Yeah, I was just looking at that page a few minutes ago, thanks.


Brian

BRIAN,

RadioShack AM/FM Radio Kit
http://tinyurl.com/2ycrp

RadioShack Catalog #: 28-179
Follow step-by-step directions to make an AM/FM radio
that plays through the included speaker.
- Change from AM to FM with a Switch
- Great for Children Age 8 to Adults
- Requires 4 "AA" Batteries

~ RHF
..
..
= = = "Brian" wrote in message
= = = thlink.net...

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=28-179

..

"Brian" wrote in message hlink.net...
I'm guessing 10-365pf would cover the majority of the broadcast band.


Brian

I bet it takes double that. He will need a "dual" 365pf cap to cover
the whole band with one cap, and no switching. With the dual cap, you
solder the two gangs together, and end up with 730 pf. But the min
value will be larger with the dual cap, and will reduce the upper
range a bit. IE: two 10-365pf caps, will give a 20 pf min, instead of
10. This shouldn't keep you from tuning the whole BC band, but if you
rig a way to switch to only one gang, you can increase your upper
range even farther. My 44 inch per side loop uses a triple 365pf
cap.It also has five smaller value gangs "maybe 25-50 pf each??" ,as
well for eight gangs total. With that cap, and a switch which I mount
of the side of the cap, I am covering from 450 kc to 2300 kc in two
ranges. My 16 inch round loop uses a plain dual 365pf cap. No extra
gangs. It covers from 500-2000kc with no switching. I really have my
doubts a single 365pf cap will cover the whole BC band. To cover the
low end, you will need more turns to tune with the small value cap.
This in turn will reduce the upper range due to the extra turns in
themselves, and also the extra stray capacitance you will see from the
extra windings. If you tune for 540 kc at the low end, I doubt you
will be able to tune 1600. I'm taking a wild stab, and guessing your
upper range might be 1000 kc or so ?? MK

"Mark Keith" wrote in message
om...
"Brian" wrote in message
hlink.net...
I'm guessing 10-365pf would cover the majority of the broadcast band.


Brian

I bet it takes double that. He will need a "dual" 365pf cap to cover
the whole band with one cap, and no switching.

Why would this be the case, when a standard AM radio covers the entire
broadcast band with a single 10-365pf variable??

Mark Keith wrote:
"Brian" wrote in message hlink.net...

I'm guessing 10-365pf would cover the majority of the broadcast band.


Brian


I bet it takes double that. He will need a "dual" 365pf cap to cover
the whole band with one cap,


Resonance in a circuit happens when inductive reactance equals
capacitive reactance. They're both measured in ohms. There is NO
magic capacitance for covering the broadcast band. The PROPER
capacitance is the one that matches the antenna/coil over it's
intended range.


So, resonance occurs at:
Xl = Xc

That can also be said as:

f = 1 / [2 * pi (sqrt LC)]

http://www.electronics-tutorials.com.../resonance.htm




mike

= = = "Brian" wrote in message
= = = hlink.net...

I'm guessing 10-365pf would cover the majority of the broadcast band.

Brian


BRIAN,

Loop Inductance of about 240 uH with a Capacitance Range of 10-365 pF
provides Full Coverage of the AM/MW Broadcast Band: 530 kHz to 1710 kHz.

AM Loop Antenna Calculator - by Bruce Carter (Version 4, 5-19-2003)
http://www.mindspring.com/~loop_ante...loop_calc4.htm
NOTES:
* Enter Edge Length (Inches) is the Distance
of One Side of a Square Loop Antenna.
* Enter Loop Width (Inches) is the Distance
between the First and Last Winding. (Depth)

Check-Out the "Loop Antenna Information Forum" eGroup on YAHOO!
LOOPS= http://groups.yahoo.com/group/loopantennas/

jm2cw ~ RHF

..

= = = "Brian" wrote in message
= = = thlink.net...

Anyone know if the variable cap, that I'm assuming is included
in this kit, will suffice for the bc band loop I want to build ?

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=28-179

Brian

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

BRIAN,

Since this is an AM & FM Radio 'Kit' it most likely uses a
"Miniature Poly-film Variable Tuning Capacitor" for AM & FM Bands
CAPS= http://www.oselectronics.com/ose_p98.htm
Part Number: BC-88

Ideal Variable Tuning Capacitor for miniature circuitry and use
as exact-duplicate replacement in current transistor receivers.
Tunes AM Band from 540Khz to 1600Khz and FM band from 88Mhz to
108Mhz. Rotates through a full 180°

Maximum Capacity:
AM/MW Antenna Section AM 21-152PF, Oscillator Section AM, 10-74PF.
FM Antenna Section FM 23-44PF, Oscillator Section FM, 14-23PF

Trimmer Capacity: variable to over 12PF.
Trimmer adjustment on rear of case.

Completely enclosed to clear polyethylene
plastic case to protect plates.
Includes calibrated dial, screw, and knob.
Small size, 3/4" Square x 1/2" Deep.


BETTER ALTERNATIVE:
AM/MW "Only" Tuning Capacitor: (Good for AM/MW Loop Antennas)
For your Loop Antenna 'Project' you may wish to consider a Capacitor
that is specifically designed to work within the Band Range of the
AM/MW Broadcast band.

"Miniature 1 Gang Poly-Film Variable Tuning Capacitor For Broadcast Band"
CAPS= http://www.oselectronics.com/ose_p98.htm
Part Number: BC-280

Tunes AM band from 540 kHz to 1600 kHz. Ideal Variable Tuning
Capacitor for miniature circuitry and use as exact-duplicate
replacement in current transistor receivers.
Works great in crystal radio sets. (Loop Antennas)
- Rotates through a full 180°
- Maximum Capacity: Single section tunes from 10-280pf.
- Completely enclosed in a clear polyethylene
plastic case to protect plates.
- Includes Calibrated Dial, Screw, and Knob.
Small size, 3/4" Square x 1/2" Deep.

REQUIRES: Loop Antenna Inductance of about 350 uH
when using a Variable Capacitor Range of 10 to 280 pf.

"AM Loop Antenna Calculator" - by Bruce Carter (Version 4, 5-19-2003)
CAL= http://www.mindspring.com/~loop_ante...loop_calc4.htm

Check-Out the "Loop Antenna Information Forum" eGroup on YAHOO!
LOOPS= http://groups.yahoo.com/group/loopantennas/

jm2cw ~ RHF

..

"Mark Keith" wrote in message
om...
"Brian" wrote in message
hlink.net...
I'm guessing 10-365pf would cover the majority of the broadcast band.


Brian

I bet it takes double that. He will need a "dual" 365pf cap to cover
the whole band with one cap, and no switching. With the dual cap, you
solder the two gangs together, and end up with 730 pf. But the min
value will be larger with the dual cap, and will reduce the upper
range a bit. IE: two 10-365pf caps, will give a 20 pf min, instead of
10. This shouldn't keep you from tuning the whole BC band, but if you
rig a way to switch to only one gang, you can increase your upper
range even farther. My 44 inch per side loop uses a triple 365pf
cap.It also has five smaller value gangs "maybe 25-50 pf each??" ,as
well for eight gangs total. With that cap, and a switch which I mount
of the side of the cap, I am covering from 450 kc to 2300 kc in two
ranges. My 16 inch round loop uses a plain dual 365pf cap. No extra
gangs. It covers from 500-2000kc with no switching. I really have my
doubts a single 365pf cap will cover the whole BC band. To cover the
low end, you will need more turns to tune with the small value cap.
This in turn will reduce the upper range due to the extra turns in
themselves, and also the extra stray capacitance you will see from the
extra windings. If you tune for 540 kc at the low end, I doubt you
will be able to tune 1600. I'm taking a wild stab, and guessing your
upper range might be 1000 kc or so ?? MK


Mark,
Gangs in the capacitor is not the issue when trying to cover the AM
broadcast band.
It is the ratio of highest to lowest capacitance that is of concern.

If I assume the highest frequency is 1710 kHz and the lowest is 520 kHz. The
ration of highest to lowest frequency is 3.29.

Square this number to get 10.82.

The ratio of high to low capacitave needed is 10.82 but this must also
include stray wiring capacitance.

A 10-365 pf capacitor has a ratio of 36.5. More than enough to cover the
band if the stray capacitance is low enough.

A 10-365 pf capacitor will work if the stray capacitance is less than 28 pf.

Once you have the a sufficient range of capacitance, you just need to make
sure your loop has the proper inductance to match that capacitance.

If you need 700 or 1000 pf to tune a loop to the AM band, then it indicates
the inductance of your loop is lower and you are just using more
capacitance to offset the condition.

--------------------

Going back to the initial question in the thread. If the tuning capacitor
supplied with the kit could cover the entire AM band with the kit's coil,
then it should also cover the entire AM band with a different coil/loop. It
is just a matter of getting the inductance right.


craigm

"Brenda Ann Dyer" wrote in message ...
"Mark Keith" wrote in message
om...
"Brian" wrote in message
hlink.net...
I'm guessing 10-365pf would cover the majority of the broadcast band.


Brian

I bet it takes double that. He will need a "dual" 365pf cap to cover
the whole band with one cap, and no switching.

Why would this be the case, when a standard AM radio covers the entire
broadcast band with a single 10-365pf variable??

Q of the circuit? Not really sure. I don't build AM radios. But in
judging from my loops, I'd be surprised to see one tune the whole band
with a single 365pf cap. I'm fairly sure neither of mine would. If I
dumped one of the gangs of my dual 365 cap on my 16 inch loop, it
WOULD NOT cover the whole BC band. When you build a loop, you build it
around the cap. IE: adjust the number of turns to give the desired
range with the cap at hand. If you can get a loop to cover the whole
band with a single gang cap, more power to you. But I'd have to see it
to believe it. It ain't happening here. MK

"Mark Keith" wrote in message
m...


Q of the circuit? Not really sure. I don't build AM radios. But in
judging from my loops, I'd be surprised to see one tune the whole band
with a single 365pf cap. I'm fairly sure neither of mine would. If I
dumped one of the gangs of my dual 365 cap on my 16 inch loop, it
WOULD NOT cover the whole BC band. When you build a loop, you build it
around the cap. IE: adjust the number of turns to give the desired
range with the cap at hand. If you can get a loop to cover the whole
band with a single gang cap, more power to you. But I'd have to see it
to believe it. It ain't happening here. MK

You're exactly right when you say you build the loop to work with the
variable cap.

I made a loop with an am broadcast oscillator cap, rather than the more
common antenna cap. The oscillator caps usually max out at something like
250 pf. I needed more turns on the loop antenna than usual, something like
14 turns on a form about 2 feet across. Limiting the distributed
capacitance with the extra turns is the biggest problem. I had to evenly
space the windings about 1/4" apart. I was able to get coverage from a
little more than 530 to 1700 kHz.

Basically, the frequency range with a fixed inductance will be proportional
to the square root of the ratio of the max capacitance to the minimum
capacitance. The distributed capacitance adds to the maximum capacitance
and the minimum capacitance of the tuning cap in the circuit. A 9:1 total
capacitance ratio will give a 3:1 tuning ratio. A 16:1 total capacitance
ratio will give a 4:1 tuning ratio.

A 10 to 365 pf tuning capacitor will tune from 530 to 1700 kHz if the
distributed capacitance can be held to below about 28 pf.

An 8 to 250 pf tuning capacitor will tune the same range if the distributed
capacitance can be held to below about 18 pf. This is more difficult
because a 250 pf tuning cap takes more turns to resonate in the AM BC band.
Wide, even turn spacing with thin wire does the job.

Anyway, those are the numbers I calculate. Hope I got it right! They seem
to be in the right ballpark. I won't try to make any attempt to measure the
distributed capacitance of a loop antenna.

I'll recommend Reg Edward's programs. These are small, ready to run
programs for all sorts of radio design problems. Among them are is the
RJELOOP3 programs for multiturn loop antennas.

http://www.btinternet.com/~g4fgq.regp/

http://www.btinternet.com/~g4fgq.regp/page3.html#S301"

Frank Dresser