Crystal Set Season (III)

2020/3/25

MW Crystal Set

As I have a 15 meter Inverted V antenna which enhance the shortwave station but not enough for MW station. Meanwhile, MW signal is really bad at my place.

For years, I got confused by strong HF stations, long story. Anyway, I got several principle to pull out MW station instead of HF stations from my MW crystal set.

Don't not directly connect your long wire to any of taps to your LC tank. If connected like this, the long wire gonna resonant with small inductance (tap to ground), then HF station domain whole tune scale.

Considering this effect, obviously the antenna should couple to LC tank via a small capacitor. "hot" taps gonna better than lower taps .

If the HF station is strong enough, the only way left is double-tuned crystal radio.

Single tuned crystal radio could not filter the strong stations out, no matter what you do.

Double Tuned Crystal radio

DTC SET ONE

Fist double tuned crystal radio tested.

I suppose there need a capacitor couple to the left tank. It could separating the stations well, but hardly heard. Only at night, it's clear enough to heard.

Deploy a series resonant tank seems would works well. The long wire antenna is not long enough, means it's capacitive domination. The antenna resistance is very low at MW band, series resonant provide a very low resistance path to ground, sound so good.

Experience is also good enough, It's much loud than DTC SET ONE. Daytime detector current attenuated to 1/10th but still clear.

Two tune coil is identical and tight coupled share same ferrite Rod. The series tuned tank need parrallen the 2x365pf air variable rotator, and an extra 360p-720p fixed capacitor parallel with air capacitor.

Key Point Of My Crystal Set

Antenna

I had a V antenna, each arm 15 meter. Single arm is pretty short for AM, I notice connected 2 arm togher is better than single arm. NOTE: lately experiment indicate it's almost identical to single ARM and would adding more capacitance to tuned tank.

I guess a 30 meter -- 50 meter Antenna could be much better, I need one.

Connected the 2 arm together at indoor end.

Ground

For MW, ground is not optional. I did make my own ground, but it's much worse than wall-power ground. It's obviously the ground is poor. Suggesting ground system need 2 or 3 2.4 meter long rod, spaced 2 meter at least, hammered to deep ground. And, the best ground is metal water pipe, which is not available at all.

Alternative ground is that burry 10 Meter copper(at least) wire (20cm or more deep). Burry a copper sheet might works I guess.

Ground is critical for crystradio.

Double Tuned

Always use double tuned crystal set, othewise, It's not possible pull out MW signal from strong HF bands, for my short MW attenna.

Couple to Antenna

Always use tap. 1/5 - 1/3 hot position. My short antenna had very large equivalent capacitance. Station move on scale follow the moving tap to hot end, very obvious effect.

I suspect tap will impact the signal strength somehow, but I'm wrong, Tapped.

Double Parallel Tuned Crystal

both coil is identical, and use same air vary capacitor.
antenna connected to 1/3 tap of first coil. And detector tapped out from second coil, 1/3 too.

Notes:

couple antenna to top of tank could not make it louder, but make it hard to tune and
Connected diode to top of tank could not make it louder either(hardly discernible). I did not notice a volume improvement, selectivity is noticeably worser.

Use 2 identical tune tank, and a dual gang capacitor, TWO tuned tank could tracking well, and make tune much easier.

Series-Parallel Double Tuned Crystal

both coil is identical, tight couple.
Detector tapped out from second coil

Notes:

No tapp one first coil.
Connected diode to top of tank could not make it louder too.

As comparison,

Use same coil make we need a very large capacitance range to cover all MW band. In my case, it need 30pF to almost 3000pF. A 360pF steps capacitor bank adds to variable capacitor to make it resonant.
As benefit, I feel like Series-Parallel double tuned crystal radio is louder than Double Parallel Tuned Crystal.
Series tuned tank selectivity is much worse than first tuned tank of Double Parallel Tuned Crystal. The good side is, finding the station is easy. Use the second capacitor find station and use series one enhance volume.

Add Antenna coil turns

Add turns to antenna coil, reduce the capacitance required, and also make the selectivity increase. I might prefer low selectivity one.

50+ meters long Antenna Journey

Pretty hard to recovery the MW signal from a 15 meter short antenna (for MW). Daytime almost nothing there except pressing the earphone so hard to ear. Even in night, the station is still not strong, depending circumstances.

It's time to build a longer antenna. I build it even though not follow the best practice at all. I run the thin wire along the tree branch, no isolation.

Not Tall, Running along tree branch, Thin isolation wire

Quick Highlight

Signal 10+ times stronger than 15 meter antenna (comparing their audio current)
Daytime receiving is quite easy
Reasonable effect for each different configuration: long wire present less capacitance, make experiment free and easy
Do not need struggle to hear, so free your mind to compare different detector, get full view of them

Effect of Different Antenna

Light Couple

50 meter antenna could light couple to tune tank via 3-5 turns another coil to produce clear voice. For 15 meter antenna, get something heard is major problem.

1SS86/1SS108 better than Point Contact 1N60

1SS86/1SS108 had much better reverse cutoff performance. While I could easily get signal under 50 meter long antenna, I found the 1SS86 actually produce louder voice than 1N60P.

Audio Impedance after Diode DID Matters

Back to 15 meter antenna days, experiment is a tough work. My Audio match box seems not provide too much value to produce louder voice. But actually, impedance after the detector is a very important matter. Using 50 meter long antenna, and 1SS86, higher impendance after 1SS86 provide louder and clear voice.

Big mistake Made

I always use a 0.22 or 0.1 uF capacitor as filter. This is deadly fault, cause of audio impendance is high, this big capacitor almost kill the clear voice totally.

Use 100p to 1000p Instead !

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