from the KD2JRT-calling-CQ-on-146-point-52 dept.
As I've said in passing before here on SoylentNews, I'm a ham radio operator, (KD2JRT - Tech). Due to a lack of time and money, I've only been able to afford relatively cheap equipment, primarily two BaoFeng UV-82s, and an external antenna mount for the car. Many of the older ham radio ops decry the Baofengs as cheaply made Chinese junk, but I wanted to see what these radios are actually capable of. Historically, I've had decent success with an external antenna and decent positioning, but I recently conducted an impromptu experiment testing the propagation characteristics with these radios and seeing how well they actually work, and perhaps creating a baseline for more in-depth radio testing in the future.
Over the weekend, I took a day trip to the southern tip of New Jersey, out to Cape May from New York City, and along the way, using a hookup from the radio to my phone, I connected my radio to the national APRS (Automatic Packet Reporting System) network, and used it to measure distance and propagation effects. Since most of the readership of SoylentNews aren't radio techs, past the fold, I'll talk a bit about Baofeng radios, APRS, digital modes, and my results.
Automatic Packet Reporting System
APRS is one of the more interesting possibilities one can do with a ham radio system. At its most basic core, APRS is a store-and-forward mesh network that is traditionally used on the 2M VHF band, and interconnected by a series of digipeaters that can use the internet to tie the entire system together. Due to its architecture, an APRS mesh exists in any location where two radios are broadcasting on the same frequency in the same general area, and APRS itself has a long history of being using in disaster recovery efforts as a low bandwidth/high efficiency reporting system. APRS information is encoded in an AX.25 UI packet (AX.25 is a level-2 protocol used in ham radio).
KD2JRT-7>APDR13,WIDE1-1:=4041.55N/07359.99W$178/039/A=-00045 Out for a drive. Msg for QSO
Broken down, this becomes:
- Callsign: KD2JRT
- SSID: 7 (Handheld Radio)
- Client: APDR13 (APRSdroid)
- Relay Path: WIDE1-1 (I want to be relayed one hop)
- Packet Type: = (Positioning data without timestamp)
- GPS Location: 4041.55N/07359.99W (40 degrees north by 73 degrees west)
- Course and Speed: 178/039 - Travelling 178 degrees at 39 knots
- Altitude: A=-00045 I'm 45 feet below sea level
- Comment: Out for a drive. Msg for QSO
Quite a bit of information in a tiny packet to say the least. Obviously my phone's altitude sensor (which was the source of the GPS information) does have some issues since I'm fairly sure I didn't drive underwater at any point in New Jersey.
It's also possible to use APRS without positional information, and to transmit digital messages from terminal to terminal, which can be relayed over the internet allowing for world-wide sharing of information, as well as post digital bulletins. Gateways exist to allow APRS messages to be sent to email, SMS, and Twitter. Unlike typical repeater systems used for voice, APRS is a simplex protocol; all messages are sent and received on the same frequency and does not guarantee delivery of a given packet. Receipt of a packet is usually in the form of digipeating; that is, a client hears its own packet back as it passes through relays on the network. In the modern era, APRS packets are also forwarded onto the internet, and are recorded by services such as aprs.fi.
Due to its nature, APRS is a perfect way to test propagation effects of radio signals from various locations. As I'm driving, the phone's GPS automatically records my position, and it's broadcast over the air to digipeaters without any intervention from myself. Now that we're got the basics of APRS out of the way, let's talk about the radio.
BaoFeng UV-82 and Antenna
The UV-82 is a simple two band handheld radio, which out of the box is capable of transmitting FM signals on the 2m VHF band, and the 70cm UHF band at up to 5 watts of power. Despite the reputation, Baofeng radios have become popular with new ham radio operators for their cheapness. For this test, I was using the Baofeng with a Nagoya UT-72 antenna magnetically mounted to the roof of the car (which acted as a ground plane) as seen in this photo.
The feed line for the antenna is run in through the tailgate, and then back to the front where the Baofeng sits in a cupholder. A data cable connects the phone (a Samsung Galaxy S6 Active) running APRSdroid. Out of the box, the Baofeng doesn't support any digital modes, so a fair question to be asked is how am I doing APRS with it. The answer is that I'm essentially treating the radio as a simple acoustic coupler. APRS/AX.25 packets on VHF are encoded in Bell 202 audio frequency-shift keying at 1200 bits per second. The radio is connected to the phone's mic/speaker jack, APRSdroid listens for the modem tones, and simply modulates its replies the same way. The radio is set to operate in VOX (essentially speakerphone) mode, and tuned to 144.390 simplex. In the future, I may buy a simple TNC that can automate this process for me.
VHF 2M Signal Propagation (or What To Expect)
Before going into any test, it's a decent idea to outline what to expect. The 2-meter band is what's known as a very high frequency band. As such, radio signals sent from the surface of the planet aren't (normally) reflected by the atmosphere, and continue out into space. As such, successfully two-way communication can only be achieved via line-of-sight communications with a maximum range of approximately 75 miles/120 kilometers under absolutely ideal circumstances assuming a relatively high receiving tower. In practice, the antenna design and power drastically influences the maximum range to a much lower number.
Without going in-depth into radio propagation theory and boring everyone half to death, there are two basic types of antennas: directional and omnidirectional. As the names suggest, a directional antenna focuses RF emissions in a specific direction, allowing you to get more bang for your buck, at the cost of focusing the beam and cutting out everything else. With a directional antenna, it's generally possible to transmit and receive upwards of 50-60 miles line of sight on 2Ms through obstructions, and there are reports of people using yagi directional antennas and handheld radios to successfully communicate with OSCAR satellites and the International Space Station in low earth orbit.
Omnidirectional antennas, like the Nagoya UT-72 instead emit signals in all directions, drastically lowing the signal strength as the power is dissipated out in all directions. Handheld radios (known as HTs) are inherently low power (also known as QRP) radios. In practice, the general rule of thumb is to expect 20-25 miles (30-40 km) at best. Within the heart of New York City, with all its obstructions and the same antenna, I usually can get a signal to propagate about seven miles from Manhattan to the W2VL repeater from the waterfront. This is compounded by the fact that the UT-72 is a compromise antenna, and uses electrical lengthening techniques to allow it to properly emanate a 2M radio signal. If I had a proper dipole antenna rig mounted to the car, I could expect far better results.
One final consideration for my mobile setup is the fact that APRS (and its base AX.25 protocol) is a digital mode. Transmission and reception is an all-or-nothing game. Unlike voice FM contacts which can usually be made out despite static (QRN ), a digital signal must be heard clearly to be successfully decoded. In other words, I'm essentially only going to be registered on the network if the signal is extremely clear. This is very much a torture test for the little Baofeng and its whip antenna.
The Raw Data
Before we go deeper into methodology, let's look at the raw data as seen on aprs.fi - track. aprs.fi only retains tracks for seven days so I'll summarize the most interesting data below.
Here's the route I drove, starting on Roosevelt Island in New York City, and ended in Cape May. Red dots represent location reports from my phone.
Testing was only conducted North->South, due to forgetting to recharge the handheld's battery, and having it die on me upon reaching Cape May.
I was heard by the following stations.
|First Heard - EDT
|Longest (tx => rx)
|Longest At - EDT
|17.6 miles 283°
|2.8 miles 151°
|6.5 miles 357°
|5.4 miles 186°
|35.2 miles 350°
|23.1 miles 257°
|34.8 miles 282°
|11.1 miles 158°
|2.1 miles 104°
|20.8 miles 193°
|16.4 miles 198°
|16.7 miles 210°
|1.9 miles 286°
NOTE 1: W2AEE in Manhattan is showing more packets than it should as I can't filter by day on aprs.fi and I was testing the APRS setup last week. In total, approximately 50 packets were received by the APRS backbone.
NOTE 2: Despite being in a vehicle, the track shows a phone icon as I forgot to change the APRS reporting symbol
Due to complications with APRSdroid, I don't have an accurate count of how many packets were sent by the handset, but I suspect its approximately 50-100 for reasons that become clear below. Furthermore, I don't have an accurate count of which stations I heard due to stupidity. Now that we've got the raw data out of the way, let's draw some conclusions, and figure out how to refine the testing methodology.
All and all, for my first serious field test with the APRS system, this is a fairly healthy result. As I set out simply to play with APRS, and not actually do an experiment when I went out on Sunday, I didn't have in-depth methodology and logging set up. I also learned a few things about how APRSdroid (or more specifically, the SmartBeaconing system) works which drastically reduced the amount of traffic I sent which degraded the results even more. Despite that, I can draw some initial conclusions from this data, and perhaps do further radio propagation tests in the future if the SoylentNews community finds these types of articles interesting.
As expected, within urban areas, my signal propagation was total garbage, only being heard 1-2 miles under best case scenarios by the APRS gateways in Manhattan. Once I got south of the island and running along the waterfront, and on the Queens-Brooklyn Expressway did the signal start being received by the N2HM digipeater in New Jersey ~10-15 miles distant. Signal propagation continued to increase as I crossed the Verrazano Narrows into Staten Island, and then onward into New Jersey. As I stated before, APRS is a digital mode, and being understood is an all-or-nothing thing; if I were using FM voice, it's likely the effective Rx distance would have been further. Supporting that data, in the relatively flat areas of Jersey Coast, I was routinely being heard upwards of 15-20 miles out by the track due to lack of line-of-sight obstructions, and at two points, I was heard over 35 miles (56 km) away by two separate digipeaters!
Unfortunately, due to a lack of IGates south of Atlantic City, I wasn't heard again until I reached Cape May, and picked up by a digipeater on the other side of Delaware Bay
One thing that partially compromises the testing results however is the APRS SmartBeaconing system. As I put in the overview, APRS includes direction and speed information as part of its transmissions. As an optimization of the system, APRS clients can use SmartBeaconing to reduce the amount of traffic they're reporting. In effect, SmartBeaconing uses dead-reckoning to determine if it needs to send a packet. If I send a position report at a location travelling south at 60 MPH, and two minutes later, I'm two miles south of the previous report, SmartBeaconing will not send a new position since I am where I can realistically expected to be. Since I had cruise control enabled, and the Garden State Parkway runs roughly in a straight line North-South past Sandy Hook, SmartBeaconing drastically reduced the amount of data I was sending. Annoying, APRSdroid still shows phantom reports it didn't send on the log of my station history; and several times, I'd see a report show up in the outbound log, but no red light on the radio saying it was sending.
For receiving characteristics, I checked the phone at each rest area I stopped at, and I was seeing a healthy list of stations show up in the tracker. However, because of the store-and-forward nature of APRS, it's difficult to tell at a glance what stations I was hearing directly, and which were being echoed by a digipeater. It's possible to determine this information by decoding the received APRS packet as repeating stations amend their call sign to the WIDE1-1 line as the packet moves through the network. I also found that I frequently didn't hear myself on the network. I believe this is due to the fact the Baofeng has a relatively slow switch from transmit to receive mode. As such, the radio was cutting itself off and not hearing its own packets come back.
For future experiments, I need to modify APRSdroid to operate as a beacon station and regularly send positional reports on a given interval, as well as log all transmitted and sent APRS packets to files so I can post-process them into a report. I also need to write a method of gaining data from the APRS-IS backbone; many APRS stations are receive only and as such I won't see my own packets if I'm heard by such a station, and possibly use the APRS messaging functionality to confirm two way operation from the network at regular intervals.
Despite the difficulties in testing, I'm pleasantly surprised by the performance of the UV-82, at least when it's using an external antenna. For the most part, I was getting results consistent with QRP operations, and was being heard at distances that my non-expert mind believe are consistent with HT/2M operation. When time permits, I'd be interested in repeating this test in the future, as well testing other types of radio. If any other SoylentNews readers are ham radio operators, and have equipment they'd like to see tested, feel free to get in contact with me via email or IRC. If people find this article interesting, I may run additional ones on radio operations. I'm hoping before the year is out to get everything together to be able to do satellite operations, and make two-way contact with the International Space Station, or one of the two OSCAR satellites currently in operation. If I can get my hands on the equipment, I may even be tempted to try for Earth-Moon-Earth communications.
I'll also be upgrading my license class to General in the next few weeks and pending money, I can also try propagation experiments in HF bands.
Until next time, 73 de KD2JRT