Today I have experimented with an
indoor transmit loop for 136 kHz.
First I attempted to measure the
efficiency of a small test loop, 6 turns and 66 cm diameter. It was
made from a 12.1 meter piece of 729-strand Decca litz wire (same that
Stefan used for his famous loading coil). Resonating with 23 nF
styroflex gave a Q of 400, ie. 0.12 ohm series
resistance. Theoretical radiation resistance (above ground) would be
about 12 nanoohms, predicting an efficiency of about -70 dB. Then I
tuned to 138.83 kHz and measured -61.0 dBm available receive power from
DCF39. Assuming a fieldstrength of 3 mV/m here, an idealized lossless
loop or Marconi should deliver +8.2 dBm. Thus efficiency of the test
loop was -69.2 dB, in good agreement to theory. This shows that there
is little attenuation of magnetic fields in the walls and the roof,
despite the thin aluminium vapor barrier above me.
To improve efficiency and power
capability, I then dismantled the little loop and reused the litz wire
for a large single-turn loop. This is about 3.5 m wide wall-to wall,
2.5 m high floor to ceiling, oriented east-west for best signal towards
Heidelberg. The loop was resonated with 83.3nF, made of 10 pieces 33nF
/ 2kV FKP-1 capacitors, in 2s-5p configuration. At 137 kHz,
each capacitor is thermally limited to about 6 A rms (30 A total). As expected, both the inductance and the Q
factor were lower (Q=72, 0.19 ohm loss). Presumably the extra 70
milliohms were contributed by absorption in steel reinforcement in the
floor and walls. The radiation
resistance of an 8.8 sqm loop should be about 0.23 microohms, thus
predicting 1.2 ppm (-59 dB) efficiency.
Before
the on air tests, I had calibrated
the DK7FC grabber receiver (180 km) using my normal LF Marconi, which
has a known -30 dB efficiency. A 136.17 kHz carrier from a signal
generator (13 dBm, 20 µW radiated power) produced a level of about -89
dB in Stefan's Eu window. During the morning hours, noise was a low
-105 dB in Heidelberg.
The big loop was then matched to 50
ohms using 16:1 turns on a ferrite ring (AL 4 uH). Fine tuning to
136.17 kHz was achieved by 0.44 uF in series to one of the cap
branches, and by slightly deforming the loop geometry. Then I carefully
applied power, expecting fireworks at any moment, or all sorts of
unwanted EMC effects due to the magnetic nearfield. To my surprise,
none of this happened. Going up to full 200 watts silently produced 32
A in the loop, with no adverse effects on electonics or radios in the
vicinity. Only the small ferrite
transformer became hot after a short while. Recalculating core
magnetisation (6V rms, 0.5 cm^2) gave 0.2 tesla peak which is way too
much. This was easily mitigated by going to 32 turns primary and 2
turns secondary.
For the on air-tests, I had
inserted 100 kohm into the Marconi uplead to eliminate possible
coupling and reradiation. Despite the higher afternoon QRN, signals
from the loop were immediately received in Heidelberg, at -78 dB and 16
dB SNR. Thus radiated power was about 0.25 mW, and efficiency -59
dB, just as predicted. To double check, I temporarily reconnected the
Marconi and transmitted a dash with full power. Due to slightly lower
load impedance (40 ohms) the TX now produced 250 W (ie. 0.25 W EMRP).
As expected, the signal in Heidelberg indeed went up by 30 dB.
During the following hour, SNR was actually good enough to marginally read
a slow Hell transmission from the indoor antenna. More pics are at
Well, so what is this good for?
Transmit during a thunderstorm? Maybe there's not much one can do with
this kind of power, but the experiment was certainly instructive, and
good fun.
Best 73,
Markus (DF6NM)
