertically polarized, very low angle "doughnut" omni-paUerm In the lower frequency HF range of 1.0 to about 3.0 MHz, the ionosphere will strongly reflect signals incident upon it at high angles above the horizon. Such an effect is called the ionosphere "sounder mode" of communications and is of military interest because it can give contact range extension during daylight hours. We enjoyed daytime QSOs with stations ranging from 100 to 500 miles distant on 160, 75, and 40 meters — stations which could not be worked using the reference \/4 monopole. Subsequent use of the DDRR on the USS Wheeling verified the same daytime range extension performance at sea on the lower HF channels outside the ham bands.

How the DDRR Works

Because many of the performance functions and much of the theory of the DDRR antenna apply equally well to the operation of other modern transmission line antennas to be discussed In Part II of this article, it is perhaps justifiable to give here some details about how a DDRR antenna works. While we are at it, we might as well give readers all the dope necessary to tailor one themselves for use in the ham bands. We're talking about an

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