[aprssig] Re: KPC-3 8.2 settings

Stephen H. Smith wa8lmf2 at aol.com
Sat Oct 22 18:11:45 CDT 2005

n7net at yahoo.com wrote:

>What is the rule of the thumb for power on APRS?  I'm
>running 5-watts into a plus 3 and I'm not doing good
>at all.  I've never found myself on findu.  So I
>assume I'm flogging a dead horse here.
>We used to run 1-watt on packet.  Has that changed?
>72 de Scott

Not sure what you mean by a "plus 3".  Are you referring to a KPC3+ TNC 
or to a "3dB gain" antenna?    Assuming the problem isn't incorrect APRS 
digipeater path setting for your area  ( see
   http://webs.lanset.com/wa8lmf/DigiPaths    for a  discussion of the 
paths issue.),  consider the following :

1)   Traditional packet was overwhelmingly operated between fixed 
stations with reasonably decent antennnas.    Typical APRS usage is from 
moving vehicles with poorer antennas that are lower to the ground.   
Further, the constantly shifting fluttering signal levels from mobiles, 
frequently plagued with multipath and phase distortion, greatly 
increases the data bit-error rate compared to fixed-to-fixed operation.   

2)   APRS runs in the non-connected non-acking UI mode, so the automatic 
retries that occured in conventional connected packet to recover from 
errors caused by collisions, mobile fades, etc never happen.   (This is 
the price you pay for the one-to-many "broadcast" nature of APRS vs the 
one-on-one connected-to-a- known-station nature of conventional packet.)

3)    Where are you located?   Reaching findu doesn't just mean being 
heard by another station.  It requires reaching, either directly or via 
digipeater hops, one of the minority of fixed stations that is gating 
off-air RF packets into the Internet  APRS server system.    If you 
don't reach one of these "igate" stations, you will never be seen on 
findu.   There are huge areas on the great plains,  intermountain west 
and other rural parts of the country where there are simply NO igate 
stations within range. 

4)   5 watts into what kind of antenna?     Bear in mind that a 
digipeater in a high location is hearing a lot more, from a much larger 
area,  than you are on the ground.  If the digi is overlooking a densely 
populated area with lots of activity, it is probably  being captured 
almost constantly by full-power mobiles running 25-50W.     In this 
case,  your 5 watts, especially into a poor antenna,  may not  stand a 
chance unless you happen to be much closer to the digipeater than most 
of the other stations on channel. 

to be received PERFECTLY before ANYTHING will be recovered from it.   
Marginal scratchy not-fully-quieted signals,  of the type so many users 
inflict on voice repeaters with underpowered handhelds running  lousy 
antennas,   simply won't work on packet.   You must be able to produce a 
solid hard-quieted signal at the distant receiver.  Roughly speaking, a 
given antenna installation and transmitter power will produce about 1/2 
to 1/3 the RELIABLE range on APRS packet that it produces on FM voice. ]]]

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com

"APRS 101"  Explanation of APRS Path Selection & Digipeating

Updated APRS Symbol Chart

New/Updated "Rev G" APRS     http://webs.lanset.com/wa8lmf/aprs
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