E2D: VHF/UHF Digital and EME

E2D covers digital operating methods for VHF and UHF, including specialized weak-signal modes for meteor scatter and Earth-Moon-Earth (EME) communication, APRS packet data infrastructure, and VHF contesting practices.

These modes push the limits of what radio communication can accomplish — MSK144 extracts contacts from millisecond meteor trail ionization, Q65 enables contacts via reflected moonlight, and APRS provides real-time position and data networking using packet radio.

MSK144: Meteor Scatter

MSK144 (Minimum Shift Keying, 144 symbols/second) is a digital mode specifically designed for meteor scatter communications. Meteor scatter uses the brief ionization trails left by meteors as they ablate in the upper atmosphere to reflect VHF signals between stations separated by hundreds to over a thousand miles. The usable reflection window from a single meteor can last only a few milliseconds to a few seconds.

MSK144 encodes a complete message in a very short burst — fast enough to fit an entire contact exchange within one or more meteor trail reflections. WSPR, Hellschreiber, and APRS are not designed for meteor scatter operation.

Q65: EME

Q65 is a digital mode designed for Earth-Moon-Earth (EME) communication. EME uses the Moon as a passive reflector to establish contacts over distances up to approximately 12,000 miles — roughly half the Earth's circumference. Q65 is optimized for the path characteristics of EME: very long delay (about 2.5 seconds round trip), high path loss (around 250–260 dB), and libration fading caused by the Moon's irregular motion.

MSK144 is for meteor scatter, PACTOR III is an HF data mode, and WSPR is a propagation beacon mode — none are designed for EME. Q65 is the correct answer.

JT65: Low SNR Decoding

JT65 is a digital mode characterized by its ability to decode signals with a very low signal-to-noise ratio — well below what the human ear can detect, and below the threshold of most other digital modes. JT65 achieves this through a combination of forward error correction coding and multitone frequency shift keying.

JT65 does not use only a 65 Hz bandwidth (the 65 refers to the number of tones, not the bandwidth), its symbol rate is not 65 baud, and it does not permit fast-scan TV transmissions. Its defining characteristic for the exam is the very low SNR decoding capability.

The modulation type used by JT65 is multitone AFSK (Audio Frequency Shift Keying). JT65 encodes data as one of 65 possible audio tones and transmits them using audio FSK — not PSK, not RTTY, and not QAM.

EME Contact Method

EME contacts are established using time-synchronous transmissions that alternate between the two stations. Each station transmits during defined time intervals (typically 60 seconds each for JT65, shorter for Q65) while the other station listens. Computer clocks at both ends are synchronized to GPS or internet time sources so that both stations switch between transmit and receive at precisely the same moments.

Storing and forwarding digital messages is an APRS/satellite technique, not an EME method. Monitoring beacon reflections from the moon is not how contacts are made — the signal is too weak and too variable for human monitoring. High-speed CW identification is not used — the Doppler shift and fading make CW identification impractical for EME.

FT8 and FT4 on VHF

When FT8 or FT4 modes are used in a VHF contest, grid squares replace the signal-to-noise ratio report in the exchange. A normal FT8 QSO includes an SNR report, but VHF contest operation substitutes the operator's four-character Maidenhead grid square locator (e.g., EM72) instead. Grid squares are used to calculate distance for scoring purposes in VHF contests and also to determine whether a contact qualifies for distance-based awards.

APRS Overview

APRS (Automatic Packet Reporting System) is a packet radio system used for real-time tracking and digital messaging. It is commonly used to track balloons carrying amateur radio transmitters — the balloon's GPS position is encoded in APRS packets and relayed through the APRS network, allowing real-time tracking on maps. APRS is not FT8, not LORAN-based, and not PACTOR.

APRS Protocol: AX.25

APRS uses the AX.25 protocol as its underlying data link layer. AX.25 is an amateur radio adaptation of the X.25 packet switching protocol, modified to use amateur call signs as station addresses. AX.25 defines the frame structure, addressing, and basic link control used by APRS and other amateur packet radio systems. PACTOR, QAM, and AMTOR are not the protocol used by APRS.

APRS Packet Frame Type

APRS beacon data is transmitted using an Unnumbered Information (UI) frame. Within AX.25, UI frames are connectionless — they are transmitted without establishing a link to a specific receiving station, which makes them ideal for broadcast-style beacon transmissions that any station can receive. Acknowledgement frames require a connected session; Burst and Connect are not standard AX.25 frame type names for this purpose.

How APRS Relays Data

APRS stations relay data by packet digipeaters. A digipeater (digital repeater) receives an APRS packet and retransmits it, extending the range of the original transmission. APRS networks rely on a series of digipeaters to carry position reports and messages across wide geographic areas. C4FM and DMR repeaters are voice repeater technologies, not APRS data relay systems. ACK/NAK relay is a connected-mode packet technique, not the mechanism APRS uses for its unconnected broadcasts.

WIDE Path Designators

The APRS packet path WIDE3-1 designates that three digipeater hops are requested with one remaining. The path system works as follows: WIDE3-3 means three hops are requested and three remain; each digipeater that relays the packet decrements the second number by one. WIDE3-1 means the packet was originally set for three hops and has already been relayed twice, with one hop remaining before the path is exhausted.

E2D Practice Questions