Hi.
For some time I have experimented with magnetic receiving loops. It was all spurred on when my Wellbrook ALA1530 started to go intermittently dead!! Mine was bought in 2006 and I have to say it was a godsend here with so much hash from SMPS and overhead cables. Sadly the construction of the early offerings of the ALA1530 left a bit to desire. The Alloy loop is either fully or partially filled with ferrite, it certainly is at the ends of the loop that is fixed onto the amplifier module. Due to stresses from wind and the plastic going brittle over time the loop cracks at the amp and it firstly goes intermittent then fails completely. Wellbrook supply a replacement amp, at a £100 it is quite pricey.
For some time I was "off the air" and had to resort to other aerials, a long wire was one and a EWE was constructed, neither came close to the loop. For a long time there had been floating around the internet a two transistor balanced amplifier that was simple and effective. I built this up with good success even though the impedance matching is far from ideal. I potted the board and the ends of the loop in an IP66 junction box and the loop has been running now for many years like this. Output is higher than the original wellbrook but is not quite as well balanced leading to a little more hash from SMPS etc but it is a hundred times better than any long wire.
Ideally a loop should be 1.2 meters in diameter and there are many different designs around, one uses copper tube as a Faraday shield with a cable running in the loop, the copper loop is open circuit at top and bottom and one end of the wire is connected to the copper loop and the other end to an RF Chip amp. I made this one up with coax but I found it didn't reject the noise as well as the balanced loop of the Wellbrook or my modified Wellbrook. At this point I hit on an idea that would use those Chinese LNA amps that are for sale on E-bay, Aliexpress and Bang good, I bought three of these for experimentation. I quickly lashed up a one meter loop with an old plastic hula hoop and used two of these amps with the output fed to a bifilar wound toroid and just fed both amps from a PP3 battery. Results were spectacular, high output and far less hash from SMPS and other sources, even my local didn't cause cross mod with valve radios but transistor sets did on many places on the band, a simple resistive attenuator cured any problems. the loop worked well over 100mHz.
I now have four meters of 10mm diameter PVC covered central heating copper pipe, an IP66 junction box and am about to start construction of a loop to replace my modified Wellbrook, the Wellbrook will be moved to my loft and will be permanently facing NW/SE, the outside loop will be rotatable by an Armstrong rotator ? I will update the thread with constructional details, pictures and circuit details over the next week or so, dependent on how I feel.
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
As can be seen in the photos, the two LNA amps required for a balanced loop, they will be fed from a 12 volt supply and will have adjustment to each amplifier to ensure balance is optimised, a voltage between 8 to 10v gives the best S/N ratio.
The looped pipe is a tight interference fit into the box and stout cable ties are used along with araldite to give further strength, a rigid plastic rod will go between the box and top of the loop for further rigidity. the LNA's, toroid and balancing pots will live in the box, once set they should not need further adjustment.
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
The Armstrong method, hopefully the mast is near the window so you can just open the window and reach out. ?
I have never tried a loop aerial, I have heard they work extremely well. Looking forward to your posts.
Frank
Hi.
Today wasn't too great but I did get into the shed and did a little bit more. I have added two BA159 Diodes back to back onto the ends of the loop and removed the SMA sockets from the LNA's also linked the earths that became redundant when removing the SMA sockets.
See the photos for the progress. Next will be to wind the toroid with a 2:1 Ratio for correct 50ohm matching.
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
Some time ago I constructed a tuned frame aerial. Seven turns of wire are wrapped around an X frame which is one square meter in size. Tuning is effected by four BB510 varicap diodes which are connected tete beche in a parallel and series arrangement. The reason for doing is to avoid demodulation of very strong signals. A multi-turn 100Kohm potentiometer is used as the tuning control. Two turns of wire for the signal take-off. The frame aerial works very well and distant medium waveband stations can be received including Radio Caroline. However, the aerial is also good at receiving static interference as well. Although some types of interference can be nulled out by repositioning the frame aerial.
So the answer to my reception requirements can be addressed by Trevor's magnetic loop solution. Another advantage this type of aerial has is that there is no tuning control and the frequency coverage is much wider than the tuned frame aerial.
Interesting to note the Philips 141U has a single loop aerial.
https://www.vintage-radio.net/forum/showthread.php?t=117841
Till Eulenspiegel.
Made loads - from 1.6 MHz to 70Mhz STLs..
Here's the 50/70Mhz one ..
Hi.
Well this wally has popped both of the LNA amps by inadvertently reversing the supply polarity. Had a bit too much on my mind at the time............. Two new ones on order ?
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
These things happen, your are not the first to do similar things, I know, been there done that. Luckily they are replaceable.
Frank
Hi.
I got the new LNA's today so once back from "up north" I'll be back to the project once again. I am slowly making an effort to keep my mind active on doing things and while in the shed pottering I definitely feel a bit less stressed, so I do think it is slowly helping me to come to terms with my loss. I was also fiddling with the TV22 project set for an hour this afternoon. So lets see how things pan out after my wee break.
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
