[OZAPRS] FW: [Fwd: 70 cm APRS frequencies]

[matthew.cook at tekelek.com.au]

Bob Bruninga wrote on 11/21/2003 09:59:50 AM:

> On Wed, 19 Nov 2003, Richard Hoskin wrote:

> 1) APRS is a SINGLE frequency tactical system for end users.  There is
no
> need or desire (in general) for other than one freq.  Everyone wants and
> needs to be "where the local action is"...  (If everyone is not on the
> same channel, then everyone is not seeing the same tactical information)
> and you dont have a real-time tactical system...

Agreed.

> 2) UHF for single UI mobile packets is MUCH WORSE than VHF due to
> triple the mobile flutter rate and 9 dB worse path loss dipole to dipole
> and 7 dB worse modem detection...

Yes but at UHF you can remove the mobile flutter with diversity receive
systems, ie two receivers and two antenna's.   You cannot get the
antenna's
far enough apart at VHF to effectively use these techniques.

I've worked on Taxi Services that use 9600 baud PSK data on UHF
(490-510MHz) for their in car job queue's.   They rely on a high power
transmit site (100W) with diversity receive systems to overcome these
issues.

> 3) APRS needs  UHF backbones to tie the long haul packets between end
> users on local LANS operating on the single national frequency.  These
> cross links and backbones CANNOT all operate on a common UHF frequency
> (hence, there is no need for a "common APRS UHF frequency))...

We wish...   With the distances between our minor and major cities this is
impractical...   The internet is our only hope..

> 4) Mobiles (think Kenwooods and HAM-HUDS) have NO NEED for LOTS of
> incoming megabytes of positions from anywhere but their local area.
They
> cannnot display it and the driver cannot ABSORB IT.  THus there is NO
NEED
> for UHF or 9600 baud to imporve the SPAM rate to mobiles...

I wish people in VK5 would see the light :-(   I like the analogy to SPAM
:-)

> 5) CONVERSLY, those with full displays and PC's are usually fixed.  THey
> DO WANT TO SEE lots of stuff.  But they are FIXED and LOCAL and so they
> have no problem "remembering" their local UHF backbone or LINK.  Again,
> there is no need for a common (mutually interferring) high speed UHF
> backbone frequency.  For best integration, it is desired that all high
> speed UHF feeds use DIFFERENT channels to completely avoid the mutual
> interfere3ce and to give dedicated full time access.

Yep... this is also a good idea

> So my conclusion after looking at this for YEARS, is that there is just
no
> need for a single coordinated UHF APRS channel.  But there is a need for
> EVERY local APRS lan to coordinate a LOCAL UHF channel (different from
> their 4 neighboring LANS) to feed high speed data to the LAN...

Thanks Bob, you make some very good points in your $0.02 worth...

73's

Matt
VK5ZMC



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