[aprssig] setting up digipeaters & I-gates

Stephen H. Smith wa8lmf2 at aol.com
Tue Jul 20 16:54:10 CDT 2010

Jaye Inabnit ke6sls wrote:
> I am most interested 
> in finding low power I-gate hardware that I can install either here at my QTH 
> or at our low level repeater site.

1)     Don't confuse digipeaters (RF-->RF relay function) with igates 
(RF<-->Internet connections).    Digipeaters are located at places with 
good radio coverage; i.e. ideally on hilltops or tall buildings.   Igate 
setups are located where you have always-on Internet access.   

2)    Unfortunately, you will NOT typically have Internet access at 
remote hill top repeater sites.    

3)    Igates are normally located at home or office locations where you 
DO have always-on Internet access.  As long as you can hear the 
digipeater(s) decently at these home or office locations with Internet 
access , the elevation/antenna height of the station doing igate duty is 
not critical.  

4)    An igate setup will consist of a radio, TNC, and a computer with 
an Internet connection running a suitable program. It could be a 
dedicated igate-only program, --or-- a  general-purpose APRS client such 
as UIview, APRSplus or the Linux-based Xastir.   These  three programs 
can all do triple duty as an end-user client producing  map displays and 
messaging, act as an igate, and act as a  digipeater  at the same time.  
Unless you live in the dream high-rent district on the hilltop 
overlooking town, the digi will probably be configured as a low-level 
WIDE-1-1 "fill-in" digi (if you enable it at all).

You do not need bleeding-edge multi-core gamers machines with HD 
displays to run APRS software.  Even old Pentium II or III "clunkers" 
will do the job decently running Win98, Win2K or a Linux distro.  This 
is an ideal use for elderly computers. For minimal hardware and lowest 
power consumption, consider older laptops. For unattended igate 
operation, you can live with small low-resolution screens that would be 
considered unusable for most of today's software.   A virtue of older 
machines is that, unlike any of today's current models,  they usually 
have REAL SERIAL PORTS without the hassle of USB<-->serial adapter 
"dongles".   [Serial ports are essential for connecting TNCs, or the 
Kenwood radios with built-in TNCs, to computers.]

5)     Note that more digipeaters can contribute to on-air congestion 
and actually REDUCE the channel capacity,   if there is already adequate 
digi coverage in the area.   This is because, unlike voice repeaters, 
APRS digipeating is not a simultaneous process.   A station makes a 
transmission, the digipeater hears it and stores it in memory briefly, 
and then retransmits it AFTER the original station's transmission. 

Even a single digipeat DOUBLES the air time each user occupies, cutting 
the channel throughput (number of packets per minute) in half.   An 
added digipeater can potentially cause each transmission to occupy a 
THIRD time slot, cutting the channel capacity to ONE-THIRD of the 
no-digipeaters scenario (assuming that the original transmission and 
both digipeats are heard over the same area; i.e. not just at a distant 

6)    Igates normally DO NOT reduce over-the-air channel capacity.  The 
bandwidth of the Internet is essentially unlimited, compared to 
over-the-air operation at 1200 baud.   In general, the more igates the 
better.  The sooner (in terms of hops) you get packets off the air and 
into the Internet, the less on-air congestion and chances for RF 

"APRS 101"  Explanation of APRS Path Selection & Digipeating


Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

NEW!    *** HF APRS over PSK63 ***

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