Microbore copper tube coils

[SurreyNick]

I came upon this homebrew replica '1929 TNT Transmitter' set while searching the web. It uses 1/4" microbore copper tube for, what I assume are, the aerial tuning coil and a reaction coil. What I'm interested to know is:

(1) How would one calculate the number of turns needed for, let's say, a MW coil, or is it simply trial and error?
(2) How effective would it be in regards to selectivity and sensitivity?

Thanks

Nick

[G8HQP Dave]

A MW coil in copperbore would be rather large! Think in terms of a hundred turns or thereabouts.

There are various formulae used for calculating inductance of a solenoid, and some appear as 'calculators' on the web. Google is your friend.

[Bazz4CQJ]

I'd guess that the circuit in the picture resonates up around 30+Mhz. The MW wave version would look like something that's been removed from a steam engine, or a high-quality liquor still?

B

[SurreyNick]

Mmm. The smallest readily available and affordable microbore copper, which would produce a self-supporting coil i.e. no former required, would be about 4mm diameter. A five inch diameter coil is certainly realistic for the breadboard TRF design I have. I will have a look on the web again, but haven't managed to find anything which provides the calculations for anything other than standard wire gauges. If anyone can point me to a calculator which would accommodate the dimensions given herein I would be grateful.
Thanks
Nick

Ignore the size of the wire/tube a close approximation will be found by using the diameter, turns and spacing using one of the online calculators. The first one on Google http://www.extremeelectronics.co.uk/...?page=isol.php has arbitary 'wire' size too.

Having a bit of fun for 4mm tube spaced 8mm centres it would be 300mm long, 170mm diameter (53 turns) to resonate at 500kHz with a 500pF capacitor, one big coil!

[turretslug]

That's one LOUD pantry transmitter- Blenheim Palace?

That's one LOUD pantry transmitter

Rather depends on what you drive it with, a SOT23 transistor will do more than needed for home use but look silly!

[Ed_Dinning]

For that size coil it has to be an 807!!

Ed

[Radio Wrangler]

If a valve is picked on matching the looks, I should be able to hear it up here...

David

[SurreyNick]

Yeah, OK.
But if I used 3mm diameter copper wire (SWG 11 gauge) and a 5 inch diameter former, married with a 300pf variable cap, then by Wheeler's formula that would give me a coil of c. 7.5 inches and 63 turns for the whole MW band wouldn't it?
Nick

[m0cemdave]

Looks about right.

The formula I have always used, which gives a close approximation for inductance of a single layer coil, is:
d^2 * n^2 / (18d+20l)
where d= coil diameter, n= number of turns, l= coil length, all in inches.

333 uH in your case

[SurreyNick]

Cool !
Now I just need to find out where I can buy SWG 11 gauge enamelled or silver-plated wire at an affordable price
Nick

[m0cemdave]

Don't forget that with bare wire or tube there will need to be some space between each turn, which will increase the overall length of the coil to be used in the calculation.
If you want high Q, spacing at wire diameter may be a good idea, which will nearly double the length.

[SurreyNick]

Originally Posted by m0cemdave

with bare wire or tube...If you want high Q, spacing at wire diameter may be a good idea, which will nearly double the length.

Fair point. In which case I might have to include a ferrite rod to reduce the number of turns. Trouble is there is no way I'm going to find a 4 inch diameter rod and I have no idea how effective a 1" rod inside a 5" diameter former will work. I could of course opt to use enamelled wire. The radio will have a reaction coil and also a variable reaction condenser with which I can adjust the Q but I will probably have to experiment a bit to see if I can find an arrangement that works at an acceptable size.
Nick

I would't bother with silver plating at MW frequencies, just polish and varnish to make it look good. 'Compo' brake pipe is cheap and shines up very well.

[SurreyNick]

Originally Posted by merlinmaxwell

'Compo' brake pipe is cheap and shines up very well.

What a fab idea! I shall definitely give that a try Thanks
Nick