Crystal Radio(I)

Summer 2016

Even though I had built some Regen reflex and TRF,  It's still interesting to build them again and again just for fun.

I'm not going to compare these earpieces sensitivity.  According 3 weeks listen, some time signal is strong, almost all earpiece is relatively aloud, and if  signal fall down, neither of them could hear anything.  ( I did had a 2200 ohm DC resistance earphone,  but it's bad compare to these phone listed here.)

Crystal Set

I start build Crystal Set from a Toroids which popular in China web forum,  R40C1, It's a ferrite materiel,   working up to 50 Mhz with   µ = 40. 

Antenna:  15 meters  random wire, no match circuits. 

Ground:  Start with None.

A initial setup  presented in following photo. Most builder use 15 mm thick R40C1, which should be same as stack of my two 7 mm R40C1. Thick one could reduce needed turns and then use more heavy Lize wire. I did not bothering  to do so ^_^. 

I do have a audio transformer ready to be tested.  Initially setup include a tapped Toroids inductor,  1N60 as detector.  Does it work?  yes. definitely.  It produce sound in first place with my SH090B.  Both antenna and detector connect via  tap.

Later on,   the most interesting thing change  to  get more volume  or pull more station out of it.  Long antenna should matched to the tank by some method, because Antenna's capacitance, inductance will affect the Tune Tank,  at least i been told so. 

Connecting the detector to top of the tank, It's suppose to get most sensitivity,  alternately,  If detector connect to some taps,  we should get more selectivity.  I try to connect the diode Detector to top of the tank,  but very disappointed about the result.  There is barely hearing  anything.   My detector Diode is 1N60, instead of IN34. 

Top connected Detector does not working, This is wired.  Using  another wind couple the long wire antenna do no good too . I  tried the "NO Capacitor" crystal, It works,  without the 365 pF variable capacitor.   This suggested the antenna or the Detector introduce a huge amount of capacitance, must be take into considering. 

    * Taps usually on the bottom of the tank, not in the top as I did.

    * The Inductor work with  the 10 pf -- 360 pF Capacitor, covering 500 khz - 1.6 Mhz,  but I did not considerate the parasitic capacitance, which is more than 40 pf,   This tonally  ruin the tuning range.

It's too late when I  recognized that this parasitic capacitance is the cause of lots of my trouble.  Top connecting diode Detector  should work well. There is no sound in my setup just  because parasitic capacitance make it can not tune to local station any more ( i can only get stations around 1.2 Mhz ).

My new experiment is  flip the inductance upside down, and  couple the long wire antenna use another variable capacitor. This  topology introduced by << A Great Teacher: The Crystal Set >>, published on QEX, Oct, 2008.

Unfortunately, It's still pretty bad to my crystal radio. I barely hear the station by hooking diode on the top of tune tank, and It's also cause by parasitic capacitance.  Connecting the 1N60 to tap of the tank works same as before.  The antenna match variable capacitor actually dominated selectivity, instead of the tune tank !!

Talk about the selectivity,  It's barely no selectivity.  The strong station take almost all of tuning range.  Even connect the detector to the  very low end of tap could not make it better.

Before move on. I decide to build an audio match box.  It's simple but looked fancy,  very convenient to change impendance which presented to tank form the detector. 

My Antenna is random wire even though i call it long wire, there is no ground at all, I connected to metal inside room. Using the wall power ground won't improve it. The station volume even reduced a little bit with a wall power ground. But the selectivity get better.

The Counterpoise hit me, Its popular idea, especially to the transmitter.  It worth a try for my radio.

1N34 forwarding voltage is around ~ 300 mV. The 1N60 forwarding voltage is around  200 mV.  But the 1N60 is a defected.  It's reverse resistance only 800 k (measuring by DVM) . Some of 1N60 is even only  300 k.  The 1N34, instead,  at least several mega ohm.  

 1N60:   200 mV Forwarding,  3 k   forward,   300 k  --- 1 Mohm   reverse.

 1N34:   300 mV Forwarding,  30 k forward,  1 Mohm - 5 Mohm  reverse.

This explain this:  while 1N60 connected to top  of tune tank, i cloud not hearing anything,  1N60 will introduce too much load, might be capacitance, ruin the tune range.  1N60 need to hook on the  very low tap to get good sensitivity. 

1N34 is much better,  sensitivity is good if connected to top of tank. 

 STRONG signal is the key for crystal set.  Selectivity and sensitivity changes with signal strength.

The counterpoise is useful. It's effect is obvious, especially for selectivity.  While signal get stronger,  Crystal set could pull out 200 uA current with counterpoise(1N60). Without counterpoise, there is  only  tens uA.  Connect the counterpoise to ground change the tune range too,  but not that far than that bad 1N60 does.

Tapped antenna and Tapped detector is isolated well.  With diode detector connected to one of the taps, different diode won't affect the tuned range.  

With both Ant and Detector connect to top of the tank, the diode is key factor.  1N34 is much better, 1N60 introduced lots of capacitance to tune tank.

What about the idea from << A Great Teacher: The Crystal Set >>?   

Yeah, tuning the antenna couple capacitor achieving  good selectivity, even dominate the whole selectivity.  This could be my fault,  the 2 capacitor need sync up, like Regen receiver.  I did not spend too much time on this. 

What i learned

• Signal condition is the first important factor to Crystal Radio, in urban area,  it could be very good.   But away the station 20 km and with lots of shadow building, it's could be hard.

• A counterpoise does help a lot.

• Diode need be selected, like my poor quality 1N60. 

Reference:

http://www.bentongue.com/xtalset/19mstry/19mstry.html

http://www.bentongue.com/xtalset/xtalset.html

Setup New Antenna

Jan @2019

Old Antenna just a relatively long wire, this new one will be Inverted V with two 15 meter leg,  resonant frequency theoretically around 5 Mhz.  This time use a insulated silver wire, might be protected from damp air.

ShortWave Crystal Radio

Spring is coming. It's winter holidays  of son's school.  I would like introduce him the little crystal radio, bonding with him by fun stuff.  Everything seems fine, He could winding his own inductor and soldering everything together.  Except one THING,  there is NO sound at all, NOW it's  embarrassing.

I recall the whole thing I did,  and IT HIT ME,  for all the time, most of strong station i received, It's Shortwave Station, instead of MW radio.

I wrote this text as a number one rule of tasting the Crystal Radio:

To ensure successful to sound a   Crystal Radio ,  don't limited your receiver to Middle Wave band only,  Use Taped Coil, scanning 500 khz to even 30 Mhz Radio,  with a powerful antenna as you can setup.  the Medium Wave need good grounding, don not count on metal pipe or your Power Cord Ground.