The Blue View - Alternative Antennas
/In the last Blue View, I talked about a long line antenna, which is what most cruising boats use for transmitting, either on the marine bands or on the HAM frequencies. As I mentioned, probably 95% of the cruising sailboats we see use a long line type antenna for their HF radios, but there are a couple of other alternatives.
One alternative is a whip antenna. This is a 23' fiberglass antenna that is mounted to a rail, usually near the stern of a sailboat. The rail is typically used as the counterpoise. The whip antenna is more often used on power boats, but we have seen a number of sailboats using them as well. The drawbacks: they are expensive (although no more so than buying and installing a couple of backstay insulators); the mounting method must be robust as a 23' pole aboard a sailboat that is rolling in a seaway will put a lot of stress on the mounting point; the transmission range is typically less than that of a properly installed long line antenna – some estimates I've seen indicate as much as 30% less.
Another alternative is a dipole antenna. A dipole antenna may be the most widely used antenna for shore-based systems. The most common version is made up of two identical length conductors placed end-to-end. The radio signal connection is made at the junction of the two wires. The dipole antenna is only resonant within a small frequency range (example: 14.0Mhz-14.5Mhz), and is tuned by adjusting the length of the two conductors. If it is operated at the tuned transmission frequency, no antenna tuner is needed – the dipole can be connected directly to the radio. We commonly transmit in the 14Mhz range, so the dipole I built was tuned for this range. I found, however, that if I connected it through my antenna tuner, it transmits reasonably well throughout the 10Mhz to 18Mhz range – maybe not quite as well as the long line, but not bad. On the other hand, within its resonant frequency band, it is superior to our long line antenna.
I built a rather simple version. I used coax cable from the antenna tuner to the junction of the two dipole conductors. As an insulator, I used a one foot section of 1x2 wood, and soldered the connections between the coax and the dipole conductors. Each of the conductors were secured with cable ties, and the entire insulator was wrapped in rigger's tape to protect it from the elements. I used a spare halyard to hoist one end of the antenna and secured the other end to the stern rail. The coax wire was led from the insulator to a cable tie on the backstay, then to our antenna tuner.
The length of the two dipole conductors should be 1/2 wavelength in total which, if you're not a radio geek, is determined by the formula: length in feet = 468/frequency in megahertz. So if you want to transmit at 14.2Mhz, the theoretical length of the conductors should be 468/14.2 = 32.96 feet, and each conductor should be half of this, or 16.48 feet. Note that this is the theoretical length. The optimal length will vary from the theoretical based on a host of variables – the type and length of feed line, other rigging, masts, angle of the dipole, etc.
To tune the antenna length, I used my radio's built in Standing Wave Ratio (SWR) meter. I was at anchor, but if I was at a marina, I would have moved Nine of Cups to an anchorage or mooring to avoid any interference from other boats or structures. I started by making the length of the dipole conductors slightly longer than the theoretical lengths, hoisted it into position and checked the SWR. The SWR will be lowest at the resonant frequency of the dipole, and to start, the center of the resonant frequency was lower than the desired frequency of 14.2Mhz. I lowered the antenna and trimmed off an inch from each conductor, then repeated the process until the the resonant frequency was centered around 14.2Mhz.
The purists out there will know this wasn't the optimal method of building a dipole. I should have used a balun between the coax feed line and the dipole conductors, or alternatively used a twin lead feed line instead of coax. There are also ways to build a multi-frequency dipole that will transmit at three frequency ranges. This was my first attempt, however, and I wanted to keep it simple. As it turns out, I was quite happy with its performance and didn't modify it.
If I didn't have a SWR meter, I would have made the lengths of the conductors the theoretical lengths and used an antenna tuner between the dipole and the radio. This is the configuration I use anyway, since I often use the dipole at other than the resonant frequency.
So what do we actually use? Most often we rely on our old long line antenna, and the dipole is our backup. Since it doesn't require a ground plane, I also dig it out when we are on the hard where the performance of the long line drops dramatically.
Well – to paraphrase my dear old dad – I've taught you everything I know about HF radios and antennas and you still don't know nuthin'.
Next week I'll move on to a totally new subject – stay tuned.