The Xtal Set Society
The Xtal Set Society
profile | register | search | main site


email us
Forums | Rap N Tap Clubhouse - Basic Crystal Radio Fun! | Oatbox radio Post Reply Send Topic To a Friend
Author Topic
gzimmerPosted - 9 November 2012 20:34  Show Profile  Email Poster  Edit Message
> My circuit is just like the one at
http://www.crystalradio.net/beginners/index.shtml

This has come up before, but that circuit has no provision to tune various stations. The tuning is fixed. In fact the diode clip lead does virtually nothing.

Better to run the antenna to the clip lead. That way the inductance in series with the antenna can be changed.

Better still to use two clip leads: On for the diode and one for the antenna.


............ Zim

Garry NicholsPosted - 10 November 2012 7:42  Show Profile  Email Poster  Edit Message
T;

I agree with Zim. You could try two different approaches:

1) Put a clip on your antenna lead in and tune the radio by moving it up and down the taps. (Ignore the antenna fahenstock clip shown in the diagram for now.) And, it will usually work out best if you clip the diode to a tap below wherever the antenna is clipped at the moment. Usually 50% up from the bottom or less (see a general rule, below, on this).

2) To the above, attach your variable capacitor between the antenna clip and ground. The cap will allow you to tune more like a "regular radio". Try different taps with the antenna/variable cap clip. Different amounts of coil used will give different tuning ranges for the cap. At night you might even hear some short wave when small amounts of coil are tapped.

Another general rule: the farther up the active part of the coil you place the diode tap, the more volume you will get. But, this is at the expense of selectivity. The farther down towards ground that you place the diode tap, the better will be the selectivity at the expense of volume. (No free lunch!)

Because there is only one tuned circuit on a crystal set like this, the selectivity will never be outstanding. You may always hear multiple stations at once, at least to some degree.

It all depends on the strength of signals in your area. After dark, strong signals my propagate in from far away by bouncing off the ionosphere, but they will likely fade in and out.

Am I being too elementary on all this? Not sure what your background might be or what you might have been reading.

Garry

homebrewPosted - 10 November 2012 9:3  Show Profile  Email Poster  Edit Message  
Garry :
The attic antenna was designed over several months. The optimal point was where adding additional spacing (6 inches) did not increase the signal strength across the Benny a measurable amount.
My aim was to get a good antenna up without blocking access to the area.
I tried one to 6 lengths of wire and ended up with a three in a triangle reflecting the roof peak shape.
A forth wire making a diamond shape did not show a marked signal increase. Two more beside it making a triangle with 6 wires brought in some good DX but was too low to allow easy movement around the attic space.
The 18 inch spacing keeps cropping up. Worked best when adding additional legs to my longwire, in ground counterpoise system designs, and old literature on flat top antennas.
With the short antenna I would use the variable in series with the antenna first.
Then add turns tapped every winding on the ground side and gator clip both antenna and gnd.
TeamongerPosted - 10 November 2012 15:6  Show Profile  Email Poster  Edit Message  
Garry,
No, not being too elementary. Pulled the antenna lead from f-clip and hooked it onto various taps, yes that does allow some tuning. When you say "attach your variable capacitor between the antenna clip and ground", you mean run it straight from antenna lead to the ground clip? Sounds like the cap lead should have a clip on it to attach to the taps along with the antenna, am I picturing that right?

t

homebrewPosted - 10 November 2012 17:16  Show Profile  Email Poster  Edit Message  
There are a couple dozen configurations for a oatmeal box set.
Putting the variable across the A/G terminals (across, parallel to the coil) is a quick and easy way to get improved performance.
Putting a gator clip on the antenna lead in will also give you additional tuning improvements with or without the variable.

Edited by - homebrew on 11/10/2012 7:39:13 PM

Garry NicholsPosted - 11 November 2012 8:11  Show Profile  Email Poster  Edit Message
T ;

I was short-cutting with the words a bit. I was suggesting that as an alternative to varying the antenna lead to various tap positions in order to tune, you could try the variable cap. It would span the "active" part of the coil. By "active" I mean length of coil from the point that the antenna clip is attached down to the ground at the bottom of the coil.

So, you could vary the point where you tap onto the coil with a clip attached to the antenna wire, and the variable cap would also be attached at that point. You could wire it up any way that you want. Attaching a wire to the variable and clipping it to the same tap on the coil as the antenna (or to the antenna clip) is a good approach. Or, you could use the same clip both for antenna and cap, running both the antenna wire and the wire from the cap to the clip.

The variable cap can be stuck down with double sided tape or Scotch/3M foam mounting squares or tape (one of my favorites). I could be stuck down on its back with the shaft pointing up or on its bottom with the shaft pointing horizontal.

You should put some sort of insulating knob on the cap shaft, otherwise your body will become part of the circuit when you touch it and it may effect the tuning.

Wiring the variable cap: I suggest wiring the frame to ground and the stationary plates (stator) to the "top" of the active coil segment. There will be solder lugs for the stator (often one on each side of the stator plates). There may or may not be a lug for the "rotor" plates. If not, attach to the frame with a clip or under a screw if there are tapped holes in the frame. WARNING ! When using screws with a variable cap, carefully check to make sure that they don't reach through and touch the plates when screwed in (use your finger tips to screw it in for this check). This will short out the capacitor as long as it is touching, and if cranked up a bit will bend the plates, which can be a bear to straighten unless you are patient and quite handy !

Varying the length of "active" coil will move the range over which the variable cap tunes either up or down on the frequency spectrum. More coil turns will lower the range and vice versa.

I recommend tapping the diode about half way between the ground and the antenna point on the coil. When you find a station, try moving the diode tap one tap at a time to see if you can improve either the volume or the selectivity, whichever one is most important for that station with the rig wired up as it is at the moment. You will likely need to retune slightly when diode tap is varied.

How am I doing?

Garry

Edited by - Garry Nichols on 11/11/2012 8:20:20 AM

_J_Posted - 11 November 2012 13:6  Show Profile  Email Poster  Edit Message
Here are some ideas.

Most oat box radios are designed as a "coil only" tuner, they tune the antenna (which is usually capacitive since it is usually too short) by adding the missing inductance in parallel (ground-antenna wise.)

When you add the additional capacitance, you need to add more coil to tune the same station. It is kinda counterproductive except the capacitor is higher Q and raises the tuning sharpness a little.

Another approach is to use a fixed or variable capacitor in series with the antenna to decouple the low Q antenna, then add back the capacitance with another high Q variable capacitor across the coil.

From there, you can look at double tuning.

John Davidson

Garry NicholsPosted - 11 November 2012 19:58  Show Profile  Email Poster  Edit Message
John (J);

I was suggesting the variable go across the coil (parallel tuning). Doesn't a cap have to go in series with a coil in order to create a situation where you have to increase the coil turns to tune the same station as without the cap?

Garry

TeamongerPosted - 12 November 2012 16:34  Show Profile  Email Poster  Edit Message  
Well I added the little variable cap from the Elenco kit, parallel across the coil to the ground. It does add a bit of selectivity, so I can tune between 680 and 810. Moving the antenna hookup along the taps changes the range a bit more. I also tried a twisted-wire "gimmick" cap, but I can't detect any differences.

Probably time to build the Elenco kit and see how it compares. Thanks again for the tips, guys.
t

Garry NicholsPosted - 12 November 2012 19:7  Show Profile  Email Poster  Edit Message
T;

Do you happen to know the value of the Elenco cap? I had assumed it was around 365 pF, but the narrow tuning range that you experience leads me to believe it might have a considerably smaller value.

The value may be stamped on it somewhere or it may be in the literature that came with the set.

I guess that you wired it between the antenna tap and the ground? You might try moving the tap point downwards towards ground if you have not already, in order to use less turns of the coil. Take the cap down with it, with the frame still connected to ground.

Is this link showing your Elenco instructions:
http://www.elenco.com/admin_data/pdffiles/MX901C.pdf

I can't make out the circuit diagram from this PDF. But it looks like the cap is what we call a "polyvaricon", which is short for polyethylene insulated variable condenser (capacitor).

If so, it is likely in a clear plastic case with poly film insulation between the plates. Looks like a multi-level sandwich! How many connection points are there to it?

Garry

TeamongerPosted - 12 November 2012 19:36  Show Profile  Email Poster  Edit Message  
Garry,
The Elenco cap is labeled CBM-223, looks like this:
http://ntfs.diytrade.com/sdp/77422/4/pd-997194/300695.html

My Elenco kit is a different one,
http://www.elenco.com/product/productdetails/radio_kits=Mjg=/crystal_radio=MTAx

There is no online manual that I can see. It uses paper fasteners and washers to make connections. On the one hand, seems a tacky way to build a radio, on the other seems a clever improvisation. I'm almost ready to test it...
t

gzimmerPosted - 12 November 2012 20:53  Show Profile  Email Poster  Edit Message
Hi Teamonger,

If you have the antenna at the top of the coil, the self capacitance of the antenna will be in parallel with the tuning capacitor and this will reduce it's tuning range.

Best to have the tuning cap between the top of the coil and earth, and then tap the antenna down the coil as much as possible.

If you then find that the set won't tune low enough in frequency, you will need to extend the coil (add more turns of wire)

........... Zim

TeamongerPosted - 12 November 2012 22:55  Show Profile  Email Poster  Edit Message  
Hi Zim,
Yes that's what I was doing, more or less (tuning cap between the top of the coil and earth). I'm getting low freqs pretty well now, would actually like to go higher. I may play with my antenna length some more.

I assembled the Elenco kit, then hooked it up to the same ground and antenna as my oatbox, and must say the results are disappointing. I hear barely perceptible mumbling that I can't make out. Turning the variable cap doesn't seem to do anything, less than what it did on my oatbox. I rechecked all the connections, and no difference. Guess I'll test the kit diode and then the coil with the oatbox setup.
t

gzimmerPosted - 12 November 2012 23:5  Show Profile  Email Poster  Edit Message
T,

> I'm getting low freqs pretty well now, would actually like to go higher.

Just move the tuning cap down a few taps so there is less inductance in the circuit.

As before keep the antenna tap as low as possible.
(too low = very faint)

...... Zim

_J_Posted - 13 November 2012 1:26  Show Profile  Email Poster  Edit Message
Thats right, Garry.
I was 'thinking' series, but didn't say it? Maybe claiming to be 'thinking' is giving myself too much credit.

John Davidson

Topic is 3 Pages Long:    1 2 3

Click Here To Close Thread, Administrators & Moderators Only.

Show All Forums | Post Reply