The 136kHz band became available to UK
amateurs in January 1998. The limits are 135.7kHz to 137.8kHz and we are
limited to an E.R.P. of one Watt. With this modest power, a lot can be
achieved.... 73kHz was available to UK amateurs who had a "Notice of Variation"
until 2003 and was from 71.6 to 74.4kHz.
LF operation is great fun!
Why not give it a try?
You'll need: A good LF receiver (or
convertor) with a narrow CW filter and bullet-proof front end. A transmitter
(see here for some ideas on building one, or
buy a Ropex). And last but not least... an aerial system. Anything that works
well on topband should do the trick but the aerial must be well insulated to
withstand the high voltages that a few hundred Watts of LF will produce. Have a
look at ON7YD's aerials
page for lots of useful info.
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 also need an ATU even to receive
properly. Use a large coil former and good insulated wire, imagine a
topband mobile whip loading coil on steroids! Usually you will need about 2mH,
use Reg's program T-ant 136 to work out the
dimensions. The circuit of my tuner is shown here, you will notice there is no
tuning capacitor, they just flash over (there could be 20kV on the aerial!).
The system is tuned with a variable inductor, or variometer.
My coil former is wound on a cylindrical
kitchen bin, and is about 350mm diameter and about 800mm tall. It is wound with
thick Litz wire salvaged from the Puckeridge LF station. I am told that the
best Q is obtained from a coil whose diameter is about 2.5 times its length so
maybe I should re-wind it on an even bigger former....
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. You could monitor the
voltage instead as the aerial impedance won't change, but it is very high so
you'll need very good components in your divider. Another alternative is to
measure the earth current which may be safer, but you would need to use an
overwind for the 50 Ohm feed rather than a tap, so that the earth current
doesn't flow down the coax outer.
For a circuit of an LF SWR bridge
see here for LX1PD's circuit based on a Drake WH7
The core I used is a small ferrite tube about
8mm diameter and about the same length. The aerial wire, with thick
insulation on it, passes through the middle giving the one turn primary. With
the values shown a 1mA meter reads about 5A full scale and it works OK from
73kHz up to 20mtrs.
Be very careful with aerial connections etc.
The voltages are very high and will arc across in all sorts of unexpected
Don't forget the receive side. Many HF
tranceivers cover 136 but their sensitivity may be poor and the front-end
filtering almost non-existent. A pre-amp and
filter ("preselector" to us old 'uns) will help greatly.
Loops or other receive-only aerials are
sometimes beneficial if you pick up a lot of noise on the main