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 Post subject: Internal Loop Antenna Construction
PostPosted: Dec Wed 06, 2017 11:59 pm 
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Joined: Mar Thu 02, 2017 1:37 am
Posts: 186
Location: Cedar Rapids, Iowa
I'm working on a Fada 148 that is missing the internal loop antenna, so I'm trying to build one. I've been trying to get the inductance at a point that I can get decent reception. After many attempts at just winding the loop that connects to the radio itself (no external antenna loop) which resulted in lackluster results, I decided to add a winding for the external antenna. This was a little better but still unimpressive. Finally I decided to carry this thing upstairs and connect the external antenna loop to a wire strung out the back door...what a difference! While I still wouldn't call the reception great, it's at least acceptable.

The resulting windings measure like this:

'Main' loop (connected to radio): 0.18mH 1.2ohms
External antenna loop: 0.02mH 0.4ohms

I intend to make the 'main' loop a little shorter for a smaller inductance...maybe about 0.15mH. So now the questions.

1. Do these values seem reasonable?
2. Does anyone have experience winding these things? I've built a form using my 3D printer:
Attachment:
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loop_antenna_003_r.jpg
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I used this at first but then switched to a small plastic coffee can...much easier to unwind and rewind. I'll be making another one in a more suitable color. I have an idea how I want to wind it but want to see if others have any experience with this.
3. What size wire should I use? And where is a good source? I think the stuff I used for my testing is either 24ga or 26ga.

Thanks for any ideas!

Rich


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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 12:19 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11518
Location: S. Dartmouth MA 02748-1225 USA
Inductance is rather critical to get the antenna circuit to track with the oscillator. Adjustment is often accomplished by puling inward two-three turns during the alignment. The factory would do the inductance adjustment of the loop by measure for the needs of the particular radio model on a jig with very little or no adjustment required.

Do you have any idea what the "Q" is of the 3D printer plastic? It could be fouling the response of the loop.

Use either a material with a known "Q" or a material like g-10 epoxy fiberglass. I would also suggest hard fiberboard, but it is porous so unless it is sealed the quality is questionable. Clear plexiglass or white PVC sheet has been proven good for coils.

I have seen the response of a radio turn to crap after the loop antenna fiberboard got wet in a cleaning process. Two days later when dry, the radio was fine.

I would be best to have a loop frame and loop itself be as large as the rear of the cabinet can accommodate, as large as a capture area possible.

YMMV

Chas


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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 12:23 am 
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Location: USA
I think you are on the right track. Remember that the loop is part of a tuned circuit. So with each change in you experimental configuration, you need to repeat the ANT/RF part of the alignment procedure. Otherwise, sensitivity will not be optimal.

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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 12:57 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 3589
Location: Near Brandon, Iowa
If you can determine the capacitance range of the variable tuning cap's antenna section, you can calculate the needed inductance of the loop antenna at the cap's midpoint value (frequency of around 1000 kHz) using the formula for a resonant LC tank circuit.

For example, a typical maximum tuning cap value is 365 pF (yours may not be typical). The minimum value will be nonzero, possibly in the 40 pF neighborhood. That would give a range of 325 pF over the entire tuning span, and a value of 200 pF (approx) at the broadcast band tuning midpoint (1000 kHz). Plug those numbers into this formula:

f= 1/(2*pi*SQRT(LC)) and solve for L, given the predetermined values of C and f from the above discussion. The answer would be the target inductance value of your loop.

It would be best if you could measure the actual capacitance of the variable cap antenna section at the
tuning midpoint (1000 kHz) rather than relying on guesswork. But you might be able to come close enough for satisfactory operation over most of the band by making "educated guesses".

Note that loop inductance is dependent not only on the loop diameter and number of turns, but also the diameter of the wire used. You should be able to turn up a recipe for a loop having a desired inductance by playing with these three variables.


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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 3:13 am 
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criageek wrote:
3. What size wire should I use? And where is a good source?

The best source might be an airloop from a junked or for-parts set. If you have one, first try it as-is. If it doesn't perform adequately, unravel the wire and use it. You might also use the junked backboard, as it would be made of the material best suited for this purpose.

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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 7:56 am 
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Location: San Jose, CA USA
lorenz200w wrote:
Note that loop inductance is dependent not only on the loop diameter and number of turns, but also the diameter of the wire used. You should be able to turn up a recipe for a loop having a desired inductance by playing with these three variables.

Although it's correct that wire size will have an effect, the effect is really quite small. Main things are number of turns and diameter. Spacing matters a little, and wire diameter hardly at all.

Where diameter can play a role is if you wind a flat coil, and the close pack the wires, then the ratio of the inner and outer diameter is affected. That may be what lorenz is thinking about.

Pretty much any fine wire can be made to work. A great free source is the degaussing coil in an old CRT color TV or computer monitor. A big coil wrapped around the CRT right behind the screen. One degaussing coil is a lifetime supply of magnet wire.

The wire you used for your first attempt looks fine, so feel free to use that again if you have enough.

You can test the resonant frequency of the tuning cap and your homemade loop as follows:

Connect an RF generator across the two ends of the loop with a 100 K resistor in series with the ungrounded lead. Connect the tuning cap across the loop as normal. Connect another 100 K resistor to the ungrounded side of the loop (same place as where the other one is connected) and run it to an oscilloscope probe (preferable a 10X probe, but a direct probe will also work, as long as the 100K resistor is in series). Now sweep the frequency of the RF generator, and note where the center of the peak in amplitude is. This will tell you very quickly if your inductance is too high or too low. Aim for about 530 kHz with the antenna trimmer on the tuning capacitor about half way compressed.

The resistors prevent the RF generator and scope probe from loading the tuned circuit too much. This way you can see the resonant frequency properly, without concern about parasitic capacitance in either the RF generator feed or scope probe.

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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 8:13 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 3589
Location: Near Brandon, Iowa
The Web has several resources that support design of a loop antenna. For example, here's one:
http://electronbunker.ca/eb/Loop_Antennas.html

I haven't needed to validate the above info, so use that particular link at your own risk. However, it does seem to do a good job of presenting all of the parameters that factor into the design of a loop antenna.


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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 3:01 pm 
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Joined: Mar Thu 02, 2017 1:37 am
Posts: 186
Location: Cedar Rapids, Iowa
Thanks to all for the helpful info!

lorenz200w - The info you gave for doing the calculations was exactly what I was looking for...thanks! Based on my rough calculations I should be at about 0.12mH, so I wasn't far off when I took a stab at 0.15mH. Then again, I have no idea how accurate my cheap little component tester is. And the article on loop antenna design looks to be a great resource. I had seen many sites discussing loop antennae but all seemed to be for 'tuned' loops...meant to be set next to a radio and tuned along with the radio. But it looks like I should be able to substitute my tuning cap for the one on the loop when doing my calculations/testing.

Tom - great advice as always! I'll probably try your resonant frequency test method to check my work. Can you explain why I should 'Aim for about 530 kHz with the antenna trimmer on the tuning capacitor about half way compressed'? I would think I'd want to shoot for 1000kHz.

I have a CRT TV sitting here waiting to be hauled to the landfill...maybe I'll take the back off and grab the wire on the degaussing coil. I want to use some wire that looks better than what I've been testing with. On my original form I designed in some grooves to help me wind it more neatly, but I think I'll leave them off when I reprint so I can get the wires as close together as possible.

On other internal loops I've looked at it appears that the 'internal' loop and the loop for the 'external' antenna may go in opposite directions. Is this true? And is it important?

Thanks again guys!

Rich


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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sat 09, 2017 1:10 am 
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Location: San Jose, CA USA
I recommend tuning the loop at the bottom of the dial. The main reason is because you know exactly what frequency it is supposed to be tuned to at the bottom of the dial. You could do it at any point on the dial if the dial calibration were highly accurate. Shooting for 530 kHz at the bottom of the dial with the trimmer in the middle of its range should be the easiest to deal with.

The trimmer has least effect at the bottom of the dial, so it is important to get the inductance right there. If you choose a higher frequency, the trimmer has more effect. You might find that by adjusting the trimmer, you can get the resonance correct at 1000 kHz or 1300 kHz, but not at 600 kHz. So choosing the bottom of the dial prevents this problem.

It's similar to adjusting the oscillator coil or padder capacitor. Bottom of the dial is the best place for both. Adjust the trimmer at the top of the dial. Then both ends of the dial end up having good tracking.

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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sat 09, 2017 3:33 am 
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Location: Ann Arbor, MI
You could find the right number of turns by trial and error. Start with a number of turns that you are sure are too many. Check the strength of a station at the low end of the dial. You can put a meter on the detector output to get an exact measurement. Then remove a half turn and recheck the strength of that station. Keep removing half turns and checking.You should soon find the number of turns that gives you the highest signal strength. Tack in enough wire to get the signal back up to maximum. Tune in a station at the high end of the band. Adjust the trimmer for maximum signal. Go back to the low end of the band and double check how many turns are correct.

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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sun 10, 2017 2:05 am 
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Joined: Mar Thu 02, 2017 1:37 am
Posts: 186
Location: Cedar Rapids, Iowa
I don't know what I'm doing wrong. I printed a new form for my loop antenna so I could get more windings on it and I'm trying to use Tom's method to find the right number of turns by connecting a signal generator and oscilloscope across the LC network, manually sweeping the signal generator, and looking for a peak on the scope. This is shown visually in several YouTube videos, including this one:
https://www.youtube.com/watch?v=G_RCyDdt2rM
I'm pretty certain I have everything connected properly, but I never see a good, smooth, obvious peak. I've tried 2 different signal generators, and I've tried it with and without the 100k resistors. When I used my cheap digital signal generator (which I can adjust in very small increments) I was able to see a very small peak, and it was at a frequency near what I would expect for the inductance I was using. I removed a couple of windings from the loop, tried again, and again saw a very small peak, about where I would expect it. But now I've removed a couple more windings and can no longer find a peak.

In any case, I never see a distinct, obvious peak like in the videos. I've tried it with a fixed capacitor as well (as opposed to the variable condenser in the radio). Any idea what I'm doing wrong??

Thanks,
Rich


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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sun 10, 2017 2:09 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 3589
Location: Near Brandon, Iowa
criageek wrote:
Any idea what I'm doing wrong??

Just a guess; but it sounds like one or more of your reactive elements is excessively lossy (low "Q").


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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sun 10, 2017 3:19 am 
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Posts: 11925
Location: San Jose, CA USA
Let's simplify the circuit, so that issues with the AVC cap or other things can't affect the resonance.

Try it with your loop wired directly on the antenna gang. Use the frame of the tuning cap for your grounded side, the the stator connection for the ungrounded side. Disconnect anything else from the stator temporarily.

There may be something else in the circuit that is preventing it from resonating. Once we get it resonating this way, then we can try reconnecting it in the normal circuit and see what changes, and trace down the fault that may be causing the problem.

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 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sun 10, 2017 3:27 am 
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Joined: Mar Thu 02, 2017 1:37 am
Posts: 186
Location: Cedar Rapids, Iowa
Good news! I had worked on this for a while so I took a break and got some nourishment, then tried again. I either had a bad connection somewhere or a bad jumper. Now I can see the peak...still not a good, steep peak, but I can see it. I started with the peak below 540kHz and worked my way up. I ended up overshooting and it ended at 549kHz, but this is just my test. And it works very well :) A fair amount of static between stations from the middle of the band up, but overall, very good.

Now I'll work on finalizing my design for the form and a mounting bracket and get them printed, then go through this same exersize. This is the wire I plan to use for the finished product:
Attachment:
loop_antenna_004_r.jpg
loop_antenna_004_r.jpg [ 247.65 KiB | Viewed 55 times ]

As Tom A. suggested, I took the degaussing coil out of an old TV and unwound the wire from it...that was a fun time...not! Based on the diameter of the original coil and the number of turns I'm estimating I have a little over 100 feet of wire to work with.

I don't have time right now but before I finish out this thread I will document the actual winding of the loop for those that find themselves in this situation in the future.

Thanks again!

Rich


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