THE LIVE WIRE

A Publication of the Blue Ridge Amateur Radio Society, Inc.
December 1998  

President: William (Buddy) Rodgers-KA4END, 19 Wintergreen Ln., Taylors, SC 29687
Vice President: Mitchell Hawkins-KD4VXP, P.O. Box 41, Pelzer, SC  29669
Treasurer: Carroll Smith-WA4WYE, 202 Agewood Dr., Simpsonville, SC 29680
Secretary: John Chism-ND4N, 139 Mustang Circle, Simpsonville, SC 29681

SPECIAL NOTICE !!    Beginning with the BRARS Inc. meeting  in JANUARY, 1999 , our meetings will be held in the Red Cross Building Conference Room just off Grove Road, across from Winn Dixie. The December meeting will be our last meeting at Greenville Technical College.

    December Meeting:    The next meeting of BRARS Inc. will be on Monday evening, December 7, 1998 ( “Remember Pearl Harbor!!” ) at Greenville Technical College, Room CE- 350 (Auditorium). We will gather for coffee and some Holiday goodies at 7:00 pm and the Meeting will start about 7:30 pm. The main business will be nominations for offices and election of officers for 1999.

    An Antenna That Works    Our feature article this month is the “Distributed Capacity Coaxial Dipole Antenna”. The publication describing the antenna was created by George- WB4N. K7UAE built and tested the design over a 5 year period. Copies of the article have been passed around
but the exact source has been lost. The copy used here was provided by Doug Gillstrap- WA4LTZ. Doug constructed an 80 meter version of the “ Coax Antenna” and was pleased with his finding that it greatly reduced “summer-time static” and seemed to offer some gain over an inverted “V”
wire antenna as well as being broad banded.

    I decided to test the concept on 10 meters as reports were coming in that sunspot activity was on the rise. As the article suggested, I used 17 feet,  2 inches of RG58A/U as the basic antenna fed through 75 feet of the same coax. Without adjustment, the antenna showed a low, flat SWR from 28.3 to 28.7 MHz. My first test transmission was on 28.480 MHz and I was more than a little surprised when S52CI - Igor of N/E Slovenia came back to give me a 5/5 report from Eastern Europe. My next contact was with AB7RS- mobile- Tracy, as he was driving to his home in Montana-- The signal report was 5/7. My “Coax Antenna” is mounted as a semi- “V” with the apex about 28 feet of the ground. My oak tree offers the potential for a much higher antenna but until I get help from a tree climber or expert archer, it will stay at 28 feet.

                               

Distributed Capacity Coaxial Dipole Antenna

    This dipole antenna takes on some very interesting characteristics unlike its predecessor, the “simple dipole”. Unlike the simple dipole, this antenna is very broadbanded. Normally the bandwidth of this coaxial dipole is around 500 kHz, with average installation considerations and an SWR of
less than 2.1 : 1. The higher SWR occuring, of course, at the band edges. The SWR will be absolutely flat when the antenna is of the proper length at the design frenquency.
    By proper choice of design frequency, one may have an SWR on one band edge equal to the oposite band edge. If this is your preference, you will want to establish a design frequency somewhere in the top half of the band. As an example, for the 75/80 meter band, the design frequency should be about 3920 kHz. At the 4 mHz end if the band, the SWR will be about 1.5 : 1
with the SWR at 3.5 mHz also at about 1.5 : 1. This is all without the aid of a “match-box”. However, antenna parameters may vary slightly from one antenna loaction to another.
    There are several reasons why this antenna is so broad. One reason being because this dipole is matched to the feedline. Another reason being that it electrically incorporates its own balum. Also, this dipole has a large circularmil area over its entire surface, thus a low Q.
    Over a 5 year period of testing, K7UAE reports a arithmatic average of 1.5 dB gain over a simple dipole cut to the same frequency and installed at the same height and configuration. From the author’s findings, this gain figure could be a conservative representation. Also, he reports a -6 dB figure due to static charge build-up common to the open wire construction of the  simple dipole. Since this coaxial dipole is completely covered by a vinyl jacket, it greatly reduces charge build-up which,when discharging, causes a popping noise in the receiver. The vinyl jacket covering is usually found on most well designed mobile antennas.
    This antenna also greatly reduces harmonics of the operating frequency. Any signal fed to the antenna which is hamonically related to the antenna’s operating range is reduced by a considerable amount as compared to a simple wire dipole. This feature, as well as other features of this antenna, amount to a substantial savings in cost for the extras such as an antenna match-box, low pass filters, balum, etc.
   
    In summary, the advantages of this antenna are:

1) broad banded,
2) almost unaffected by environment,
3) positive gain with reference to a common dipole under the same relative conditions,
4) greatly reduced harmonics,
5) substantial decrease in static build-up, and
6) essentially non-directional.

   

Construction Hints

    For antenna lengths see accompanying chart. For illustration purposes the 80 meter antenna will be used. It is suggested at this time that one may use any 52 ohm coaxial cable for both the contruction of the antenna and the feeding of it. The common choice of cables that meet this requirement are RG58A/U or RG8/U. It may be noted that RG58A/U is a very good choice
in that is the least expensive. As for attenuation in this coax, within the HF bands, losses are considered to be negligible. Also, with this antenna, one may use RG58A/U at the maximum legal power limits without fear of feed-line break down.
    For 80 meters, 120 feet is the total length. Measure 30 ft. 6 inches out on each side of center and solder the inner conector to the outer conductor of the coax. This forms the 52 ohm matching section and the balum. When completed, one will want to waterproof these points of the antenna well. Next, at both ends of the antenna, the inner conductor must be shorted to the outer conductor. Waterproofing of the ends is not necessary at this time as the ends may need cutting later for tuning purposes.
    At the center of the antenna, remove one inch maximun of the vinyl jacket, 1/2 inch each side of center. Cut the shield in the center all the way around the coax. Do not cut the insulation or the center conductor. Form 2  leads with the shield. These leads are the feed points of the antenna. Next
connect the feedlilne to these leads by soldering the center conductor of the feedline to one shield and the shield of the feedline to the remaining shield. When this is compleated, waterproof the best you know how. As for feedline length, random length may be used. However, lengths of 57 ft., 67 ft., and 103 ft. are about the optimun for 75/80 meter use. The same coax should be used for antenna and feedline. Separate feedlines must be used for each antenna.
    Next, the antenna is erected. After erecting the antenna, measure the SWR and trim the antenna to length at the desired frequency. The SWR will  be an absolute flat 1.1 : 1 when the antenna is at the proper length at the desired frenquency. Be sure to solder the ends of the antenna ( shield to
center conductor) before measuring the SWR and also after completion.  Finally, waterproof the ends. This completes the installation.
    Follow the same procedure for antennas of other bands. Refer to the chart for proper dimensions. It may be added that this antenna is designed for use as a “Flat Top” or as an “Inverted V”. George, WB4N

    Editor’s Note: I have enjoyed my year as Editor of The Live Wire  but am quick to admit that I am running low on ideas for feature articles. To save me from that worry, Teri Hawkins - KF4WVD with some assistance from her husband, Marty - KF4HTL has agreed to take over as Editor (s)
with the next issue of the official BRARS Inc. newsletter.
    Please help her by providing material for the newsletter.. Several of you helped me by providing write-ups on subjects of special interest to you and this allowed us to share your interest with the rest of our membership. Please continue this with subjects of your choice. Tell us about a recent
project for an amplifier or antenna. Explain to us how this coax antenna works. Tell us about packet communications. Keep us informed about FCC and ARRL activities. Anybody got some good Ham Radio jokes? What have you seen on the Internet about amateur radio that would be of interest? I’m
sure that if we all make contributions, Teri will be able to provide an informative, useful newsletter.
    I want to thank Jeff -WA4EFT and others around his home that helped by getting the letter from the printing service, labeling envelopes, folding the newsletter, stuffing and stamping the envelopes....several hours  of work each month. While thanking those that helped, lets not forget John - ND4N, who provided up-to-date labels for the mailing list and and the minutes of previous meetings. Also thanks go to Jonathan -KE4ZVU, who manages our website and sends the newsletter by E-mail to those that prefer E-mail to snail mail.    73 Carlisle - WA4ULE