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Best Antenna for Crystal Set Worked All States BC challenge?




 
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Author Topic: Best Antenna for Crystal Set Worked All States BC challenge?  (Read 3481 times)
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N1BCG
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« Reply #25 on: October 26, 2018, 03:05:02 PM »

The tuned loop works FB at any convenient height above ground. I tried it with great results on a 5’ mast with a rotator on a tripod. I doubt that there would be much improvement with additional height given that BCB signals either travel along the Earth’s surface (day) or arrive from the ionosphere (night). No need to reduce that distance by 50’ ;-)

I usually use it indoors, and despite the chicken wire within the walls, it still seems to work effectively. You can use a portable radio with signal meter to check signal strengths inside vs outside.
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PA0NVD
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« Reply #26 on: October 26, 2018, 03:15:46 PM »

Hi Tom
What about using the aluminium rim of a bicycle wheel?
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w3jn
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« Reply #27 on: October 26, 2018, 05:40:14 PM »

Is a crystal radio connected to an amplified antenna still a crystal radio....?
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« Reply #28 on: October 26, 2018, 05:58:23 PM »

I don't think it is valid for worked all states challenge, but still nice to experiment with. And simple low cost wide band without tuning.
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« Reply #29 on: October 26, 2018, 07:52:36 PM »

speaking of amp concoctions, being sometimes lazy I have adapted the venerable "video distribution amplifier" to RF preamp use. They are not nice like the above and sometimes need a bit of hacking to get above 10-15MHz, but they are a dozen for a dollar these days. Some were made for early VGA computer analog signals. Those have better bandwidth, but it's merely a matter of removing or downsizing a few HF-reduction caps that the dist-amps used to keep the bandwidth contained  to that required.
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K1JJ
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« Reply #30 on: October 26, 2018, 10:45:33 PM »

Nico:  Yep, an aluminum bike wheel would make a FB loop.  I'll give them a call and see if they have a pair of junk ones.

Johnny:  I think if the active matching board is set at unity gain, it would be a reasonable fudge.  

Clark: Then probably at this low frequency, height will not effect the take-off angle barely at all. The only reason to get it higher is to get it in the clear away from noise. In this case, I'll bet locating it on a small 10' tripod in the middle of the antenna field away from everything would be the best bet.

So essentially, there are four adjustable parts to the system to help select a station for best S/N:  1) The rotatable loop for signal level discrimination,  2) The antenna matching at the loop and/or antenna peaking in the radio  3) The diode detector circuit peaking   4) The variable coupling movement between the two stages.  That's quite a lot of variation.

Another thing to consider is audio. The set will do OK using suitable headphones, but a speaker will probably be too low for weak signals or pleasure listening. I plan to add a small audio chip to drive a speaker when needed.  The DXing can be done roughing it on the headphones.

I laid out 250' of #10 antenna wire today on the ground. This reference antenna will be stretched between the 50'er at the house to the top of the 100'er which is grounded in a radial field. I'll bet this system would make a great LF antenna.  I'll make a small pi-network to test it on my station receiver.  I'm curious how strong BCB signals will be compared to the 160M dipole at 190'.  We shall see.

After talking with Bob/W1RKW, for the crystal sets we both decided to start with standard cylindrical coils, single layer that are able to slide in and out of each other. Will hold off on the Litz wire for now. Another thing is substituting a Schottky diode for a germanium for less voltage drop. Must test.

T


* Bike Wheel.jpg (35.22 KB, 550x550 - viewed 31 times.)
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« Reply #31 on: October 26, 2018, 11:43:39 PM »

Hard to find, but the most effective detector diode is the backward diode. Almost an order of magnitude better. Essentially it is a tunnel diode where the current reverse peak is at zero volts, It has virtually no voltage drop, in the mV range. Someone has friend in the military radar world? Or microwave labs? There they are sometimes available
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K1JJ
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« Reply #32 on: October 27, 2018, 12:26:57 AM »

Hard to find, but the most effective detector diode is the backward diode. Almost an order of magnitude better. Essentially it is a tunnel diode where the current reverse peak is at zero volts, It has virtually no voltage drop, in the mV range. Someone has friend in the military radar world? Or microwave labs? There they are sometimes available


I will axe around. Anyone have one of these diodes here?


BTW, thanks for posting the picture of the built broadband matching circuit board.  That will be my first go-to project once I build the loop. EZ as pie.


T


* Broadband loop amp 1.jpg (2246.7 KB, 2592x1944 - viewed 41 times.)
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VE3ELQ
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« Reply #33 on: October 27, 2018, 07:07:43 AM »


I will axe around. Anyone have one of these diodes here?


I might have some, not sure. Years ago a police officer friend told me how they confiscated police radar detectors and smashed them before disposal. I asked for and received a box full, 20 or 30 which I salvaged for parts.  They all used a pellet shaped microwave diode detector in a cavity mixer behind a feed horn. How does one measure these diodes to determine what they are?? I have a bunch of them.  Can mail them if of any use.

73s Nigel

Edit:  On further reading they might be zero bias schottky in PK37 package see link.
https://www.semigen.net/zero-bias-schottky-detector-diodes/
If so they may not work at low freqs, dont know, then this link may be of interest.
http://213.114.131.21/_pdf/DIODES/zero_bias_schot8.pdf


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PA0NVD
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« Reply #34 on: October 27, 2018, 09:36:31 AM »

Hi Nigel
I don't think that are backward diodes. Most probably it are the gun diodes or the mixer diodes. There is no need for backward diodes in a radar gun.
These things are very nice to make 10 GHz transceivers for ham use. I used the "door opener" type of gun radars for transceivers about 40 years ago and we could span easily up to 30 miles using wide band FM
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« Reply #35 on: October 27, 2018, 10:16:24 AM »

This guy claims better performance using a zero bias mosfet as the detector. Just FYI.
http://www.arrl.org/files/file/Technology/tis/info/pdf/culter.pdf

73s  Nigel
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« Reply #36 on: October 27, 2018, 02:13:59 PM »

http://aldinc.com/pdf/ALD110800.pdf

Nigel:  Very cool.  A quad FET array that will run with no external power with zero voltage drop as a detector.   I must try it.

It reminds me of taking a Model-T car and adding in modern parts.  Like adding solar panels and an electric motor.  

I suppose if we were to stick with original, it would mean using a real galena catwisker.  

The article also mentions adding a toroid with 9 turns to kill IMD caused by TV and FM stations.


Between the broadband loop, FET detector,  coil designs, audio level enhancements, etc., there's a lot of optimization and experimentation fun to be had. I'll bet one of these 75M receivers will work FB on 75M AM. I wonder how many AMers have made QSOs using a crystal set on AM?

T
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N1BCG
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« Reply #37 on: October 27, 2018, 02:35:51 PM »

A MOSFET detector? Sheesh.

Here's the rig you should be building:



:-)
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« Reply #38 on: October 28, 2018, 01:44:25 PM »

Yep, just like a "Foxhole" radio. The prisoners in Eu WWII were quite ingenious.


Bob/ W1RKW has been doing some research.  Turns out the 5082-2835 Schottky diode IS the best  (0 dB loss) - but only if a tiny 10uA bias is applied. A small button calculator battery could be wired in and last a very long time.  This is as good as the MOSFET array! I don't have any info on the back diode yet, but 0 dB is as good as it gets.   (A 1N34A is about -3Db loss and most diodes are worse from there)

Here's the diode tests and other ideas.   Look at the 20 Mhz column at the bottom of the page under "Experiments With Diode Detectors:"

http://www.techlib.com/electronics/crystal.html


T
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« Reply #39 on: October 28, 2018, 01:50:33 PM »

Tom,
Take a look at this high performance crystal set
https://hackaday.com/2016/09/16/high-performance-crystal-radio/

Chris uses an audio amp in most of the video but does demonstrate using audio straight from the detector in his overhead cone speakers.  Gets respectable volume from them.

The selectivity from his set is amazing.

rkw

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« Reply #40 on: October 28, 2018, 02:22:42 PM »

Bob,

Wow, that really is some system..  He uses four tuning circuits total.  I like how the floating bandpass filter, with nothing physically connected to load it except coupling, is inserted in the middle.  Selectivity sounds almost as good as a tube radio with I.F tuning.  The multiple hi-Q stages are the key. And no power used at all.  (Or pick up another 3 dB by using the biased Schottky diode.)


Back to Litz wire and ABS 4" forms.   I like how the coils sit out away, in the clear, from everything for best Q, least coil pattern interference and best selectivity.  He really has optimized it to the best set I've seen to date.  When he turns that tuner coil 90 degrees, it is absolutely dead.  Clean.

It really paid to do our homework first before starting anything.

https://hackaday.com/2016/09/16/high-performance-crystal-radio/

The schematic is at 13:41  -  It is clear enough to figure out.   Note that the antenna unit tuning variable cap is two-ganged  and the sections are used individually as a series tuner and as a parallel tuner.  Designed for a long wire, in his case about 250'.   Imagine further signal discrimination when adding a rotatable loop.

I believe he is picking up a higher Q by using smaller coil inductances and larger C ratio with that huge 4-gang 500 pF sectioned cap - more than with the usual sets I've seen.. Compare his small coils (less turns) to the Plexiglas crystal set with those huge pi-wound ones.  Less L, more C, lighter coupling, less loading = higher selectivity.

Coils mounted on 10"? insulator pillars using nylon hardware. Capacitors floating on Plexi. Selectable audio transformer taps.  He squeezes every uV out of that set.  Well thought out.

And he is even tapping enough power from the bandpass filter (coupling loop) to drive his digital readout the proper way.

T
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Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #41 on: October 28, 2018, 03:06:01 PM »

That's a good observation on the small coil windings.  I didn't catch that. That's good to know.

I like how he uses a rotatable coil on his antenna tuning  unit which is cool section by itself and is controlled by a knob so his body influence is reduced or eliminated and all the sections use 6:1 reduction gearing on the variable caps.

He did mention that he has a switch to select the detector diodes. I'm not sure what is gained by using 2 in parallel.  Why not just use the better performer unless the tuned circuits play a roll when selecting the diodes. Maybe there is a give and take between the diodes and tuning networks.  It's curious.    

One question I have, I'm going with a long wire and how critical at MW would it be if the wire kind of zig zagged. I'm thinking the straightest, longest and most vertical run would be best but my situation prevents me from doing that.  I can get length but I'd have to zig zag some amongst my natural supports.

I'll be starting my winding next weekend. Hope to have a roll of Litz by mid week.
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« Reply #42 on: October 28, 2018, 03:32:09 PM »

OK Bob -

What gauge/ source of Litz are you getting?   His coils are so simple, might as well use Litz, WTF.

I'd like to try that Schottky diode and see zero loss.  I just KNOW that's where I'll end up anyway... :-)

He mentioned doing the BCB challenge - to log as many stations as possible over a week, etc.  I wonder what is the most effcient way to do this? Obviously we will have to either wait for them to ID or use a computer data base to figure it out. The loop null might help.

Yes, tuning with long rods is a great idea to minimize hand capacitance.

I was actually thinking of mounting the coils up high like that before I saw his video, in addition to experimenting with high C, less L.  Good to see it has already been proven out and saved time.

As for your antenna....   just figure you want at LEAST 1/4 wave (or even 1/8 wavelength) out there for the lowest freq to feed against ground. This will give you 20-50 ohms to play with, depending on ground efficiency.   IE, the most important thing is to get the wire stretched out even though it may not be perfectly straight.   Obviously on 900 Khz the wire will need to be 300' or so, so you will be dealing with 10-20 ohms for the lowest freqs. So bottom line is just like LF band, get as much wire out there -  as long as it is reasonably stretched out. No spider webs, even though it will add some capacitance hat effect.. If it folds back on itself too much it will simply mean increased pattern cancellation.  Bear in mind that the AM radio antenna on a car is only a few feet long and matched well -  with VERY poor ground to work against and works "decent" ... So you will have a huge advantage with even a 100' straight wire in the trees fed against a good Earth ground.

T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
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« Reply #43 on: October 28, 2018, 05:10:37 PM »

Tom,
I ordered 150 feet of 660/46 Litz off of EBay. i'm going for big Q.  It was the least expensive source I could find and seems to be the most readily available place for a variety of Litz.

What I have noticed over the years with DX BCB, that stations don't seem to ID as often as they should especially those that are automated.  So one could wait a while to get call letters.  Even  WTIC doesn't do the obligatory ID.  How they get away with it, I don't know.  But it is what it is.  It's part of the hunt.

OK on the length of wire and fold back.  I think I'm ok in that respect as far as short wavelength goes. Longer wavelenths might suffer but I'm thinking since the exit out of the shack is at ground level and will essentially spiral out up and out from that point to the highest point I might be OK.  I can string the wire in a nice wide open and expanding semi circle which will be close to 200 feet.  As far as ground goes, I was think of just using the electrical service ground. It's a few ground rods located near the shack and attached to the cold water pipe in the shack. We have a water well that is fed by plastic.  What do you think about an independent ground at least for the crystal set?

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« Reply #44 on: October 28, 2018, 11:55:31 PM »

To aid in identifying stations, check out this website that shows the day and night patterns and power for AM stations in the U.S.:

http://www.nf8m.com/patternmaps.html

Many stations stream live (with some latency) so you can confirm what you’re hearing after narrowing the possibilities.
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« Reply #45 on: October 29, 2018, 07:52:35 AM »

A small button calculator battery could be wired in and last a very long time.  

Now that's cheating..  Cry   S'posed to be completely self powered..

Nigel
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« Reply #46 on: October 29, 2018, 08:12:15 AM »

If you don’t need a lot of current then you could also build a second crystal set with a 1N34 to use as the power source. Longwire and tuning circuit for peaking output and electrolytics for filtering.

Do that on a larger scale and you could power an ocean liner just on radio waves. Here’s proof: https://en.m.wikipedia.org/wiki/The_Big_Broadcast_of_1938
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« Reply #47 on: October 29, 2018, 09:23:05 AM »

If you don’t need a lot of current then you could also build a second crystal set with a 1N34 to use as the power source. Longwire and tuning circuit for peaking output and electrolytics for filtering.

Seriously though I had the same thought.  Tune a second RX to the strongest RF signal in your location whatever that is.  Harvest the energy to power a second Epad mosfet audio amp.  Note the spec sheet applications first listing. 
And quote:
With these zero threshold devices, an analog circuit with multiple stages can be constructed to operate at extremely low supply or bias voltage levels. For example, an input amplifier stage operating at 0.2V supply voltage has been demonstrated.
unquote

Be a fun engineering exercise.  Ya I know we're driving you nuts so will shut up.

73s  Nigel

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« Reply #48 on: October 29, 2018, 09:51:04 AM »

If you don’t need a lot of current then you could also build a second crystal set with a 1N34 to use as the power source. Longwire and tuning circuit for peaking output and electrolytics for filtering.

quote:
With these zero threshold devices, an analog circuit with multiple stages can be constructed to operate at extremely low supply or bias voltage levels. For example, an input amplifier stage operating at 0.2V supply voltage has been demonstrated.
unquote

Just to make things clear, are we are allowing the introduction of modern solid state devices into this "crystal set", but we have an "unwritten" restriction that it must be self-powered?  This would lead me to believe that we might even add a nickel cadmium rechargeable battery to the mix (but lithium ion batteries are forbidden, because they are just too new).  All the power is still coming from the broadcast station, yes?   Throw in a small switching power supply and you could charge not one, but a bank of nicads.  Then you could power a FB "Hi Fi" audio amp, not a measly .2 volt device.  Let's hear that Hi Fi broadcast signal all over the house!  

Sounds like a fun engineering contest is about to unfold!

I can recall back in the '50s when I was about 8 years old, I made a crystal set, but found that an output transformer would take the impedance down to a level that would drive my headset or a small speaker very nicely.  The UTC LS-55 worked best!  Then my dad came home from RCA and gave me several 2N109 engineering sample transistors, before you could buy them in the store.  A two-stage audio amplifier was then powered by three jars with a nail and a copper strip, filled with vinegar.  I found I had enough sensitivity that I could dispense with the outdoor antenna, and just clip one side of the input tank to the finger stop on the dial telephone, using the AT&T free antenna wires.  Listened to Jean Shepard (K2ORS) on WOR after midnight every night!



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N1BCG
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« Reply #49 on: October 29, 2018, 10:02:20 AM »

The schematic is at 13:41  -  It is clear enough to figure out.

Here's a screenshot in case you missed it (it lasts for just a moment) [#1].

I've also found that using two diodes for full wave rectification dramatically increases voltage. This is done by A.C. coupling the tuning components to the junction of the diodes. [#2]



* CrystalRadio.jpg (189.51 KB, 857x581 - viewed 46 times.)

* IMG_9479.JPG (1889.31 KB, 4032x3024 - viewed 24 times.)
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