SHORTWAVE CRYSTAL RADIO SET
By Patrick Smith (GW0VMR)
Introduction
Heres one of (hopefully) many! constructional ideas by the clubs QRP expert, Patrick - Here detailing how to build the simplest radio circuit out - the Crystal Radio Set, forget these ones you buy in Tandy (can't get them now anyway!) or Maplin, just gut an old radio for parts, get a plastic tube (or loo-roll) for the coil, and you're away.
Updated - May 30th 2001 - Added revised text
I
have always liked crystal sets as they suit my simple mind, and the history of
them is also very absorbing.
The
development of crystal sets came to an abrupt halt with the invention of the
valve. Although sets are still sold as electronic education sets, most of the
circuits use the basic layout with the resulting poor performance, but there is
a big upsurge in crystal set design now mostly due to the internet!
So
being a fiddler, here is my attempt at a set with fair selectivity that will
tune from 5Mhz to 15Mhz aprox!
The
main inductor is link coupled to the aerial and the diode detector is tapped
down the coil, this is to try and stop, minimise loading effects from the aerial
and diode, changing the "Q" of the coil which would result in poor
selectivity which is important to have on the SW bands.
You
can get some idea of how good the Q of a tuned circuit is if you own a GDO?
Wind two coil’s that have different formers. Now with L1 and a VC see
how far away you can obtain a dip with the GDO.
With L2 and the same VC set to the same frequency as L1 was, see how far
away you now can obtain a dip. If
L1 dips at 3inches and L2 at 5inches, L2 has the better Q.
If you are into ATU building this is a good test but of course it will
not tell you the value of Q.
L1
is 15 turns of 1.5mm electrical wire with the turns spaced apart by about 4mm
L2
is the coupling coil, 5 turns 1.5mm
over the bottom end of L1
For
a former use a 50mm diameter plastic tube about 110mm long.
I use empty bathroom sealer tubes, which have very good RF properties,
failing this use a toilet paper cardboard tube
For
your base, find a bit of wood about 4 inch's by 8 inches, this will give you
lots of room to play "no
attempt to miniaturise this set has yet been done” Now if you want a de-lux
model, varnish the wood first.
I
fixed the coil, capacitor to the base with a hot melt glue gun but any other way
is fine.
Everything should fit together like this:
Use
drawing pins pushed into the base as connector pads for all the joints, these
will hold heat from your iron for some time so be careful of dry joints.
The
output impedance of the diode detector is about 100k ohms but I find no benefit
matching this down to 2k for my old F J Brown headphones.
High impedance phone's are now highly collectable, If you don't have a
pair look for a set called DLR type 5 these are fairly low impedance phones but
work really well on crystal sets and they are still cheap at under a tenner.
( Birkett are now selling them for £5.).
Failing this, a cheap crystal earpiece from "JAB components"
will work ok, buy two and try to make a head phone set?
The
diode is a germanium type OA90/OA91 or similar.
Buy a few and experiment to see which is the best. You could also try an
old germanium transistors connected as a diode.
The
tuning capacitor is any type with a value of 270pf up to what ever you have and
the little plastic POLYCON types are ok to use.
So
there you are, a SW RX for power cuts, or just to have fun with. Play with the
circuit and change it as much as you like.
It would be really good to have a crystal set DX competition, so let HQ
know what you have heard.
Good
luck and best of DX