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HF Bicycle Mobile Antenna
One of my goals since purchasing a Specialized brand bicycle was to run some sort of mobile Amateur radio station while while transversing the bike paths of Montevideo and beyond. Using a VHF handheld was certainly an option, but it is hard to work DX on two meter FM. It was clear that HF was the logical choice.
To work HF, I needed an antenna for my recently purchased Yaesu FT-817ND transceiver. Searching the Internet found little useful information on home brewing such an antenna, so I decided to design my own. With a well stocked junk box, you should be able to duplicate this antenna for as little as $5.
You will need the following items to build this antenna:
Start by removing the flag from the pole and cleaning any remaining glue. Sand down any remaining rough spots, as they will make winding the coils that much more difficult.
Prepare the spoke nut by filing down the outer diameter to about 1/8". Since they are generally made of brass, a small file and drill should make short work of this task. Lightly sand and tin the outside rim of the nut flange, as we will be soldering a wire to it in a latter step.
The hardest part of building this antenna is drilling the 1/8" hole in the end of the 1/4" fiberglass pole. You do not have a lot of room for error, so take your time and do it slowly. I found that a Dremel type tool is almost perfect for this task. Do not worry if you mess up on the first try, just cut off the section and try again. It took me three tries before I got it correct. Once the hole is finished, epoxy the modified spoke nut in place.
While the epoxy is setting, cut off the non-threaded end of the broken spoke and file down any rough edges remaining. Again, I used my Dremel tool with excellent results. Heat the end up in a propane torch until it is almost red hot. Quickly insert the tip into the plastic bead. The easiest way to do this is by setting the bead on a heat-resistant surface and pressing the spoke through the hole. Do not let the end of the spoke extend past the edge of the bead! We want this smooth so as not to poke out your eye while in use.
You will now need to drill two holes in the flag mounting bracket. One is for the RCA jack, and one is for the standoff. The size and location of each will depend on your actual hardware. Check out the picture to the right for some ideas on the best location.
Place the 1/4" rubber grommet over the pole and move it all the way down to the bracket. Strip about 3" of enamel from the #16 gauge wire and tin the length of it. Attach the standoff to the bracket and solder the end of the #16 wire to it. Wrap the remaining tinned wire around the pole, above the grommet, until you reach insulation. Solder the tinned wires together for added strength, being careful not to melt the grommet.
Begin winding the wire over the pole as tight and uniformly as possible. The use of a small variable speed electric drill is a great time saver, and makes the job much easier. Take your time, as the smoother this job goes, the better your antenna will perform. The actual number of turns is not all that critical, but you should have at least a 20.5" long coil when finished.
When you get to the end of the coil, strip and tin approximately 3" of the wire. You may want to enlist the aid of a small clamp or some tape to secure your winding while doing this step. On my antenna, I placed a small piece of copper tape I'd purchased many years ago at All Electronics around the pole, wrapped the tinned wire around it, and quickly soldered the wire and tape together. If you do not have such tape, you might be able to get by without it by just tightly wrapping the wire around the pole. The main thing to keep in mind is to maintain the tightness of the coil around the pole.
Strip and tin about 3" of the #24 gauge wire, wrapping it around the remaining copper tape. If you did not use tape, you might try wrapping it around the pole and then butting it against the #16 gauge joint. In either case, solder the wires together, taking care not to make too large of a solder joint as we will be sliding a clamp over it in a future step. Wind about 75 space wound turns around the pole until it just reaches the tip. Strip and tin about an inch of wire, and solder it to the previously tinned rim of the spoke nut. Take care not to get solder inside the treads or you will not be able to attach the spoke spike.
Mask off both ends of the antenna and spray the entire length with clear enamel paint. I gave my antenna four coats, allowing them to dry between applications. This will help keep the top coil in place once the antenna is finished.
When the enamel is completely dry, take some fine sandpaper and remove a strip of insulation from the bottom coil along its total length. A Dremel sanding drum works wonderfully for this job, but you can do a fairly good job by hand if you are careful. The purpose of this is so we can create a variable inductor by sliding a shorting line up and down the length of the coil.
Next, drill a hole in the center of the plastic equipment foot just slightly larger than the outside diameter of the bottom coil. In my case, the OD of the bottom coil was 0.35", so I used a 5/16" drill and enlarged the hole a bit with the Dremel. You want a snug fit on the coil, but not so tight as to impede movement of the foot. Once that is finished, file down one side of the foot so that it resembles the letter "D" when viewed from the end. In the very center of this flat, drill and tap a perpendicular hole for the 6-32 thumbscrew. Tin the 6-32 nut and solder the end of the 36" Teflon wire to it. Epoxy the nut to the edge of the foot, aligning the hole so that the threads are aligned. This is easier if you insert the screw into both threads and tighten it. Click on the diagram above for a more detailed view of the modified foot.
Once the epoxy has set, remove the screw and tin the very end with a small bit of solder. This will make it easier for the coil to connect to the shorting lead.
Slide the foot over the coil and place it at the top of the bottom coil. Wrap the Teflon wire around the bottom coil about three or four times in the opposite direction that you wound the coil. Trim the wire to length and solder the end to the standoff.
Finish the antenna by placing some heat shrink tubing over the center joint. Solder a small piece of wire from the center pin of the RCA jack to the standoff. Insert the spoke spike in the top, and you are finished! ;-)
Tuning the Antenna
Tuning the antenna is actually quite easy if you have a noise bridge, network analyzer, or SWR meter. Simply adjust the position of the clamp over the section of the bottom coil for the best match. You can also get fairly close to the proper setting by adjusting the coil for the highest amount of background noise.
To make tuning easier in the future, you may want to measure the the placement of the clamp for a given frequency so that you may QSY quickly. Click on the graphic to the right for a sample plot of length verses frequency for my antenna while installed on the bicycle.
In practice, I have found that tuning the antenna for the highest amount of noise and using the SWR meter of the FT-817ND to zero in on the proper tuning point is not only fast but surprisingly easy. SWR does vary somewhat with the position of the rider, so it is best to do your adjustments while at least sitting on the bicycle.
The lowest usable frequency of this antenna can be extended by adding more turns to the bottom coil. Earlier models of this antenna tuned down below 10 MHz by adding approximately 120 turns (4 inches) of wire to the bottom coil. This could be extended further by adding even more turns. Unfortunately, adding this much wire makes the antenna quite a bit heavier and unwieldy. No doubt the efficiency will also degrade as more wire is added.
I use RG-174 coaxial cable to connect the antenna to the rig. This smaller cable is quite a bit lighter and easier to work with than RG-58. With a run of less than 10 feet, the additional loses (~.25 dB) incurred by using RG-174 are not enough to worry about.
When initially testing this antenna, I found that there was some RF on the outside of the the coax which caused some interaction while tuning the antenna with a noise bridge. Wrapping the coaxial cable around a split ferrite choke near the feed point solved the problem. A similar solution can be achieved by coiling the coax half a dozen or so times into a tight loop near the feed point.
Field Test Modifications
After a few weeks of using this antenna, I found that the adjustable coil was not adhering to the fiberglass very well. To fix this problem, I applied liberal amounts of "Super Glue" to the length of the coil. Be sure to take your time, and have plenty of towels and acetone available to clean up the inevitable mess you will make. :-) Once the glue has saturated the windings, set it aside until it has completely dried. A light sanding of the coil completes the job.
Over the last few months, the combination of hills, bumps, and wind has put a lot of stress on the rather cheap metal clamp used to secure the antenna to my bicycle. This stress is significantly reduced by attaching two lines from the seat of my bicycle to the central joint of the antenna. The use of fishing tackle spinners and mono-filament fishing line makes an almost invisible antenna guy which is easy to connect and disconnect. These lines also aid in the lateral stability, reducing the amount of antenna movement due to wind turbulence.