Matt, N3PAY - HAM radio hobbyist

How does VHF propagation get measured?

Thanks to Jim KX0U for showing me vhf.dxview.org. It's a live map that colors the country by how far VHF signals are getting out right now — greens and yellows where things are normal, oranges and reds where the band is open. That's a great picture, but it left me wondering: where does that data actually come from? Nobody is out there with a signal generator sweeping the whole country every hour.

The trick: use the traffic that's already there

The VHF map doesn't run its own beacons. Instead it listens to a network that's already broadcasting all day long — APRS, the Automatic Packet Reporting System.

APRS stations sit on 144.390 MHz (here in North America) and send short packets: position beacons, weather data, messages, and digipeater IDs. Thousands of them are transmitting around the clock. Every one of those packets is a tiny, timestamped, geolocated propagation test that somebody already paid for.

How a packet becomes a data point

Here's the chain:

  1. A station transmits a packet. It contains the sender's callsign and, usually, its position (or a known fixed location).
  2. Another station hears it and gates it to the internet. These are I-gates (Internet gateways). When an I-gate copies a packet off the air, it forwards it to APRS-IS, the internet backbone of the APRS network, and stamps it with its own callsign and location.
  3. Now you have two endpoints. The packet knows where it came from (the sender) and where it was received (the I-gate). Both have known coordinates.

That's the whole measurement. A packet that traveled from station A to I-gate B, received in the last hour, means 2 meters supported at least that A-to-B distance at that moment. Most hops are short and local. But when the band opens — tropospheric ducting, a temperature inversion, sporadic-E — I-gates start hearing stations from hundreds of miles away, and those long paths light up.

From dots to a footprint

The map takes all those received paths from the past hour and smooths them into a color-coded footprint. If the longest paths being copied in your area are 50 miles, you're green. If I-gates near you are suddenly pulling in packets from 300 miles out, that region goes orange to red. It's not predicting propagation from solar numbers or a model — it's showing you what actually got through, reverse-engineered from ordinary APRS traffic.

Why this is clever

The same idea powers a lot of ham propagation tools, but with different source data:

In each case the insight is the same — you don't need a dedicated beacon network if you're willing to treat the everyday traffic as your beacons. On VHF, APRS packets are perfect for it: short, frequent, geolocated, and already flooding the internet through I-gates.

So the next time your position beacon gets gated by a station three states away, you weren't just showing off — you were a data point telling everyone the band was open.

Sources