[aprssig] Appalachian Trail Golden Packet Digipeater

Dave Baxter dave at uk-ar.co.uk
Wed Apr 8 08:49:37 EDT 2009

Hi Bob...

You're going to need a bigger battery!

Those capacity ratings are only for a very low current draw.  (C/20 from
memory, like 375mA)  The more current you pull, the lower the available
capacity is, and it falls quite rapidly too as the load current rises.

If you had a good regulating DC-DC converter in line, you could keep the
radio happy, while sucking the battery dry, but again, that's more to
carry, and more inefficiency too.

I'd have thought doing the AT trial on an alternative (quiet) frequency
with low power, say 5W and "good" antenna's (wire or co-ax co-linear's
hooked up a tree, or under a kite etc) would work well.  A D7 with the
affore mentioned jelly cell will do about 5W out, and as you say, is a
proven bit of kit.   Plus, it all packs away in a small space.   I'd
even opt for a 17AH battery, but that's just my thinking.

The solar panel (or wind genny?) as a charging backup would be a good
idea, but again, to make it work reliably, a "Tiny" 1AH battery isn't
going to cut it, as with small capacity batteries and "alternative"
energy sources, you do need a charge regulator or you'll cook the
battery.  Jelly's don't like being overcharged at all, almost as much as
they hate being over discharged.. 
Of course, if you can drive to the site, none of this is a problem!


Dave G0WBX.

> -----Original Message-----
> From: Robert Bruninga [mailto:bruninga at usna.edu] 
> Sent: 08 April 2009 03:19
> To: 'TAPR APRS Mailing List'
> Subject: Re: [aprssig] Appalachian Trail Golden Packet Digipeater
> The battery required to run a D700 as the digipeater for the 
> AT trail event for 5 hours has been estimated and a photo 
> about 60% down the page:
> www.aprs.org/at-golden-packet.html
> The D700 draws about .6 amps with mild audio and with a 20% 
> TX duty cycle on the TX, about 4.5 amps or less at medium 
> power (10W).  This adds up to about 1.5 amps average power.  
> SO for the hike-up mountain sites, you will need to either 
> carry a 7.5 AmpHour battery or just a tiny 1 amp-hour or so 
> battery and a
> 1.5 amp solar panel (or any mix inbetween).
> For this first attempt we only want to run D700's since their 
> TNC/Radio interface is a fixed, optimized known quality. What 
> we want to avoid is the 10-to-20 dB loss in weak-signal 
> performance seen on many existing APRS systems that were 
> plugged-n-played without any adjustment of tones, levels, 
> skew, deviation or quality.  A D7 or D700 will decode another 
> D7 or D700 with only a single LCD "S" meter segment lit.  But 
> sometimes you can see nearly  FULL scale packets not decode 
> because they are horribly distorted and skewed.  We don't 
> want any of those unknowns on this first golden packet event.
> With the radio and battery and a lightweight fiberglass 
> push-up mast and antenna, the complete digipeater system can 
> weigh under
> 15 lbs.  And fortunately, so far, of the 15 stations, only 
> two have to be hiked in, and only one other one has to be 
> carried 66 feet vertically up a ladder.
> Enjoy the hike.  But remember, this is a good demo of how 
> easily an emergency APRS digipeater can be fielded for events 
> or emergencies.
> Bob, WB4APR

More information about the aprssig mailing list