HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

You should have the schematic in your mailbox by the time you get this
message.

If I am to translate the annotation correctly (and it is obviously in
error - R10 and R12 are not the pair being changed), then Input Z
somewhere in your frequencies of interest (you've named several) runs
around 10 Ohms with the switch itself attenuating your signal by 6 to
10dB. This, of course, says nothing of the abysmal match to the
antenna whose Q will be buried in potter's field.

Hi Richard,

You are correct, the resistors being changed at R11 and R12, sorry for
the error. To correct the statement on the schematic, consider R11 and
R12 as being changed from 10 ohms to .2 ohms.

I'm not sure what the 'abysmal match to the antenna whose Q will be
buried in potter's field' statement is about. With my antenna being in
the 2 ohm impedance range, and the receiver being at 10 ohms (I'll use
your figure), how can the match be abysmal? Granted, it's not anywhere
near ideal. Have you assumed I was using a parallel tuned loop?

Since power transfer is the goal, and the antenna has a lot of ability
to reject out of band signals, it was my hope to use the antenna
itself as the only (purposely) tuned circuit in the system. Wouldn't
converting the antenna impedance to a more traditional 50 ohms with a
toroid, and then having a second toroid to convert it back down to 10
ohms also be destructive to the antenna Q and lossy?


I gather that the switch operates as a baseband quadrature/synchronous
detector. It looks suspiciously like it will either short the input,
or leave one half dangling, but I trust you got the schematic from a
good source. I won't bother to try to verify the logic.


The logic is good, trust me, the receiver as shown outperforms many
much more expensive ones on the market currently. It is based on the
receiver shown at:

http://www.qrp2001.freeserve.co.uk/qrp2001rf.gif

It's design has been around for awhile, the first prototype was built
in 2000. While I disagree with the use of resistive matching, it
should be ok at VLF as the signals are usually quite large there to
begin with. For a simple receiver, it is the best answer. Although
ideally, each switch should have it's own 1 ohm variable resistor for
absolute best results...the purpose being to swamp out the dynamic
switch series resistance differences.

Note the receiver has no rf amp, it isn't needed. The gain is provided
by some low noise op amps, and no rf stage is needed. The QRP2001
receiver is designed for 100 KHz to 30 Mhz, but it is only rated down
to 1.8 MHz. However, it's worst case sensitivity is .4uV for 10 db
sinad.


The roll-off frequency of the amplifier(s) is at the bottom of the AM
band, it would do better to track the oscillator frequency about one
seventh below so that AM signals are depressed below WWVB instead of
competing with it. Increase the caps from 470pF to 3300pF.


Agreed. Thanks.

The 470 pF value was chosen for a wide band spectral display from DC
to 96 KHz, which is about the best that generic PC sound cards can do
today. If the panoramic view of the surrounding spectrum wasn't
necessary, these caps would be much larger in value. As it sits
however, it's nice to have a panoramic view of the surrounding
spectrum, so the caps might not be changed.

For my purposes, they don't need to be nearly as small as they are,
but the original design was for HF...where a wiew of the surrounding
spectrum was handy.

The reason why you want low front end Input Z is to satisfy the
amplifier topology (the gain will be roughly half what it is probably
specified at). This could have been done better in half a dozen
different ways with the same active parts.

The problem here is some bozo marked the input "antenna" and removed
the necessary follower amplifier that would have been fed by the
antenna which would in turn feed this circuit through the transformer.
This sucker, as drawn, is going to be deaf, deaf, DEAF. If you hear
WWVB it will be by virtue of their strong power, not by any quality of
design here.


WRONG, but I think I understand your error. BTW, the 'bozo' was me::.

Again, I think you've assumed it would be fed with a parallel loop
resonant antenna.

And, no active components are needed for outstanding performance.

It is possible that the antenna circuit might need to be tuned with
passive components, but that possibility needs further evaluation.

To unplug this design's ears and make up for the massive goof, add a
FET follower. Load the FET drain with a 2.2 Ohm resistor and make
sure you couple the signal through a large cap feeding the
transformer. Also, bias the FET on with a hi Z divider so you don't
wipe out the Q of the antenna. I will bet that even the proposed "I"
and "Q" paths are mislabled or missapplied.


OK, this is a fair request...from someone without experience in this
type of receiver. However, in reality, an rf stage of any sort is
unnecessary.

Again, I think you're trying to match a 2 K parallel tuned loop to the
relatively low impedance of the receiver input. I noticed you said
'follower'. Which, I think means unity gain, but is used for impedance
matching.

Any active component before the audio op amp is STRONGLY
DISCOURAGED in this type of receiver. This includes back to back
diodes as the receiver switches can handle 4v p-p. It also includes
varicap tuning diodes. A front end rf amp should be avoided at all
costs, it can only degrade the performance of this type of receiver.

The receiver has incredible immunity to strong out of band signals,
much more than any superhet. The nature of the beast is that the
quadrature detector cancels them out by (effectively) NOT reinforcing
them. The desired signal is however very strongly reinforced. Since we
don't need an rf amp to get good sensitivity, any active component
before the load capacitors is strongly discouraged.

Even at 60 MHz, the performance is only slightly degraded, and that is
due to the inability of the analog switch to enable and disable fast
enough to allow reception there. There is a commercial application
using this technology that implements receive and transmit with analog
switch method and it does not have an RF amp.

Of course, there is a trade off. In order to obtain immunity from
strong adjacent out of band signals, we give up the ability to reject
harmonics. Harmonics are only attenuated 6 db....so a tuned input must
be used if the antenna cannot adequately prevent harmonics of the
receiver frequency from entering the receiver.

In theory, a minimum of 6db enhancement is available because there is
no mixer, so there is no conversion loss!

The susceptibility to harmonics is a small trade off. Remember, a
receiver of this type is wideband, needs no conventional mixers, no
detector diode, no transformers, no crystal filters, no IF amps, has
no conversion loss and no major non-linear components...thus offers
outstanding performance with the cost to build very much reduced
(relative to a superhet). What you don't see on the schematic is the
incredible sound of the receiver audio which is clean and crisp...it's
not quantifiable by bench measurements however.

I've put some links to web references of this technology at the end of
the message.

In the meantime, I'd appreciate comments regarding the issue of how
well the loop will feed the antenna input for the modified receiver
schematic I sent you by email.

Regards,

T

PS:I hope Dave is still with us. Although he probably left when he
assumed I measured the input impedance with an ohm meter:: Dave, are
you able to concede that the input impedance of the receiver might be
around 2 (or 10) ohms now?


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

If you want read up on this type of receiver, I can recommend the
following:


http://www.qrp2001.freeserve.co.uk/contact.htm

http://www.flex-radio.com/

Flex-Radio makes the quadrature based SDR-1000 transceiver. For a very
detailed explanation (without heavy math) of the detector theory,
check out the QEX article, part 1 at:

http://www.flex-radio.com/articles_files/SDRFMP1.pdf

And, there are independent product reviews for the SDR-1000 at:

http://www.flex-radio.com/articles_files/index.htm

Dan Tayloes NC2030 high performance signle band transceiver is
detailed at:

http://www.qslnet.de/member/df7tv/nc...es_2004_10.pdf

The complete schematics for the NC2030 are at:

http://www.norcalqrp.org/nc2030.htm

Note that the NC2030 uses the same type of detector, but does not use
a sound card and does not rely on a computer at all. It is a stand
alone transceiver.

There is also a 9Y4 who home brewed a complete transceiver, details
at:

http://9y4ar.tripod.com/tayloe_mixer.htm

Although slightly off topic, a low power ssb/cw exciter can be made
just as easily as the receiver using the same analog switch
technology. The process is simply the reverse process of the detector.
To see how simple a high quality transmitter is, try:

http://www.w1tag.com/Phasing.htm


..

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

On Thu, 27 Oct 2005 06:37:17 GMT, Owen Duffy wrote:

On Mon, 24 Oct 2005 18:03:49 -0400, TRABEM wrote:


Assume the wire diameter is a conservative thick 2mm.


Assume nothing Reg.

Well T, reading through the thread you seem to be real short on
relevant information (ie you don't adequately anticipate the
information people need to answer your questions), then very ready to
deal abruptly with people who make the wrong assumptions about the
context.



Owen,

You're right of course. Although there is a balance needed as the
complete details would fill a small book...no one would read it to the
very end.

Check the thread a little later in it's history, I think there has
been additional pertinent information goven.

Enjoy.

T

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

May I ask, what is with the almost fanitical adherence to Q?

TRABEM wrote in message ...
On 26 Oct 2005 09:27:44 -0700, "K7ITM" wrote:

I could perhaps scan the relevant pages of the references I
mentioned...


No, I can get them att he schools library I think. thanks for the
offer.

How are you planning to couple your 2-ohm load to your loop without
doing really bad things to its Q?

Is it better to convert the loop to a higher impedance just to feed it
into the house? It appears that anything I do is going to knock the
heck out of the antennas Q though.

I have not decided whether to mount the receiver at the antenna yet,
or whether to run the twisted line directly into the house from the
antenna (since it's a short run). Most likely it will have a short run
of cat 5 cable going from the antenna to a 1 to 1 toroid transformer
located in the receiver. The only selectivity for the receiver will be
the antenna itself. The receiver is very small, and uses very little
power, so it's pretty feasible to mount the entire receiver at the
antenna and run a balanced line feed of the audio into the house.

(And just what sort of detector do
you have that represents a 2 ohm load?)

Is it better to convert the loop to a higher impedance just to feed it
into the house?

It's an analog switch input, modified by my neighbor that gave me one
of them. The switch vendor says the switch series resistance should be
around 3 ohms, but it measures around 2.5 ohms. Probably is a little
lower than expected due to the integrating capacitors (.1 uF) which
are hung on the output of each switch. The .1's go to ground.

I measured it twice, once with a 1:1:1 isolation transformer and once
with a 6:1:1 isolation transformer....The tester looses accuracy at
low impedances, so we repeated the measurement with the generator
feeding the high impedance side of the a transformer also. I got
nearly the same reading after correcting for the transformers
impedance step down value, since both readings agree pretty well with
the switch vendors ratings, it's very likely that the receiver input
impedance is around 2 ohms.

(And just what sort of detector do
you have that represents a 2 ohm load?)

Cheers,
Tom

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

Hooey Boy, are you gonna get an earfull now! How dare you ask something
like that! You just evoked the wrath of flag, momma, and God! har..

J

"Fred W4JLE" wrote in message
...
May I ask, what is with the almost fanitical adherence to Q?

TRABEM wrote in message
...
On 26 Oct 2005 09:27:44 -0700, "K7ITM" wrote:

I could perhaps scan the relevant pages of the references I
mentioned...


No, I can get them att he schools library I think. thanks for the
offer.

How are you planning to couple your 2-ohm load to your loop without
doing really bad things to its Q?

Is it better to convert the loop to a higher impedance just to feed it
into the house? It appears that anything I do is going to knock the
heck out of the antennas Q though.

I have not decided whether to mount the receiver at the antenna yet,
or whether to run the twisted line directly into the house from the
antenna (since it's a short run). Most likely it will have a short run
of cat 5 cable going from the antenna to a 1 to 1 toroid transformer
located in the receiver. The only selectivity for the receiver will be
the antenna itself. The receiver is very small, and uses very little
power, so it's pretty feasible to mount the entire receiver at the
antenna and run a balanced line feed of the audio into the house.

(And just what sort of detector do
you have that represents a 2 ohm load?)

Is it better to convert the loop to a higher impedance just to feed it
into the house?

It's an analog switch input, modified by my neighbor that gave me one
of them. The switch vendor says the switch series resistance should be
around 3 ohms, but it measures around 2.5 ohms. Probably is a little
lower than expected due to the integrating capacitors (.1 uF) which
are hung on the output of each switch. The .1's go to ground.

I measured it twice, once with a 1:1:1 isolation transformer and once
with a 6:1:1 isolation transformer....The tester looses accuracy at
low impedances, so we repeated the measurement with the generator
feeding the high impedance side of the a transformer also. I got
nearly the same reading after correcting for the transformers
impedance step down value, since both readings agree pretty well with
the switch vendors ratings, it's very likely that the receiver input
impedance is around 2 ohms.

(And just what sort of detector do
you have that represents a 2 ohm load?)

Cheers,
Tom

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

On Thu, 27 Oct 2005 12:23:46 -0400, TRABEM wrote:
I'm not sure what the 'abysmal match to the antenna whose Q will be
buried in potter's field' statement is about. With my antenna being in
the 2 ohm impedance range, and the receiver being at 10 ohms (I'll use
your figure), how can the match be abysmal?

Even allowing for the values you offer (they are wrong) you have a 5:1
mismatch. Your actual mismatch is Q times that.

Granted, it's not anywhere
near ideal. Have you assumed I was using a parallel tuned loop?

Tom has already carried the water describing what your antenna Z looks
like. It is orders of magnitude greater by virtue of unloaded Q. You
want to aspire to make your receiver match that as closely as possible
- this design does not. What you have is a heavy, heavy load that all
but wipes out the advantage of Q.

Since power transfer is the goal, and the antenna has a lot of ability
to reject out of band signals,

Not any more.

it was my hope to use the antenna
itself as the only (purposely) tuned circuit in the system. Wouldn't
converting the antenna impedance to a more traditional 50 ohms with a
toroid, and then having a second toroid to convert it back down to 10
ohms also be destructive to the antenna Q and lossy?

2 Ohms, 10 Ohms, 50 Ohms are all still very trivial in comparison to
what the antenna has to offer. At the danger of introducing an
analogy, you have a 1.2 liter high performance race car which needs to
turn 14K RPM for 300 HP and you've put it to hauling a 40 foot fifth
wheel trailer. You are not even going to pull that load a foot before
you burn out the clutch.

Although
ideally, each switch should have it's own 1 ohm variable resistor for
absolute best results...the purpose being to swamp out the dynamic
switch series resistance differences.

You are arguing precision at the wrong end of the scale. Any
additional resistors are burning signal up and any appeal to
technicalities has been lifted from other applications that don't even
come close to this situation. I've been designing with these switches
for 25 years, and for very small signals.

Note the receiver has no rf amp, it isn't needed. The gain is provided
by some low noise op amps, and no rf stage is needed. The QRP2001
receiver is designed for 100 KHz to 30 Mhz, but it is only rated down
to 1.8 MHz. However, it's worst case sensitivity is .4uV for 10 db
sinad.

This is comparing apples and donuts when the menu only offers steak.

First, all these glowing accounts of excellent performance come from
European sources where VLF is far more common, and those services pour
up to a MW into the air. Your fillings would work just as well.
Second, your glowing reports are about HF characteristics with
conventionally sized antennas. When you attempt to extrapolate this
to VLF, you are not carrying the decimal point of inefficiency to the
left as you drop down in frequency.

Again, I think you've assumed it would be fed with a parallel loop
resonant antenna.

You haven't described anything else, and no appeal to series resonant
is going to resolve Q going down the toilet. Loss is loss no matter
what topology. This was the point of discussion with ESR.

And, no active components are needed for outstanding performance.

And yet you were the first to offer some builders have experienced
miserable results. You are about to join that pouting crew. On the
other hand, as I've suggested, you may still get WWVB with all these
problems - even wrist watches do. In that eventuality you have no
real basis of comparison and your only feeling would naturally be one
of wonder and awe. An aw shucks glow in the eyes does not translate
to a marvelous DX receiver.

Again, I think you're trying to match a 2 K parallel tuned loop to the
relatively low impedance of the receiver input. I noticed you said
'follower'. Which, I think means unity gain, but is used for impedance
matching.

Exactly.

Any active component before the audio op amp is STRONGLY
DISCOURAGED in this type of receiver. This includes back to back
diodes as the receiver switches can handle 4v p-p. It also includes
varicap tuning diodes. A front end rf amp should be avoided at all
costs, it can only degrade the performance of this type of receiver.

This only applies in the face of strong signals being applied to such
an amp. Yes, you have guaranteed that with the abysmal match and all
these Cassandra forecasts come true.

The receiver has incredible immunity to strong out of band signals,

This comes only from Q. This is a baseband receiver which means it is
open to all frequencies. Thus the necessity of a hi Q passive front
end (you killed it). What you call immunity is a product of dynamic
range capability and what circuits that follow this detector.

The nature of the beast is that the
quadrature detector cancels them out by (effectively) NOT reinforcing
them.

No, you've gotten very poor instruction on the qualities of this type
of detector. I was designing them 35 years ago and they are used in a
bajillion TVs. Absolutely every one of them has front end
electronics.

The detector neither cancels nor reinforces, it provides a phased
output. The detector also has its points of failure too, but when all
the necessary pre-conditions are met, it has many more features and
immunities. You have described none of these.

What you don't see on the schematic is the
incredible sound of the receiver audio which is clean and crisp...it's
not quantifiable by bench measurements however.

This is simply absurd. The very qualities you describe are measured
every day and are the purpose of this style of detection's use.
However, as you describe them, you still give the appearance of not
knowing what to do with the "I" and "Q" channels. Therein lies the
difference.

I would also offer, that among all your attached references, much less
your own discussion, absolutely nothing is said about the "I" and "Q"
channels. These outputs (why two?) are pushed into a black box, and
one AM signal emerges which begins to argue: what is being detected?
and where? I especially like these ace buster questions because it is
overwhelmingly obvious that no one actually knows what the "I" and "Q"
channels are for. They can be put to work without any more fuss than
amplifying them, but instead they are pushed into equations, software
and black boxes.

What is worse, I have yet to see anyone actually offer what forms of
modulation can be detected - there is a serious gap of research in all
these articles you've offered. For so many that have come here to
breathlessly announce the miracle of the Tayloe mixer, to a person,
they don't even know a fourth of what could be done.

I've put some links to web references of this technology at the end of
the message.

All very nice commercials, and one offers the nuts and bolts of the
practical detector that must have missed your attention as it
contradicts with:
"First, the RF input signal is bandpass filtered and applied to
the two parallel mixer channels."

In fact, and as I've experienced through 30+ years of their design,
there are filters all around.

Your having snubbed the antenna Q violates this first premise. Then
we look at the "I" and "Q" channels, the only way to achieve what you
describe as the marvelous characteristic of
The receiver has incredible immunity to strong out of band signals
is achieved by conventional filtering just like in the superhet. In
fact one of your references employs a nine pole Butterworth. Another
design has cascading filters out to eleven poles. This being a
baseband converter has simply shoved the filtering into the AF band.
Out of band performance only applies for those who cannot hear
dog-whistles. In short, a technological shell game. Effective,
certainly, but not unheard of - direct conversion was the original
form of receiver.

Going further, the QEX article accurately describing the Tayloe
detector describes the purpose of the capacitors in the circuit you
sent me. Problem there is that your lowered RC constants are running
out at 1µS for samples being taken at a much slower rate. Result is a
serious droop is occurring.

schematic I sent you by email.

Didn't get it. My Kill filters barely let your last schematic
through.

73's
Richard Clark, KB7QHC

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

Hi Trabem

I've been trying to follow this thread because I like to play with tuned
loop antennas for broadcast band reception.
I missed the part about how much this 20 meters of #2 copper cable with
its support weighs. Your project sounds Serious. That antenna must weigh
close to 500 pounds
The loop antennas I've been building are large diameter coils of smaller
wire. I recognize that the type antenna I build arent acceptable for your
consideration. But, I do have some experience with using a low freq loop
in the city. If you are located near man made noise, it is very likely
that resonating the loop doesnt result in highest Signal/Noise ratio.
Perhaps you already have experience with Low Freq loops and can tell me
about your experiences. I am interested in learning.

Jerry



TRABEM wrote in message ...


You should have the schematic in your mailbox by the time you get this
message.

If I am to translate the annotation correctly (and it is obviously in
error - R10 and R12 are not the pair being changed), then Input Z
somewhere in your frequencies of interest (you've named several) runs
around 10 Ohms with the switch itself attenuating your signal by 6 to
10dB. This, of course, says nothing of the abysmal match to the
antenna whose Q will be buried in potter's field.

Hi Richard,

You are correct, the resistors being changed at R11 and R12, sorry for
the error. To correct the statement on the schematic, consider R11 and
R12 as being changed from 10 ohms to .2 ohms.

I'm not sure what the 'abysmal match to the antenna whose Q will be
buried in potter's field' statement is about. With my antenna being in
the 2 ohm impedance range, and the receiver being at 10 ohms (I'll use
your figure), how can the match be abysmal? Granted, it's not anywhere
near ideal. Have you assumed I was using a parallel tuned loop?

Since power transfer is the goal, and the antenna has a lot of ability
to reject out of band signals, it was my hope to use the antenna
itself as the only (purposely) tuned circuit in the system. Wouldn't
converting the antenna impedance to a more traditional 50 ohms with a
toroid, and then having a second toroid to convert it back down to 10
ohms also be destructive to the antenna Q and lossy?


I gather that the switch operates as a baseband quadrature/synchronous
detector. It looks suspiciously like it will either short the input,
or leave one half dangling, but I trust you got the schematic from a
good source. I won't bother to try to verify the logic.


The logic is good, trust me, the receiver as shown outperforms many
much more expensive ones on the market currently. It is based on the
receiver shown at:

http://www.qrp2001.freeserve.co.uk/qrp2001rf.gif

It's design has been around for awhile, the first prototype was built
in 2000. While I disagree with the use of resistive matching, it
should be ok at VLF as the signals are usually quite large there to
begin with. For a simple receiver, it is the best answer. Although
ideally, each switch should have it's own 1 ohm variable resistor for
absolute best results...the purpose being to swamp out the dynamic
switch series resistance differences.

Note the receiver has no rf amp, it isn't needed. The gain is provided
by some low noise op amps, and no rf stage is needed. The QRP2001
receiver is designed for 100 KHz to 30 Mhz, but it is only rated down
to 1.8 MHz. However, it's worst case sensitivity is .4uV for 10 db
sinad.


The roll-off frequency of the amplifier(s) is at the bottom of the AM
band, it would do better to track the oscillator frequency about one
seventh below so that AM signals are depressed below WWVB instead of
competing with it. Increase the caps from 470pF to 3300pF.


Agreed. Thanks.

The 470 pF value was chosen for a wide band spectral display from DC
to 96 KHz, which is about the best that generic PC sound cards can do
today. If the panoramic view of the surrounding spectrum wasn't
necessary, these caps would be much larger in value. As it sits
however, it's nice to have a panoramic view of the surrounding
spectrum, so the caps might not be changed.

For my purposes, they don't need to be nearly as small as they are,
but the original design was for HF...where a wiew of the surrounding
spectrum was handy.

The reason why you want low front end Input Z is to satisfy the
amplifier topology (the gain will be roughly half what it is probably
specified at). This could have been done better in half a dozen
different ways with the same active parts.

The problem here is some bozo marked the input "antenna" and removed
the necessary follower amplifier that would have been fed by the
antenna which would in turn feed this circuit through the transformer.
This sucker, as drawn, is going to be deaf, deaf, DEAF. If you hear
WWVB it will be by virtue of their strong power, not by any quality of
design here.


WRONG, but I think I understand your error. BTW, the 'bozo' was me::.

Again, I think you've assumed it would be fed with a parallel loop
resonant antenna.

And, no active components are needed for outstanding performance.

It is possible that the antenna circuit might need to be tuned with
passive components, but that possibility needs further evaluation.

To unplug this design's ears and make up for the massive goof, add a
FET follower. Load the FET drain with a 2.2 Ohm resistor and make
sure you couple the signal through a large cap feeding the
transformer. Also, bias the FET on with a hi Z divider so you don't
wipe out the Q of the antenna. I will bet that even the proposed "I"
and "Q" paths are mislabled or missapplied.


OK, this is a fair request...from someone without experience in this
type of receiver. However, in reality, an rf stage of any sort is
unnecessary.

Again, I think you're trying to match a 2 K parallel tuned loop to the
relatively low impedance of the receiver input. I noticed you said
'follower'. Which, I think means unity gain, but is used for impedance
matching.

Any active component before the audio op amp is STRONGLY
DISCOURAGED in this type of receiver. This includes back to back
diodes as the receiver switches can handle 4v p-p. It also includes
varicap tuning diodes. A front end rf amp should be avoided at all
costs, it can only degrade the performance of this type of receiver.

The receiver has incredible immunity to strong out of band signals,
much more than any superhet. The nature of the beast is that the
quadrature detector cancels them out by (effectively) NOT reinforcing
them. The desired signal is however very strongly reinforced. Since we
don't need an rf amp to get good sensitivity, any active component
before the load capacitors is strongly discouraged.

Even at 60 MHz, the performance is only slightly degraded, and that is
due to the inability of the analog switch to enable and disable fast
enough to allow reception there. There is a commercial application
using this technology that implements receive and transmit with analog
switch method and it does not have an RF amp.

Of course, there is a trade off. In order to obtain immunity from
strong adjacent out of band signals, we give up the ability to reject
harmonics. Harmonics are only attenuated 6 db....so a tuned input must
be used if the antenna cannot adequately prevent harmonics of the
receiver frequency from entering the receiver.

In theory, a minimum of 6db enhancement is available because there is
no mixer, so there is no conversion loss!

The susceptibility to harmonics is a small trade off. Remember, a
receiver of this type is wideband, needs no conventional mixers, no
detector diode, no transformers, no crystal filters, no IF amps, has
no conversion loss and no major non-linear components...thus offers
outstanding performance with the cost to build very much reduced
(relative to a superhet). What you don't see on the schematic is the
incredible sound of the receiver audio which is clean and crisp...it's
not quantifiable by bench measurements however.

I've put some links to web references of this technology at the end of
the message.

In the meantime, I'd appreciate comments regarding the issue of how
well the loop will feed the antenna input for the modified receiver
schematic I sent you by email.

Regards,

T

PS:I hope Dave is still with us. Although he probably left when he
assumed I measured the input impedance with an ohm meter:: Dave, are
you able to concede that the input impedance of the receiver might be
around 2 (or 10) ohms now?


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

If you want read up on this type of receiver, I can recommend the
following:


http://www.qrp2001.freeserve.co.uk/contact.htm

http://www.flex-radio.com/

Flex-Radio makes the quadrature based SDR-1000 transceiver. For a very
detailed explanation (without heavy math) of the detector theory,
check out the QEX article, part 1 at:

http://www.flex-radio.com/articles_files/SDRFMP1.pdf

And, there are independent product reviews for the SDR-1000 at:

http://www.flex-radio.com/articles_files/index.htm

Dan Tayloes NC2030 high performance signle band transceiver is
detailed at:

http://www.qslnet.de/member/df7tv/nc...es_2004_10.pdf

The complete schematics for the NC2030 are at:

http://www.norcalqrp.org/nc2030.htm

Note that the NC2030 uses the same type of detector, but does not use
a sound card and does not rely on a computer at all. It is a stand
alone transceiver.

There is also a 9Y4 who home brewed a complete transceiver, details
at:

http://9y4ar.tripod.com/tayloe_mixer.htm

Although slightly off topic, a low power ssb/cw exciter can be made
just as easily as the receiver using the same analog switch
technology. The process is simply the reverse process of the detector.
To see how simple a high quality transmitter is, try:

http://www.w1tag.com/Phasing.htm


.

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

Richard Clark wrote:


This is comparing apples and donuts when the menu only offers steak.

What kinds of steak are on the menu? I haven't had lunch yet and I
could go for a nice Chateaubriand.

By the way, some kinds of donuts have apples in them. Apple fritters I
think they call them. In a way they're both apple AND donut - since you
brought it up.

ac6xg

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

OK Richard,

I like to think I'm not stupid, but reading your last message.......

That being said....

I just went through and reread every single word in this entire thread
including all the replies and I have the following questions:

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

I have a series resonant loop with moderately large conductor wire and
reasonably high Q capacitors. It's tuned to resonate at 60 KHz.

A series tuned circuit is a low impedance circuit, so 60 KHz signals
from the antenna are passed to the receiver and other frequency
signals are attenuated as the X(C) or X(L) on either side of the
resonance frequency attenuates them. Using Reg's rjeloop3 we get 25.4
uH for the loop inductance, and it resonates at 60 KHz with 286,700
pF. It also gives us the X(L), which is 9 ohms and (presumably) the
X(C) is -9 ohms. It estimates Q at 221. The 2 reactances are equal and
opposite, they cancel each other out. This leaving us with the
effective R(ac) for the loop inductance and the effective R(ac) of the
resonating C. The sum of these 2 resistances gives us the net
impedance of the loop. I know that the ESR of the cap is in the
milliohm range and the same for the wire, but I don't know the actual
impedance of loop. For a parallel tuned loop, Reg's program gives me a
value of 2K ohms. I don't really know what the value is for a series
tuned loop such as I will have.

Why does this make my antenna look like 2 K ohms impedance? I was
(admittedly guessing) that it looked more like 2 ohms. Maybe 2 ohms
isn't right, but can the impedance of a series tuned loop made out of
number 2 copper and low ESR caps be 2 K ohms??

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

My receiver measures around 2.5 ohms input impedance, verified by 2
different test methods...maybe that's not absolutely accurate. But,
you said it should have looked like 10 ohms instead. I accepted your
number however, let's say it's a 10 ohm impedance input from now on.

I believe that the antenna and receiver should be designed for maximum
power transfer which means making the antenna and the receiver front
end equal with regard to impedance. Is this correct? Is maximum power
transfer indeed my goal?

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

Regarding the integrating caps C value:

Yes, I understand I've lowered the effective series resistance (by
modifying the input circuit) that charges and discharges the
integrating caps. And, yes, I know different values of caps will be
needed. And, NO, I haven't addressed that issue yet. But, thank you
for pointing it out to me. I had thought these caps should be greater
than 1 uF and possibly larger. At this time, it's not a priority as
Winter is coming here and I have to get the antenna installed and
tuned. Fine tuning the receiver will come in the Winter when VHF
quiets down and the snow is 10 feet deep in the woods. For now, the
priority is making sure the antenna is all set before the snow flies.
Thank you again for reminding me that the integrating caps need to be
a different value. Since I might have to have a different input
circuit, or even an tuned circuit in the front end, I don't want to
address this issue now.

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

So, how do I fix it??

I'd be perfectly happy redoing the input circuit for 50 ohms input
impedance and putting some selectivity back into the front end if
that's what it takes to have the receiver function. My idea of
matching a 2 ohm impedance antenna to a 2 ohm impedance receiver
without any tuning at all (other than the loop antenna) was just that,
an 'idea'. If it doesn't work, then it doesn't work. But, on the
surface, it seems reasonable. It appears the major problem here is
that you think my antenna impedance is 2K and I think it's 2 ohms (or
less). So further discussion is a waste of time and bandwidth (until
this issue is resolved).

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

My primary question is about the impedance of a loop antenna made out
of 20 meters of #2 copper in series with each other using low esr caps
and tuned to resonance at 60 KHz.

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

schematic I sent you by email.

Didn't get it. My Kill filters barely let your last schematic
through.


Not sure what I did to deserve an honored position in your kill file.
I confess to being stubborn and cranky, but I don't think I was
disrespectful or made inappropriate comments. I won't email you
anymore schematics.

Obviously, if I had all the answers, I wouldn't need to ask here. If
you could shed some light on the series tuned antenna impedance and Q,
it would help me to make forward progress and I'd appreciate it. Where
have I gone wrong?

TNX,

T

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

On 24 Oct 2005 16:57:07 -0700, "K7ITM" wrote:

I'm puzzled. My copy of rjeloop3 suggests the Q will be about 200 at
60kHz with a 9mm wire diameter, and you'll see about 2kohms when it's
resonated. Are you not taking the output across the ends of the loop
(across the capacitor)?


No, you are describing a parallel tuned loop, aren't you Tom,

That is NOT what I'm trying to build.

I am planning a series tuned loop, which is C in series with L and the
output is taken across the unused loop terminal and the unused cap
terminal.

I think it should be called a series tuned loop, shouldn't it?

I know I suggested a whole bunch of times that your 2K loop impedance
sounded like a parallel tuned loop value and you keep insisting that
my series tuned loop will have an impedance of 2K ohms.

You also told me that "Tom has already carried the water describing
what your antenna Z looks like". I suggested that perhaps Tom and you
thought I was referring to a parallel tuned loop and said several
times that it was a series tuned loop.

Then you ranted on and on or maybe I ranted........

Did Tom and you not hear me when I said it was a series tuned loop or
did I not make it plain enough.

Isn't the impedance of a series tuned circuit LOW at resonance??? It
was when I went to school.

If I've err'd, please let me know how.

Thank you.

T




PS:

And, yes......I expect the Q to be cut in half if I attach a receiver
and a loop with identical impedances to each other. I call it loaded Q
and it's a necessary evil if one doesn't want to resort to electronic
(active component) impedance matching.

Put another way, if my receiver had a 50 ohm input impedance and my
loop had a 50 ohm output impedance (with a Q of 100, unloaded), I'd
expect to have a (net) Q of 50 after the receiver was connected to the
antenna.

Reg's software tells me I have a Q of around 221. I assume that's net
Q for the loop itself (unloaded). If my receiver is made to have the
same Q as the loop, then I expect the loaded Q to be around 110 after
they are connected together.

I know you mentioned an active buffer amp to transform impedances. No
doubt this would help to keep the loaded Q up, but I'd like to avoid
any active antenna preamp/rf stage if possible......as previously
discussed.

And with a skin depth of about 0.01" at 60kHz
in copper, certainly 3" diameter soft copper pipe would have the lower
resistance. You might have some trouble finding soft copper pipe,
though. But even hard copper pipe should have a low RF resistance.
"Reference Data for Radio Engineers" (or "Reference Data for Engineers"
in newer incarnations) has lots of good info for figuring out things
like RF resistance of copper wire. I assume your welding cable doesn't
have strands that are insulated from each other like Litz wire.


I thought about litz, and it probably would have been cheaper than the
copper welding cable I bought. But it's fragile in the outdoors and
breaks easy when the wind blows it especially in long spans like I am
going to have. Rather than encase it in some sort of protected sheath,
I decided to use the welding cable.


Consider that Q is energy stored divided by energy dissipated per
radian (1/2pi of a cycle). Then the net Q will be 1/(1/Q(inductor) +
1/Q(capacitor)). So if the cap and inductor have the same Q, the net Q
will be half that. And if you put a resistive load across the
coil+cap, that will dissipate power and lower the Q further.


I think I understand that now and understood it before you explained
it. But, thank you.

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

Thanks Reg,

I'm trying.

But, I wanted to make sure you know that I didn't mean any disrespect
when I commented that you should 'assume nothing'. It was short, but
not in any way to snub you.

I write this only after a comprehensive review of the entire thread.
IN the process oif that review I realized another member thought it
was a hostile comment...so it occurrent to me that you might have
thought so to.

And, in fact, it was not my intention to convey that message when I
wrote it.

If any offense was taken, please accept my apology.

I don't know if you've been following this thread or whether you have
ducked and are laying low.

I'm having a great deal of difficulty understanding how a series tuned
loop can have an impedance of 2K ohms, but Richard has reiterated this
over and over and in fact seemed to take offense when I suggested that
the loop impedance should be much lower.

So, I'm not sure who is right::

Like I said, I'm tryin'

Regards,

T