Superloop receiving antenna for 136kHz
Date 10/4/01 Updated February 2003
See bploop.pdf. for interesting designs for receiver loops by Jim Moritz, M0BMU, I will be trying out Jim's method of coupling a loop to a feeder, which I suspect will be better than the one described below. Watch this space
About a year ago I made a receiving loop for 136kHz using computer ribbon cable housed in plastic waste pipe. It wasn't very successful. It would appear that the low Q caused by the construction of the ribbon cable was the problem. I have since made the G3LNP loop although I had difficulties with the amplifier. I finished up using a low impedance pick-up loop without the amplifier - this worked reasonably well but it did lack sensitivity.
I
decided to redesign the plastic waste
pipe special because if its inherent
weather resistant structure This
time I made it twice the size and used
Litz wire from the Crawley club source.
This antenna turned out to be very good
and outperformed all other loops built
previously. Furthermore, this design
does not need an amplifier. A
description of its construction follows.
The loop housing comprises 34mm plastic waste pipe forming a square loop, 2m per side as shown on the right. The corners are joined using 90 degree elbow joins. The structure is held in place with an aluminium T section comprising a 45mm pole and an angle top piece.
The loop itself comprises 10 turns of 2.5mm Litz wire and a single turn pick-up loop. The loop is assembled by cutting 11 x 8.3m lengths of wire and feeding them through a single 2m length of waste pipe. One 90 degree elbow is fixed to the end of the first 2m length of pipe after first feeding the wires through.
The wire bundle is then fed through the second section of pipe and a second joint added. The third section of pipe and joint are then added. Construction should be done with the plastic pipes laying on the ground - at this stage the whole structure is very floppy.The wires are then fed through the fourth length. This is where it gets a bit tricky. The final 90 degree elbow should have two slots cut in it on the outside part of the elbow so that the wires can be fed through and still be clear of the pipes when they are fixed in. When the wires are fixed in place the first and the fourth lengths of pipe can be pushed into the elbow joint.
The
fourth elbow forms the bottom part of
the loop (shown right) is strengthened
by fixing it to a piece of
weather-proofed plywood using standard
waste-pipe wall-mounting brackets. This
plywood base also provides a method of
fixing the loop to the support mast.
The loop is fixed to the support mast and the top T section using appropriate sized U bolts.
Each of the wires has now to stripped of insulation and tinned. They also have to be identified using a multimeter so that they can be connected together to form a continuous 10 turn loop. The 11th turn, the pick up loop, is connected to the receiver coax feed. I used connector blocks in the prototype because of the rather experimental nature of the project. They are sprayed with spray grease to prevent corrosion. You could solder the wires together with a shrink wrap cover.
The main loop is resonated with a mixture of 6 x 150pF fixed capacitors and a 500pF variable (meshed about two-thirds) giving a resonating capacity of around 1200pF. The whole of the bottom of the loop is wrapped in plastic sheet, held in place with clothes pegs, to keep out the rain
If you can't find Litx wire you could use PVC insulated wire. The Litz wire used here has a wire diameter of approximately 1.5mm; the diameter including the insulation is 2.5mm. If you use electrical wire check that it has an insulating thickness of at least 0.5mm because, with this construction the insulator material determines the wire spacing. The wire should also be fairly soft and flexible to ease construction.