[aprssig] APRS Success to FIXED stations and IGates

Bob Bruninga bruninga at usna.edu
Sun Sep 16 11:42:18 CDT 2007

Here is an important concept that people must also learn about the basics of the APRS FM channel:

APRS is optimized for best throughput for mobiles.  This means we try to make all digis use only one time slot for all outgoing copies of the packet.  If we had all digis hold off until all other copies were duplicated, then the channel would be 3 to 12 times more congested!

This means that collisions always occur if you are equidistant between any two digis.

But since mobiles usually move, there will not be any stagnant problems in the network that will keep one from being seen in the surrounding area on average.

> when I was running the WIDE1-1,WIDE2-1 
> I very seldom showed up on findu.com.  
> Many times I would hear myself digi'd...
> turns out it's always the KI4ELU-7 digi

If an IGate can hear two digis equally, then that IGate will ALWAYS hear a collision from those two digipeaters.  and will NEVER hear those packets.  This is why care should be taken in the antenna siting for all IGATES.  That is, all IGate antennas that can hear multiple digipeaters should be situated such that no two digis are heard with equal signals.  

Lower the antenna, move it to the other side of he house, move it 2 feet, bend it, do what ever is necessary to make sure that one digi is at least 10 dB stronger than all others.  If you do not do this, then the IGate is going to miss a lot of packets due to collisions.

Usually, there is enough variance (up to 60 dB between a near and a far digi) in the local area for a typical station so that it is rare for two digis to be heard with equal strength.  But if they are, the user must apply what he knows about basic VHF FM doubling to understand that this is not good in a contention based system like APRS.

IGate antenas usually do not need to be high and all-seeing.  Just use the simplest antenna that can hear the closest digi best.  This will make sure that that digi always wins on any collision.  ANd he IGate hears every packet.

Hope this helps.

Bob, Wb4APR

More information about the aprssig mailing list