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Old 28th Jul 2020, 5:30 pm   #41
Bazz4CQJ
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Default Re: ATU vs. Preamp vs. Active Antenna

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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Old 28th Jul 2020, 11:10 pm   #42
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Default Re: ATU vs. Preamp vs. Active Antenna

First you make a different sort of pipe-bending machine:

You want three identical wheels. Each about three or four times the diameter of your tubing. Make them out of almost anything. Steel or aluminium are fine, even a hardish nylon like 66 will do.

You want them wide enough to machine into them a concave profile that takes about 1/3 of the diameter of the tubing. Bore out each wheel from opposite sides to take a ball race, so you'll need six ball races.

Make axles to take the ball races. Make a pair of parallel frames to take all three axles with the races/wheels assemblies between them. Two wheels will run on the bottom side of your tubing, the third wheel is above the tubing and its axle goes through slots. A moving frame holds the ends of this axle and is pulled down by a screw adjuster.

You can set the moving wheel so the straight tube just passes through , touching all three wheels. Move the middle wheel down a little and push-pull your tubing to and fro. This will curve your tubing slightly. Move the wheel a little more and keep pushing-pulling until your tube takes on the curve you need. Concentrate and tighten up on areas with not enough curve. Take care to form the curve in one plane.

With a bit of care you can make tubing into a full, smooth circle.

Want to do a different size tube? Make three new wheels to suit the new diameter.

It's actually easier than it sounds you are not forcibly bending in one go, you are increasing bend very slightly in stages. It's a bit like snake charming. the eventual carve is sneaked up on.

David
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Old 29th Jul 2020, 12:58 am   #43
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Default Re: ATU vs. Preamp vs. Active Antenna

Thanks for those notes, which I shall keep on file. Maybe when the garden has gone back to sleep, I'll find time to have a go. Perhaps if I said I was repairing bicycle wheels, then I could get a Government grant

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Old 29th Jul 2020, 8:41 am   #44
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Default Re: ATU vs. Preamp vs. Active Antenna

''First you make a different sort of pipe-bending machine:''

Or you could just hire one from Jewsons - very cheap.
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Old 29th Jul 2020, 9:48 am   #45
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Default Re: ATU vs. Preamp vs. Active Antenna

The ones in the hire shops are great for tight bends, but don't do it for making large radius circles. I was making a 2m dia loop of 28mm copper pipe for a transmitting loop.

It still needed one joint around the loop because of the max length I could get. Solder joints are extra losses, so I butt jointed and welded the copper. Borax as flux and 1.5mm copper wire from twin-and-earth as filler rod. File flat and polish.

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Old 29th Jul 2020, 10:47 am   #46
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Default Re: ATU vs. Preamp vs. Active Antenna

What is the advantage of making the magnetic loop element into 25 - 28 mm thickness pipe rather than using 1 or 2 mm thin wire?

I have seen a Chinese made Active Magnetic Loop MLA-30 on eBay, and its element was 1mm thin wire.
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Old 29th Jul 2020, 11:43 am   #47
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Default Re: ATU vs. Preamp vs. Active Antenna

Not much advantage for a receiver. Huge advantage for transmitting.

With a typical loop well under a half wavelength in size, resonated to the wanted frequency with a capacitor, the effective radiation resistance can be sub-Ohm. So you neew the losses at the wanted frequency to be less than this. Due to skin effect, pipe is most efficient. Copper is low resistance and you do need plenty of surface area.

If you can only get the RF frequency loss resistance down as low as the radiation resistance, then 50% of your power heats the loop. So you want to get well below the radiation resistance. So big diameter tube, and copper-welded joint. Solder is not so good. My variable capacitor was made of plates copper welded into slots in the pipe.

Serious mechanical engineering.

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Old 29th Jul 2020, 2:13 pm   #48
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Default Re: ATU vs. Preamp vs. Active Antenna

Way back in post #7 Bazz referred to LZ1AQ:

http://www.lz1aq.signacor.com/

I bought one of his amplifier kits and made up a loop from 2.5mm cable (I think). It is cleverly fed by Cat 5 cable, one of the pairs being used for the balanced signal and others for power and a switch signal which allows selection of either the loop or a short vertical. It's amazing sometimes to switch between the two aerials. I find the system very effective in reducing RFI across a wide spectrum.

LZ1AQ goes into a lot of theory on his site which I find very plausible. He comes from an academic and practical engineering background and has clearly spent a lot of time on research. He does consider the benefits of a copper pipe loop as against a simple wire and does a thorough analysis of the needs of the all important preamplifier. There are also a lot practical recommendations. All very well worth reading.

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Old 29th Jul 2020, 4:01 pm   #49
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Default Re: ATU vs. Preamp vs. Active Antenna

Firstly, I think it is best to completely separate all discussion of receiving loops and transmitting loops. The complexity of transmitting loops is so much greater, perhaps in part because of the very high voltages that come in to play?

Re LZ1AQ; yes I think his site is a "Must see" source of info. I think he published his first designs back around 2011 and now he runs his own business selling them. His 2011 design may well be an easier amplifier to build than a Wellgood.

Re the material for the loop; my understanding is that is that you want to try and keep the inductive reactance of the loop to a minimum, and this is especially true as you go north of 2MHz. The reactance is directly proportional to the inductance and the inductance of a single multistranded wire is about 5 times higher than a 1" tube. The choice of material can also affect the RF resistivity. Other materials options beside tube include using the braid of "monster" coax cable.

Of course, even if we separate the receiving loops from transmitting loops, there's still the issue of whether to choose resonant loops or non-resonant loops. The net is full of info...it's picking out the wheat for the chaff that's a problem.

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Old 29th Jul 2020, 4:25 pm   #50
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Default Re: ATU vs. Preamp vs. Active Antenna

Having a Faraday screen (yes, a real Faraday one) on a receiving loop can be an advantage, if the area around it is full of e-field interference.

This can be done by having the loop made as a ring of coax. THe ends of the inner used as the loop and going to the preamp, while the braid is cut short a little at both ends and the loop of braid is NOT completed, but gets a single point connection to a local earth.

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Old 29th Jul 2020, 8:42 pm   #51
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Default Re: ATU vs. Preamp vs. Active Antenna

Quote:
Originally Posted by Bazz4CQJ View Post
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 .

B
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.
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Old 30th Jul 2020, 10:29 pm   #52
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Default Re: ATU vs. Preamp vs. Active Antenna

Quote:
Originally Posted by David G4EBT View Post

It easily outperforms a 66ft end fed wire aerial in terms of signal strength and much lower noise level.
David, were there to be cold and wet afternoon one day, it would be very interesting to see what results you got by substituting the copper pipe loop with a loop of a single length of connecting wire. I suspect that on the low frequency broadcast bands, the difference might be very small, perhaps undetectable. Whether that would still be true even at 3.5MHz is less clear, but I seem to recall you no longer have an Rx for that frequency?

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Old 31st Jul 2020, 9:17 am   #53
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Default Re: ATU vs. Preamp vs. Active Antenna

Great info. Thank you.

Would Delta Loop element work as well as the circle shaped element?
Not having to bend pipes into perfect circle, Delta shaped element would be easier to implement?
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Old 31st Jul 2020, 10:00 am   #54
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Default Re: ATU vs. Preamp vs. Active Antenna

The cross-sectional area of a loop sets how much flux it captures, the length round the periphery sets the losses.

A circle gives the best ratio. Losses are only important on transmit.

For reception, all you need to worry about is not losing too much area.

Circles don't have to be perfect.

A circle in self-supporting pipe comes into its own on receive if mounted on a rotator of some sort. It isn't so much as peaking the wanted signal because the peaks are broad and soft, it is being able to plant a sharp null on an unwanted signal that is very useful.

David
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Old 31st Jul 2020, 5:58 pm   #55
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Default Re: ATU vs. Preamp vs. Active Antenna

What can be useful here is 22mm blue PVC water pipe, intended for underground use. Builder's merchants keep huge coils of it, conveniently of ~1m diameter.

It's cheap and it weighs little, so a single loop of it gives you the structure for use with a rotator, even if you use it with no more than a single strand of wire passing through it.

David; are you really sure that at higher frequencies (>2MHz) loop "thickness" i.e. wire v tube is not significant on receive?

I note that in some articles on loops, the term "fat loops" has crept in .

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Old 31st Jul 2020, 9:24 pm   #56
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Default Re: ATU vs. Preamp vs. Active Antenna

LZ1AQ looks at many of the issues of loop design here:

http://www.lz1aq.signacor.com/docs/e...arison-v10.pdf

He makes the point that you only need the last word in loop construction (low inductance etc) if you have the good fortune to live in a very low noise, rural location.
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Old 31st Jul 2020, 10:31 pm   #57
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Default Re: ATU vs. Preamp vs. Active Antenna

Quote:
Originally Posted by IanG3XYV View Post
He makes the point that you only need the last word in loop construction (low inductance etc) if you have the good fortune to live in a very low noise, rural location.
That's very interesting; I do live in the sticks, but we have overhead telephone lines carrying broadband and overhead power cables too. In the past, my Wellgood magloop was better than my horizontal wires (of various kinds). But I recently cut down some trees which created the space to put up an Inverted Vee for 80m and that aerial is outperforming the magloop (same noise level, better signal level) on that band.

However, as the magloop could be rotated, it still has potential, but I do need to squeeze everything I can from it for it to compete better against the Vee. If I recall, LZ1AQ suggests that magloops will perform better at height, which is not a universally agreed issue, though I personally suspect he's right and mine could be higher.

B
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