On Mon, 12 Sep 2005 21:31:52 -0400, TRABEM wrote:


It has occurred to me that the antenna itself has a great deal of
selectivity, yet some loop users still report front end overload from
AM broadcast band and other megawatt LF rf sources.

Look for "antenna effect". Most likely the broadcast signals are
connected directly to the pickup wiring, which works then as an
ordinary "electric" antenna.

One way would be to make the pickup loop of coaxial cable. Bend the
end of the cable back to a convenient place on the cable to form a
loop, solder the end (both centre connector and shield) to the shield
at that point. At the opposite end of the loop cut the shield and now
you have a nice electrostatic shield around the centre conductor.

Paul OH3LWR

On Mon, 12 Sep 2005 21:52:01 -0400, TRABEM wrote:

But, if the switch is anywhere near linear, you would not want your
gain stage before the switch, would you?? I can't see using an rf amp
at the antenna that just creates non linearity when you could use a
nice quiet audio amp op amp on the far side of the analog switch.

If you use a huge amplification on a single frequency, you can end up
with stability problems, due to unexpected feedback paths, such as a
direct conversion receiver with a lot of audio gain started to
oscillate, when the loudspeaker sound vibration was connected back to
some front end component that was microphonic.

Putting a (possibly switchable) preamplifier between the selective
loop and the mixer would allow some gain to be done at non-audio
frequencies, thus, reducing the risk for stability problems.

Paul OH3LWR

Hi T,

I think you missed the point. With a tuned antenna, what comes out of
the antenna+tuning is NOT broadband! Frequencies not so very far from
the one you are tuned to will be greatly attenuated. With that sort of
input, you should be able to get by with even a 16-bit converter, if
it's linear (such as a decent delta-sigma is).

For example, a square loop antenna one meter on a side, at 150kHz,
tuned to resonance with a capacitor, should have a Q around 300. That
means it's about 0.5kHz wide at the 3dB points, and will be down about
20dB at 2.5kHz away from center, and 40dB down at 25kHz away.

-- I just read your other reply to my other posting in this thread,
where you worried about broadcast band overload. I suspect that if you
have that, it's because, as someone else said, the antenna is acting as
something other than a loop for that frequency. It's important to keep
the loop balanced with respect to ground. I'd strongly recommend
against a "shielded" loop unless you understand just why you are doing
that. The shield becomes the antenna, and as such, it must be
symmetrical... Also, for the antenna you described, about 3 meters on
a side and large wire, expect the Q to be even higher and the bandwidth
narrower. I think you'll find the resonated impedance to be more like
a few kohms for a single such turn. Then, use a good balanced FET
amplifier to get to a low impedance to drive your transmission line.

By the way, I would note that the switching detector/mixer/converter in
the schematic you showed is not as good as the usual current
implementation of the H-mode mixer, because the channels of the FETs
doing the switching in the one you gave a link to operate at a voltage
which depends on the instantaneous signal amplitude, if I read the
schematic right, and since the channel resistance is a non-linear
function of that voltage, the detector will not be strictly linear.
The H-mode mixers operate one end of the switches at a constant
voltage, and of course the other end when the switch is on must be very
close to the same voltage. I expect (and I think the practical
experience is) that the H-mode mixer will be more linear. Do you know
what the third order intercept for your mixer is supposed to be? I'd
be pretty surprised if it was better than about +45dBm.

But even so, even if you DID have a broadband antenna, you can find op
amps, and you can make amplifiers with discrete parts, that have
distortion products more than 120dB below the level of signals in
excess of a volt at the amplifier output, in the LF frequency range.
In other words, the distortion products will be less than a microvolt,
with one volt output signals. You don't need to run that preamp with
any appreciable voltage gain, so you're handling some pretty big input
signals. And the best of the 24-bit delta-sigma ADCs shouldn't be far
behind that. (I wish I could do that well at 50MHz!--we do make a
23-bit ADC that samples at up to 20MHz, but it's a bit pricey for what
you're trying to do.) As others have pointed out in this thread, the
atmospheric noise is so bad at LF that the antenna doesn't have to be
very efficient to capture enough signal to be useful for receiving.
Unless you are practically next door to a transmitter operating on a
frequency near the ones you care about listening to, dynamic range
isn't likely to be a big issue at LF. [Your other posting suggests
that folk DO have troubles with other signals. I'd go looking for
answers about WHY before jumping to conclusions about what to do about
them.] That's a far cry from the case at HF.

Cheers,
Tom

On 13 Sep 2005 14:21:59 -0700, "K7ITM" wrote:

Hi T,

I think you missed the point. With a tuned antenna, what comes out of
the antenna+tuning is NOT broadband! Frequencies not so very far from
the one you are tuned to will be greatly attenuated. With that sort of
input, you should be able to get by with even a 16-bit converter, if
it's linear (such as a decent delta-sigma is).

For example, a square loop antenna one meter on a side, at 150kHz,
tuned to resonance with a capacitor, should have a Q around 300. That
means it's about 0.5kHz wide at the 3dB points, and will be down about
20dB at 2.5kHz away from center, and 40dB down at 25kHz away.

-- I just read your other reply to my other posting in this thread,
where you worried about broadcast band overload. I suspect that if you
have that, it's because, as someone else said, the antenna is acting as
something other than a loop for that frequency. It's important to keep
the loop balanced with respect to ground. I'd strongly recommend
against a "shielded" loop unless you understand just why you are doing
that. The shield becomes the antenna, and as such, it must be
symmetrical... Also, for the antenna you described, about 3 meters on
a side and large wire, expect the Q to be even higher and the bandwidth
narrower. I think you'll find the resonated impedance to be more like
a few kohms for a single such turn. Then, use a good balanced FET
amplifier to get to a low impedance to drive your transmission line.

By the way, I would note that the switching detector/mixer/converter in
the schematic you showed is not as good as the usual current
implementation of the H-mode mixer, because the channels of the FETs
doing the switching in the one you gave a link to operate at a voltage
which depends on the instantaneous signal amplitude, if I read the
schematic right, and since the channel resistance is a non-linear
function of that voltage, the detector will not be strictly linear.
The H-mode mixers operate one end of the switches at a constant
voltage, and of course the other end when the switch is on must be very
close to the same voltage. I expect (and I think the practical
experience is) that the H-mode mixer will be more linear. Do you know
what the third order intercept for your mixer is supposed to be? I'd
be pretty surprised if it was better than about +45dBm.

But even so, even if you DID have a broadband antenna, you can find op
amps, and you can make amplifiers with discrete parts, that have
distortion products more than 120dB below the level of signals in
excess of a volt at the amplifier output, in the LF frequency range.
In other words, the distortion products will be less than a microvolt,
with one volt output signals. You don't need to run that preamp with
any appreciable voltage gain, so you're handling some pretty big input
signals. And the best of the 24-bit delta-sigma ADCs shouldn't be far
behind that. (I wish I could do that well at 50MHz!--we do make a
23-bit ADC that samples at up to 20MHz, but it's a bit pricey for what
you're trying to do.) As others have pointed out in this thread, the
atmospheric noise is so bad at LF that the antenna doesn't have to be
very efficient to capture enough signal to be useful for receiving.
Unless you are practically next door to a transmitter operating on a
frequency near the ones you care about listening to, dynamic range
isn't likely to be a big issue at LF. [Your other posting suggests
that folk DO have troubles with other signals. I'd go looking for
answers about WHY before jumping to conclusions about what to do about
them.] That's a far cry from the case at HF.



Hi Tom,

I read your message above about 3 times now AND read all the other
posts in this thread over again.

After all of this, I must say that I'm very much in agreement with you
although I know little about the linearity of the particular analog
switch used in this configuration.

When I started this, I was paranoid about out of band signals mixing
and creating problems. Read posts from almost anyone using an LF
preamp or presenting a design for one and they will almost certainly
contain warnings about overload and mixing byproducts.

So, I wanted an almost unattainable filter on the front end, without
realizing in fact how much attenuation the antenna I hope to build
will have for out of band signals. You are exactly right, there is
probably no need at all for a tuned input in the receiver since the
antenna tuning will be so sharp.

Maintaining HI-Q in the antenna should be the primary goal I think
rather than worrying about the input filter parameters!

I disagree about the rf preamp however, Bill Ashcock says I shouldn't
need one at all as long as I keep the Q high in the antenna and feed
it into a balanced line to get to the shack. Bill says some of the
guys who have single turn loops 40 feet per side or larger have so
much signal, they have to attenuate. I'd like to start out without a
preamp unless it is really needed.

By the way, the antenna might not be balanced.....but if it's fed
through a balun on each, the feedline is balanced. And, the feedline
can be simple twisted wire pairs....which goes a long way towards
reducing stray rf pickup and makes the feedline cheap and the losses
low.

If I feed the antenna into a balun and run that to another balun at
the receiver, isn't that the same as having a traditional center
tapped loop in terms of 'balance'?

I'll try to find some software for loop design and see what the loop
looks like in terms of impedance and then decide how to couple that
directly into the receiver without a front end filter (just simple
impedance matching).

Thanks so much to you and everyone who wrote regarding this, and for
the nudge in the 'right direction'. I feel a lot better now regarding
the plan of attack than I did just 2 or 3 days ago!

Regards,

T

PS:I like your estimate for my loop impedance. If it's 1000 ohms as
you think it might be, I can handle that step down just fine with a
balun or 2.

I hope it turns out to be true:

On 13 Sep 2005 14:21:59 -0700, "K7ITM" wrote:

But even so, even if you DID have a broadband antenna, you can find op
amps, and you can make amplifiers with discrete parts, that have
distortion products more than 120dB below the level of signals in
excess of a volt at the amplifier output, in the LF frequency range.
In other words, the distortion products will be less than a microvolt,
with one volt output signals. You don't need to run that preamp with
any appreciable voltage gain, so you're handling some pretty big input
signals.

With some kind of vertical antenna (possibly with some capacitance)
which is small compared to the wavelength on LF, it is going to have a
large capacitive reactance. There will also be some antenna input
capacitance from input to ground, so essentially there is a capacitive
voltage divider formed by the antenna capacitance and input
capacitance and the amplifier is trying to tap off that voltage. So
the amplifier really needs current gain, not voltage gain i.e. a high
input impedance and a manageable (50 ohm etc.) output impedance.

Paul OH3LWR

You can find a good program to estimate loop inductance and some other
parameters on Reg Edwards' web pages. He has a link in many of his
postings on this group and the r.r.a.antenna one. The impedance comes
from the Q and the fact that you are resonating it--or at least it's
presumed that you are resonating it. So if it has a Q of 300 and the
inductive reactance is 50 ohms, the resistance when resonated is
300*50=15000 ohms, for example. That's why folk like to use preamps at
the antenna: transform that high impedance down to a low impedance
that's easy to send along a transmission line. Seems to me that if
they are having trouble with intermod distortion in the preamp, the
preamp isn't designed properly. It's not terribly difficult to get
very low distortion at LF these days. By the way, if you build a
really big loop and have so much signal you can attenuate it, that
gives you a chance to lower the Q and increase the bandwidth: if what
you want to listen to occupies much bandwidth, you don't want your
antenna to filter out the information you want to listen to!

I'd suggest you read an antenna book like Johnson and Jasik, or the
antennas chapter of King, Mimno and Wing's "Transmission Lines,
Antennas and Waveguides." They will make it a lot clearer why you
might want a balanced loop. You don't need a grounded center-tap to
make it balanced--just make it very symmetrical.

Cheers,
Tom

On 15 Sep 2005 01:14:37 -0700, "K7ITM" wrote:

You can find a good program to estimate loop inductance and some other
parameters on Reg Edwards' web pages. He has a link in many of his
postings on this group and the r.r.a.antenna one.

Yes, I found it day before yesterday and was stunned to see the loop
impedance change so much as the loop is moved off frequency. I want my
antenna to cover 50 Khz to 200 Khz and the antennas impedance will
vary from 6K ohms to 1 K ohm, quite a LARGE range.

The software was very helpful and enlightening.

I was hoping to feed the antenna to the house over twisted pair line
laying on the ground. This requires a balun to make the low impedance
line balanced.

I think the antenna should be balanced as well, which helps in
elimination of out of band signals that might overload the preamp. I
think, but aren't positive that the balanced antenna is necessary to
eliminate the 'antenna effect' which allows the antenna to pick up
other signals that it wasn't designed for just because it's a piece of
wire hanging in free space.

So, my plan was to build a balanced loop and feed it to the house with
a balanced feedline.


The impedance comes
from the Q and the fact that you are resonating it--or at least it's
presumed that you are resonating it. So if it has a Q of 300 and the
inductive reactance is 50 ohms, the resistance when resonated is
300*50=15000 ohms, for example. That's why folk like to use preamps at
the antenna: transform that high impedance down to a low impedance
that's easy to send along a transmission line. Seems to me that if
they are having trouble with intermod distortion in the preamp, the
preamp isn't designed properly. It's not terribly difficult to get
very low distortion at LF these days. By the way, if you build a
really big loop and have so much signal you can attenuate it, that
gives you a chance to lower the Q and increase the bandwidth: if what
you want to listen to occupies much bandwidth, you don't want your
antenna to filter out the information you want to listen to!


It's not likely that I will ever want to listen to SSB or any other
wider band modes, but I did consider putting in a resistor to kill the
Q if I ever wanted to do this.

I'd suggest you read an antenna book like Johnson and Jasik, or the
antennas chapter of King, Mimno and Wing's "Transmission Lines,
Antennas and Waveguides." They will make it a lot clearer why you
might want a balanced loop. You don't need a grounded center-tap to
make it balanced--just make it very symmetrical.


No, it's pretty clear that I want a balanced loop. Several lowfers
made strong suggestions that I should not waste my time building
anything that wasn't balanced, and I couldn't agree more.

Cheers,
Tom

Tom, I hope to put up a high Q, but relatively large loop. I expect to
have a very large signal output to the receiver. Signal strength of
received signals will probably NOT be an issue.

Because the signal will be relatively high level, I would like to
resist using a preamp at all.

While I was playing with filters last night, I tried to design a
filter that would also convert impedance from 6 K down to 100 ohms. It
became impractical with a balun BUT, a filter that transforms
impedance seems to kill 2 birds with one stone.

However, performance sucks real bad with filters of higher impedance
and with any filter that attempts to make a large or moderate
impedance transformation.

Is there any other means of converting impedance with out an active
amp (using passive components)? Since I have a big signal, I can
sacrifice some signal strength as long as the losses are not to great.

Maybe I should start another thread?

Regards,

T

Yes, this might be good for another thread...

Given the limited LF bandwidth you're interested in, I can't imagine
that making a transformer would be all that difficult. Not trivial,
certainly, but far from impossible. A 5:1 turns ratio will give you a
25:1 impedance ratio. You'll want to use a core material that doesn't
introduce distortion. A transformer like that also gives you a way to
keep the loop loading balanced. But--are you going to put the tuning
capacitor at the loop, or do you have in mind putting it, say, at the
receiver end of the feedline? If it's at the loop, how will you adjust
it? And just what size signals do you expect to get? One of the nice
things about LF/VLF is the predictability of signal strengths. Also,
beware of worrying a lot about feedline impedance. How long will your
feedline be, in wavelengths? If it's, say, 0.05 wavelengths at 100kHz
(and THAT's 150 meters long!), does it really make much difference that
it's quite a different impedance than the antenna? And...what IS the
impedance of the line, at that frequency? It may well be a bit
different from what you calculate for the line at 10MHz. What would
happen if you fed your one turn loop with 100 feet of "300 ohm
twinlead" or "450 ohm ladder line" and just tuned it at the receiver
with a capacitor across that line? Small transmission line wire size
would ding the Q some, but would that be an issue? I'm just
speculating here, and maybe someone with direct experience with that
sort of feed will offer suggestions.

One thing to keep in mind here is that the LOOP construction will
almost certainly be the most challenging and expensive part, for a big
loop. And--you may really not NEED THAT big a loop! More signal also
means more atmospheric noise, and you won't improve signal:noise ratio
just by getting more of both signal and noise. Anyway, once you have a
well-constructed loop, it's relatively easy to play around with
different feed systems--preamp and remote tuning, straight feedline,
whatever.

Cheers,
Tom

Hi Tom,


Given the limited LF bandwidth you're interested in, I can't imagine
that making a transformer would be all that difficult. Not trivial,
certainly, but far from impossible. A 5:1 turns ratio will give you a
25:1 impedance ratio. You'll want to use a core material that doesn't
introduce distortion. A transformer like that also gives you a way to
keep the loop loading balanced. But--are you going to put the tuning
capacitor at the loop, or do you have in mind putting it, say, at the
receiver end of the feedline? If it's at the loop, how will you adjust
it? And just what size signals do you expect to get? One of the nice
things about LF/VLF is the predictability of signal strengths.

OK, I just ran some numbers for a 6000 ohm to 100 ohm toroid
transformer at 190 Khz. I didn't have to deal with the secondary at
all, because the primary has to be so large...I never made it past the
primary! Perhaps I made an error in the calculations? The transformer
has to present about 6000 ohms of inductive reactance, which is 16.6
millihenrys. Even on a large high mu core, I'd have to wind 400
turns!!!!!!!!!!! With that many turns, the losses would be big, and
would still have to wind a secondary (although it would much much
smaller). Did I make a mistake in the calculations?

I don't mind going to the antenna to tune it-lowfer signals don't
change frequency much.


Also,
beware of worrying a lot about feedline impedance. How long will your
feedline be, in wavelengths? If it's, say, 0.05 wavelengths at 100kHz
(and THAT's 150 meters long!), does it really make much difference that
it's quite a different impedance than the antenna? And...what IS the
impedance of the line, at that frequency? It may well be a bit
different from what you calculate for the line at 10MHz. What would
happen if you fed your one turn loop with 100 feet of "300 ohm
twinlead" or "450 ohm ladder line" and just tuned it at the receiver
with a capacitor across that line? Small transmission line wire size
would ding the Q some, but would that be an issue? I'm just
speculating here, and maybe someone with direct experience with that
sort of feed will offer suggestions.


Was hoping to use twisted wire which can be homebrewed or cat 5...it's
cheap and available. I think the twisted wire runs 80 to 90 ohms
impedance. I considered that I might just tolerate the mismatch since
the run was so short....but it's such a big difference, I am not sure
the input filter in the receiver will react as expected.

One thing to keep in mind here is that the LOOP construction will
almost certainly be the most challenging and expensive part, for a big
loop. And--you may really not NEED THAT big a loop! More signal also
means more atmospheric noise, and you won't improve signal:noise ratio
just by getting more of both signal and noise. Anyway, once you have a
well-constructed loop, it's relatively easy to play around with
different feed systems--preamp and remote tuning, straight feedline,
whatever.

Are there any other ways to convert the impedance without big losses
or without resorting to an active preamp? I'd like to avoid the preamp
if possible, especially since I expect a good output voltage from the
big and relatively high Q loop.

T

Well, the 8:1 turns ratio transformer at
http://www.jensen-transformers.com/datashts/110khpc.pdf would almost do
it for you. It falls off a bit at the high end. But with it, you
could go down to less than 10Hz. That might be fun. What core
material were you using in your calcs?

I really think you're being too cautious about preamps. But again, my
point is that you can put the effort into the loop, put it up, and then
play around with different feed methods. If you do use a feed line and
leave the tuning capacitor at the antenna, you won't notice any
significant loss in the line. Reg will try to "sell" you one of his
programs for looking at how the line transforms the impedance, but I'd
just use a Smith chart program myself. Much more educational, to me,
than just seeing numbers. If you want to try tuning at the receiver
end, use heavy wires in the line: their resistance adds to the
resistance of the loop itself, lowering the Q. But with the capacitor
at the loop, the circulating currents stay out there, for the most
part. You could add a small variable at the receiver end, to do fine
tuning. You may well discover that's an advantage...it's a pain to try
to tune the loop without being able to listen to the signal...

Cheers,
Tom
(about to bow out of this...time for you to go try some things.)