To the editor: I was intrigued by the recent item on SSB whip antennas (“Asked and answered,” Issue 126, Nov./Dec. 2002). I have had quite a bit of experience using a Shakespeare 17-foot model 5390 single-sideband whip antenna onboard a sailboat.
One of the major technical points provided by this whip is its self-resonance at 12 MHz. The antenna tunes quite easily from 4 to 27 MHz. I have also had quite a bit of experience with insulated backstays. I have loaded them from the bottom and from the top! When you have one cut to a specific frequency, they are hard to beat. However, if it is cut to work in the 8- or 12-MHz range, they are more difficult to tune and produce a comparable signal in the 4-MHz band. In this case, you must invest in a good antenna tuner; you can’t get by with a simple antenna matcher.
Somewhat along this same line, I would like to explain the radio setup I used when I raced Luan Two in the Tampa Bay to Isla Mujeres Race this past May. The basic radio installed in the boat is an Icom M800 marine SSB. It is used for both voice and email. However, for this race I wanted to be able to use an automatic position reporting system (APRS) program. This is an amateur radio application that sends a digital signal that contains latitude and longitude data as well as six other data fields. The purpose was to keep my first mate, N4TFP, informed of our location so she could call the other spouses and relate our progress. I have a powerful base station with antennas on a tower behind my house.
Due to the complexity of running a navigation program fed from a GPS receiver, and also requiring the APRS software to use the same information on the laptop, I decided that it was too complex to start and stop the APRS operation whenever we needed to talk with race officials. Therefore, I temporally installed a second radio: a small ham transceiver that would transmit the APRS information back to my home base. I did not want to install a second 17-foot whip antenna, so I used a co-ax switch that swapped the output from the M800 to my Kenwood 430. All of the voice schedules were at the top of the hour, so I adjusted the APRS software to transmit on the half-hour. During the five days of the race, the APRS system transmitted 137 position reports, one every hour, which were all received at home base. Any time of day or night, my wife could walk into our radio room and see a sailboat symbol displayed on the chart on the computer monitor. It was very similar to what the crew and I saw on our laptop monitor. The APRS software not only displayed the lat/long coordinates but also computed the speed made good over the last hour.
Obviously the 17-foot whip antenna did an excellent job as a radiator. The key point here is the digital signal output of the ham rig was only 30 watts, as compared to the M800, which produced about 90 watts on voice. The APRS frequency was 10.151 MHz lower sideband, while all voice contacts were on 8 MHz upper sideband. These two frequencies were close enough to the resonant frequency of the whip that I could use the simple antenna matcher.
Jim Johnston lives in St. Petersburg, Fla. He is a rear commodore of the Seven Seas Cruising Association.
