Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Dec Sat 16, 2017 12:33 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 22 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Internal Loop Antenna Construction
PostPosted: Dec Wed 06, 2017 11:59 pm 
Member

Joined: Mar Thu 02, 2017 1:37 am
Posts: 194
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:
loop_antenna_001_r.jpg
loop_antenna_001_r.jpg [ 248.61 KiB | Viewed 392 times ]
Attachment:
loop_antenna_003_r.jpg
loop_antenna_003_r.jpg [ 230.53 KiB | Viewed 392 times ]
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


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 12:19 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11539
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


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 12:23 am 
Member
User avatar

Joined: Dec Wed 25, 2013 7:57 am
Posts: 1814
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.


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 12:57 am 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 3608
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.


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 3:13 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 25739
Location: SoCal, 91387
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.

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 7:56 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11950
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.

_________________
Tom K6VL


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 8:13 am 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 3608
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.


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Thu 07, 2017 3:01 pm 
Member

Joined: Mar Thu 02, 2017 1:37 am
Posts: 194
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


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sat 09, 2017 1:10 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11950
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.

_________________
Tom K6VL


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sat 09, 2017 3:33 am 
Member
User avatar

Joined: Mar Sun 01, 2009 10:27 pm
Posts: 4245
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.

_________________
Tom


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sun 10, 2017 2:05 am 
Member

Joined: Mar Thu 02, 2017 1:37 am
Posts: 194
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


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sun 10, 2017 2:09 am 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 3608
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").


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sun 10, 2017 3:19 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11950
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.

_________________
Tom K6VL


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Sun 10, 2017 3:27 am 
Member

Joined: Mar Thu 02, 2017 1:37 am
Posts: 194
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 187 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


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Tue 12, 2017 2:28 am 
Member

Joined: Mar Thu 02, 2017 1:37 am
Posts: 194
Location: Cedar Rapids, Iowa
I thought I was ready to call the loop antenna complete...it looks good and works pretty well:
Attachment:
loop_antenna_010_r.jpg
loop_antenna_010_r.jpg [ 215.7 KiB | Viewed 120 times ]
Then I realized that I don't have the power cord installed yet. The hole for it is in the area I've circled in red here:
Attachment:
loop_antenna_013_r.jpg
loop_antenna_013_r.jpg [ 236.42 KiB | Viewed 120 times ]
So now I'm looking for ways to deal with this. I see a few options, in no particular order.

1) The left end of the chassis is open so there is room to run the power cord along the side of the cabinet and in the side of the chassis. I don't like this idea because there is no way to provide strain relief.
2) Drill a new hole in the chassis. There is room for another hole just to the right of the mounting screw threads you can see at the lower left of the back of the chassis.
3) Redesign the loop antenna frame to pull the left end in making it narrower, or pull the bottom up making it shorter, or both.
4) Cut a notch in the loop antenna tab that is covering the hole, providing room for the power cord to go through it.

I think my favorite idea is #2...drill a new hole. This solution also moves the power cord farther from the antenna windings which has to be a good thing.

I'd like to coat the loop antenna with something to keep all the wires in place and provide a little more protection. So I'm looking for suggestions for what to use for that.

Let me know your thoughts!

Thanks,
Rich


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Tue 12, 2017 2:31 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11950
Location: San Jose, CA USA
The loop windings should not really be against the chassis, so the fact that you're concerned about routing the power cord means there is an additional problem. Having the loop against the chassis both changes its inductance and capacitively couples it to the chassis, both of which probably have a bad effect on reception. Usually the loop was mounted with spacers to keep it at least 1/2" away from the chassis. When you do that, you also have plenty of room for routing your power cord.

[edit] Looking more closely at your pictures, you might have just enough clearance between chassis and loop (maybe 1/4"?) that the loop will work OK, but not quite enough for the power cord. I'd still be tempted to move the loop back a little, but if your cabinet won't accommodate that, then rerouting the power cord does seem reasonable.

_________________
Tom K6VL


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Tue 12, 2017 2:59 am 
Member

Joined: Mar Thu 02, 2017 1:37 am
Posts: 194
Location: Cedar Rapids, Iowa
Thanks Tom - I designed the antenna mounting bracket to leave a small gap for just that reason...to keep the windings off the chassis.
Attachment:
loop_antenna_015_r.jpg
loop_antenna_015_r.jpg [ 174.91 KiB | Viewed 114 times ]
It's about 1/4" at the left end, which would be enough room to put the cord in, but it would be bent at a sharp angle to go through the hole. The antenna does already extend outside the cabinet by about the thickness of the antenna frame, but I could easily make a new bracket that provides a little more gap, which would result in the antenna extending farther out the back. Perhaps I could make a back cover for the cabinet that would cover everything.

Rich


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Tue 12, 2017 3:00 am 
Member
User avatar

Joined: Mar Sun 01, 2009 10:27 pm
Posts: 4245
Location: Ann Arbor, MI
Just a nitpick, but the original loop would most likely have used cotton covered wire. I think that they called double cotton covered wire.

_________________
Tom


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Tue 12, 2017 3:05 am 
Member
User avatar

Joined: Mar Sun 01, 2009 10:27 pm
Posts: 4245
Location: Ann Arbor, MI
If you cut off some of the plastic at the top of the antenna you could mount the whole thing higher. Perhaps then the cord would more easily come out under the antenna.

_________________
Tom


Top
 Profile  
 
 Post subject: Re: Internal Loop Antenna Construction
PostPosted: Dec Tue 12, 2017 3:59 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11950
Location: San Jose, CA USA
I'd go with at least 1/2" spacing. I don't think I've ever seen one mounted closer than that. Some are more like 1" away. The chassis changes the behavior of the loop: the ferrous metal changes the inductance, and the fact that the metal is a conductor generates eddy current losses in the loop (so even being too close to a nonferrous chassis is not good).

_________________
Tom K6VL


Top
 Profile  
 
Post New Topic Post Reply  [ 22 posts ]  Moderators: Marcc, Norm Leal Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: AJJ, Exabot [Bot], radiodale1, Ralph 100, spsquires, Yahoo [Bot] and 18 guests



Search for:
Jump to:  
cron


















Privacy Policy :: Powered by phpBB