SSB HF Mobile Antenna - Build Your Own - Part 2
Now that you have finished the bottom mast and the coil for whatever band that you are interested in, it’s time to get the rest of the antenna finished.
Hat or Telescope?
Here you have a choice. You can either build a capacity hat to increase the radiation efficiency or you can just make a telescoping whip to tune the antenna without using the capacity hat. The use of the capacity hat on 160, 75, 40 and 30 meters will improve the radiation efficiency of the antenna and so if you really want the max out of your kilowatt antenna, a capacity hat will help a lot. It also will help the increase the efficiency on the other bands as well.
Make Capacity Hat
To start with, you must make up some pipe caps with a 3/8″-24 bolt and matching nut soldered into them just like you made for the top and bottom of the mast. Once you have this done, then you solder them to a short piece of pipe and solder the caps to it just like you did the bottom mast, following the same precautions as outlined in the first half of this article. Figure 1 shows a side view of the capacity hat before the ring is installed. The spokes can be made out 1/8 or 1/4 inch hard drawn copper tubing and the ring can be made of soft drawn copper tubing as it is easier to bend. You may be asking, “Where are the dimensions??”
Well, since it is your antenna you can make it as large as you want to or as large as your XYL will stand for. Generally speaking, the larger the better, but one larger than 2 feet becomes a little bit more than you want to have hanging up in the air at 55 mph or higher.
Cut to the same length, and then drill 4 holes to match the rod diameter at 90 degrees to each other in either the pipe caps or the mast section between the caps. Once the rods are inserted into the 4 holes, solder them in place. At this point, you can either put the ring on or not, but if you don’t, at the power levels that will exist with a kW mobile, you run the risk of corona discharges from the tips of the rods. This could cause injury if someone was unfortunate enough to be too close to a rod when you were transmitting. It could also start a fire in a shrub or other vegetation. This is a good enough reason to use the ring on the capacity hat. So, better to attach the ring.
The Ring
To mount the ring on the rods you will need 4 T connectors of the appropriate size to match the pipe that makes up the spokes and ring. Be sure to clean the rod ends and the T connectors so they will solder well. Push the T connectors on the rods, but do not solder them yet. Next, put the end of the ring pipe into one of the T connectors and gently bend it until it forms a semicircle and matches up with a T connector on one of the other rods. Be sure when you cut the pipe at the second T connector you have enough pipe to fit into the T connector easily. After you have both ends of the ring pushed into the T connectors and everything looks lined up, fire up the torch and solder the T connectors to the two rods and the ring to the T connectors. Continue in this manner until you have all parts of the ring soldered in place. By soldering one section in at a time, you can get the ring in place with a minimum of grief and it will come out a nice circle or a reasonable facsimile thereof. If it doesn’t come out a perfect circle, it will work just as well. My first attempt at this looked like a watermelon, but it worked fine. However, since this is your antenna and you have to look at it, how close to a perfect circle you want your ring to be is up to you.
The Top Whip
Now, assuming the capacity hat is done, the top whip is next. For this section, you need two sections of pipe. One of which must be able to slide in and out of the other. Again, the preparation of a pipe cap with a bolt soldered in it is required. Of course, the bolt must to be 3/8″-24 in order to match the rest of the system. The sections of pipe used do not need to be the same diameter as the lower sections, so use a pipe cap the bolt will fit into and then apply a reducer to allow smaller pipe.
The next step is to split the larger diameter pipe so that by using a small hose clamp you will be able to adjust the length of the top section for tuning. The length of the cut in the outer pipe is going to depend on how stiff the pipe is, but generally if you make two cuts at 90 degrees to each other, about 3 inches will suffice. The top of the smaller pipe needs some sort of anti-corona ball on it For this, a pair of small pipe caps soldered together with a hole drilled in each one, then soldered on the end of the small pipe will do just fine.
The adjustable whip and a tapped loading coil will handle just about all of the tuning problems that might show up. Of course, you don’t have to use both methods, but it will enable you to tap the coil for more than one band and fine tune the antenna with the adjustable top section.
Water Drain
If you do make your whip adjustable, the last thing that you will need to do is drill a small hole in the bottom of the outer pipe to allow any water that gets into it to drain out easily. Slide the smaller pipe into the larger one and clamp it with an appropriate hose clamp and the top section is done.
Matching Network
Now, assemble and mount the unit on the vehicle. It is almost ready to tune up and use. All that remains is the matching network. What! you don’t have a matching network? Well, a matching network is needed to simulate 50 ohms to your kW linear and make the antenna work better. So, attach a shunt coil from the coax center conductor to ground and then tap the antenna connection up from ground until you find the lowest VSWR at your operating frequency. Once this point is found, it will usually does not need to be changed and will cover an entire band. 75 and 160 meters are the only exceptions. Figure 2 shows the whip top section and Figure 3 is the diagram for the matching coil.
Tuning
Tuning up the antenna is no different from tuning up a commercial antenna, except two methods of tuning are available if you made the top whip adjustable. If the tip is adjustable, it will enable you to tune the antenna on more than one band by tapping down on the coil and setting the tip to a different length. It is more efficient to have separate coils for each band but tapping one coil will do fine for several bands.
Improve Conductivity
After you have gotten the SWR down as low as you can, the last thing to do to increase your antennas efficiency is to improve the conductivity of the spring at the ball mount. Fortunately, this is not too hard to do.
The average antenna spring consists of a coil of spring material and a length of braid that runs from the top to the bottom of the coil. Over the passage of time, this braid will corrode and disintegrate. When this happens, you end up with a small intermittent coil at the base of the antenna. Of course, this does not help your signal at all, as the addition of a small amount of inductance will certainly change the antenna tuning. To get around this, a length of copper flashing is needed to form a loop from the base of the spring to the top of the spring. This allows the spring to flex without any effect on the antenna tuning. It also can carry more current with less I2R losses than the coil. See Figure 4.
On the Air
Now your antenna is finished and should produce a respectable signal to be heard without melting your coil. If you want to, coat your antenna with any type of preservative to keep the corrosion down. One ham used large diameter shrink tubing to cover his base section and top whip section. He put the coil inside some large plastic containers and sealed them up. No matter what method chosen, you should protect your antenna from the elements.
All Can Benefit
You do not have to have a kilowatt to benefit from this antenna. Any mobile hF rig will benefit from the larger conductors and increased overall efficiency. So good luck with your “own-made mobile antenna!”
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 14th March 2024