Projects :: Mountain Bike Mobile

Radio and bikes have never easily worked together. Cycling apparel and cycle add-ons lack any kind of loop points to attach a hand-held radio. If you can stuff your radio in a pocket, chances are you cannot access it whilst moving, and the aerial will be at the wrong polarisation; and probably poking you in the nose! This was the problem I was facing, and this is my solution.

I decided to investigate the External link  Klick Fast system as used by the UK emergency services. The first challenge was to adapt their Motorola (TETRA) radio mounts to something that could be used with Amateur Radio transceivers. If only they would all standardise...

The Yaesu FT2D uses two sturdy stainless screws to attach its belt-clip (to the battery), and that allowed me to drill a couple of small holes and securely attach the Klick Fast radio mount to the battery. A separate experiment with a Baofeng GT3 required the construction of a small aluminium converter plate (not shown here). Here is a photo of the slightly modified Klick Fast Motorola TETRA mount attached to my FT2D:

Klickfast FT2D

I also added a screw-together jacket mount through part of the pocket on my flossy cycling jacket to hold the radio:

Klickfast Jacket

This helps hold the radio in a suitable position for those times of the year where you need a jacket (most months in the UK!). It also has the effect of making people look twice to ensure you are not the Police! The downsides with this solution are: (i) the weight pulling down on the left side of the jacket; (ii) you cannot see the display and do not know which band people called via unless they state they are on VHF or UHF; (iii) the jacket mount is covered when wearing a ruck-sack for shopping. The solution to (iii) was to fit the webbing-style Klick Fast mount to the straps of the ruck-sack and attach the radio to the strap (not pictured). The solution to (ii) may involve a cricket bat!

What about those times of year when it is nice enough to ride in a T-shirt? The solution to that was to install a handlebar mount to the bike. This picture details a handlebar extender to lift the radio and front light away from the busy and angled handlebars (the RG316 will be explained later):

Klickfast Handlebars

We can now view the screen to see which band people called via, and/or run APRS to beacon our location. Except for one major obvious problem: the rubber-duck aerial is now at 40 degrees from the horizontal and picks-up little. A new solution was needed for the aerial. The well-known tat-bazaar came to the rescue: 135 degree SMA male to female adapters from China. A proof-of-concept of moving the aerial and adding a replacement ground plane was required. This was duly created by stripping down some 2.5 millimetre-squared twin-and-earth mains wiring. Three pieces of the thicker wire were spun straight with a drill (and vice) before being bent at one end and soldered to the body of the SMA adapter. A couple more SMA adapters were needed to space the ground-plane wires away from the radio:

Ground-plane PoC

In an earlier test of this system, the light was in the middle with the radio to the right. The right-most wire annoyingly clanged off the top of the handlebars. Future tests of a new ground-plane adapter (detailed below) required the re-arrange to avoid catching on my gloves as I moved my hands.

Ground-plane PoC

With the aerial now vertical and operating over a ground-plane, this proof-of-concept test worked quite well - to a point. I found the APRS beacons needed some help from the QTH station (Yaesu FTM-400XDE plus TinyTrak4 TNC with Xastir) acting as digipeater. There are local digipeaters at Barkway Ridge, Ampthill, and Maddingley; but none of them can hear the APRS packets from the hand-held when cycling around Biggleswade/Southill/Shefford/Henlow/Clifton. At the time of testing, I was using a Comet SMA3 aerial prior to it malfunctioning with an open-circuit. Another issue was the need to duck-under low-flying trees and the proximity of the aerial to one's eyeballs/nose. I asked Aunt Sally for my thinking-head so I could up with a better solution; and make a better ground-plane adapter...

What if ... ?

What if I fitted an SMA bulkhead connector to the retro-fit rear mudguard and installed the aerial behind me? In creating a brand new ground-plane adapter system, I had sourced a couple of female-female SMA bulkhead connectors. The spare was duly installed in the plastic mudguard with four brass M2.5 bolts (thanks to Chris G3XIZ for having a stash of random M2.5 nuts, bolts, and washers).

Mudguard SMA

After sourcing suitable RF spares, a coaxial lead was made with RG316 and two right-angled SMA connectors. The lead was long enough to reach from the rear to the front of the bike, with a little extra for the steering. The image below shows the mudguard fitted with one SMA and one weather-proof tie-wrap. Two ties were fitted by carefully cutting-out rectangular holes in the plastic with a suitable knife. This was not the easiest thing to photograph!

Mudguard coax

Here is a close-up of the underside with the right-angled SMA attached prior to weather sealing. I must again thank Chris G3XIZ for a few brushes of Liquid Tape to seal the connectors. Whilst I try not to ride in the rain, UK weather can catch you out!

Mudguard coax

Here is the mudguard fitted back to the bike with the RG316 run along the underside of the frame. Weatherproof tie-wraps secure the coax to the brake cable.

Mudguard coax2

A loose weatherproof tie-wrap helps hold the RG316 as it follows the brake/gear cables. Those with solid hydraulic brake lines could tie the RG316 more securely.

Coax cable

The spare RG316 is loosely looped around the handlebar extender to allow movement when steering. Here it is attached to the radio:

FT2D handlebars

Am I being horrid making you wait until the end to see the aerial system? Here is it:

Mountain Bike Mobile aerial
Click on the image to load a larger version (in a new window).

The ground-plane adapter is made from a female-female SMA bulkhead adapter and four 250 millimetre lengths of 2 millimetre thick brass rod. Whilst not exactly quarter-wave (at 2 metres), they are the easiest lengths to source. The rods were placed in a vice by approximately 4 millimetres and bent over to create a stable hook-end for soldering. A spare SMA-male adapter was fitted to the bulkhead prior to gently clamping the assembly in a vice. The brass rod was held in place with a "helping-hands" whilst they were soldered with a 45 Watts soldering iron and proper Lead/Tin solder. You may need to file the underside after soldering to ensure everything is level. Those with a proper metal bending machine may be able to bend the brass rod closer to the end. The outer-ends of the brass rod needed bulking-up in order to facilitate the installation of the stylish Lego parts. Compression covers for brake/gear cables (pictured below) were just the right size. They were cut-down and soldered on, then filed flat to ensure a snug fit with the Lego prior to applying a suitable glue. Florescent polyethylene beads might make suitable end protectors if you can source four and not 100! Be aware you may need to drill larger holes and beads made from resin will crack and break. I tried resin beads first, then switched to Lego!

An SMA male-male coupler is required to connect everything up. The aerial is a Diamond SRH771.

Cable ends

An 8 mm spanner is carried to facilitate fitting/removal in case of leaving the bike unattended for a length of time.


Testing of this set-up with APRS proved the fill-in digipeater is still required. Whilst digipeated packets from the main repeaters were received with ease, they still could not hear packets from the bike. Testing of simplex VHF/UHF and working through the local VHF/UHF repeaters is still pending. The low position of the aerial compared to a vehicle mounted aerial, and the meatbag in front of it, all compromise its performance. A larger aerial may help, however, no-one appear to manufacturer a longer aerial with an SMA mounting.

Future improvements: movement of the suspension seat-post allows the saddle bag to push the mudguard down slightly; and the lumps and bumps of the roads can send the mudguard flapping up and down. A couple of straps made from stainless-steel that run from the mudguard to the spare block-brake mounts may be required to cure that. This may also allow a heavier aerial to be used.

Page updated: 4th October 2019

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