Manned Experimental Propagation Transmitters

A technical description of MEPT Equipment operated by ZL1BPU

The MEPT concept is very useful in determining the extent and nature of radio propagation. Not only does regular, long term operation of low powered transmitters provide a means of studying propagation, it also provides dependable signals for stations experimenting with reception and antennas. These transmissions are very low power - just sufficent to allow detection and measurement with the most sensitive 'waterfall' type computer software.

Since in most countries Amateur operation of Beacons (unmanned continuously operating transmitting stations) is not permitted, these propagation experiments must use manually controlled equipment. Operation is frequently announced (and reception reported) on the 'QRSS Knights' mail group.

The ZL1BPU MEPT transmitters operate using the special purpose callsign ZL1EE. The transmission location is Waiuku, Franklin, 50km south of central Auckland city, at 37 14.54' South 174 44.66' East [RF72is].

HF MEPT

The HF MEPT is multi-band capable (160 - 10m), but is usually to be found on 30m, where it is received world-wide. It is a bit more sophisticated than many MEPTs, being based on a commercial SSB exciter, the JRC NMA-73D, and a commercial amplifier, the companion JRC CAR-45B. (These are components from a 10kW airport transmitter!) This means that the signal source for the MEPT is audio, which is generated using a wavetable (.WAV file). The amplifier is very linear and will handle messages with multiple concurrent tones with no visible IMD products.

Although the NMA-73D is frequency agile, the synthesizer steps are rather coarse, 100Hz, so to operate in a tight slot of the band, the frequency is adjusted by using .WAV files with carefully chosen audio frequencies!

The JRC Power Amplifier is a recent upgrade (August 2014). This MEPT operates 24 hours/day and (apart from when I'm away) has done so for about five years.

This MEPT operates as a test bed for newly developed MEPT techniques. It is currently running a very small embedded controller (recycled Linksys NSLU2 NAS) running Linux. This can be configured and controlled via the local network (and potentially the internet), and currently runs a cron script which contains both WSPR and QRSS3 messages (.MP3 files) stored on a memory stick, and played out via a sound dongle. Work is underway to add a web interface for easy upgrade and control. Other controllers are planned.

Red letters in the table identify items in the rack photo to the right of the table. Click on this photo for a much enlarged view.

HF MEPT Rack - Click on image for larger view
Frequency range: 1.6 to 30 MHz. 100Hz steps, OCXO reference.
A Control Unit: Home-brew remote-control unit connected by parallel
cable to the Exciter. It provides frequency control, mode,
power level and manual PTT, and is the entry point for
serial link PTT control and audio from the computer via
a line transformer.
B Exciter:
Japan Radio Corp NMA-73D SSB exciter.
100Hz spaced channels, PLL synthesizer.
All synthesis from single reference, 1MHz OCXO.
Operates in USB mode with audio tone input.
* Photo of JRC NMA-73D Exciter
* Interior view of JRC NMA-73D Exciter
C Power Amp:
JRC CAR-45B highly linear high voltage solid-state
amplifier capable of 100W output.Typically running
1W output as MEPT. This recycled box also contains
the 20V and 50V power supplies for the amplifier,
control relays and metering. The cross-needle meter
is genuine JRC.
* Photo of JRC CAR-45B amplifier
* Photo of partially completed amp rack unit
* Close-up of SWR/Power Meter (10W & 100W ranges)
D Signal Source:
(1)Toshiba 300CDS laptop, Windows 98, AC operated.
ZL2AFP VMEPT software with WAV file source.
The software acts as timer and WAV file player, and
also controls PTT.
(2) Recycled Linksys NSLU2 NAS operating OpenWRT
Linux. Uses a cron job and beacon script to transmit two
MP3 files every 10 minutes. The first file is a WSPR
message, the second QRSS3. The system is remote
configurable (via network) and uses SNTP for accurate
message timing. Uses audio VOX PTT. TX power level
independently set in script for WSPR and QRSS modes.
Message: Two mode, QRSS3, 4 min message, 10 min period.
'ZL1EE' in sequential MT-Hell, 1s dots; five diagonal
stripes in 6dB steps; 'ZL1EE' in CASTLE mode,
5s dots (see below). Option (2) adds WSPR message.
Antenna Tuner: Homebrew L Network, 1.8 - 30MHz.
Uses roller inductor and 100mA RF ammeter.
Antenna: 6m vertical, homebrew all-metal whip at 2m AGL,
with several ground-level radials.
* Photo of antenna
Power Supply: AC operated about 60VA total (never mind efficiency!).
Powered from 250VA UPS with 40AH battery.


A grabber view of the signal received on 30m by VK2DDI

Spare HF MEPT

For short-term experiments, another MEPT is used. This uses an FEI FE-5650A Rubidium Synthesizer, and can operate from 160 metres to 20 metres. It is especially useful for standard frequency and calibration transmissions, since the carrier frequency is known and traceable to 1 ppb (1 e109).
  • The MEPT uses a Codan 7727 HF transceiver converted into a linear amplifier, and is capable of 20 Watts continuous operation.
  • For operation up to 150 MHz, an alternative DDS synthesizer (Novatech 409B) can be used. This has a TCXO reference.
    A 100W amplifier is available for 2 metre use.

The synthesizers are controlled either by DOS software written by ZL1BPU or a newer Windows version, depending on the computer used. This MEPT operates on request, and is frequently used for frequency measuring practice, since the frequency resolution is 10 mHz and the source is traceable to NIST via GPSDO.

MF MEPT

The MF MEPT is a single-band (630m) 10W transmitter, of deceptive simplicity, but flexible capability. It consists of a computer controlled synthesizer driving a keyed CMOS intermediate amplifier (100mW), which can be used stand-alone, up to 50MHz, but for this purpose drives a 10W single-band PA. The Synthesizer is USB powered and controlled.

The transmitter usually sends visual QRSS3, but can also send WSPR or dual-mode (as in the example below). This MEPT operates on request.

Frequency range: 450 - 470 kHz. 1Hz steps, synthesizer crystal controlled.
Synthesizer:
QRP 2000 synthesizer designed by DG8SAQ (Si570 chip)
USB powered. Synthesizer has a divide by 32 stage for LF/MF,
and is thus capable of operating from 110kHz to 320MHz.
Intermed Amp:
CMOS 74HC240 capable of 100mW. Based on a design by
N7KSB. The buffer is powered at 5V, operates from 110 kHz to
54 MHz, and is keyed by the synthesizer.
Power Amp:
A pair of IRF510 FETs in Class D push-pull, at 12V, driven by
CMOS gates. RF sensing used to remove power when drive is lost.
10W output.(Switching circuit not shown in photo below).
* Interior view of 10W amplifier
Signal Source:
Asus EEEPC netbook, Windows 7, AC operated.DG8SAQ
USB control software with script file containing pattern.
Message: Two mode, QRSS3, 3 minute message, 10 minute period.
WSPR message, followed by arrow and 'ZL1EE' in sequential
MT-Hell (see below).
Antenna: Marconi vertical, 7m high, base loaded from a metal tuner hut.
Three heavy up wires and three contiguous 30m top wires.
Eight underground radials up to 30m long.
Antenna efficiency about 1%. Best DX 3500km!
Power Supply: AC or 12V operated.


MF 10W transmitter (left), intermediate amp (centre) and DG8SAQ synthesizer (right)


A grabber view of the signal received on 630m by VK2DAG
Note, this is a dual mode (WSPR and QRSS3) message.


Copyright M. Greenman 1997-2014. All rights reserved. Contact the author before using any of this material.