E2D

VHF/UHF digital, APRS, EME and meteor scatter

Choose the right weak-signal mode for meteors or the Moon, and read APRS frames and digipeater paths.

11 pool questions1 on the exam

Syllabus: Operating methods: digital modes and procedures for VHF and UHF; APRS; EME procedures; meteor scatter procedures

Pick the mode for the path: MSK144 meteors, Q65 Moon

Meteor scatter bounces signals off a short-lived ionized trail. MSK144 is designed for it. EME (Earth-Moon-Earth, moonbounce) bounces them off the Moon. Q65 is designed for it.

Pick the mode for the reflector: MSK144 for meteors, Q65 for the Moon.

RememberMeteor scatter = MSK144. EME = Q65.

JT65 digs a many-tone signal out of the noise

JT65 is a weak-signal mode: it decodes signals at very low signal-to-noise ratio. It uses multitone AFSK, one of many audio tones per symbol. Its sibling modes FT8 and FT4 swap the signal report for your grid square in a VHF contest exchange.

Many tones, very weak. The decoder finds the stair-step in the noise.

RememberJT65: very low SNR, multitone AFSK. VHF contest FT8/FT4: grid square replaces SNR.

EME contacts run on the clock, taking turns

The echo from the Moon returns about 2.5 seconds later and is faint. So stations do not chase echoes. They agree on time-synchronous transmissions that alternate between the two stations.

Long delay, weak echo: both stations follow a clock and take turns.

Moon distance384 thousand km

Round-trip delay2.56s

RememberEME = clock-locked, alternating transmit and receive periods.

APRS beacons are AX.25 UI frames

APRS (real-time tracking, such as balloons) uses the AX.25 packet protocol. A beacon is an Unnumbered Information (UI) frame: just sent, with no connection and no acknowledgment.

APRS = AX.25 UI frames: broadcast, no connect, no ACK.

RememberAPRS = AX.25, beacon = UI frame. Tracks balloons.

Digipeaters repeat APRS packets. WIDE3-1 = 1 hop left.

APRS stations relay data with packet digipeaters. In WIDE3-1, the first number is the hops requested (three). The second counts the hops remaining (one), going down by one at each digipeater.

WIDE3-1: WIDE3 asks for up to three hops. The 1 is how many are left.

Where the packet isafter digi 2

RememberWIDEn-N: n hops requested, N hops remaining.

Loading lesson…

Go deeper

Compendium articles on the ideas in this lesson, beyond what the exam asks.

Check what stuck

Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

Practise E2D →