The Antenna

The CCD antenna is now available for the amateur hands.

Harry Longerich W4ANL

Rte 1, CV 9 Fredericksburg TX 73624

What is ihe 48-46 antenna? It consists of 48 pieces of wire interconnected with 46 capacitors. The technical name is "Controlled Current Distribution" antenna, or CCD, If s been around for a long time in the commercial broadcast field, and recently it was configured for use on the amateur bands. An excellent description and analysis of Ihe CCD antenna is available in both References I and 2: these are worthwhile reading.

Rather than making a dipqle out of two continuous lengths of wire, die CCD antenna uses capacitors to vary the phasing of the currents in the antenna to ensure thai the most possible current is distributed to all sections of the antenna. These capacitors are inserted in series with the elements, about one every yard, for the complete length of each dipole element. In die 48-46 antenna each side of the dipole is made Lip of 23 capacitors and 24 sections of wire, each 35 inches long. The dipole is center-led with open wire line via a transmateh. The 4S-46 antenna uses a new approach to mounting the capacitors, and 1 also give some hints on how to build a simple RF detector to make sure the antenna currents are properly distributed.

Why consider a CCD antenna? Well, it has several distinct advantages over the conventional dipole:

Fig. Lùcite of Lejan insulator with capacitor,

(1) It has about 3 to 4 dB gain over a conventional dipole at the basic design frequency.

(2) There are minimal high voltage loops on the antenna.

(3J There arc no end effects of any conscquencc, t4) The antenna may be erected in any manner, shape, or form—whatever suits your real estate.

(5) The entire antenna radiates, A dipole has about 67% radiation efficiency. The CCD approaches 97%.

long, with a ]-3/2-inch pigtail at each end. making cach piece a total of 38 inches overall. \ used Te 11 on™-covered wire, 16-gauge stranded. I used covered wire because here in Texas we have occasional severe weather, and precipitation static can be a problem. The covered wire reduces this type of static. Because there are no high voltage loops on the antenna, using covered wire does not cause undue losses.

In order to simplify measuring the wire to 35 inches, I built a simple jig on

"When conditions are right, / can copy European stations 5 x 9 plus on 75 meters.

Fig. Lùcite of Lejan insulator with capacitor,

(6) Ihe CCD antenna, when used with an antenna system tuner, will have a gain on the fundamental design frequency and will operate on all higher amateur bands. It will not work properly on frequencies lower than the design frequency.

(7) The CCD antenna may be erected close lo the ground without seriously affecting the efficiency of the radiated signal.

(8) This antenna is very quiet when used in the receive mode.

(9) Construction is straightforward (though time-consuming- 96 solder connections arc involved in building this type of antenna).

{lO)The feed-point impedance will vary between 275 and 325 ohms, and will require the use of a balanced line or a 6:1 balun when used with a coaxial feed line.

Construction details

Initially, I built a 7 MHz CCD tenna. Though it is not difficult, construction does take some time, l started by cutting 48 pieces of wire, 35 inches a piece of 1-inch by 2-inch wood, drove in two nails. 35 inches apart, and bent the wire at the 35-inch dimension. 1 judged the I - 1/2-inch pigtails. Alter cutting and stripping the 4ft lengths of wire. I cut 49 picecs of LucSte™ tyou can use Lexan™ if it's available) and drilled them as shown in Fig, 1, The Lucite insulator provides a means to connect adjacent wire elements as well as a way to mount the series capacitors.

When ifounting the capacitor, it is important to provide sufficient capacitor lead slack to relieve stress on iL Obtaining the capacitors shouldn't be a problem, I bought 60 390 pF silver micas rated at 200 volts, with a tolerance of 5%. They cost nine cents each at a surplus out lei. The 200-vol I rating will safely handle the legal amateur power level, One reason for buying 60 capacitors was to insure that I would have 46 capacitors that were within the 5% tolerance rating. Nine capacitors did not meet the requirements. Using polyester capacitors is better, but they arc more expensive. Ii possible, buy capacitors with long leadv, it simplifies connecting them to the wire elements, Once the capacitors were mounted and connected to the wire elements, I covered them and the leads with RTV compound. This provides an excellent weather seal.

Check the current flow

Before you erect the antenna in its final location, install it about four feet above the ground. Remember, it tk>es not have to be in a straight line; install ii lo suit your real estate. When you have erected the antenna ai the four-fool level, use a simple RF snitfer {Fig. 3) and excite the antenna with 3-5 wans of power. Start at the feed point and work to either end of the antenna. At the feed point you will have some indication of current. It makes no difference what the reading is; whatever it is. use that as the reference reading. Work your way to the end of the antenna. As you approach a capacitor the reading will decrease a bit, but as you travel to the end of the w ire element the reading should again increase. If a reading should drop off alter moving away from the capacitor, you have a problem in that section, ft could be a bad capacitor, broken lead, or poor solder connection. Correct this before proceeding further with the measurements. Remember: When making these measurements, do not change the power output of the exciter.

Installation

Once you have ascertained that the current tlow is uniform throughout the entire length of the antenna, you are ready to install it on your real estate. The length of the feedline is immaterial but, of course, the shorter the better The lowest point of my 3-5 MHz antenna is four feel high The center of the antenna is about 35 feet, and it is mounted among 40-foot trees, through branches and close to buildings. When conditions are right, I can copy European stations 5 x 9 plus on 75 meters. The 7 MHz antenna is also among trees, and 1 can work VKs, 5x8-9, using 10« watts when propagation conditions warrant.

This is exciting

One of the problems of living in Centra! Texas is that ground conductivity is very poor To overcome this, I installed a #16 copper wire, buried one inch in the ground, and followed the exact pattern of the erected antenna. Initially, when I excited the antenna with 5 watts, the RF sniffer indicated a reading of 29 microamperes. With the ground wire in place and 5 watts of excitation, the RF sniffer indicated 33 microamperes at the ends of the antenna. What this (Did me was that the antenna was passing more current, and the radiating efficiency had increased. Reference 2 states that the use of capacity-loading disc screens at the ends of the antenna w ill increase the radiation resistance, improve current distribution, and almost completely eliminate end effect. The end caps [ use are illustrated in Fig. 2, and they arc mounted vertically, about three Icel above the ground wire. The resonant frequency of the antenna was lower, so i eliminated one section from each end and the frequency was about 6.995 MHz. I again excited the antenna with 5 watts and

390 pF capacitor on last section of antenna

Circle made from 1/16" Brass welding rod

8 35" lengths of number 16 wire

F ig* 2, End capacity hat [or 7 MHz CCD antenna; two required.

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