MF Operation at DF0WD
by Wolfgang Buescher, DL4YHF
last updated: January 2013.
During initial reception tests on the UK's former 500 kHz band,
it became obvious that the 'station ground' (connected to mains ground)
carried a lot of noise, most likely from switching mode power supplies,
compact fluorescent lamps, and similar. The receiver made a buzzing sound,
and weak amateur radio signals were completely masked.
An important feature of this LC parallel configuration is its ability to act
as a transformer. The antenna, and its counterpoise ("Earth") are completely
isolated from the noisy "mains" ground.
Primary coils (60 turns) and secondary coil (2..3 turns, with red plastic coated wire)
are separated by about 1 cm. For simplicity, foam plastic from an iso-mattress (sic!) was used.
The original 'receive-only' setup consisted of a short wire, which was connected to the topmost tap of the primary coil (largest impedance). To connect longer wires without degrading the 'Q' (which helps to keep the strong MF broadcast signals away from the receiver), different taps were made on the primary coil. They will later be used to find the best impedance match for a longer transmit antenna.
With the above receive-setup, crossband contacts with various stations from the UK and Ireland on the 'old' band around 500 kHz were made.
The loading coil shown above was turned into a variometer (to make it suitable for transmission with a low power level), and a few homebrew accessories were added:
In the foreground, from left to right:
The variometer consists of the old loading coil wound on a plastic tube with 15 cm OD,
and a smaller inner tube with 11 cm diameter. The wire for the inner (rotatable) coil
is old 'Tensolite' wire (most likely Teflon covered, silver plated Litz wire).
The loading coil compensates the capacitive reactive part of the antenna impedance (-j * 1 kOhm here),
by connecting +j * 1 kOhm in series. The result is purely resistive, and consists of the ground loss,
environmental loss (trees), coil loss, and a tiny bit of radiation resistance (in this case, way below 1 Ohm).
The output tap on the 'impedance transformer' (integrated in the SWR meter housing) was set for 26 Ohms
to achieve the lowest possible SWR, thus with a 30 watt transmitter it should be possible to push 1 Ampere
into the antenna wire. It was.
The transmitter used for the initial tests was built in a rush (before discovering that the old IC706 can be used as an exciter for 472 kHz):
By accident, it was found that the author's old IC706 would transmit around 472 kHz.
It's not sure which of the modifications in this radio's former life did the trick, but quite certain
it is just a matter of configuration (these "modifications" used to be on mods.dk or other sites).
Anyway, the external MF "power" amplifier will remove harmonics, so this less-than-ideal waveform is not a problem.
10 Vrms on 50 Ohms is approximately 2 Watts of RF, but voltage and 50 Ohm impedance are too large to drive the power MOSFET's gates directly.
The complementary push/pull driver was left unchanged. Note that the 22 nF capacitors parallel to the MOSFET gates
are not a typo: Together with the rather low inductivity of the step-down transformer (trifillar wound ferrite toroid),
they form a resonant tank (with low Q) near 475 kHz.
The output power could be increased with more secondary turns on the output transformer, if a larger ferrite toroid was used
(in the author's prototype, an FT114A-61 was used because nothing else was found in the junk box). Through the secondary
taps, the output power can be selected without sacrificing the PA's efficiency. When tested with 14 turns secondary,
the PA delivered 30 watts RF, and consumed 3 amperes DC input current. This was at the amplifier's clipping point,
i.e. more input drive didn't significantly increase the output power anymore. For CW, this is acceptable, and SSB isn't
an option on MF (even though it would be technically possible with this amplifier, when "moderately driven").
With a 'linear' power amplifier (in addition to a 'linear' exciter, such at the IC-706 in this case)
all kinds of soundcard-based modes are possible, including those with a non-constant RF envelope.
Sidenote: Similar effects can be seen on shortwave, when operators overdrive their transmitters
in PSK31, trying to squeeze 100 watts average power out of a 100-watt radio ;o)
With the IC706 (HF transceiver), using 30 .. 40 W from the amplifier shown above, and 1.2 A antenna current, reception was reported through the
WSPR network from a number of receivers in western europe (DL, F, I, PA, ON, G, GM, EI).
Even during daylight conditions, the MF signal with an estimated ERP of 500 mW (which was later found to be much less (*) ) was copied
at GM4SLV in Shetland, over a 1000-kilometer distance.
In winter time, the ionospheric absorption drops faster, and 'DX' signals appear from the east before sunset. With a moderate ERP, and place not plagued by local QRM, contacts were made in relatively slow CW (but not QRSS) with Malta (9H) and Romania (YO).
In January 2013, radio amateurs in other European countries got access to the 630 meter band, like the UK (requires an NoV) and the Netherlands. This will hopefully boost activity beyond 2013-01-01, when the first stations in PA, and one hour later G, GW, GM, and EI got 'on air' just a few minutes after their local midnight.
When it became obvious that the ERP was 'much lower than expected', and some stations had difficulties
to copy DF0WD on MF, a new final amplifier was built. To use the existing '13.8 V, 20 A' power supply, a more efficient design
was required. The choice was that, instead of operating the MOSFETs in the 'linear' region (which resulted in a poor efficiency),
the RF power output stage should run in switching mode, and produce a maximum power of approx. 150 Watts;
ideally from a DC supply of not more than 15 A (at 13.8 V) to have some margin for the IC-706 (exciter) from the
station's '20 Ampere' power supply. With a DC input power of approx. 200 W, and an RF output of 150 W,
the design goal was "at least 75 %".
DM4TR MF Grabber: 472-479 kHz live spectrogram by Thomas, DM4TR, in JO61DE
DK7FC MF Grabber: by Stefan, DK7FC, in JN49IK
Grabbers in Birmingham: A long-lasting service for LF and MF by Dave, G3YXM
TF3HZ live spectrograms (covers a part of the 630 meter band)
The Shetland Grabber: 472-479 kHz live spectrogram by John, GM4SLV (temporarily offline?)
WSPR Spot Database for MF and LF ("old" interface but imo easier to use)
The RSGB LF Group (at Yahoo), also used by MF operators
GW3UEP's site devoted to 500 kHz CW with a variety of homebrew CW transmitters
Operating Portable on Medium Wave by Finbar, EI0CF - Amateur Radio at it's best !
G4WGT Multi-Grabber page: now includes the new 630 meter band.
Temporary 500 kHz (or 475 kHz) grabber by Rik, ON7YD
WebSDR at the Universtiy of Twente; with gapless coverage from 0 to 29 MHz .