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


.