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TeamongerPosted - 6 November 2012 12:18  Show Profile  Email Poster  Edit Message  
I just completed my first "simple" oatbox radio, pretty much following the plan on crystalradio.net. I used 12 taps instead of 8, one every 4 turns, a total of 50 turns. (I thought the additional taps might get me more stations.) My antenna is old speaker wire, dangled out my third floor window almost to the ground. My ground line (also speaker wire) goes out the same window, attached to the metal fire escape. I'm not sure it's a good ground.

The only station I can recognize is KSFN, a local Chinese broadcast out of Piedmont at 1510 AM. I can hear this on all the top 6 taps, also a more distant station in background. On the bottom 6 taps I can still hear it, but much fainter. Does this mean the frequency range of my radio is extremely narrow? Is there anything I can do to expand the range with this "simple" design?

I'm using an earphone that came with an Elenco crystal radio kit that I haven't assembled yet. It works much better than a similar looking white earphone I bought from Antique Electronics Supply, which seems to be a dud. I also bought my clips and diode from them.

I plan to put my antenna on the roof, and probably lengthen it by splicing in more speaker wire.

Now if I can only learn to understand Chinese. Oh well, some of the music is nice...

t

homebrewPosted - 6 November 2012 15:31  Show Profile  Email Poster  Edit Message  
It works like it originally did in the teens, designed to pick up a local. A better ground might give you sharper tuning but in today's higher dendity radio environment you need a variable capacitor in the circuit.
I have a cheap set of metal shelves and a bed spring that both work better than a short indoor wire antenna on simple circuits.
You can try several grounds alone and hooked together, house electrical gnd from a wall outlet plate screw or copper plumbing pipes.
On a oatbox set, metal shelf antenna, and house grounds I can usually pick up half of the eight locals, usually run together but tunable.
How close are you to the station? Is there a chance that it would be considered a boomer at your location?
Your current ground might work good as a antenna with a different ground.
Oatbox sets with minimal A/G connections do wierd things occasionally. Reversing the A/G connections, using each one by it's self as antenna or ground, or reversing the diode sometimes gives surprising results.
A oatbox by itself is limited but fun to play with. The coil can be used as a basic antenna tuner for other sets later or as part of a trap if needed.
_J_Posted - 6 November 2012 20:5  Show Profile  Email Poster  Edit Message
Welcome T!

The better antenna will probably expand your stations, but the oatbox doesn't separate stations very well as you will see.

John Davidson

TeamongerPosted - 6 November 2012 21:46  Show Profile  Email Poster  Edit Message  
Hello
TeamongerPosted - 6 November 2012 22:16  Show Profile  Email Poster  Edit Message  
Hi guys, thanks for the replies. I doubled length of my antenna and put it on the roof, and now I mostly get a different Chinese station (lucky me) at 1400, KVTO out of Berkeley. Only 1000 watts, but pretty close to me, as was the other one at 1510 which is 2400 watts at night. So it appears I have moved somewhat down the band, is that normally what happens when the antenna is lengthened? I'd like to get down to 810 for a high power station like KGO, is there any way to "point" it down that way? One problem may be, I have several high risers around my three story apt building.

I tried using the fire escape as antenna instead of ground, that didn't work at all. Then I turned the diode around, didn't seem to change anything.

I think my Elenco kit has a variable capacitor of sorts, maybe I could add that to my circuit. Where would it go on a simple radio, between the ground and earphone? Or would I have to go to the "advanced" two-coil design?

Having fun, appreciate the advice.
t

homebrewPosted - 7 November 2012 6:49  Show Profile  Email Poster  Edit Message  
Variable in series with the antenna first.
And parallel to your coil second.
Changing the antenna length changes it's natural tuning frequency. Adding a variable in series will do the same and give you a bit of control.
It sounds like your ground is not good enough to allow the coil taps to tune properly.

Edited by - homebrew on 11/7/2012 6:51:32 AM

Garry NicholsPosted - 7 November 2012 8:23  Show Profile  Email Poster  Edit Message
Hi T;

I can offer a few general rules regarding frequency coverage:

-- More wire on the coil will lower the range tuned, loosing coverage at the top of the band.

-- The more a variable condenser is closed (plates meshed) the lower will the tuning go.

-- The opposites of each of the above are true.

-- Same with the antenna and ground wires. Lengthening either will lower tuning and vice-versa.

Any sort of "ground" conductor will act (also) as an antenna until the point where it actually goes into the earth. Just because we call it a ground does not keep it from picking up signals just as a run of any type of conductor hooked to the antenna terminal. In effect, the set is "tapped in" along the antenna, with the base of the antenna being where the "ground disappears into the ground".

What type of material is your building and roof made out of? This may be effecting the antenna/ground, which sound like they are close to it or touching it.

On the "white earphone", do you know if it is a crystal or ceramic earphone or is it a magnetic or dynamic earphone? If you give us a part number we can check it on the supplier's web site.

Crystal earphones, also called ceramic earphones (look like big ear plugs) can be flakey. There are some so-so connections in them that don't always work right. I have some good sounding and sensitive ones from Mouser, but every now and then they get weak or suddenly cut out all together. I wack them in the palm of my hand or tap them on the desktop (sometimes fairly hard!) and they spring back to life.

May I also suggest eventually moving to two earplugs wired in series. That always seems significantly better to me on my sets.

Garry
neary Syracuse, NY

homebrewPosted - 8 November 2012 6:36  Show Profile  Email Poster  Edit Message  
On a oatbox set without a fixed or variable capacitor in the circuit you should be able to still pick up the locals, run together and each slightly tunable to max using the taps. I would guess that the ground circuit is not up to par.
krystalloPosted - 8 November 2012 7:24  Show Profile  Email Poster  Edit Message
Hey All,

I agree that a better ground (and antenna) might help.

Also the fact that the ground parallels the antenna isn't ideal either(signal cancelling effect?).

PERHAPS less coupling to the antenna via a twisted wire "gimmick" capacitor might be something to try and / or "less" ground ( counterpoise).

The other thing about many "Oatbox" plans out there on the net is that the taps are evenly spaced. Generally in the 1920's commercial tapped sets there are "coarse" taps at every 10 turns and "fine" taps at every turn or two turns.This allows a MUCH greater resolution of net coil inductance value and thus (in theory), better separation of stations .

IMHO,the evenly spaced tapped Oatbox is a fun SIMPLE set, but also a set that is pretty quickly and easily "left behind" in favor of a variable capacitor tuned radio (which should work significantly better than a tapped set , even in it's simplest form).

K

krystalloPosted - 8 November 2012 7:25  Show Profile  Email Poster  Edit Message
Hey All,

I agree that a better ground (and antenna) might help.

Also the fact that the ground parallels the antenna isn't ideal either(signal cancelling effect?).

PERHAPS less coupling to the antenna via a twisted wire "gimmick" capacitor might be something to try and / or "less" ground ( counterpoise).

The other thing about many "Oatbox" plans out there on the net is that the taps are evenly spaced. Generally in the 1920's commercial tapped sets there are "coarse" taps at every 10 turns and "fine" taps at every turn or two turns.This allows a MUCH greater resolution of net coil inductance value and thus (in theory), better separation of stations .

IMHO,the evenly spaced tapped Oatbox is a fun SIMPLE set, but also a set that is pretty quickly and easily "left behind" in favor of a variable capacitor tuned radio (which should work significantly better than a tapped set , even in it's simplest form).

K

TeamongerPosted - 8 November 2012 16:1  Show Profile  Email Poster  Edit Message  
Well I changed my ground to an outlet center screw; bought a long one and added a nut so it still holds the faceplate on. I alligator-clipped ground wire to the excess sticking out. I also use that outlet for TV, etc. Does that sound safe?

Suddenly instead of stations around 1400-1500, I was getting KNBR 680, again on all my taps. I may try reducing antenna length again, to see if that brings me up to 740 or 810.

To Garry: my roof is ticky-tack epoxy stuff, yes my antenna is just lying on it, with the end loosely coiled around a vent pipe. I don't think it's grounding out there, since the wire is insulated. The white earphone is a P-A480 from tubesandmore.com, definitely supposed to be for crystal. I think it's just a dud, but I'm not worried because the earphone from the Elenco kit works very well.

So K, what is a twisted wire "gimmick" capacitor? Where would it go in the circuit of the simple oatbox?

Thanks again guys,
t

Garry NicholsPosted - 8 November 2012 20:54  Show Profile  Email Poster  Edit Message
Hi T;

Not sure what you mean by roof is "ticky-tack epoxy stuff". Beside lying on the roof, I guess it runs up the side of the building? What kind of construction is the building? (This all has to do with if the building is absorbing energy. A wood frame building would likely be the least offensive.)

If the vent pipe is metal, you might want to try an insulator at the end of the wire (or just nylon/plastic string) and attach that to the vent pipe. Coiling the insulated wire around the vent pipe would make a capacitive connection to the pipe. Radio frequency energy is very rapidly varying AC (alternating current) and it has properties over and above DC (direct current).

The amount of capacitive connection would depend on how much insulated wire is in contact with the pipe. If it is plastic, don't worry about it!

Try wacking that earphone. Don't be shy. It doesn't work now! It may spring to life and then you can compare it to the other one, or hook them up in series and use two.

Garry

krystalloPosted - 8 November 2012 22:48  Show Profile  Email Poster  Edit Message
Hey T,

A "gimmick" capacitor is made from two INSULATED wires twisted together for a certain amount of twists ( 2 to 40 twists ???) per experimental results.

Note there is NO connection of the metal conductor in one wire to the other- only the INSULATION is twisted together.

The gimmick can go in any number of spots in the set, but for now try it wired in series in the antenna feed line and/or the ground line.

K

homebrewPosted - 9 November 2012 7:17  Show Profile  Email Poster  Edit Message  
Tar and anything black should be avoided but I've used black insulated wires and PVC pipe without any noticable reduction in performance.
The roof and sides of the building will **** the life out of the signal. Experimenting with my attic antenna I found that 18 inches was the optimal spacing between wires and the house framing materials. It was about the same for my inside antenna and the walls. Lead ins to the house are unavoidable lossy areas usually but keep the ant and gnd leads separated from each other as much as possible.
You should be able to pick up all three of your stations maybe overlapping some. For the higher frequencies tap the coil length shorter and connect the diode to a tap close to the center.
Changing the antenna length should not effect the tuning that much. Longer antennas usually pick up more signal strength when tuned.
It is completely possible to miss a weak station on a oatbox set without a ones tap option. Adding tapped single turns to the gnd side of the coil would probably be worthwhile.
Garry NicholsPosted - 9 November 2012 7:41  Show Profile  Email Poster  Edit Message
homebrew;

When you say, "18 inches was the optimal spacing between wires and the house framing materials", I think that you are saying that spacing greater yields no further improvement, while spacing closer shows degradation. Have I got that correct?

T;

homebrew brings up a good point. If the ground and antenna wires run parallel to each other, even if spaced considerably, there will be little or no pickup along that distance. That is because the currents on one wire will be going the opposite way as those in the other, and the fields around each will mix and cancel (or partially cancel, depending on the magnitude and phase of the currents in each wire).

This would be a concern where the parallel situation is a significant distance compared to the total length of completely separated antenna and ground wires.

This does not mean that you should not try it if it is all you can do. I'm trying to offer optimized choices for situations where you have more than one way to do things.

Also, the "gimmic" cap that Krys mentions can be thought of as a "fixed position" variable capacitor. More twists = more capacity and vice-versa. I think that you said you had a variable capacitor. You can use that instead if you wish.

But it might be used to better advantage tuning the Oatbox set. I'm not sure what circuit you are using for the Oatbox. I find two different hookups when I check online. One PDF ends: XSOB1-manual-050108.pdf and another with a "072706" date in the URL.

Are you able to point us to a link to the circuit that you are using?

Garry

TeamongerPosted - 9 November 2012 15:46  Show Profile  Email Poster  Edit Message  
My ground now runs to a socket outlet screw, so not parallel anymore. The building is wood frame. I might be better off with elevated antenna with insulated end, but my splice is rather fragile. Might want to get a new single wire. I used 22 gauge hookup wire for my coil, would the same make a good antenna?

I will try that gimmick cap, and foil with wax paper that someone suggested. When you say "in series in the antenna feed line", you mean between antenna and coil?

My circuit is just like the one at
http://www.crystalradio.net/beginners/index.shtml, except I have a few more taps.
t

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