A NEGLECTED ANTENNA WITH POSSABILITIES.

 

USCGC McLane

As a radio operator in the United States Coast Guard it became my lot to serve on the USCGC McLane out of Brownsville, Texas.The McLane was 125 feet long and 25 feet wide and operated on HF frequencies from 2 MHz to 32 MHz with 180 watts of CW power.While serving on the McLane we went as far south as the Yucatan Peninsula.During the time when we were underway we needed to maintain communications with the Coast Guard Radio New Orleans NMG which at times was close to 1500 miles away.This was very successfully done with one antenna called an H antenna.

The H antenna started at the yard arms down both sides of the ship to 10 foot support poles at the stern and there was a wire crossing a few feet from the mast thus connecting the two wires running from the yard arm.This antenna interested me so I studied it and fount that the two lines coming from the mast were insulated from the single line which ran from the port side of the stern up to the insulated support line from the yard arm across to the insulated support line from the starboard side and then down to the starboard stern of the ship.A single wire ran from the antenna down to an insulated feed through on the rear bulkhead which went into the radio room and thus connected to the transmitter.After drawing the antenna I realized that it was just an old off-center fed Windom antenna that was formed in a U shape.

The Windom was a common antenna in the 1930’s but it is not selective against low level  spurious radiation so with the number of TV sets growing in the early  1950’s  TVI became the ham’s nightmare.  With its lack of selectivity making it hard to prevent TVI the Windom fell into disuse.While underway the McLane had no problem with TVI in the middle of the Gulf of Mexico so the antenna worked very well.

The antenna requires a very good RF ground. This will not only improve the operation of the antenna system but it will also help prevent stray RF inside the shack while transmitting. One reason the antenna may have worked so well on the McLane was the ocean is the ultimate ground.

The antenna is a half wave dipole at the lowest frequency used. It is fed at the 300 Ω point (.34 of total length on one side and .66 of the total length from the other end) with a single conductor, twin lead or ladder line. (The feed point of the H antenna was much closer to the center maybe .25 to .75.)

A Windom antenna is said to work on the fundamental frequency and the even harmonics so the 1.8 MHz antenna would work on 3.6 MHz, 7.2 MHz, 10.8 MHz, 14.4 MHz, 18 MHz, 21.6 MHz, 25.2 MHz, and 28.8 MHz.This would make it on or close to the 160, 80, 40, 30, 20, 17, 15, 12 and 10 meter bands. On this type of antenna the antenna tuner acts as a real antenna tuner, unlike that of a coax fed antenna, and can make up for the difference.The frequencies used by the McLane were not as close to the even harmonics of the lowest frequency used as the ham band frequencies are to 160 Meter. The lowest frequency we used on that antenna was 2686 KHz so the antenna needed to be about 174 feet long, 49 feet longer then the ship, by forming a U shape the ship could accommodate the antenna.

Most modern HF transmitters use class A, B, or AB, AB is the most common, because SSB requires a linear amplifier and linear amplifiers have a very low spurious output so if a low pass filter is used between the antenna tuner and the transmitter output there should be very little problem. Also most TV’s are now used on cable or dish which are not as susceptible to TVI as they were in the earlier day’s television sets which were attached to antennas on the roof of the house.

Many hams feel constrained when it comes to putting up antenna because of limited room but as described above a very effective antenna was used by the United States Coast Guard on the very very limited space of a 125’ ship. If you can put two poles on the roof of your house which are parallel to the back fence and run a single 260 feet long wire starting at your back yard fence up to one pole then over to the other pole and then down to the back fence again you should be able to put up an antenna which could be used on any desired frequency from 1.8 MHz to 30MHz.

Please use the comment section below to let me know what you think.


HTML Codes

 

Share on Facebook



This entry was posted in Ham Radio and tagged , , , , , , , , , , , , , , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *