AM loop antenna for my receiver

[RolandSWL]

Hello!

Just throwing out another idea. You could use an  NTE618  varactor diode and a simple variable dc supply to control the capacitance of the tuned loop. This way you would eliminate the problems of a remotely mounted mechanical capacitor.

I don't have the particulars of exactly how to do this, but, someone else here might be able to shed more light.

Roland............

[W4AMV]

Lay them out neatly side by side with say a spacing of several wire diameters. The space between windings is not to critical... but scramble winding unless precisely done, increases the capacitance per turn and lowers the resonant frequency of the loop. Taken to far, your loop will never resonant as it will appear as a capacitive element.

[W4NEQ]

One problem with varactors outside is thermal stability.  If your resonated Q is high enough to get good output, you're going to have to stay on top of the tuning.

Chris

[MorrisSWL]

Lay them out neatly side by side with say a spacing of several wire diameters. The space between windings is not to critical... but scramble winding unless precisely done, increases the capacitance per turn and lowers the resonant frequency of the loop. Taken to far, your loop will never resonant as it will appear as a capacitive element.

Thanks for that info.  I'll wind the wire neatly in a flat plane.

MorrisSWL

[MorrisSWL]

Hello!

Just throwing out another idea. You could use an  NTE618  varactor diode and a simple variable dc supply to control the capacitance of the tuned loop. This way you would eliminate the problems of a remotely mounted mechanical capacitor.

I don't have the particulars of exactly how to do this, but, someone else here might be able to shed more light.

Roland............

Already ordered the mechanical variable capacitor and want to keep it as simple as possible.  Thanks for the thought though.

MorrisSWL

[MorrisSWL]

Finally completed the AM loop antenna for our Onkyo TX-8011 receiver.

The frame is made out of two pieces of 3/16" x 1-1/2" x 20" wood, ripped off a clear 2x4.  I notched the center of the strips half way to form an X.  This made the four sides of the square 14" long.  

The 365 pf variable cap that I bought from ebay has 3 threaded machine screw holes on the shaft side.  That allows mounting the cap to the frame of a radio with the shaft sticking out.  The problem with this setup is that the threads appear to be metric and I only have US standard screws in my junk box.  Also, the screws have to be a precise length.  Too long and they will hit the capacitor plates. --- My fix was to cut a piece of sheet metal and epoxy the "bottom" of the cap to it.  The sheet metal has two holes which I can then bolt to one leg of the wooden X frame using machine screws and nuts.  A much better mounting method with no chance of a screw hitting the capacitor plates.

VERSION-1:
I used a flat ribbon cable and soldered the ends to form a loop with 18 turns, then mounted the loop on the X frame which was a perfect fit.  Soldered the ends of the loop to a 365pf variable capacitor that I bought from ebay.

I positioned the loop next to a battery powered portable radio tuned to 1460 which is the main station my wife wants to listen to.  The loop would not resonate at 1460.  I moved one lead from the cap to the different turns and found that 7 to 8 turns would allow comfortable tuning of the loop to 1460 and another station at 1540 that my wife also wants to listen to.  I was not interested in tuning lower frequency stations.

The ribbon cable that I used had a tiny gauge wire that looked smaller than 30 gauge wire.  The soldered ends broke a couple of times so I gave up on using the ribbon cable.

VERSION-2:
I used the same wood frame and cut 1/8" deep notches on the ends.  The notches are spaced 1/8" apart and I cut the notches using a hacksaw.  There was room for 10 notches since the wood is actually 1-7/16" wide.  I wound 10 loops using 22 gauge wire and connected the ends to the 365pf cap.  I used 10 loops since the loops were spaced 1/8" apart as opposed to being very tightly spaced when using the ribbon cable.  I turned out that the 10 loops are just about right.

I had also drilled 5 holes 5/8" below the tuned loop on each leg of the frame.  These holes are spaced 1/4" apart.  I threaded 22 gauge wire through these holes and had a 5 turn coupled loop.

Connected the coupled loop to the TX-8011 and adjusted the cap so that station 1460 came in fairly clearly.  But there was still just a hint of static.  I tried using 1, 2, 3 and 4 turns for the coupled loop.  One turn yielded nothing and the signal got stronger as I added more turns up to 5 turns.  So I drilled one more hole in each leg of the frame below the coupled loop and added a sixth loop.  Now station 1460 came in clearly without any static.

I tuned the receiver to station 1540 and there was a small amount of static.  I adjusted the variable cap and 1540 came in clearly.  

So now I'm able to receive stations 1460 and 1540 clearly but I have to adjust the cap to do so.  I tried placing the cap next to the receiver using about 6 feet of coax but all I heard was static and adjusting the cap did nothing.  So having a remotely adjustable cap was a failure.

Currently, my wife can only listen to station 1460 and she doesn't want to get out of bed to adjust the cap to hear station 1540.  Besides, the cap adjustment is pretty fine to obtain resonance on either station so it would be too much for her to do every time.

I'm wondering if removing one loop from the tuned loop will allow receiving station 1540 clearly without having to adjust the cap.  But even if it does, I would have to run 3 wires from the tuned loop to the receiver so my wife can throw the switch when she changes stations.  Do you think there's a possibility that this would work?  I'm guessing it won't so no sense even trying but thought I'd ask what you think.

What I documented is only a small part of all of the tests I did to get my loop to work.  I spun my wheels trying all kinds of things over two days to get it to ultimately work.  I heard a lot of static over those two days  .   So I hope the info above will help someone to get their receiver to receive weak AM stations.  

MorrisSWL

[W4AMV]

What Fun Morris. The battle you are up against is achieving high sensitivity and hence high Q in the main coil. Hence it must be tuned. The fact that many of your tries did not work out is due to the inductnace value of the main loop being incorrect and the difficulty of resonating. I suspect the value of L is 300-600 uH, quite large. Need to think about how to solve your problem without re tuning the loop C. It may be that a better solution is to go with a long wire outside as high and as long as possible. Cannot recall but I assume there are reasons you dismissed that idea. Again, the problem with the loop is it is a small capture area, must have high Q, and therefore the need to resonant it to the desired frequency. 

[KB2WIG]

2 seperate loops with fixed caps??

klc

[W1AEX]

Morris,

I was facing a similar problem with a bedside radio in my house. I like to listen to two non-local AM broadcast stations, one in New York City and the other in Boston. Living in the north central part of Connecticut put me out of their listening areas with the usual little loops that come with radios these days. I've played with high Q tuned loops and they work very well but installing one next to the bed would have brought howls of protest from my wife. I also determined that the biggest problem I had with the bedside radio was digital noise from the cordless phone that shares the table the radio is on. Because I still wanted the phone nearby, the solution was to move the antenna out of the interference.

I've had very good success using PVC pipe to build an 18" x 18" frame to wind 6 turns of wire onto, using electrical tape at the corners to hold the wire to the frame. The loop is un-tuned, covers the entire BCB and is fed with about 25 feet of twisted wire that runs from the radio, under the carpet, and over to a closet where the loop is oriented for best reception of the two stations. For even better reception I've also placed the loop in a window across the room, but the closet location works fine for my purposes. There's nothing magical about the 18" frame size, that was just a convenient size that could be concealed. You could make it bigger or smaller, but I imagine the greater the capture area is the better it will work. I didn't experiment much with the number of turns on the loop, but again, more turns may work better on the lower end of the BCB but since you are primarily interested with the top end of the band it probably won't help much.

If an outdoor antenna is possible at your location, you might want to try a broadband 85 foot loop. What I built is a pair of simple K9AY type bi-directional loops. There's no terminating resistor, so one loop receives north-south and the other receives east-west. They work beautifully on the AM BCB and in fact they do an excellent job below the AM BCB and as high as 40 meters. They're quiet and hear quite well. This past week I built and installed a 2N5109 broadband preamp at the feedpoint of one of the loops and that has added a whole new level of performance across the spectrum. I'm adding another preamp to the other loop as soon as I scrape together the parts. Details for building the outdoor loop as well as a few YouTube videos of the loops in action can be seen here:    

http://www.w1aex.com/loop/loop.html

The only deviation from the plans on that page would be that you would need to wind a transformer for the feedpoint of the radio to transition from the balanced high impedance terminals to the unbalanced RG-6 feedline. That's easily done but it does add one more step.

Good luck with your project,

Rob W1AEX

[MorrisSWL]

What Fun Morris. The battle you are up against is achieving high sensitivity and hence high Q in the main coil. Hence it must be tuned. The fact that many of your tries did not work out is due to the inductnace value of the main loop being incorrect and the difficulty of resonating. I suspect the value of L is 300-600 uH, quite large. Need to think about how to solve your problem without re tuning the loop C. It may be that a better solution is to go with a long wire outside as high and as long as possible. Cannot recall but I assume there are reasons you dismissed that idea. Again, the problem with the loop is it is a small capture area, must have high Q, and therefore the need to resonant it to the desired frequency.

Yup, it was frustrating at times but fun and satisfying after it started to work.  The reason I didn't try a long wire is that I lost the loop antenna that came with the TX-8011, and the user manual says that the supplied loop antenna should be connected in addition to a long wire antenna.  So I had to build a loop antenna regardless.

Thanks,
Morris

[MorrisSWL]

2 seperate loops with fixed caps??

Now that would be a "real" challange!  The tuning of the variable cap needs to be so precise that I can't imagine how getting the proper values of two fixed caps would work.

Thanks for the thought though,
Morris

[MorrisSWL]

I've had very good success using PVC pipe to build an 18" x 18" frame to wind 6 turns of wire onto, using electrical tape at the corners to hold the wire to the frame. The loop is un-tuned, covers the entire BCB and is fed with about 25 feet of twisted wire that runs from the radio, under the carpet, and over to a closet where the loop is oriented for best reception of the two stations. For even better reception I've also placed the loop in a window across the room, but the closet location works fine for my purposes. There's nothing magical about the 18" frame size, that was just a convenient size that could be concealed. You could make it bigger or smaller, but I imagine the greater the capture area is the better it will work. I didn't experiment much with the number of turns on the loop, but again, more turns may work better on the lower end of the BCB but since you are primarily interested with the top end of the band it probably won't help much.

That's a very interesting PVC AM loop antenna.  Just what I was wondering if it would work without all of the turns being in a flat plane.  I have a few questions:

1. I see you used 25 feet of twisted pair from the loop to your receiver.  When I tried about 7 feet of twisted pair from my loop, all I got was loud static.  Luckily I had about 7 feet of small diameter coax (like about 1/8" diameter) and when I used that, my loop started working.  Any secret to using twisted pair?

2. How is that loop connected to your receiver?  Do you have a factory supplied loop antenna at the input terminals?  If you don't have any external antenna connected, will your receiver receive any AM stations at all?  I ask since our TX-8011 won't receive any AM stations at all without an external loop antenna connected.  I lost the one that came with the TX-8011.

If an outdoor antenna is possible at your location, you might want to try a broadband 85 foot loop. What I built is a pair of simple K9AY type bi-directional loops. ...snip...

Very interesting info!  Thanks for sharing that.  I want to try building an untuned PVC loop like you did before I try any external antenna.  My goal is to receive AM 1460 and 1540 static free just using the digital tuner in the TX-8011 without my wife having to adjust the variable cap.

Thanks much for all of your info,
Morris

[W1AEX]

1. I see you used 25 feet of twisted pair from the loop to your receiver.  When I tried about 7 feet of twisted pair from my loop, all I got was loud static.  Luckily I had about 7 feet of small diameter coax (like about 1/8" diameter) and when I used that, my loop started working.  Any secret to using twisted pair?

2. How is that loop connected to your receiver?  Do you have a factory supplied loop antenna at the input terminals?  If you don't have any external antenna connected, will your receiver receive any AM stations at all?  I ask since our TX-8011 won't receive any AM stations at all without an external loop antenna connected.  I lost the one that came with the TX-8011.

1. Nope, no secret at all. I grabbed 50 feet of wire, folded it back on itself to make a pair of 25 foot leads and then used my drill to create the twist. The twisted pair feedline seems to work quite well and ignores the noise from the phone while doing a decent job of carrying signals from the loop back to the receiver. In fact, if I disconnect the loop and leave the feedline connected the receiver doesn't hear much at all, which tells me that the twisted pair is doing a decent job of not acting like an antenna.

2. My receiver has two push terminals on the back for a balanced feed loop just like the picture of the one that Pete provided in the thread. The receiver has no internal antenna at all, so whatever you hook up to those terminals is all you've got for reception on the AM broadcast band. The little plastic loop they provided worked fine for strong local signals, but the feedline was too short to allow it to be placed far enough away from the noise field created by the phone. My first attempt involved lengthening the feedline to the little stock loop, and that did work, but the larger PVC frame loop is far superior and captures much more signal making reception of the distant stations more reliable.

Rob W1AEX

[KB2WIG]

" Thanks for the thought "

Well, it was a 'duh' moment on my part.  

A bit better idea. Some trimmer caps after you find the sweet spot.

FWIW, RG8X has 25pf per foot.......  ya could just cut and cut untill you find the right spot.


klc

[MorrisSWL]

Success!  Now able to receive the two AM stations (1460 and 1540) that my wife wants to listen to, static free.  This is a long post but thought that it might be interesting reading for folks who love radio.

Since having the loop antenna on the wall and placing the variable capacitor by the receiver didn't work, I tried varying the amount of inductance instead.  Basically I have a spdt switch at the receiver which selects the amount of the wire loop used.  Here's what I did:

I decided to mount the loop antenna on the bedroom wall close to the TX-8011 receiver, instead of inside of the closet.  I wanted the antenna as close to receiver as possible.  The antenna is mounted with the X frame vertical and horizontal which makes it look like a diamond on the wall.  Two 6 penny finishing nails in the wall support the horizontal frame.  With the closet bi-fold door open and the window curtain, the antenna is not visible.  With the bi-fold door closed, the antenna is visible but looks rather "neat" on the wall. (to electronics folks anyway)  

I took 3 five foot lengths of 22 gauge wire and twisted them together.  Connected one end of the twisted wires to a small spdt toggle switch with black in the center, purple and gray on the outside terminals.  I looked around for a small plastic project box to mount the switch but didn't have any.  Before making a trip to Radio Shack, a thought hit.  I scrounged through the grandchildren's toy box and found a small plastic car just the right size.  Removed the bottom of the car to remove the wheels.  Drilled a 1/4" hole through the roof for the spdt switch and another small hole through the rear license plate for the wires.  Mounted the switch in the car and it looks rather neat with a small toy car on top of the CD/DVD player (which is on top of the receiver).

Drilled 2 small holes about 1/4 inch apart in the bottom portion of the antenna frame and strung the 3 twisted wires through the holes for strain relief.  The wires continue up to the horizontal leg where the variable cap is mounted.  The black wire connects to one terminal of the variable cap, the purple wire to the 10th turn of wire (the full loop).  This allows tuning AM station 1460.

Now connecting the gray wire to the correct place on the loop to optimally receive station 1540 was a challenge.  Remember that there are 10 turns.  First I stripped a small portion of the insulation off at loop 9 and connected the gray wire to it.

With the switch set to loop 10, I adjusted the variable cap to receive 1460 clearly.  Then set the receiver to station 1540 and there was some static.  Flipped the switch to select loop 9 and most of the static cleared but there was still a little static.  By playing with the variable cap, I determined that a little more inductance was needed to receive 1540 clearly.  Judging from how much I had to turn the variable cap, I estimated that maybe 1/4 turn more was needed.  So I stripped a small portion of insulation to obtain 9-1/4 loop.  Connected the gray wire to that point.  And wow, 1540 came in clearly!

To recap, I set the switch for turn 10 and adjusted the variable cap for clear reception on 1460.  Then I tuned the receiver to 1540 and I heard a little static.  I flipped the switch and 1540 came in clearly.  That's so sweet  .

The moral of this story is that even if I managed to get the remote capacitor to work, it would have been a royal pain for my wife to fine tune it every time she switched from 1460 to 1540.  It's a good thing that the remote cap method didn't work.  Switching inductance in and out works and a remote switch is so much easier to use.  Just flip the switch without fiddling with a variable cap.  If one strips the insulation off the entire last turn of the loop, it would then be possible to use a small clip to slide on the wire to really fine tune a second station.  If one used a multi-position switch, I guess it would be possible to have a position for every station one wants to receive.

Hope you enjoyed the read and hope it helps someone,
MorrisSWL

[W1AEX]

Nicely done!

[KB2WIG]

Great job!! Its nice to see a bit 'o construction.

Now we have to shame you into getting licensed.


klc

[MorrisSWL]

Nicely done!

Great job!! Its nice to see a bit 'o construction.
Now we have to shame you into getting licensed.

Hi W1AEX and klc,

Thank you very much for your kind words.  
Also, I appreciate every post that everyone made to offer help and suggestions.  So thanks to each and everyone of you too.
Very nice forum here with nice folks.

Best regards,
Morris

[Opcom]

These receiving loop construction accounts are encouraging. Thanks for sharing the particulars!

2 seperate loops with fixed caps??

Now that would be a "real" challange!  The tuning of the variable cap needs to be so precise that I can't imagine how getting the proper values of two fixed caps would work.

Thanks for the thought though,
Morris

Well, the old timers used to tune their TRF receivers.. I worked with a 3-stage one at an antique radio show and with no mechanical coupling between capacitor shafts and no calibration it would have been difficult if not for 50KW AM stations. Maybe that's why old people were perceived as cranky!! It musta drove them nuts.

[MorrisSWL]

These receiving loop construction accounts are encouraging. Thanks for sharing the particulars!

Welcome.  Do post details of your setup if you build a loop (or already built one).

[Opcom]

I'm just reading and learning from examples at this point. I'm beset by noise in the city.

[John K5PRO]

In the late 1970s I built a square loop from wood. It was about 3 feet across and had 4-6 turns of litz wire. A 365 pf meat slicer was in series. Alongside it, i had a second single turn loop of RG174, with the center cond soldered to the shield and a cut in the shield. It worked fantastic, with my old Scott receiver at the time. Hi Q so it had to be tweaked for each station, but I did a lot of DXing before I got rid of it. It covered the entire BC band. Now I wish I hadn't tossed it.