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2018-11-15, 23:01:49
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Author Topic: Crystal radio weird reception - 1 Mexican station  (Read 440 times)
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Thought i'd write this down, because of the odd reception.
I've been looking at crystal sets again, just to see about coil design and especially to try basketweave designs.
Firstly though, I wanted to quickly try out my Ground and antenna, to check all was well.
The Ground is a copper pipe in the earth outside, with a 10ft 18AWG wire to my bench.
The antenna, is a 26awg wire up and across the adjoining rooms ceiling, total length approx 30ft. Nothing fancy, tried and tested years ago.
Germanium diode, olde worlde telephone earpiece, Coil is on a 2.2" PVC pipe, with 65 turns and taps at 5, 18 and 30 turns.
To the design on this page: http://www.techlib.com/electronics/crystal.html
The other circuit built to test is similar, but using 22AWG.

Here's where it gets odd.
I live near Tulsa, OK, a few miles South.
Here's the radio locator page for the general area and stations:  https://radio-locator.com/cgi-bin/locate?select=city&city=74401&state=OK&x=0&y=0
In the 'old' days a few years ago, simple builds would get the local big stations easily, KRMG and KFAQ, both 50,000W Clearchannel megablaster things. Also religious radio stations and could tune in at nights to Chicago, Southern Texas, Nashville etc.
But NOW, all I get is 1 Mexican station  ;D
Listening and trying to understand, I heard 'Los Americas' several times, before/after commercials etc.
Looking that one up: https://radio-locator.com/info/KMUS-AM?rd=427890
It's again in Tulsa, but only 7000W.
OK, so, perhaps the antenna is more directional or something...except, last night both circuits could only receive 'Los Americas' and the same is true today. That station only puts out 250W at night !!!!!
How in the heck can the main 2 blasters and any local stations be getting nowhere, yet I can clearly hear something with 1/4 the power of a microwave oven ?
It certainly doesn't present itself as a swamper, being 50 miles away and with so little power.

Something is wrong, but am stumped on what it is. Changing the taps all over the place just moves that station around.
Mind you, if you are Mexican and/or like Mexican music, i've built exactly the right thing !

 :-\


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ʎɐqǝ from pɹɐoqʎǝʞ a ʎnq ɹǝʌǝu
   
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If there's a swamper you can normally detect them by replacing the tank circuit with an RF choke. This desensitizes the circuit and opens up all the higher frequencies simultaneously with a few humps here and there. If the station is very close to you then it'll still be playing away in your earpiece.
   

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If there's a swamper you can normally detect them by replacing the tank circuit with an RF choke. This desensitizes the circuit and opens up all the higher frequencies simultaneously with a few humps here and there. If the station is very close to you then it'll still be playing away in your earpiece.
Sounds like he's already doing that, inadvertently.    :D   

Maximize Q to maximize selectivity.

Maybe of interest:

https://www.youtube.com/watch?v=cBEhtzGcYW4


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"The easiest person to fool is yourself" -- Richard Feynman
   
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Selectivity...that was the word I couldn't remember  C.C
Thanks for the hints and tips guys. Am aiming to change the layout over the next few days. Including different coil designs. For once, I won't mind the shouty religious radio stations one little bit.
I especially like that coil within a coil approach in the video TK.
"Spirit radio mode"  ;D

I might well try the aerial caps. Presumably pF types between 5 and 50 ?


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ʎɐqǝ from pɹɐoqʎǝʞ a ʎnq ɹǝʌǝu
   
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Built one of the 'Australian' crystal sets this evening.
It's certainly a nice looker of a design and the different connections were worth trying out.
My second coil in the middle didn't seem to do anything, so a 0-512pF variable cap went in series with the aerial. That worked !
I was able to listen to......*drum roll*.....the Mexican station  ??? 


Edit: Rewatching your vid TK, I only now realised the inner coil is part of the winding of the outer coil and why the instructions said to solder to the 2 separate pieces that connect the inner coil to the outer coil. Also, I used 65 turns on the outer coil, not 30.
 
« Last Edit: 2018-10-28, 05:43:41 by Slider2732 »


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ʎɐqǝ from pɹɐoqʎǝʞ a ʎnq ɹǝʌǝu
   
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Here's a variant, on the no pF tuner line of thinking.
A coffee jar lid, with 30 turns of 28AWG, center tapped in case of altering it to the Australian twin coil design later. The thinking being to decrease the turns, but variably increase the inductance with a piece of ferrite.
It works fine.
A large donut ferrite piece sits in the middle and the only single station I can ever receive at the moment comes in just as strongly as with any other design.
A TV yoke ferrite ring worked and looked somewhat impressively overkill, but had to be held 1/2" or so above the base of the coffee lid, for strongest reception. 


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ʎɐqǝ from pɹɐoqʎǝʞ a ʎnq ɹǝʌǝu
   

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Frequency equals matter...


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