TheRadioBoard

do you have a ground connection its not on the first scematic

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31 years old now playing with crystal sets since 12 years old tweaking and tinkering and sharing ideas ever since...

No ground connection. There is only the loop and nothing else.

COOL BEANS !! IME TAKEING NOTES!!
thanks {DANIEL}

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31 years old now playing with crystal sets since 12 years old tweaking and tinkering and sharing ideas ever since...

@Bob: Just used your spreadsheets. After I found that I have to adjust the result field - B27 - by myself in accordance to the used number of sides, the results was 190µH. Although I wrote before that the inductance is about 210µH, I found now that the 190µH must be correct. The explanation is easy: while I made the estimation of the loop inductance, the whole receiver was yet not finished and the loop was connected to the circuit by two long litzwires of about 80cm each. I guess that these wires added enough capacitance which lead to the found deviation.

Thanks for pointing out the error. The calculation formula for cell B27 should be:
=B19-B25

Then it will adjust for the number of sides automatically. I missed that when I added the formulae for loops with different numbers of sides. I'll fix and re-upload the spreadsheets. But anyone who has already downloaded, can just make that change themselves.
<<Edit: spreadsheets have been fixed and reposted.>>

Regarding the actual inductance of your loop, I had a similar situation. I originally used a cheap LC meter which gave a value that was too high. It seemed plausible because the tuning range was about right, but that was a result of the high self-capacitance. I later found that the meter was heavily influenced by the high DC resistance of the loop. That's when I used the other method to measure the inductance, with the side benefit of also finding the self-capacitance. I guess I should clean up that spreadsheet and post it as well.

Well, actually I should have found that correction by myself

I don't have a L-meter, my multimeter can only measure C. I calculated the L from the received station frequency and the measured C of the variable cap. For measuring the capacitance, I removed the connections from the aircap to the circuit, thus the parasitic capacitance of the wires and the loop could not be measured. Inductance was simply calculated from the measured value.

Now I can assume from the spreadsheet results, that the capacitance added by the long wires must have been about 43pF

Hi,

Just wanted to share my loop antenna receiver too .
First of all I would like to thank mak1939 for showing me this post and Brösel for his tips.
When I built the first loop I had only western electric 509w headphones, so, I could hear only 3 stations but anyways I was very excited that some loop like this could work at all!



After that I received USI headphones from Scott and built impedance matching trasnformer (I used mak's idea for the box http://theradioboard.com/rb/viewtopic.php?t=4024, he didn't mind). I didn't do a proper DX'ing but can easily count now at least 13 stations at night, wow!
I also built another bigger loop using Litz wire from mak, hoping for a better performance.. My observation using litz wire:
1) tuning became sharp, I can easily miss a channel, so I have to tune very slowly now.
2) my first impression is that using litz wire didn't really improve the performance I think, I hear some more interfereing channels on a background but thats it, no magic.
3) it became sensitive to my hand capacitance, so, once I take my hand off the tunning knob the frequency changes too.
In my plans, I would like to improve the circuit further by using some better quality capacitor and diode (will be glad for some suggestions/offers), maybe then my litz wire antenna perform better..
What I also noticed to my surprise is that a loop antenna receiver is not a proper name i would call it rather a transceiver! when I have a second "normal" radio tuned to the same frequency as my crystal radio and when I talk into my sound powered headphones I can hear my own voice on another radio! The max range is around 40 cm. Have you also tried that?
I am bit puzzled how it works. I understand how it works as a receiver: LC tank resonates at a specific frequency, creating high impedance and thus preventing the signal with the same frequency from going into ground (if there is one) or dissipate. Diode followed by a low-pass filter converts RF signal to baseband audio signal that drives the headphones. But how it works as a transmitter? The diode again acts as a mixer (http://makearadio.com/tech/files/ndams.pdf) that modulates the carrier signal with an audio signal..but how carrier signal being generated, by the same audio signal, but how? Looking at schematic I have problem of visiolize the signal flow and modulation process. I think due to the diode I have a DC audio signal that hits the LC tank and causes it to resonate at some specific frequency and creates a carrier that is then somehow mixed with AC(or DC?) audio signal, right?

I would really appreciate your help,
Aydar.

p.s. what site do you use for posting photos?
p.p.s when you press to zoom the photos you would need to wait couple of second before a bigger photo will be shown.

That's a great vernier dial ... where did you find it ???

Tony

That's a nice looking radio! If tuning is sharper, the Litz is working better than you think. The Q must be high because it sounds like the audio from the phones, into the non-linear diode, is exciting the LC circuit and generating an rf signal. Very cool!

73,

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http://kr1s.kearman.com/
http://qrp.kearman.com/

Just a change in tuning selectivity on the Litz loop indicates higher Q and less circuit resistance. It appears that the next levels of signal in your location are way too low to pick up. You can try:
1. Moving the loop one diameter away from walls and floor.
2. Couple a external single wire antenna with some height to the loop thru a few turns on the frame.
3. Improve the variable - My experiments indicate that it takes a huge improvement in the capacitor quality to hear a improvement in volume.
Two AA5 variables hooked in a balanced ground circuit worked better than subbing in a high Q TRW variable.
4. Diode tap - Litz 50% point, solid wire 10, 50, and 70 percent points
5. With the loop on a card table put a large piexe of rabbit wire or screen on the ground below the table and hook it to the house electrical ground.

p.s. what site do you use for posting photos?

Hi Avdar,

Sign up at http://photobucket.com/, post your pictures there, then copy and post your photo link in your RadioBoard message.

Ming

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I love crystal radio!

Hi,

wow, this old thread comes alive again And you've done a nice looking loop.

First answering this one here:

I got it from Jackson Brothers http://www.mainlinegroup.co.uk/jacksonbrothers/index.htm#catalogue, it is the 50mm Dial Drive Assembly 6:1 (4080-S, 4080-B). You can get in touch with them using the email adress given at the website.

Anyway I should mention that those dials have no stop-mechanism in - so if your variable cap is at the outer positions you can easily further turn the dial. This does not cause any harm but makes the dial readout somewhat useless.


Since the time of introduction I reworked this loop set providing a larger loop, changing from the 150 x 0.04mm litz to a much more rugged 270 x 0.071mm litz wire with double cotton layer protecting the litz and using one of those zero-voltage MOSFET as detector after experimenting with several diodes. I just realized that it seems I never introduced this updated set here, but you can have a look at it and its description following this link:
http://www.crystalradio.us/crystalradios/2009-6.htm#Ralf%27Br%C3%B6sel%27Siemieniec

If you are using litz I would better avoid taping since this will lower the Q. However, lowering the Q might not be that significant since the Q value of loops is not that large anyway. For my loop I assume the resonance impedance being around 250k, and even if I switch to another litz such as 630 x 0.04mm this would most likely not result in a clear improvement due to self capacitance of the loop. Assuming your loop is in the same range and having an input impedance of the transformer of 200k, tapping the loop cannot yield in a much better impedance matching as what you already have. Maybe start to play with different diodes first to get a good matching here, I guess a FO-215 would be a good candidate (for more infos I recommend reading Dicks website: http://www.crystal-radio.eu/engev.htm.

For being less sensitive against hand capacitance you can connect a plastic shaft to the variable cap which allows for tune the variable cap having more distance to it. I also think that adding a vernier dial might be a good idea, using my loop without it makes tuning very difficult.

Good luck

Brösel

Hi Aydar,

The transmitter effect is interesting, and there is a reasonable explanation for the cause.

When you are receiving an AM signal using a diode detector the carrier of that AM signal is already present in the circuit. The diode responds to the carrier and the AM modulation to produce the audio output plus some DC.

Now when you speak into the sound powered earpiece it is acting as a microphone driving audio signal back into the audio output of the diode detector. The diode responds to the audio input and the modulated carrier from the received signal resulting in your audio being modulated onto the carrier of the signal you are receiving. The remodulated carrier signal is present at the antenna where it can couple or radiate to another receiver.

The audio modulation on the receive carrier is weak because the received carrier is weak to begin with and there is a lot of inefficiency in the diode detection circuit.

If you have a real strong local AM station then you could play a couple of games to see if the effect I described explains what is going on. For instance, you could tune your crystal receiver to the strong local station then tune the second receiver to what would be the second harmonic of the strong station frequency. Now try talking into the sound powered earpiece and see if you can hear the remodulated second harmonic of the carrier signal. The local station would need to be very strong for you to detect the remodulated second harmonic (especially if you have a high Q tuned circuit in your detector), but it is worth a try if you are curious.

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Überdyne: An extraordinary small force.
Kevin

"No stop" is good in the situation where you attach the vernier dial to a capacitor already having a geared (vernier) drive. The vernier dial drive-shaft needs to fully turn a number of times (equal to the gear ratio on the variable cap.) to allow 180 degree movement of the rotor.
"Overall turns ratio" is the product of both vernier ratios.

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Thank you all for the reply! I will definitely will try the things you mentioned!

I have to admit that without my wifes help the copper wire loop antenna would probably not look so nice (with the litz wire loop I cheated - didn't make the wholes but made windings over nails..), at the end (after an hour or so) my wife became completely bored but nevertheless helped to finish it


To Kevin: thanks a lot for the hint! That slipped my mind and was bothering me a lot(!), of course, while I transmit my crystal radio still acts as a receiver (!) and this how I get my carrier! I can't really try your proposal on second harmonic, what I did try is the following, tuned to a frequency with no signal, just noise, and check on the second radio if I still hear my voice. As a results, I could still hear my voice but not as strong as on frequencies where I recieve some radio stations. I suppose that even though there is no audible signal there must be some station, one with the same frequency that is just not strong enough to drive my headphones (and the second radio), right?and that "invisble" station probably "gives" me the weak carrier signal..
What I also noticed is that when I put my second "normal" radio close to my crystal set, tuning the crystal radio seems to boost or to fine tune the signal for the second radio and may also suppress it, I guess the loop antenna acts as some sort of a rejecter..when I put both my loop antenna receivers close to each (and parallel to each other), tuning one radio changes the received signal frequency for the second crystal radio..


Meanwhile I was doing some research on FO-215 diode Brösel mentioned and found very interesting article http://www.lessmiths.com/~kjsmith/crystal/dtest.shtml (if you can't open it this is another link from cache http://webcache.googleusercontent.com/search?q=cache:3CoAU1mACrYJ:www.lessmiths.com/~kjsmith/crystal/dtest.shtml+&cd=1&hl=de&ct=clnk&gl=de), where the author writes "Interestingly, I find that an old Russian D18 diode has very similar characteristics to the FO-215 and so much also be placed into the "Holy Grail" class of germanium diode. Take your pick! ". And on a russian crystal radio website another author compares Russian diodes and says the same thing about D18 http://forum.odlr.ru/showthread.php?t=4932 (in Russian only). I am very pleased that our (mother) russian diodes can be considered as "holy grail" Next month I will be visiting my parents and was thinking of buying some, they are really cheap for 60 dollars you can get 2200 diodes! if someone is interested let me know, will be glad to share. I don't think I buy 2200 diodes (thats the min amout to buy from that internet store) so I will keep digging and maybe will find something more appropriate for the amount and the price. Also I think of buying some Russian balanced armature transducers for myself http://www.quartz1.com/price/model.php?group=212&ext=6605%20%D0%91 and http://forum.odlr.ru/showthread.php?t=13587&page=4 and http://forum.odlr.ru/showthread.php?t=13587&page=5, supposedly to be a "holy grail" for russians..well, lets see if they can beat my USIC, maybe in a month I will let know

Aydar.