The 501-504kHz band became available to UK
amateurs in March 2007 by special research permit only. Operators are now
limited to an E.R.P. of 10dBW or 10Watts. With aerial efficiencies of the order
of 2% a 500W transmitter would be needed.
In 2013 is is hoped that we will get the new
472-479kHz band, as agreed at WRC2012, with 5W EIRP.
If you don't have a Notice of Variation (NOV)
then you will be unable to use 500kHz until the new band comes in, as no more
NOVs are to be issued.
Any modern transceiver should receive on
500k. A narrow CW filter and bullet-proof front end are useful (we are right
next to the MW broadcast band here!).
A power mosfet should make a good PA,
consider a linear design so that you can use modes like Wolf and PSK. I have
managed to modify my Icom IC735 to act as a drive source, though it isn't very
stable. Jim has designed a mosfet TX and Roger
GW3UEP has several versions of his simple
QTX transmitter for 500 or
472kHz at up to 100W.
When it comes to aerials, anything that works
well on topband should do the trick but the aerial must be well insulated to
maintain a high Q. Have a look at
page for lots of useful info. It's was written for 136kHz but the
principles hold good for 500.
The earth could just be the water-pipes plus a
few radials or as many as you can muster, it does make a
difference. You will need to build an ATU. Coil efficiency is not all that
important as the power levels are low and you can easily compensate for coil
losses by turning up the TX a little. Usually you will need about 100-300uH.
The circuit of my tuner is shown here, you will notice there is no tuning
capacitor. The system is tuned with a variable inductor, or variometer. There
are two reasons for this, one is that you need to get your ATU outside so that
you don't lower the Q by running the aerial wire into the house, and
high-voltage capacitors don't like getting damp. The other is that the voltage,
even with a few Watts, will be quite high on the - electrically short - aerial
and you would need a very good capacitor.
The variometer consists of a small coil of
about 10 turns, which can rotate inside the main coil. As it rotates it either
adds to, or subtracts from, the inductance of the main coil. In my tuner the
main coil is about 5" diameter and the variometer coil about 3" diameter. The
coil will need about 60 turns if it is the same size as mine, assuming your
aerial has a similar capacitance! (about 400pF). The overwind at the base of
the coil should be three or four turns, chosen to match to 50 ohms. You may
want to break the earth at point X to keep the aerial system's earth separate
from the shack earth, it sometimes helps to reduce received noise.
Apart from an HF SWR bridge between the TX
and the ATU (which reads rather low but gives some indication of match) the
most valuable tuning aid is the ammeter in the aerial. Thermocouple meters are
antiques these days but are a very good way of monitoring the aerial current,
which is what matters! I have made a current meter based on a current
transformer which works just as well, circuit below.
The core I used is a small ferrite EMC
clip-on core about 1" long. The aerial wire, with thick insulation on it,
passes through the middle giving the one turn primary. With the values shown a
50uA meter can be calibrated to 1A full scale but any reasonably sensitive
meter could be used with suitable changes to the resistors.
A 10m vertical
will need an aerial current of 4.7A to radiate 10W (according to
G3NYK's spreadsheet), so for 5W it would be 3.3A
(4.7 x 0.707).
The meter will need calibrating so that you
can accurately measure your aerial current. It is a requirement of the licence
that you are able to verify that you are not exceeding the EIRP
To calibrate the meter I placed it in series
with a 50 ohm dummy load and connected it to my 50W 500k transmitter. Using the
formula I = root P/R we can see that the 50 Watts should produce a current of
1A, about a fifth of the current we're going to have to measure in the
Of course we can't rely on the transmitter as
an accurate generator so we need to measure the voltage across the dummy load
with a 'scope. The peak-to-peak reading is then multiplied by 0.3535 to convert
it to RMS and this figure is put into the formula V/R = I to work out the
actual current in the load. N.B. Always measure the dummy load with a good
digital multimeter before you start, many are not exactly 50 ohms. Use the
measured value in your calculations.
If you don't have an accurate oscilloscope
you can make a voltage proble. Connect a signal diode from the "hot" end of the
dummy load to a 100nF capacitor whose other end goes to the earthy side of the
dummy load. The voltage across this capacitor will be the peak RF voltage on
the dummy load. To convert to RMS multiply it by 0.707. This assumes that you
have a good high-impedance multimeter (any digital one should be fine) and that
your TX output is a nice clean sine-wave.
For 50W into 50 ohms you should get 50Vrms
Adjust the variable resistor in the current
meter to read the calculated current. If you have a variable power supply you
can change the power of the TX to calibrate the meter at a few different
levels. A good range for most aerials would be 0-5A.