"craigm" 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. 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.

Of course. I've already noted that.

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.

If you say so. It would depend on the spacing of the wires to a large
degree.
I'm just saying most all the loops I've built so far need a wider
range cap.
If they are getting by with a single 10-365 cap, I'd like to see the
loop, the size, winding spacing , etc..I bet it will be quite
different than the ones I build. Most of my loops are the standard
solenoid type loops. The spacing varies, but on my 44 inch per side
loop, the ratio is pretty wide, maybe 5 to 1.
It's smaller on my small loop, as it uses thick wire, that is closer
together as far as the ratio.

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

The stray capacitance of my large loop is appx 9 pf. "assuming
rjloop3.exe is fairly accurate as a calculator. It seems to be." .

There is no way in heck a 10-365 pf cap would cover the whole BC band
on that particular loop. I can tune up to about 2300 kc on the upper
end if I use a single low value gang. "I have a switch". According to
my calculations, if I used only a single 365pf, my lower limit would
be appx 810 kc. I calculate a dual 365pf to drop down to about 580 kc.
I calculate needing 1220 pf at 450 kc. And this is pretty close to
what I'm using. As you can see, yea, I'm using a multi-gang cap, which
is switchable to allow a low value for the high end, but my loop also
covers a wider range than just the BC band. 450-2300 kc in two ranges.
My 16 inch round loop uses a dual 365 pf cap, which I assume is maybe
20-30 min-730 high value. With that particular loop, it covers from
500 to 2000 kc.
I built a loop a while back for another poster of this group. It was a
diamond loop on a 30 inch frame. It used a multi-gang cap with a dual
365 pf, and a bit extra in three other small gangs. That loop covered
540 to 1830 kc.

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.

Of course.

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.

Maybe so, but all the loops I've built need that range, and I always
build the loop around the cap at hand. I haven't looked at the loop
he's considering, but it must be quite a bit different than the box
solenoid type loops I've been building. Maybe a smaller pancake type
loop? I'd have to look at it.

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

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.

It's quite possible I guess, if the loop specs fits that low a range
to allow whole band coverage. None of mine do though. BTW, by whole
band coverage, I am including the new upper range also to 1700 or
whatever it is... All mine go higher than that. My large loop is a
diamond, 5 turns, 44 inches per side. The PVC cross support is 5 ft
across. It's here in the room and rotates. It's a kick butt loop. Very
sensitive, and very balanced. I'm not saying a loop can't be made to
cover the whole band with a single 10-365. I'm just saying that for
the average "box" type loops many will try to build, I don't see it
happening. I guess just general box/diamond loop info from what I see
here...:/ MK

I'm not saying a loop can't be made to
cover the whole band with a single 10-365. I'm just saying that for
the average "box" type loops many will try to build, I don't see it
happening. I guess just general box/diamond loop info from what I see
here...

In my experience, a single 10-365pf cap has gotten me full MW coverage most of
the time on homebrew loops, just not all of them. I have a couple of MW loops
that need a switched-gang set-up like you mentioned, to get a fuller range.
But, I also have a big (nearly 3 feet across) quilting-hoop-frame loop, twelve
turns tightly spaced, with a single, tiny 10-365pf cap. Goes from nearly 500 to
just below 1850. I didn't go through any detailed formulae when I was just
experimenting with the big loop frame (I've got it put together like a Kiwa
loop, it will turn in azimuth and altitude, very helpful for nulls) so I must
have lucked out.
But those smaller 365 caps have also been perfect for crystal sets, with full
tuning range of MW, so I always figured they'd work for loop antennas in the
same range (if you've matched the inductance right) and so far, nine times out
of ten, they do.
Linus

"Mark Keith" wrote in message
om...
If they are getting by with a single 10-365 cap, I'd like to see the
loop, the size, winding spacing , etc..I bet it will be quite
different than the ones I build. Most of my loops are the standard
solenoid type loops.


http://www.mindspring.com/~loop_antenna/amloop2.htm

Mark Keith wrote:

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.


Sure, but being the cap is a semi-fixed limited range device, you will
vary the number of turns in the loop to come up with the usable range.
The loop should be built around the cap. It's the part that is
unchangable within it's range. MK


True to a point, but I'm sure a tap or two on that coil may be
switched in or out as required. The gangs on a capacitor could be
switched in and out too, in conjunction with varying the portion of
coil used.


mike

"Brenda Ann Dyer" wrote in message ...
"Mark Keith" wrote in message
om...
If they are getting by with a single 10-365 cap, I'd like to see the
loop, the size, winding spacing , etc..I bet it will be quite
different than the ones I build. Most of my loops are the standard
solenoid type loops.


http://www.mindspring.com/~loop_antenna/amloop2.htm

That is a standard box loop. But I do note his high end is only 1650.
Also his cap is a 500 pf, not a 365 pf. Still not too bad for one cap
I guess. In my case, I would have still used a dual 365, as I like
mine to cover 160m also. "up to 2000 kc" With his loop, he states he
is tuning the low end using only appx 365pf or so. Myself, I would
have probably taken one turn or so off, and had a higher top range up
to nearly 1800kc, and still cover the low end using the full 500 pf.
MK

GrtPmpkin32 wrote:

I'm not saying a loop can't be made to
cover the whole band with a single 10-365. I'm just saying that for
the average "box" type loops many will try to build, I don't see it
happening. I guess just general box/diamond loop info from what I see
here...

In my experience, a single 10-365pf cap has gotten me full MW coverage most of
the time on homebrew loops, just not all of them. I have a couple of MW loops
that need a switched-gang set-up like you mentioned, to get a fuller range.
But, I also have a big (nearly 3 feet across) quilting-hoop-frame loop, twelve
turns tightly spaced, with a single, tiny 10-365pf cap. Goes from nearly 500 to
just below 1850. I didn't go through any detailed formulae when I was just
experimenting with the big loop frame (I've got it put together like a Kiwa
loop, it will turn in azimuth and altitude, very helpful for nulls) so I must
have lucked out.
But those smaller 365 caps have also been perfect for crystal sets, with full
tuning range of MW, so I always figured they'd work for loop antennas in the
same range (if you've matched the inductance right) and so far, nine times out
of ten, they do.
Linus

The traditional single gang 10-365 cap' was used to tune the oscillator
in a MW radio, not a front-end preselector. The oscillator usually
operated at 455-Khz (I.F.) *above* the desired frequency. This would be
about 995-Khz for the low end (540-Khz) of the band. That's why a
variable cap' with a maximum of 365-pf is not really low enough
(practical) when you want to use it as a tuning cap' for a MW loop
antenna or preselector. This is because it has to tune down to the
actual lower limit of the MW band (540) instead of the receiver's
oscillator frequency (995) at the low end.


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"starman" wrote in message
...

The traditional single gang 10-365 cap' was used to tune the oscillator
in a MW radio, not a front-end preselector. The oscillator usually
operated at 455-Khz (I.F.) *above* the desired frequency. This would be
about 995-Khz for the low end (540-Khz) of the band. That's why a
variable cap' with a maximum of 365-pf is not really low enough
(practical) when you want to use it as a tuning cap' for a MW loop
antenna or preselector. This is because it has to tune down to the
actual lower limit of the MW band (540) instead of the receiver's
oscillator frequency (995) at the low end.


The 10-365 pf section of the tuning cap is used to tune the antenna loop
circuit (and/or the RF amplifier circuit if it's a three gang tuner). The
oscillator section usually runs around 5-185 pf or so..

"starman" wrote in message
...

The traditional single gang 10-365 cap' was used to tune the oscillator
in a MW radio, not a front-end preselector. The oscillator usually
operated at 455-Khz (I.F.) *above* the desired frequency. This would be
about 995-Khz for the low end (540-Khz) of the band. That's why a
variable cap' with a maximum of 365-pf is not really low enough
(practical) when you want to use it as a tuning cap' for a MW loop
antenna or preselector. This is because it has to tune down to the
actual lower limit of the MW band (540) instead of the receiver's
oscillator frequency (995) at the low end.



I don't think so.

If you have a single gang cap in an AM radio, then it is tuning an antenna
coil. If you have an oscillator in a traditional radio, then you have a
superhet and will see a two or three gang capacitor.

The most frequent thing I've seen for the AM broadcast band is a dual gang
capacitor with the oscillator section having about 75% of the capacity of
the antenna/RF section.

When you see a dual 365 pF cap used in a superhet, you will also see a pad
cap in series with the oscillator section so that the oscillator tracks at
the needed 455 kHz offset.

With a 265 uH coil and a parallel capacitance of 10 pF it resonates at 3093
kHz.
With a 265 uH coil and a parallel capacitance of 365 pF it resonates at 512
kHz.

This is more than enough for the AM broadcast band. However in real life one
gets some stray capacitance due to wiring.

Adding 20 pF for stray capacitance, we get.

With a 265 uH coil and a parallel capacitance of 30 pF it resonates at 1785
kHz.
With a 265 uH coil and a parallel capacitance of 385 pF it resonates at 498
kHz.

Still, this is more than adequate.

Given the right inductance and keeping stray capacitance low, 365 pF is
enough.

If you need more capacitance, it probably means your stray capacitance is
very high and you reduced the number of turns in the loop (inductance) to
offset that problem and then added more variable capacitance to cover the
low end of the band.

craigm

"Brenda Ann Dyer" wrote in message


The 10-365 pf section of the tuning cap is used to tune the antenna loop
circuit (and/or the RF amplifier circuit if it's a three gang tuner). The
oscillator section usually runs around 5-185 pf or so..

The older analog tuning stereo's are a great source of multi-gang
caps. The one I'm using now, came from a big monster kenwood reciever
from the 70's. Three 365pf stages, and 5 more small value stages.
"maybe 50-100 pf??" Not sure...
Great cap for any loop, if you want a wide tuning range. Some stereo's
use two 365 pf gangs, and three smaller gangs. Those are good too, and
will provide for sure all band coverage, and then some. I used one of
those on another loop I built recently. Now, if I ever see an ancient
stereo with analog tuning, toasted or not, I grab it. Just the cap is
worth the hassle of dragging it home. There is at least one source on
web for a good variety of caps that are good for loops. One place in
particular has quite a few types/values, etc. You can do a search for
variable capacitors, and it should come up. MK

CM,

For 'common' Box Loop Antennas in the 16" to 48" size range:

The Common Mistake.
Many people use Hook-Up Wire (Insulated and Stranded) when
building Loop Antennas with very close spacing of 1/8" or
less between the windings. The result is usually a Loop
Antenna that will NOT Tune the 'full' AM/MW Band 540 kHz
to 1700 kHz with a single 365uf Variable Tuning Capacitor.

The Better Idea.
They should have simply used "Magnet Wire" (enameled single
solid) with a 1/4", 3/8" or 1/2" Spacing between the Windings.
This usually "Results" in a Loop Antenna that will Tune the
'full' AM/MW Band 540 kHz to 1700 kHz with a single 365uf
Variable Tuning Capacitor.

IMHO: Using LITZ Wire with the "InDoor" Loop Antennas in this
size range and with the 'wider' Spacing can produce a Higher
"Q" and is worth the extra money.

FWIW: For Loop Antennas that use a one to two turn "Coupling
Coil" which is about 75% to 80% of the size of the Main Tuning
Loop Antenna and place inside of the Main Tuning Loop works
better {Tunes Sharper} and has a Higher "Q".

For more about Loop Antennas Check-Out the YAHHO! eGroup:
"Loop Antenna Information Forum"
http://groups.yahoo.com/group/loopantennas/

iane ~ RHF
..
..
= = = "craigm" wrote in message
= = = ...
"starman" wrote in message
...

The traditional single gang 10-365 cap' was used to tune the oscillator
in a MW radio, not a front-end preselector. The oscillator usually
operated at 455-Khz (I.F.) *above* the desired frequency. This would be
about 995-Khz for the low end (540-Khz) of the band. That's why a
variable cap' with a maximum of 365-pf is not really low enough
(practical) when you want to use it as a tuning cap' for a MW loop
antenna or preselector. This is because it has to tune down to the
actual lower limit of the MW band (540) instead of the receiver's
oscillator frequency (995) at the low end.



I don't think so.

If you have a single gang cap in an AM radio, then it is tuning an antenna
coil. If you have an oscillator in a traditional radio, then you have a
superhet and will see a two or three gang capacitor.

The most frequent thing I've seen for the AM broadcast band is a dual gang
capacitor with the oscillator section having about 75% of the capacity of
the antenna/RF section.

When you see a dual 365 pF cap used in a superhet, you will also see a pad
cap in series with the oscillator section so that the oscillator tracks at
the needed 455 kHz offset.

With a 265 uH coil and a parallel capacitance of 10 pF it resonates at 3093
kHz.
With a 265 uH coil and a parallel capacitance of 365 pF it resonates at 512
kHz.

This is more than enough for the AM broadcast band. However in real life one
gets some stray capacitance due to wiring.

Adding 20 pF for stray capacitance, we get.

With a 265 uH coil and a parallel capacitance of 30 pF it resonates at 1785
kHz.
With a 265 uH coil and a parallel capacitance of 385 pF it resonates at 498
kHz.

Still, this is more than adequate.

Given the right inductance and keeping stray capacitance low, 365 pF is
enough.

If you need more capacitance, it probably means your stray capacitance is
very high and you reduced the number of turns in the loop (inductance) to
offset that problem and then added more variable capacitance to cover the
low end of the band.

craigm

..