THE OCTOPUS

A 4-Band Antenna For Portable Use

by

Geoff Haines, N1GY

            My participation in the world of Amateur Radio has lead me in many directions. From DX to Disaster Recovery, I have used my meager skills to do many things in Ham Radio. The most enjoyable of these is the building of the simple gadgets and accessories for my hobby.

            Recently, I had one of those ideas that would make some people call my sanity into question. If a "Fan Dipole" is possible, and a "Hamstick Dipole" is not only possible, but also frequently used, then why not combine the two! Now those of you with seventy foot towers with multi-band Yagis on top who never leave their linear behind can leave the room. This idea is for those who like to go somewhere quiet like the beach or a park and set up a modest station for an afternoon of radio fun. It also works for those of us who are ARES members and have to set up a working HF station in the middle of a parking lot after a natural disaster like a hurricane.

            Using the concept of a horizontal fan dipole, I drew up a design for a "Hamstick" type dipole taken to extremes. The basic plan was to find a mount that could be mounted on a mast that could accept up to eight "Hamstick" type antennas radially around the mount in the horizontal plane. Since I have often found the perfect parts for my projects at the local home improvement chain stores, I looked there again.

            Almost as soon as I got to the electrical department, my search was over. An eight sided electrical box of the type used to mount overhead lighting was ideal. It had eight sides, a top and a bottom and would be relatively easy to modify. The punch-out slugs only affect one direction of the antenna mounts and this is easily solved with pairs of fender washers that have been drilled out to 1/2" internal clearance. The punch-out slugs in the top and bottom of the box allow the entire mount to be slipped over the top of the 3/4" diameter top section of masting. I used an angle bracket bolted to the bottom of the box to allow me to clamp it to the mast. The next problem was how to mount the antennas. Checking the hardware aisle I found 3/8"-24 bolts in the fasteners section. The long nuts for the other side of the mount were harder to find, but by going to truck stops, where they usually have a CB parts wall, Radio Shack, where one can still find a few parts, and raiding my junque box, I eventually came up with eight stud mounts to attach to the central hub.

            Four of these stud mounts were mounted directly to the hub, and four were mounted with insulating washers. The four insulated mounts were wired to a single SO-239 type connector mounted to the bottom of the hub box. Each insulated mount has a grounded mount 180 degrees away from it. The connections from the insulated mounts to the SO-239 were made with ring terminals of appropriate size soldered to a wire harness which connected to the SO-239.

            The next step was to install four pairs of "Hamstick" type mobile antennas cut for four different bands on the mount. With each pair, one is connected to an insulated mount and the other is installed in the grounded mount opposite to the first. I chose to use Valor Pro-Am style antennas because the 3/8" x 24 connection of the whip to the helically wound lower shaft of the antenna makes them easier to store and assemble than the set screw arrangement of the Lakeside produced antennas. This is not to say one is better than the other.  I have purchased both makes before and indeed use the Lakeside made antennas on my mobile rig where storage is not an issue.

            Since we now have four dipoles all connected to the same coax, a little explanation is in order. When the radio transmits RF up the coax to the antenna array, it sees all of the antennas as unsuitable except the one that is most resonant to the frequency being transmitted. Therefore, all of the RF energy is directed automatically to the dipole that is closest to resonance at that particular frequency. This is the way a fan dipole works, and what we have here is a fan dipole that is, in effect, turned sideways.

            There are two ways to make sure that the transceiver sees a 50 Ohm load. One is to mount and raise one pair of antennas at a time, and tune each pair to present a 50 Ohm load. Very time consuming, but it does work. Each pair will have to be retuned to some extent because of slight interactions between the pairs of antennas, but eventually all can be made to work. The second way is to use an manual or automatic antenna coupler or tuner like an LDG Z-100. This was my personal choice. Once the antenna array goes up, I don't want to have to take it down until I am ready to go home.

            The "Octopus" Array is mounted on a telescopic mast home-brewed of aluminum tubing available from several sources. The telescopic feature allows the array to be positioned from a low position for NVIS type operations and conversely it can be extended to twenty two feet for more DX like activity. The low position should be above eight or nine feet off the ground for safety's sake. No one likes to run into the sharp end of an antenna.

                        Like the "Fan Dipole", the "Octopus Array" is a compromise. If time were no object and funds unlimited, there are antenna systems that work better. However, when your antenna system has to ride around in the trunk of your car, waiting for the next ARES callout, or you just have one afternoon to devote to catching some fun DX at the beach, this antenna will do the job well enough. A decent enough antenna is better than no antenna at all.

             To store the array, just remove the "Hamsticks" and put them back into whatever bag they are normally stored in. The mast is telescoped to its minimum length and stored the same way. You can leave the hub on the mast or store it separately, your choice. The initial set up only takes about 15 to 20 minutes and once done, you are finished with antenna work until its time to go home. There are no radials to lay out (and trip over) and when disassembled, it all fits in the trunk of your car.

            This antenna array also allows the use of a VHF/UHF antenna mounted above the hub on the very tip of the mast. Therefore, with just one mast you can provide communications over a broad range of amateur bands with a minimum of effort. Sounds like the amateur radio we all know and love.

            In testing this antenna array, I initially raised it to about eleven feet above ground. The radio and autotuner combination had no trouble getting a good match on any of the four bands which were 75 Meters, 40 Meters, 20 Meters and 15 Meters. During an all day display of amateur radio at a local American Red Cross Center, I set up the mast and antenna to its full height of 26 feet. I had no trouble at all in checking into the Maritime Mobile Service Net on 20 meters and similar success in making contacts on all of the bands except 75 meters which is usually pretty dead during the daytime. I was able to get a good match on all four bands, so I have no doubt that it will work on 75 meters when the band is open.          If you decide to try this approach, you can substitute your favorite bands of choice. You could build a two or three band version just as easily. In terms of overall performance, the array appears to work just as well as any single band "Hamstick" type dipole. The main advantage is that when you want to change bands you just switch the radio to the new band and give the "tune" button a jab. Two seconds later you are ready. No going out to the mast, lowering it down to six feet and removing one set of antennas to substitute another pair. All of that could easily soak up 15 or 20 minutes, and you still have to raise the mast back up to the proper height.

            The "Octopus" resides in the back of my SUV, along with the telescopic mast and its base. also home-brewed. When deployed, the base sits under one of the tires on the SUV and keeps the mast vertical  until the guy ropes are deployed. Then I can drive the car away and set up the rest of the station in a more convenient spot. Total time from the initial set up of the mast and antenna to being on the air ready to catch some DX or assist after a disaster is about 30 minutes. It can be less if I use the radio mounted in the car instead of a separate station set up like the "Radio in a Box" pictured in "Up Front" in  the October 2005 issue of QST. On the other hand, using the "Radio in a Box" means that the vehicle is available to do other things while the station continues to be operated.

            Keep on building. Whether an antenna, an accessory for the shack, or a whole radio, building it yourself is what makes this hobby so much fun and so special.