Quote:
Originally Posted by Bazz4CQJ
I have a Wayne-Kerr bridge which I used to measure L. I'm fairly sure the Wellbrook loop is aluminium tube about 25mm diameter and I suspect that making a 1m loop like that in Al would need some "proper kit" to do it. My expertise in pipe bending is limited to 1/4" stainless steel, and I suspect there is little read-across between the two
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The Wellbrook loop is 1 Metre in diameter and made from 20mm diameter aluminium tube.
When I made my Gary Tempest inspired 'Wellgood' clone I considered making the loop from 22mm copper tube as I have a 22mm bending spring, but I didn’t fancy forking out £15.00 for a 3M length of 22mm copper tube as I had two pieces of 15mm two metres long to hand. The only advantage of 22mm tube would be that it would be more robust, but 15mm copper tube is quite robust enough.
I made a simple bending jig from scrap chipboard and wooden offcuts then annealed the copper tube to make it easier to bend by heating it to cherry red with a propane blowlamp then allowing it to cool down. (The tube would have bent without annealing, but during bending it ‘work hardens’ making any slight adjustments to the shape a bit tricky). I bent two halves of the loop, pulling the bending spring through the tube with an attached steel wire a stage at a time to ensure that the tube was supported by the spring throughout its length as the bending progressed.
The same process could be used to bend annealed 22mm copper tube using a bending spring, but I didn't think it would make any difference performance wise. Anyone who feels disinclined to bend a loop of copper tube could just use eight 45 degree end-feed ‘elbows’ to make an octagonal copper tube loop.
I joined the two halves of the loop at the top with an end-feed soldered coupling and at the bottom end, with an acetal rod insulator that I turned to shape on my lathe. I used a piece of scrap 12mm thick white plastic chopping board on which to mount the amp and the loop, along with ‘U’ bolts to attach the loop to a 28mm diam mast. The loop is secured with two 15mm plastic pipe clips at the top and bottom as shown in the picture below. Low density polyethylene chopping boards can be cheaply bought from catering equipment suppliers. For example, 12mm thick x 300mm (12”) x 450mm (18”) £5.99 +VAT at the time of writing:
https://www.nisbets.co.uk/hygiplas-s...ing-board/j252
The connections to the copper loop were via soldered tags secured with self-tapping screws into the copper tube and acetal insulator. Once assembled, set up and tested, the connections can be waterproofed with self-amalgamating tape. This two-transistor version similar in design to the Wellbrook performs just as well as the Gary Tempest 5-transistor version I built back in 2016.
I erected it in the loft of my garage in May last year. It's connected to a KB valve radio. I use it most days and it performs really well.
Pic 1) Loop bending jog for copper tube.
Pic 2: Loop joined up.
Pic 3: Acetal insulator.
Pic 4: Amplifier mounted to loop.
Pic 5: Loop installed in garage loft.
It easily outperforms a 66ft end fed wire aerial in terms of signal strength and much lower noise level.
Incidentally, I wonder if anyone has seen or bought, one of the Chinese 100kHz to 30 MHz 'MLA' ('Mega Loop Antenna') widely available on ebay from UK suppliers at prices around the £35.00 mark. EG:
https://www.ebay.co.uk/itm/Ring-MLA-....c100677.m4598
The loop itself is small gauge stainless steel and is shown coiled up rather like bandsaw blades are coiled up till unpacked. Looks to be about 1 Metre diameter when unfolded. They're powered from a USB charger which isn't supplied. (They don't come with the mounting post or hardware). Not a great outlay. Lots have been sold by several sellers. The spec can be seen at the link. No claims are made about the gain compared say to a dipole, but local radiation noise suppression is stated to be 'up to 30 dB compared to 'long-haul' ['long-wire'] antennas'.
To comply with forum rules, please confine any remarks about the antenna itself and obviously no discussions about the seller, who is only one of many offering the 'MLA' for sale.