Bicycle mobile VHF antenna an AC9LF special

I don't have a very long commute to work. Usually it only takes me around 6 minutes to get to any place I need to go. I live very close to grocery stores, hardware stores and my place of work. That said I really don't have any road time to justify putting a mobile radio setup in my car at the moment. I still wanted to do some kind of mobile operating and my focus shifted to be bike.

One of my favorite things to do in the summer is ride my bicycle. Sometimes the rides are a little long and if I don't have a riding partner can be a bit dull after a while. I considered combining amateur radio and bicycling. It's been done, and some setups are very impressive and eye catching. Not something I am interested in. I would like my setup to be fairly low profile, yet effectively be able to hit the local repeaters around the city running low power on a small handheld transceiver.

Reading on the various ways others have mounted antennas on bikes I found two main ones. Either mounted on some kind of bracket on the back of the bike or attached to the 6 foot pole of a fiberglass safety flag.

The safety flag setup is usually done with a wire J-Pole twin lead design such as this one here:
http://www.bikexprt.com/bicycle/antennae.htm

I thought I could go a little lower in the profile as this antenna is pretty high. I may still have a go at it sometime but for now I decided on trying to use a quarter wave antenna to cover the 2 meter band. I wanted to also try using the bike frame as the counter poise to this antenna. I read that the frame typically does not make for a good counterpoise but I wanted to try it anyway. There seems to be enough material on the bike to act as one on 2 meters and up.

I was looking into ways to mount this antenna. Some people constructed their own brackets but most just attached the antenna to a rear rack mount. Usually the antenna was attached with some kind of connector like an NMO or SO-239 mount which was mounted through the rear rack mount on the bike. I looked at these as well as antennas on Amazon. Most of what I found didn't appear to be tunable. Meaning that you could not pull out the antenna element and trim it to resonance for the frequency range you want to use it on. This may be necessary, especially when attached to an odd structure like a bicycle. It is by no means a car roof. They also seemed a little pricey for just a stainless steel rod of a certain length to be screwed on to a mount.

I did spend a little bit of money on a rear rack mount for my bicycle from REI, (I know right, I'm supposed to be cheap!) Well the thing cost me a little over $1.00 after my 20% discount and the dividend from what the XYL must have spent some money on the previous year or two. I mounted this to the bike and not having any commercial antenna to put on it I built one with what I had laying around.

Playing the role of Macgyver I pieced together an antenna from a small block of scrap wood to be used as the center insulator, a scrap section of copper tubing, a length of scrap coax with an SMA connector from an old cell phone antenna, the left over BNC cap from the bitx 40 kit, a bolt, a washer, a nut, zip ties and finally a generous portion of hot glue.

I drilled a hole about the same diameter of the copper tube in the wood, almost but not all the way through. I drilled a second hole about halfway through the wood to fit the bolt I had. The head of the bolt would rest at this half way point. A smaller hole was drilled for the bolt to go through the wood and through the rear reflector attachment hole in the bike rack. The bottom side of the rack was sanded down to remove the paint to make electrical contact between the bare aluminum rack plate and the bolt.

Sanded aluminum area - AC9LF

The copper pipe was inserted into the wood and the center conductor of the RG-174 coax was soldered to it at the base protruding from the wood. The shield was clamped to the wood with the screw head and makes electrical contact with the frame through the nut at the bottom. The coax was wrapped around the bar on the rack about 10 times to make an attempt at a choke. A small piece of ferrite was clamped on at the feed point as well. The effectiveness is currently unknown. The coax was secured to the bike frame up to the handle bars with zip ties and the feed point covered in hot glue. The glue was built up a bit at the base of the antenna to help secure it structurally to the piece of wood. There is no other mechanism securing the pipe other then friction and glue.

HOT GLUE!!!! - AC9LF

Choke  - AC9LF

Tuning a copper pipe to resonance takes some time... I cut off 1/4" sections at a time with a pipe cutter. The starting length was about 19.25". The end length is around 17". I was able to get an SWR of less then 1.5:1 across the 2 meter band with some variation when I mount the bike. I also appear to get low SWR on a usable part of the 70cm band. The plastic cap is to help prevent possible impalement if I were to somehow fall on the antenna portion. Chances are, this wont happen. If it does it is more likely the antenna will bend or break off instead.

Finished junky antenna - AC9LF

I'm using an old camera bag secured to the handlebars with zipties to hold the HT.

I still have to take it for a test spin to see if it falls apart. I have some extra zip ties to secure it to the frame if it does fall off during cycling. Future improvements may be to add a connector to make it easier to remove when needed. I'm excited to give this a go.