[aprssig] Re: motorcycle mobile tracker

[Ray McKnight]

All of this is good.
The point of the original post was that the
antenna was flexing under wind strain and
potentially not as effective.  TRUE.

The fact that its mounted on a motorcycle isn't important,
except that on a M/C you are likely fighting many other
factors that conspire against your otherwise good signal.
Bad/poor ground, low height, etc etc.  If the antenna
flexes, it does become less efficient.  The problem is that
most antennas DO flex.  We're talking about the loss imposed
by the disparity between transmit and receive polarity in signals.
The receive antenna is mounted one way (usually vertically) and
the transmiter has the problem of a flexing antenna which can
ultimately cause up to 20db of loss due to cross-polarization
differences.

It happens on cars, motorcycles, airplanes, etc.  
What some have tried, to varying success, is use a guy
wire to limit or prevent flexing.  Seen frequently on larger
HF antennas, a piece of fishing line is used in FRONT to
prevent the antenna from flexing from wind pressure.  It
should be strong enough to resist the wind pressure/antenna
loading at top speed but be thin enough to break if the antenna
hits something solid, preventing loss of the antenna or damage to
the vehicle/antenna mount.  It should also be easily removed
for parking, etc.  Just on a M/C you might not look too "cool"
with a guy wire on your antenna, your mileage may vary...

----- Original Message ----- 
From: "Stephen H. Smith" <wa8lmf2 at aol.com>
To: "TAPR APRS Mailing List" <aprssig at lists.tapr.org>
Sent: Saturday, May 20, 2006 23:30
Subject: [aprssig] Re: motorcycle mobile tracker


> apratt at bestbits.org wrote:
> > I have been trying to set up a tracker on my motorcycle. I have a 25W
> > Yaesu radio and a 1/2 wave antenna. The range is great, but it only
> > works when I'm moving under 55 miles an hour. The antenna's not stiff
> > enough - it flexes and my outgoing signal drops. I'm in the market for
> > a new antenna or some way to stabilize the one I've got.
> >
> >   
> 
> 1)    Bear in mind that ALL of a data packet MUST be received ABSOLUTELY 
> PERFECTLY for ANY  data to be recovered.   A pop of noise or a momentary 
> fade or flutter that is negligible on voice will fatal to a data 
> packet.  You MUST have a noise-free solid hard-quieted signal at the 
> receiver for reliable data operation.   APRS is even worse than standard 
> packet in this respect, since it doesn't use any kind of ACK/NAK 
> handshaking protocol to retransmit missing or defective packets.   
> 
> 
> 2)   You need enough signal margin so that even with 10-15 dBs of mobile 
> flutter (typical variation for a mobile), the signal will remain fully 
> quieted.     [ The instruction manual for the Kenwood D700 essentially 
> confirms this fact by stating you can't expect reliable data operation 
> until signals deflect the S-meter bar full-scale. ]
> 
> 
> 3)   As a result of these facts, roughly speaking, for a given transmit 
> power and antenna, you can expect about *ONE-HALF* to *ONE-THIRD* the 
> reliable range on packet data that you get on voice. 
> 
> 
> 4)   The transmit power of the pocket tracker is about the same as an 
> unlicensed FRS walkie-talkie. I.E the Pocket Track can be expected to 
> have about ONE-THIRD the range of an unlicensed FRS radio on voice!!
> 
> 
> 5)    Further, the 1/4-watt Pocket Tracker simply can't compete with 
> normal handhelds, let alone full-sized mobiles. It's tiny signal doesn't 
> stand a chance on a busy channel where even handhelds are transmitting 
> with *10-20* times the power and many mobiles are using *200 TIMES* the 
> power (50W) of a PT !!!!  Bear in mind that even through the channel may 
> sound empty to you on the ground, a digipeater in a high location is 
> constantly hearing many many signals you don't hear.  Your puny signal 
> has to cut through all this stuff to be heard and repeated.
> 
> 
> 
> 
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