"craigm" wrote in message ...
"starman" wrote in message


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.

In my case, it's because I want a real wide range. My stray
capacitance is low, but I do use only 5 turns for a 44 inch per side
loop.
I can tune up to 2300 kc using a single low value gang. I want AM-BC,
but I also need the 160m band up to 2000 kc. Thats why I design mine
the way I do. But I would always buy the biggest caps I could get,
even if I didn't need it. The price difference isn't that much. The
extra gangs could come in handy on a different loop...IE: one web site
has about 10 various types of caps...One they sell is multi-gang, and
will add to 1500 pf total...Thats the one I would buy, if I was going
to buy one from that page. Might as well get your moneys worth, even
if you don't need it all...:/ Use a switch, and you can use it for LW
down to a certain freq in addition to MW, but still have a low value
for the high end. I have some fixed caps to clip in to drop down into
longwave on mine. "to about 175kc". So all total counting those, I
cover from 175 to 2300 kc. Myself, I would never buy a single gang, if
I could get a dual for not too much more. I like free ones out of old
stereo's the best...:) If I didn't need 160m, I would use more turns,
and design for LW/AM-BC, instead of AM-BC/160m.. It would be slightly
more sensitive down low, using more turns. I may build another monster
loop just for LW eventually...MK

craigm wrote:

"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

I was thinking of a superhet with a single gang cap' to tune the
oscillator and no front-end preselection tuning, which would require
another gang on the cap'.


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"starman" wrote in message
...
craigm wrote:

"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

I was thinking of a superhet with a single gang cap' to tune the
oscillator and no front-end preselection tuning, which would require
another gang on the cap'.


I've never seen a superhet without an antenna tuning stage (at least not
before the new garbage which is PLL tuned and has no tuned antenna stage,
and therefor also has no gain). All the older superhet designs, whether
tube, transistor or IC (analog and almost all portable digital tuners) has a
tuned input.

starman ) writes:

I was thinking of a superhet with a single gang cap' to tune the
oscillator and no front-end preselection tuning, which would require
another gang on the cap'.


If they make them, they are going to be rare.

If you don't have front end tuning, the receiver is going to overload
on strong signals. If the IF is too low compared to the signal frequency,
you also will never know which signal you are receiving is the one you
want, and the image frequency that you don't want.

If you've got a receiver with a low IF frequency, but front end tuning
not ganged to the local oscillator tuning, you may be able to null
out the image frequency, but you will have to keep adjusting both knobs.
It would be easy to mistune the front end tuning, and tune in the image
frequency. That's why all receivers have ganged tuning, at least after
it was invented decades ago.

If the IF is higher in frequency, of course one can use other techniques.
Put the IF in the HF range, and the image response will be MHz away, and
the front end will not need constant tuning. That lead to the separate
front end tuning in the sixties, where it only needed peaking every so often.
For limited range receivers, such as for only the ham bands, a suitably
high IF could mean that one could use bandpass filters at the front end,
ie they tuned a fixed 500KHz or so segment, and did not need tuning
as you crossed the band.

Or put the IF above the shortwave frequencies, and you have more leeway.
There, the image frequency is the other side of the IF, so one could use
a low pass filter, with a cut-off of 30MHz, though that still means
the active stages before the first IF filter see a 30MHz range of frequencies,
which may lead to overloading. At least some receivers, once first IFs
went that high, allowed for a low pass filter and some sort of preselection,
so you could choose.

Michael

Michael Black wrote:

starman ) writes:

I was thinking of a superhet with a single gang cap' to tune the
oscillator and no front-end preselection tuning, which would require
another gang on the cap'.


If they make them, they are going to be rare.

If you don't have front end tuning, the receiver is going to overload
on strong signals. If the IF is too low compared to the signal frequency,
you also will never know which signal you are receiving is the one you
want, and the image frequency that you don't want.

If you've got a receiver with a low IF frequency, but front end tuning
not ganged to the local oscillator tuning, you may be able to null
out the image frequency, but you will have to keep adjusting both knobs.
It would be easy to mistune the front end tuning, and tune in the image
frequency. That's why all receivers have ganged tuning, at least after
it was invented decades ago.

If the IF is higher in frequency, of course one can use other techniques.
Put the IF in the HF range, and the image response will be MHz away, and
the front end will not need constant tuning. That lead to the separate
front end tuning in the sixties, where it only needed peaking every so often.
For limited range receivers, such as for only the ham bands, a suitably
high IF could mean that one could use bandpass filters at the front end,
ie they tuned a fixed 500KHz or so segment, and did not need tuning
as you crossed the band.

Or put the IF above the shortwave frequencies, and you have more leeway.
There, the image frequency is the other side of the IF, so one could use
a low pass filter, with a cut-off of 30MHz, though that still means
the active stages before the first IF filter see a 30MHz range of frequencies,
which may lead to overloading. At least some receivers, once first IFs
went that high, allowed for a low pass filter and some sort of preselection,
so you could choose.

Michael

The most common tube type AM radios (5-tubes) used a two gang tuning
cap' for the front-end tuning and oscillator but I think there were some
cheap models with no front-end RF amp' or preselection tuning. The
antenna was connected to the mixer which had some gain to off set the
lack of an RF amp' stage.


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"starman" wrote in message
...


The most common tube type AM radios (5-tubes) used a two gang tuning
cap' for the front-end tuning and oscillator but I think there were some
cheap models with no front-end RF amp' or preselection tuning. The
antenna was connected to the mixer which had some gain to off set the
lack of an RF amp' stage.



Armstrong's first superhets didn't have a tuned RF stage, but those radios
were designed to operate at frequencies too high for the TRFs of the WW1
era. Something like 3 mc, if I recall. There wasn't any interfering
signals at the image frequency.`

I'm not aware of any commercially available AM superhets without preslection
and such radios would have a big problem with images 910 kc above the
intended frequency. The images would include other stations in the
broadcast band, the old Loran buzzsaw, hams on 160 meters, etc. Few AM band
only radios had RF stages, and most of those were simple RC coupled stages
for extra gain, but not selectivity. But, as far as I know, they all had a
tuned loop antenna or antenna coil.

Frank Dresser