Motor Cycle Antenna for APRS on 2m

After having APRS on 144.575 Mhz,  the  VHF allocation for the mode in New Zealand, for some time in my car,(ZL1BTB-10)  I decided it would be nice to try it on my motorcycle, ( ZL1BTB-9) so I looked around to see what I have available , I  used a tiny track 3plus APRS  modem  from Byonics  , a junk sale purchased GPS  unit  with  rs232 @  4800 baud  .12 v DC , and a FDC  Chinese  brand  small vhf  hand held radio that covered the amateur radio frequencies we use here  in New Zealand , The radio had a switch able  RF output of 0.5 and 5 watts  . The radio  had its own battery pack but I opted to use a  small  switched mode 12v battery eliminator designed to clip onto the back of the radio in place of the battery as i have had the radio go flat on long rides ,, The battery eliminator had a male cigarette  plug on it so I connected a female socket directly  to the 12 v battery of the motorcycle  ;
    I decided to use a quarter wave ground plane on the rear right hand side of the bike and fed with RG58  50 ohm coax terminated in a BNC cable at the user  end , The system was assembled , tried  and  did work to a limited extent  ,I could be seen riding around the city with quite good accuracy and providing a nice track on the APRS.Fi .com  website .  However I did  have an intermittent problem with the Tiny Track 3 locking up after a while .  At  first I thought it was RF getting into the modem causing lock up etc  I tried rf bypassing , ferrite chokes , positioning of the coax , modem ,radio and  rider , but to no avail. The small 3 terminal voltage regulator  was coming to hot to touch and switching  the TT3 off . It was eventually traced to the voltage supply on my 1973 Norton commando battery charging supply being too higher voltage , most old British motorcycles have a very simple voltage regulating system ,the rectified  DC from the motorcycle alternator is clamped at 15 v by a big grunty 50W Zener diode to prevent the battery getting  over charged  ???  this high voltage was proving too much and the tiny 3 terminal regulator  in the tiny track 3 was going haywire . I ended up by running the modem and gps  off a small 12v gel cell and have never had a problem since  ,its a little bit of a nuisance but I can live with it as it works well.

I was never happy with the RF  performance of the quarter wave set up and 5 W . I felt it should perform better than it did .Now I know a motorcycle frame is not exactly a substantial ground even at 2 m  and with the rider bolt upright ahead of the antenna , shielding 180 degrees of radiation ( assuming it was omni directional )  it just did not perform as I would have liked .  The next was to try a 5/8 wavelength vertical mounted in the same position . So one was made up and I even spent much time tuning and matching using my Agilent  5062A  VNA  . Although much  better than the quarter wave I felt it still should  perform better than it did . (Given my previous experience with 5/8 whip antennas) .  It just shows to show you despite excellent rf matching to an antenna , if other parameters are not taken into account  ( ground plane etc ) it can still operate like a piece of wet string ....

                                       

After some thought on the matter, I figured the motorcycle frame was insufficient to allow the ground plane dependant antennas to operate efficiently, so I did some  internet research on ground plane independent antennas, This  harked back to the old standard  vertical dipole . That is what I needed. Centre feeding it was  put in the too hard basket ,so it looked like endfeeding was the way to go.  All I had to do was match the 50 ohm  unbalanced feed to the balanced dipole antenna high impedance end , Now this can be done several ways ,i.e  tapped tuned resonant circuits  feeding one end  of the high impedance point . easily done but can become a little bit of a mechanical nightmare , hiding away tuned circuits and capacitors from the elements  and holding the 38 inch stainless tapered whip by the base as well  and then  attaching the whole lot to the frame .   I figured the easiest and probably most robust system to build was an end fed Zepp for 2m  , Now the ubiquitous  " J pole " of UK  fame is based on the configuration . Non radiating quarter wave stub as an impedance  match , feeding  the high impedance end of a resonant dipole , so I looked up the usual rf  equations for antenna construction and made up a  half wave on 144.575 MHz,   fed with a quarter wave stub , all made of stainless steel and brazed onto the screw on antenna base to mount it to the motorcycle frame. The usual 19.5 "tuned lines   and 39" whip  brazed together . when it was finished it looked exactly like a J pole but I didn't fold over the dipole radiator as bandwidth was not an issue but wind resistance was ... !

 

I soldered  the coax feed to the appropriate points up from  the base of the  stainless steel antenna  using  commercial  Duzal  acid based (Zinc chloride ) soldering flux ' and adjusted thus to achieve a good low swr  match  as shown on the Agilent 5062A , (overkill I know , but since I have it .its just as easy to use it,)  I went  for a ride on the  bike to try the performance of the new antenna.  Things went great , until I came to the first corner when apon rounding, the bike engine cut out briefly , restarted only to let  out a  loud backfire , It wasnt until quite a few corners later did I realise it was RF getting into the non sophisticated electronic ignition ( Boyer  Brand) . Every time I went around a corner the tiny track would send a data position to the radio and the resultant rf  packet  killed the ignition briefly .  Then of course it came back on when the RF  stopped , muffler full of unburned gasses ...boom !! . Id never had this on the bike before, even when running HF  mobile  10W  ;  RF getting into the wiring  !,      I repositioned the  coax ,still did it!  ,  Changed radio position on the tank , High power? ,  it  still did it .Then  switched to  low power and it minimized it  . So i stopped and thought ,   RF on the coax ??, it has to be ! so I looked at the antenna , its a tuned ( balanced line feeding a balanced antenna ) system ,  therefore it should have  a balun to keep RF  flowing down the  outside of the coax and radiating in to the bike wiring ,  now if anyone tells you a J pole does not  require a balun,  I will challenge that statement from first  principles .

                                                                                                                                                    

 

A simple 1:1 current balun was constructed  using a small red t series amidon core of 25 mm O.D  and with  5 turns of rg174 ( teflon) coax tightly close wound and held with a cable tie.  This coax was  connected to the matching points on the base of the antenna . Off  for a ride , and no backfires!!  I went for my usual ride out to the local lakes and back and noticed immediately a much better  APRS.fi.com  track on the computer , with "hits"  on just about all the corners of the route . This was not happening with the previous  quarter wave  or 5/8 wave antenna , 

When you mount your  j pole . never  electrically connect it to the  frame or  pipe supporting  it  . In fact  if you can ...have some sort of insulating medium separating it up  some inches  from the metal mount . If you connect the base of the j  pole to the metal mounting pipe,  the induced RF currents  from the end of the parallel matching section will induce Rf  into the  metal pole and your  radiation  pattern will go skew wiff and deviate  from the hopefully somewhat doughnut pattern of a vertical dipole and you will loose antenna  gain ..  .. 


 I have been using it for quite some time now and with much better results  all round .  The 19" tuned line puts the base of the 2m dipole  up high above the bike and less of it is shielded by the rider , the rider being somewhat ahead of the whip  shields it from a lot of wind and it tends to stay upright  than bend back ( thats why I didnt opt for J pole configuration the folded whip is higher wind resistance and will bend back from the vertical.

 ,The  simple 2 m dipole antenna is probably the most gain antenna your going to get on a motorcycle , so i don't think I can really improve on its performance.... its working well so ill leave it and keep it simple                  . try it  you'll like it !!

                                                                                    click to return to HOME PAGE