[aprssig] Mobile Power Output

Stephen H. Smith wa8lmf2 at aol.com
Sun Feb 4 23:56:07 EST 2018

On 2/4/2018 11:05 PM, Michael Barnes wrote:
> I was mentioning to a local ham I was interested in putting APRS in my truck. 
> (I'm still digesting all the great comments from my previous query.) He said 
> I will need a minimum of 50 watts and the frequent transmissions will pretty 
> much eliminate the possibility of other 2 meter operations while I have the 
> APRS running. This news is certainly disconcerting. My original intention was 
> to use a Raspberry Pi with a TNC-Pi connected to a Kenwood HT with an 
> external antenna. Having to add another full sized mobile radio and running 
> that much power with all the other radios in my truck might be an issue.
> So, is the need for 50 watts realistic?


> I do live in a rural area, but other 2 meter use seems to get by quite well 
> with handheld radios.

The difference is that......

_ ALL of a packet MUST be received PERFECTLY before ANYTHING can be recovered 
from it._

Standard AX.25 packet has NO forward error correction that can fix corrupted 
packets at the receiving end.   You must deliver a "full-smash" 
completely-quieted signal to the far end for successful decodes.    The kind 
of  hissy, popping, fluttering, not-fully-quieted signal that so many users of 
underpowered hand-helds inflict on voice repeaters just won't work on packet data.

You MUST have enough excess signal level that even with random 15-20 dB drops 
in signal level (typical mobile flutter) the signal will remain in full quieting.

Roughly speaking, given the same transmit power and antenna gain, the reliable 
range on packet will be about one-half to one-third what you would get on voice 
under the same conditions.  Consider the range you would expect with a 
hand-held on simplex voice. Now cut that by a half to two-thirds.

[This assumption is based on typical mid-western flat-lands conditions where 
digipeaters are seldom more than 50-150 feet above ground level; i.e.  a 
horizon-grazing non-line-of-sight path between the mobile and the digipeater.   
In the real west, where digipeaters are often on mountain tops *THOUSANDS *of 
feet above the users, it is not uncommon for low-power mobiles to be copied 
50-75 miles away. ]

As far as desensing the voice radio:
1) Disable smart beaconing with it's incessant rapid-fire transmissions and 
beacon only once every three minutes or so.
2) Use the highly-compressed "Mic-E" data format for your transmissions. The 
Mic-E coding can burst out a complete position report in less than half a 
second.  The effect in the other radio is just a fractional-second hiccup in 
reception that is quite easy to live with.   (Or for total isolation, try to do 
your voice operation on UHF as much as possible.)
3) Unobtrusive short-burst Mic-E operation puts a premium on radios that can 
keyup and settle on frequency really fast. And then unkey back to receive just 
as fast.  Most hand-helds are really poor at fast-turn-around RX-toTX and 
TX-to-RX operation.

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

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