Medium wave - back to the roots

Motivation:

My affinity to radio started when - already a teenager - I managed to listen to  "The voice of the GDR" from the former German Democratic Republic on medium wave one evening back in late 1989. I was using my new "ghetto blaster" which offered FM and AM (i.e. medium wave) reception.

In the following months I discovered radio stations from all over Europe.
Further, I went into the "business" of sending reception reports and receiving QSL cards (always by snail mail these days).

In early 1991 I could afford my first receiver with shortwave coverage, a Grundig Satellit 500.
I gradually discovered the broad spectrum of short wave, but never lost touch with broadcast listening on medium wave.

These days I am looking back in melancholy, as the last german broadcast station has shut-down service on medium wave by end of 2015.

My interest in medium wave became virulent again, when the frequency range from 472 .. 479 kHz was assigned in 2012
in Germany for ham radio use on a secondary basis.

Receiving:

Back in the early 90s I used an active indoor antenna for medium and short wave reception, which worked quite well.

Pretty fast I discovered that in today's environment I could only receive QRM that way.
There are a lot more "electronic devices" around in operation in a typical german household these days, compared to the pre-Internet era.

So far my optimum receive coverage could be achieved using

- a PA0RDT mini-whip receive antenna.
  There are several kits available in the Internet, even fully assembled antenna and power-feed units.
  But you may also set up the PCB yourself by milling a single-sided copper-plated board.

- a standalone SDR receiver solution, the so-called ARM Radio.
  It is based on an Micro-controller evaluation board with display and touch-screen.


So far it shows great performance at a low price (< 40 USD) and it is superior to e.g. my good old Alinco DX-77 TRX.

The image below shows the receive coverage (630 m band, WSPR) using the ARM Radio with the mini-whip.




A second plausibility check for your receive rig are none-directional beacons ( NDB ).
These beacons - used for aeronautical direction finding - transmit between 190 kHz and 535 kHz.
They may be used for propagation experiments as well.

Here is an overview of the NDBs that I can receive at my location (just a quick band scan):


Transmitting QRPp:

As a decent engineer, It took me months of brain-storming, simulation and several emails exchanged with Stefan, DK7FC.
Finally, I decided on setting up a small vertical loop.

The loop has a trapezoid shape with ca. 16 m circumference.
It is made up of 13 parallel braided copper wires of 0.5 mm each (so it is a loop with 1 winding).
Resonance is achieved using ca. 5 nF in parallel to the open loop.
With an additional 1:13 balun I could transform the loop impedance up to ca. 50 Ohm for coax feeding.

   

Using the Magnetic loop calculator by DG0KW, I calculated the loop "gain" as -38 dBi.

When I had the antenna carefully tuned to the WSPR TX frequency of 476 kHz, the next important question came up:
Which transmitter to use ?

OK, there is my proven Ultimate 2 with 200 mW output, so far used "barefoot" on 21 MHz and 28 MHz.

It does support 475 kHz operation as well, putting ca. 200 mW out.
But 200 mW into a -38 dBi antenna:  That is is real QRP (or better "QRPp")

With 200 mW (23 dBm) TX output and the above simulated antenna gain,
I have calculated an equivalent isotropical radiated power (EIRP) of ca. -20 dBm EIRP (= 10 W EIRP)

10 W of radiated power, that seemed to me too much QRPp, even using WSPR.
Honestly speaking, I did not expect too much WSPR spots with this setup.

This is the result after 6 weeks of operation using 10 W EIRP (200 mW into the 16 m loop) on 475 kHz:



Mileage highscore:

WSPR spotter my equivalent isotropically radiated power
distance
mileage in km per Watts EIRP
DH5RAE
10 W
439 km
43 900 000 km / W EIRP
DL-SWL
10 W 435 km
43 500 000 km / W EIRP
DJABR
10 W 423 km
42 300 000 km / W EIRP



Upgrade from QRPp to QRP:

In the next step I set up a 25 W amplifier and later on a 40 W amplifier.
The image below shows the first iteration step of the 40 W variant :



With that amplifier I have gained ca. 23 dB.
So the estimated output is now ca. 7 dBm EIRP (5 mW of radiated power):

The extra power is a real improvement:



Top ODX:

WSPR spotter my equivalent isotropically radiated power
distance
mileage in km per Watts EIRP
UD3SBG 5 mW 2 378 km 475 600 km / W EIRP
OH8HTG/B
5 mW
2 013 km
402 600 km / W EIRP
SM2DJK
3 mW
1 787 km
595 667 km / W EIRP

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