Last night I got home with a few hours of daylight to spare (thank you very much Summer Solstice) so I decided to throw a quick and dirty doublet together for Field Day. Like most hams, I have a few (unless you ask my XYL, then it's hundreds) boxes of random "junk" laying around that I use from time to time to experiment, so I pulled out a few bits and pieces to put the antenna together. In case you've never built a doublet, here are my reasons for doing so and the theories that I employed.
There is a lot of confusion surrounding the terms "doublet" and "dipole," so let me wade into the murky waters and explain the difference as I see it. Technically, there is none. They are two names for the same thing. However, the difference, in common parlance, lies in how you feed said antenna and whether the antenna is cut to resonance for a target frequency.
Typically, a "dipole" is a resonant antenna for a given frequency and is fed with coax. There are many purveyors of 40 meter, 20 meter, 10 meter, etc. dipoles that include a PL259 connection at the center connector to attach your coax. These dipoles are typically resonant near the center of the given bad and give adequate bad with to allow the ham to operate anywhere on the bad without the need for a tuner. Try to load them up on another band then their design frequency however, and noise, smoke, silence, and cursing often occur.
"But I have a fancy built-in tuner in my rig, so I can load that 40-meter dipole up on 6 through 160, right?"
Maybe, but the impedances of other bad on a resonant dipole can be amazingly high. So, in short, it's not a good idea to test fate. There are however tuners that will allow you to load up on other bands, but the signal losses of doing so offer drastically diminishing returns. In other words, you might be able to tune that 40-meter dipole up on 160-meters, but it might work as well as loading up a Soup Can.
A Doublet (again, I'm speaking of "Doublet" as is generally understood. Both "dipole" and "doublet" mean the same thing), is a little different. The basics are the same. You generally cut a doublet antenna to be a half wave long on the lowest frequency you plan to operate. Impedances on other bands will still be amazingly high, but the losses will be much lower and turn our "Soup-Can" into a usable, although compromised, antenna. How? I'm glad you asked.
By using ladder line or open wire feed-line (two wires separated by spacers) the signal losses are dramatically reduced. The ladder line is then fed into a Balanced Line Tuner (or a 4:1 Balun) and then coax to your rig. A Balanced Line Tuner like the CG Antenna Ltd. Remote Auto Tuner is placed where your ladder line meets the ground and powered by a battery (in my case an SLA battery which is charge by a 5W Solar Panel (Pics coming soon) or by a "Bias Tee Power Injector" such as the MFJ-4116. When the auto tuner senses RF energy it quickly measures the SWR and makes the necessary adjustments to put it back in a usable range. Remote auto tuners like the CG can seamlessly deal with the wild impedance differences between bands and allow you to operate on 6 -meters one minute and 40-meters the next. So now you can pick a Saturday and knock out that DXCC you were thinking about getting someday. Maybe, but there are a few other things to consider.
Our 40-meter Doublet and subsequent Balanced Line Auto Tuner are doing their job singingly, but it is important to realize exactly what they are doing. Like a 40-meter resonant dipole, our antenna will project the bulk of it's RF energy 90° to the antenna in both directions, in other words, off the sides of the antenna. This is assuming that the antenna is flat-top and at least 1/4 wave (33 feet or so) above the ground. When we move dow to 20-meters, our antenna will still put the bulk of it's energy out at 90°, but as we go higher in frequency (17-meter, 15-Meter, 12-Meter, etc.) our two predictable lobes will start to break up and lose energy. by 15-meters, there will essentially be 4 lobes at 45° each. I wish I had the ability to plot this out for you, but at the moment I don't. Suffice it to say, that you must consider the trade-offs when ever you deploy a multi-band antenna.
Wow, that was a lot of theory to get to my application. So, I decided to deploy a lightweight 66' (half wave on 40-meter) doublet fed with 300Ω Ladder Line. I built this antenna to use for Field Day 2013, but it will also be used regularly when operating portable with my Hendricks PFR-3. That being the case, I wanted it to be pretty bullet-proof without adding too much weight. Also, I wanted to save the nice 500' spool of antenna wire I have to complete an 80-meter full-wave loop this summer. Luckily, my fellow Ham and father recently unloaded a spool of hefty speaker wire on me.
I found this Wireman 814C in a box and drilled the holes a little wider to accommodate the wire. I soldered everything up, added some shrink-wrap (in case it stays up longer than planed) and put it away for the weekend.
As you might notice, I need to buy a heat gun as my lighter really charred the shrink wrap, but at least it's still functional. I'll report back on how the antenna performs, but in the meantime, I hope I've given you some food-for-thought on dipoles vs. doublets.