For a little bit I’ve been struggling with poor performance on my bicycle mobile station.  It was an intermittent fault.  Sometimes it’d work great, other days the FT-290R II would complain bitterly about a SWR issue, and receive performance would be abysmal.  But then I’d set off anyway, get a block away, and the problems just disappeared.  Or the thing would be working perfect, and I’d get down the road and it’d stop working.

Damn frustrating.  Intermittent faults such as these are the worst kind to try and locate.  I thought of all kinds of possibilities, but the one thing I hadn’t considered was the antenna.

Performance had been pretty patchy ever since the weekend before LCA.  It was on the Saturday that somewhere between Annerley and Milton, I lost the ¼ wavelength stainless-steel whip that I had been using.  So I spent that evening rigging up a SO-239 socket so that I could use the commercial antenna I had; a Nagoya NL-77BH that I bought at BARCfest in 2008.

I rigged that up, and on the Monday I did successfully make a contact from the bicycle on my way to LCA, but it was patchy.  I did find a few glitches, so fixed those, and Friday I made a contact in the afternoon, but it was still pretty hit-and-miss.  Not the consistent behaviour I got out of my ¼ wave at all.  Okay, maybe the coax is damaged.  Tried different leads, no dice.  Recently I bought a front basket for the bicycle, and so I could put the FT-290RII in there.  Ran coax back to the antenna, last Wednesday afternoon and Thursday morning it worked beautiful.  However Monday it gave me no end of grief.

Suspecting that the weight of the radio pressing down on the BNC terminations may have damaged that section of coax, I grabbed a length of RG195 and terminated it with BNC connectors.  Still no good.  Using the SWR meter in the FT-897D, the impedance match was out by miles.

Today I had another look.  I took the antenna off the bicycle and placed it on a mag-mount antenna base, and placed the base in the centre of an open garage door.  So big ground plane, not much different to most cars.  Checked SWR, still through the roof.  Tuned to the Mt. Cotton repeater on the FT-897D, no signal.  Pulled out a hand-held, perfectly clear 5/8 signal on its original rubber-ducky antenna.  As I was unplugging the antenna base, I watched the signal strength suddenly shoot up and the radio crackle to life when the shield was disconnected (leaving just the centre pin).  I had noticed a dead short before, but thought it was the antenna mount on the bike… something was up.

So, I grabbed a bit of solid copper wire, a PL-259 plug, and some offcut insulation.  I made a new ¼ wavelength antenna, cutting it initially at 60cm.  Swapped it for the NL-77BH and the performance was beautiful.  Check SWR, and yes, it’s high, but then again, 60cm is waay too long.  I estimated about 51cm and folded the wire over at that point, twisting the excess around the body of the antenna.  Signal strength immediately went up two S points, and on checking SWR, it was significantly reduced.  I moved it back to the bicycle where I tweaked it further.

Once happy, I cut off the excess, used pliers to fold the end sharply and soldered the folded end to the body of the antenna to prevent it hooking anything.  Then used some heat-shrink tubing to finish it off so there were no sharp ends to poke eyes out with.  The antenna provides a good match from 144 right through until 148 MHz at 30W using FM.

I haven’t tried a contact on the bike yet, nor have I got any pics to share, but the radios seem happy with it, and it appears to be hitting repeaters in the area once again, including Ipswich.  Given it’s a good 30km between The Gap and Marburg (as the crow flies) with some decent hills to boot, that’s not bad going.

It would appear the additional complexity of these high-gain commercial antennas comes at a significant cost, they don’t like getting shaken to bits on the back of a bicycle.  I’m not sure how repairable the commercial antenna I have is, it may be a case of throw the thing out, at which case I think any love affair I had with commercial mobile whips might be over.  At least my ¼ wave antennas can be made for <$20 in about 10 minutes from parts I can buy in town, versus spending >$50 and having to wait for it to arrive in the post.