Loop Antenna Crystal Radio

A loop antenna crystal radio is of interest on account of its selectivity and portability. But, with respect to signal strength, it is no match for a crystal radio using an external vertical wire antenna and earth.

If signal strength could be considerably improved with increased loop size, then its portability would be compromised.

As usual, 2 configurations are possible - parallel-tuned-series-fed and series-tuned-shunt-fed.

1. Parallel-tuned-series-fed version 

Parallel-tuned-series-fed Loop Antenna Crystal Radio
2. Series-tuned-shunt-fed version

Series-tuned-shunt-fed Loop Antenna Crystal Radio
The loop antenna was rigged up using a 4-legged stool as a makeshift frame. The stool was laid on its side on the table, making it convenient to wind the loop on its legs.

The 4-legged stool used as a makeshift Loop Antenna frame
Both the versions were tested with the plane of the loop oriented towards the local 612 kHz, 200 kW AM broadcast station.

Signal strength was the same with both, though a far cry from another crystal radio using a vertical wire antenna and earth.

Current-operated 'S' Meter for a Crystal Radio

An 'S' meter, for a crystal radio with high-impedance phones, is generally a high-resistance microammeter connected across the phones. In other words, a sensitive voltmeter that does not bypass headphone current significantly.

On the other hand, for a crystal radio with low-impedance phones, it's a low resistance microammeter connected in series with the phones. That is, a sensitive ammeter that does not drop headphone current signnificantly. A voltmeter was discovered in my junkbox to build such a meter for my crystal radio which uses low-impedance balanced-armature phones.

On removing its series and shunt resistors, the resistance of the 1mA FSD movement turned out to be only 60 Ω and hence quite suitable for the purpose. The existing graduations on the meter face were removed and replaced with graduations 0 -10 using a lead pencil.

Crystal Radio S-Meter - Schematic
The meter was already mounted on a square bakelite flange. The flange mounting holes served as terminal mounting holes, obviating the need for an enclosure.

Crystal Radio S-Meter
The result - an interesting weekend project and a handy crystal radio test aid.

Motor problem

It was just another day at the plant.

A colleague from the neighbouring department called to say he had a problem.A replacement three phase induction motor, drawn from the stores and wired, just refused to start.

Our exchange :-

Voltage okay? 
Yes, all 3 phases terminal voltage 415V okay.

Maybe an overload or single-phasing. Does the overload relay trip?

In that case it has no winding!
You crazy?!

Okay, I'll check.

A little later the telephone rang again.

How did you guess?!

It turned out that someone in the motor rewinding department had sent an unwound motor back to the stores instead of a rewound one!

Some thoughts on shack earthing

The main purpose of mains-supply-earthing (third pin) in the shack is to blow the fuse in the event of a live-chassis fault.

Protection against static build-up / lightning strike is another matter altogether.

As a SWL in the mid-1970s, the antenna I used with a Philips transistor portable receiver was a long wire at a height of about 50 feet. One summer afternoon, the receiver front end (AF117) blew right after a loud crackle of static.

Then again, my first tube homebrew CW rig had a 3-pin mains supply plug. The antenna was a straight dipole 50 feet high. Operating on a summer afternoon, I received a jolt through my Junker CW Key and survived to hear the crash of thunder from a nearby lightning strike. I immediately yanked the twin-line feeder and threw it on the floor (upper floor of my 2 storey house). After a few moments I was surprised to see the arc from the banana plugs to the cement floor.

Those days, in our sparsely populated area, my antenna was way above other surrounding structures. During thunderstorms the static build-up on my 2m ¼ λ ground plane antenna would cause a whine in my 2m receiver as it dissipated through the front end coil to ground.

Conditions are totally different now, with my 2 storey house surrounded by high rises, cell phone towers and other structures with lightning arrestors. Problems of static build-up and lightning are a distant memory. I have not heard that static whine for years, even though I still use that ¼ λ ground plane antenna.

And my shack earth is just mains-supply-earth (third pin)!

Of course, if you live in the countryside, the lightning arrestor at the feeder entry-point, with a separate earth bonded to the mains-supply-earth, is a must.

While on the subject of bonding – a friend’s shack and attached bath had a separate safety earth, not bonded to the mains-supply-earth. His 2m antenna was mounted on a metal mast which was also separately earthed.

For reasons not known, the shack earth was open when the water heater developed a ground fault. The path of the fault current was through the floating earth wire, 2m rig, coax feeder braid, antenna mast and finally to ground, resulting in a coax cable fire. A parallel path also caused considerable damage to a Drake AC4 power supply.

Fortunately he had the presence of mind to pull the main breaker before he doused the fire. 

Rugged 807 & 1625 tubes

The good old 807 and 1625 tubes can take a real beating.

One night decades ago, when I was working DX on CW with the shack lights off, the room suddenly lit up. The cathode-keyed 807 final was white hot! I let go of the key and cut the power.

Since I was monitoring the exciter signal, I was unaware of a break in the drive and had continued to send.

After the rig had cooled and the fault rectified, it was back to normal operation again. I still have that tube.

Lightning jolt

It was a typical summer afternoon in the year 1977.

I was working CW on 40m - my homebrew EL84 rig feeding a straight dipole antenna with telephone drop wire as feeder.

Meanwhile the sky became overcast and there was a continuous rumble as I continued to work.

All of a sudden I received a jolt through the CW key followed by the crash of a thunderbolt.

Without a thought I yanked the feeder off its sockets and threw it on the floor. My heart missed a beat when I saw the arcing between the banana plugs and the cement concrete floor.

The following morning's local newspaper headline was 'Lightning stuns football players'. It was a direct hit at the nearby football field. Fortunately only a couple of them had fainted and there were no casualties.

Drake TR-4C - receive problem

The old workhorse seldom gave problems on transmit.

A frequent receive problem was the receiver going dead on switching back to receive from transmit.

The problem was solved by replacing custom C145 (4 electrolytic capacitors housed in a can) with individual capacitors.

Capacitors drying out was the root cause.