APRS & the right antenna for the job

I’ve been running several variations of an APRS I-Gate for the past 6 months. I’ve focused my efforts on learning two different software platforms available: APRSISCE32 & Direwolf. The latter being...

aprsigate

I’ve been running several variations of an APRS I-Gate for the past 6 months. I’ve focused my efforts on learning two different software platforms available: APRSISCE32 & Direwolf. The latter being suited for a Linux operating system (specifically NOOBS on a Raspberry Pi 3). I’ve used a combination of different radios/scanners/antennas/computers, trying to figure out what worked best. Basically I wanted to “hear” as many packets as I could & upload them to the net. My goal was to reduce APRS network congestion in the area. My experience with these two different software’s is that APRSISCE32 is a much more ‘full featured’ client compared to Direwolf, although you can use Xastir with Direwolf, but I’ve never tried as my experience with Linux is just starting. APRSISCE32 has a great user interface if you are looking for something that is a bit more interactive while your using it. Direwolf is just a small stand alone application you can run on say like a RaspPi & forget about it. I’m not sure what the decoding differences are between the two, I’ve seen statements around the net that certain soundcard TNC programs decode better than others. I do know that with Direwolf, I seem to have a bit more packets uploaded, but then again that could’ve just meant that at the time I was using Direwolf, there was high activity in my area. I’ve only ever tried AGWPE as the soundcard TNC, so I can’t attest to any of the others, if you have experience with other packet engines and they seem to work better, please let me know!

I finally hit 3,000 “positions packets heard directly” at which time, APRS.fi supposedly calculates your ‘receiver range estimate,’ although I’m not too sure how accurate it is. (although I’m not complaining about the 93 mile estimate!). I think APRS has a lot of potential that hasn’t been tapped yet, it allows a real-time user interface with constant updates, messaging to others hams, GPS position reports, it feeds the VHF tropo maps, all sorts of cool stuff!

I started to notice that as I’ve been upgrading to better VHF/UHF antennas, it seems as if my APRS packet reception has gone down. (When I say ‘better’ I basically mean more gain). I’ve been comparing my packet reception to some other hams in the area whom I’m familiar with and their setup, and my station performance was lacking for some reason. Was it a setting in the software? Was my volume set to high? Was I receiving APRS collisions over the air because of my exposure to multiple digipeaters? Or could I be using the wrong kind of antenna for the job? I started to think that maybe having a collinear antenna that has a compressed signal might not be the best antenna for the job. I actually live just about at the apex of an 800ft hill in South Austin, I have a pretty good overview of the city from my roof. Austin is kind of like a bowl, the city is surrounded by hills. The hill country starts due West of me near Dripping Springs, which is probably why I don’t receive APRS packets from out West – check out my packet received map for July 2016:

aprsjuly2016

When I first started out on VHF/UHF, if you’ll remember or look back to my older posts, you’ll note that I started with a J-Pole, then went to an X-30, GP-3, even tried an Arrow 4 element beam (but I really wanted an Omni-antenna for APRS). Now I’m all the way to a Tram 1480 collinear antenna which has 2 5/8 wave 2m elements in phase which in effect ‘flattens’ the signal going out on the horizon. After some extensive googling time on the net, I finally found this little statement that had me concerned about the antenna I was using for APRS:

“One thing to consider, in order to get greater gain in an omni-directional vertical antenna, the pattern is compressed in elevation……If the antenna will be on the top of a hill overlooking a valley a high gain omni-directional antenna *may* have less gain down into a valley than say a lower gain omni, due to the elevation pattern, depending on the height of the hill.”

Now granted I’m definitely not on the biggest hill in Central Texas by any means, but the majority of my APRS reception should be from about 0 miles – to about 50 miles I would say. Sometimes further when ducting occurs (as seen on http://aprs.mountainlake.k12.mn.us/). So who is to say that if I switched to a less gain antenna that I’d actually pickup more packets? Maybe I should make a ground plane antenna devoted to 144.390 rather than using a dual band antenna….. Some options to consider. Other hams have even suggested attempting to move my antenna to a different location, or even mounting it on the ground, but I also use this antenna for FM communication, so up on the roof is the best compromise.

I’m going to try a few different things, a few different antennas, to see if I can improve my packet reception any. Not that I’m dissatisfied with my current setup, but I’m starting to understand the physics of antennas a bit more 🙂 There’s a bit more to it than simply just picking the biggest antenna you can afford/install, although generally that rule probably works for most people, lol! Really the question when thinking about what antenna to install should be “What do I want to do with this?” I mean I could build a 30 element 2m beam if I wanted to pickup APRS packets from Africa, but that’s not exactly my goal 🙂 . I’ll post an update after I’ve had plenty of time to evaluate a ‘lower’ gain antenna. Yet another burning experiment in the back of my mind!

73

K5ACL

 

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