Crystal Radio Receivers.

[ionburn]

My addition to the pot:

I used a long wire in the late 70's / early 80's and remember it must have been tuned as I got a specific French station using only a diode and earpiece (it was confusing me as I got little else with a crystal radio setup). This indicates to me that tuning the aerial may be utilised to good effect.

I also think that like with other systems, unless great care is taken with design more components mean more losses. In my book usually simple is best!

[G6Tanuki]

I dabbled with crystal-sets in the 1970s. I found the following:

1] Aim for the highest-possible 'Q' in the tuned-circuit. 'Q' can be thought of as gain! Big air-spaced variable capacitors worked better than the JB "Dilecon" types or miniature Polyvaricons. Some plastics make horribly-inefficient coil-formers. I found plastic 1.5-inch waste-pipe to be good, 2-inch PVC drainpipe, waxed toilet-roll cardboard cores etc not-so-good.

2] The L/C ratio of the tuned circuit was important. Coils are lossy (their windings have resistance so lowering the Q) - instinct would therefore suggest using a coil with very few turns and lots of "C" to resonate it - but that produced a tuned-circuit that was rather broadly-tuned. You needed to balance "sharp" tuning [lots of L, not so much C] against low losses [lots of C, not so much L] i found a rough figure of aiming for "1 Picofarad of capacitance for each Metre of wavelength" and cutting the coil accordingly worked well.

3] The detector should present the lowest-possible load to the tuned circuit. if you're using low-resistance 'phones [something like military DLR5s ] you need to tap the detector down the tuned-circuit.

4] Coupling the antenna to the tuned-circuit is a compromise - you want 'tight' coupling for maximum energy-transfer, but loose coupling to stop the antenna from damping the tuned-circuit! I did various experiments with winding the antenna coupling-coil on a separate length of pipe that fitted over the main tuned-circuit and which could be moved up and down to vary the coupling.

5] When it comes to antennas, get them as long and high as possible. A 200-foot longwire at 40 feet was brilliant. I used the central-heating radiator in my bedroom as the 'earth'.

Done sensibly a crystal-receiver can perform surprisingly well: from the Wales/Shropshire borders I could easily receive Luxembourg [208M as I'm sure you remember] after dark. I built a 9V battery powered 3-transistor amplifier [BC109, AC128/AC176] which would give impressively-loud and good quality loudspeaker reception of Radio 1 [247M] during the daytime; people hearing it found it hard to believe how simple it was!


Keep on experimenting...! I've got a 49-metre-band shortwave crystal-set here - fed with a 100-foot longwire I can sometimes hear the sunday-morning free radio stations around 6200-6300KHz!

[AC/HL]

Back in the crystal set/early valve days, 100 feet was the standard to be aimed for. A variable capacitor was often fitted in the aerial lead for matching to the receiver. The aerial was simply fed through a window frame using an Ebonite tube as an insulator, nothing fancy.
DLR5s were commonly available. They're actually quite low impedance, but very sensitive to make up for it. Oddly, the earth on it's own can act as an aerial, anything to get a signal to detect. I once had reception without a diode, but it was a very local station.

[Herald1360]

I believe that 100ft figure was something vaguely legal limitish related to the PMG's Wireless Receiving Licence conditions.

It's still only about a tenth of a wavelength on medium wave.