"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
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