Short Wave Coil

[Hartley118]

The puzzle remains as to how it is that your original Slinky steel coil gives reasonable results at MHz frequencies.

Skin effect perhaps? - and the flat wire Slinky coil has a pretty big surface area, maybe compensating for the lower conductivity of steel compared with copper?

Martin

[pwdrive]

The big TRF looks complicated, has a look of multi-waveband and mains powered as well      I note some metal valves in it.     I prefer the glass versions myself.    If any metal ones fall into my hands, I get rid of them as soon as possible.     I eventually converted my HL2 receiver to an EF91 receiver as EF91s are very cheap and plentiful.
Bob

Yes, mains powered from a separate PSU, doesn't look pretty does it, it was one of my experimental lash ups, it looks more complicated than it is, it's just an RF amp and Regen detector with a mixer in between and a bit of audio afterwards, two of the valves on there could be dispensed with, one was used temporarily as an experimental osc for the mixer, the other to give some umph to the audio.

In the main I did not use that osc, I used my bench sig gen instead to set the band, the advantage of that receiver was it's constant tuning rate and range on any shortwave band you wish, the tuning range of the detector was 500kHz centered on 3.25MHz that made it ideal for SSB or CW reception on any band as it also included a small value band spread capacitor.

That set up could be done with 1.4 volt battery valves or even ECC82's running at 12 volts or dedicated 12 volt valves, the metal valves and other bits were all I had to hand, have to say though the metal valves are good, a good reliable RF screen unlike a lot of EF89's etc, anyways performance was excellent, received JA Hams on a couple of occasions and VK also, one VK was only running 10 watts. Pity I never got it all tidied up.

The Regen detector was configured as a Hartley.

Anyone recognize the chassis?

Lawrence.

[Sparks]

It never even crossed my mind that it would not work when I was making it. As far as I was concerned, steel is a good conductor, and it is in coil form which gave it inductance. Connect it across a capacitor, and I could not see how it could fail to work.
TRF - What is the oscillator for? I thought you only needed one in a superhet!
Bob

[pwdrive]

You suggested it was a TRF not me....

That said, the radio frequencies are tuned, yes you can class it as a superhet but not in the sense of a fixed IF and ganged RF/Osc as one would normally expect.

What range does the Slinky cover?

Lawrence.

[Sparks]

It was the word "regen" that made me think TRF. I have no idea what range my coil covered, but I know it was SW because I was picking up stations from the other side of the world. The set had an EF39 in it, followed by a 6V6.
Bob

[Sparks]

Here is the first version of the coil with the EF39 in the middle.   The 6V6 output is lying on the bench.     After I found that it worked OK, I built it neatly with the valve outside the coil.
Bob

[Mike Watterson]

steel is a good conductor

Actually it's very poor, though RF has a skin effect, so a polished and varnished coated surface or silver plating makes a huge difference. The higher the frequency, the thinner the better layer needs to be. The reason for Litz wire is partly to avoid skin effect (many separately insulated strands), it's also used because it's easier to "wave wind", less slippy and more flexible.

Magnetic materials also have a deleterious effect on RF circuits and can increase losses.

"Q" is "quality factor". Parallel resistive leakage in capacitors, series resistance in capacitors or coils, magnetically induced eddy current losses, dielectric losses (anything that heats up in a microwave oven has likely high dielectric loss, and plastic/ceramic/glass that stays cold is low loss) all reduce Q, which is bad. If Q is too high (you deliberately want losses, wider band etc) then a series or parallel resistor is added, depending on circuit.

Thus 4,000 Ohms headset not only gives more level than a 600 Ohms on a crystal radio, it gives sharper tuning. A crystal earpiece may need 10,000 Ohms across it to fix frequency response as well as provide a DC load for the diode / crystal detector. The Valve or ZN414/MK484/TA7641 IC or JFET has sever megaohms input impedance so is little load (High Q) on the tuning coil, whereas an ordinary transistor might be 2,000 to 20,000 Ohms input impedance and thus Q is very low.

This why a valve superhet with only 2 IFTS can perform better than a four IFT transistor set. Also the older IFTs are high Q and the miniature pot core IFTs may be very much lower Q.


Surprisingly despite fashion for it, Gold isn't that good a conductor though better than steel, it's advantage is lack of corrosion, though if there is any dampness it increases corrosion of other nearby metals, especially tin. Very many metals are better than Iron, Steel, Nickel etc. Tin is far better than most steels but so much worse than copper, that only copper power and audio lines should have tin plating, copper for RF should be silver plated, or polished and varnished. Gold plating copper PCBs is bad practice for RF, due to the intermediate plating required. Usually silver is used with a protective coating.


Poorer Q "damps" the response, wider band, less sensitive. Higher Q gives narrower tuning and more signal. High Q is like a frictionless pendulum in a vacuum chamber. Very low Q is like a rusty pivot for the pendulum in an oil bath. A compass needle in air is high Q, takes ages to settle. One in a liquid is "damped", low Q.

Slinky springs are used as MW or LW crystal radio coils because they are cheap, readily available and un-insulated so fixed capacitor and a metal strip as a "tap" can tune. However a copper coil on a glass bottle with the enamel rubbed off with emery paper/ nail board/wet& dry will work better. Or 1.5mm solid copper wire, polished, varnished and then the connection area sanded will also be far better.

[Sparks]

The longer I live, the less I seem to know!      
Bob

[Mike Watterson]

The longer I live, the less I seem to know!      
Bob

Pure wisdom and a sign of knowledge.

The teenager thinks they know everything.

There are huge areas of Electronics I don't know and have now decided I won't learn. Instead I'll write books and fiddle with the Electronics I have.

[Sparks]

I am a writer as well      I went for years getting everything rejected.   Sometimes, I wondered why they didn't send a book of rejection slips to cover my next dozen manuscripts!        Then I hit on the much sneered at "vanity publishing," where I pay for them to be printed exactly as I like them, and sell them myself.    6 printed books, and quite a few e-books.      Amongst my many other duties, I was lumbered with the job of editor of the ship's newspaper on Union-Castle Mail steamers 46 years ago and that got me interested in writing!
Bob