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Re: Loop Antenna for WWVB LF Receiver

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  • redsp
    ... I ve been doing some LTspice simulations on this. I think I need to add some passive circuitry to apply a DC bias to put the differential input on the
    Message 1 of 27 , Feb 24, 2013
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      --- In loopantennas@yahoogroups.com, "jpopelish" <jpopelish@...> wrote:
      >
      > --- In loopantennas@yahoogroups.com, "redsp" wrote:
      >
      > > You've given me something to think about. What about the inductance of the secondary? How does that impact the primary circuit? Wouldn't that affect the tuning of the primary?
      > (snip)
      >
      > According to the data sheet, the 200 turn winding has at least 80mH inductance. That means a single turn primary will have only 1/40,000th of that or 2uH inductance. This gives you an idea of the core they used, since you know roughly the size, and that it produces about 2uHy with one turn.
      >
      > But you can add as many turns as you wish to the primary to raise the primary inductance and lower the turns ratio.
      >
      > You can also tune the inductance with capacitance. This might work best with a series capacitor between the loop and the primary, and a parallel capacitance across the secondary of the transformer.

      I've been doing some LTspice simulations on this. I think I need to add some passive circuitry to apply a DC bias to put the differential input on the edge of triggering, so I included this in the simulation along with the parasitic capacitance of the chip input. The antenna has a loop inductance of about 90 uH. In LTspice the transformer turns ratio is indicated by attaching the corresponding inductance on the secondary (L1*N^2). I'm not sure how a real transformer works in this regard, inductance of primary and secondary.

      At very high turns ratios the parasitic capacitance creates a resonance with a null when looking at the tuning capacitance on the antenna. If that gets too close in frequency to the peak for the signal, it greatly disrupts the Q and the bandwidth making it very twitchy to tuning. Using the secondary capacitance alone to tune the response seems to drop the output voltage significantly with readings of -dB rather than some +60 to +70 dB when tuned with the primary capacitor. I'm not familiar with analysis of this circuit to understand why a null is created on the primary cap, but not the output...

      The optimum balance seems to be a turns ratio around 25 to 33. In this range the difference in tuned output is in the 1 dB ballpark, but if the secondary capacitance drifts 1 pF the offtuning creates anywhere from -4 to -6 dB attenuation. Oddly at the higher turns ratios where the tuned signal is strongest the attenuation is such that the offtuned signal is weaker than at lower turns ratios. I'm not sure how realistic is a 1 pF drift with temperature, etc. of the inductor, PCB traces and the IC input.

      Even odder is that if I add some capacitance to the secondary side in an attempt to swamp out the parasitic capacitance, the circuit has a higher Q and becomes *more* sensitive to that same 1 pF change!!!

      I think I am a bit concerned with the capacitance of the secondary coil. If it has much capacitance I will have to run the coil with an even lower turns ratio.

      No wonder they call it *complex* math.

      Rick
    • John Popelish
      On 02/24/2013 05:30 PM, redsp wrote: (snip) ... (snip) If you upload your simulation file (not raw file), I will take a look and check out your simulation, as
      Message 2 of 27 , Feb 24, 2013
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        On 02/24/2013 05:30 PM, redsp wrote:

        (snip)
        > At very high turns ratios the parasitic capacitance
        > creates a resonance with a null when looking at the
        > tuning capacitance on the antenna. If that gets too
        > close in frequency to the peak for the signal, it greatly
        > disrupts the Q and the bandwidth making it very twitchy
        > to tuning. Using the secondary capacitance alone to tune
        > the response seems to drop the output voltage
        > significantly with readings of -dB rather than some +60
        > to +70 dB when tuned with the primary capacitor. I'm not
        > familiar with analysis of this circuit to understand why
        > a null is created on the primary cap, but not the
        > output...
        >
        > The optimum balance seems to be a turns ratio around 25
        > to 33. In this range the difference in tuned output is
        > in the 1 dB ballpark, but if the secondary capacitance
        > drifts 1 pF the offtuning creates anywhere from -4 to -6
        > dB attenuation. Oddly at the higher turns ratios where
        > the tuned signal is strongest the attenuation is such
        > that the offtuned signal is weaker than at lower turns
        > ratios. I'm not sure how realistic is a 1 pF drift with
        > temperature, etc. of the inductor, PCB traces and the IC
        > input.
        >
        > Even odder is that if I add some capacitance to the
        > secondary side in an attempt to swamp out the parasitic
        > capacitance, the circuit has a higher Q and becomes
        > *more* sensitive to that same 1 pF change!!!
        >
        > I think I am a bit concerned with the capacitance of the
        > secondary coil. If it has much capacitance I will have
        > to run the coil with an even lower turns ratio.
        (snip)

        If you upload your simulation file (not raw file), I will
        take a look and check out your simulation, as best I can.

        Maybe I'll be able to suggest some improvement.

        --
        Regards,

        John Popelish
      • redsp
        ... I uploaded the simulation files to Antenna_LTspice.zip One is just the basic antenna circuit with the parasitic capacitance on the IC inputs. The other
        Message 3 of 27 , Feb 26, 2013
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          --- In loopantennas@yahoogroups.com, John Popelish <jpopelish@...> wrote:
          >
          > If you upload your simulation file (not raw file), I will
          > take a look and check out your simulation, as best I can.
          >
          > Maybe I'll be able to suggest some improvement.


          I uploaded the simulation files to Antenna_LTspice.zip One is just the basic antenna circuit with the parasitic capacitance on the IC inputs. The other includes some circuitry I am adding to make the digital input work for low level AC signals.

          I've done some more work with the simulation and I've also been doing some reading on tuned circuit and I think I understand it better. I can calculate the frequency of the various artifacts that I see now. To calculate the resonance peak, I need to reflect the secondary side capacitance to the primary side using the square of the turns ratio. Adding this to the primary capacitance and using that with the primary inductance gives me the frequency of resonance. The null in the primary cap (C1) voltage is just the resonance of the secondary side. When the frequency is reached where the parasitic cap (C2) resonates without the primary cap, the result is that the voltage on the L1 side of the transformer matches the exciting voltage and the C1 capacitor sees no voltage, so a null.

          This is really a red herring and I was just confused by it. When this null gets close to the primary resonance, it means the secondary cap is getting very large and the primary cap is getting small. In the limit of using no primary cap the secondary cap will create a resonance but it won't have the advantage of the transformer turns ratio I expect and the resulting peak is not nearly as high as it could be. This limits the turns ratio to around 75 before the circuit becomes self resonant with the secondary parasitic capacitance.

          I was seeing higher Q factors when I used a little secondary capacitance. But this doesn't really improve the signal strength at the output a lot. My real circuit has some passives to provide DC biasing so the digital input on the receiver will be most sensitive to the antenna signal. I included these components in the simulation file antenna.asc while antenna_transformer.asc only includes the transformer. These components were made high impedance, but still have some affect on the Q and max signal strength, limiting it to around 70 dB in the simulation.

          In the end the best way to operate this circuit is with a turns ratio around 25 to 33 and with a minimum of capacitance on the secondary. This still produces a decent gain without the response being excessively peaky and overly sensitive to small changes in the secondary capacitance.

          The coils I ended up with have 300 turns so I will be wrapping some number of turns on the primary to adjust the turns ratio. So that I can experiment with this, I expect to have multiple coils for the primary, 1, 2, 4 and 8 which will let me combine them in 15 combinations to produce turns ratios between 20 and 300.

          My main concern is just how much variation I will see from temperature effects on the circuit once it has been tuned for 60,000 Hz. It may be very twitchy depending on how sensitive the parasitic capacitance is to temperature.

          BTW, I've learned a lot about LTspice doing this design, not to mention tuned transformer circuits.

          Rick
        • John Popelish
          ... I took a look at your tuned transformer circuit and have altered it to illustrate a few concepts. I changed out the voltage source in the primary loop
          Message 4 of 27 , Feb 26, 2013
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            On 02/26/2013 07:15 PM, redsp wrote:

            > I uploaded the simulation files to Antenna_LTspice.zip
            > One is just the basic antenna circuit with the parasitic
            > capacitance on the IC inputs. The other includes some
            > circuitry I am adding to make the digital input work for
            > low level AC signals.

            I took a look at your tuned transformer circuit and have
            altered it to illustrate a few concepts. I changed out the
            voltage source in the primary loop with a current fed, field
            coil, as a signal source. I also added an inductor in the
            primary loop to represent the antenna coil. The field coil
            is very loosely coupled to the antenna coil (K = 0.001), to
            represent the magnetic field the antenna will pick up from
            space, without the field generator coil having any
            significant effect on the tuning of the antenna inductor.

            I also rearranged the current transformer, so that the
            secondary is a fixed inductance (as a prefab CT will have)
            and made the primary inductance a function of the primary
            turns count (that you will add).

            I am working with a very clunky Windows simulator in a Linux
            system, so I did not have time to clean up everything and
            make sure all your measure commands still made sense. But I
            hope the concepts I introduced are clear enough to be useful
            for you.

            I uploaded the file to the Popelish folder as
            Antenna_transformer_w_loop.asc

            If I get the chance, I may work on it a bit more.

            Let me know if this makes any sense to you.

            --
            Regards,

            John Popelish
          • redsp
            ... I have wondered about the way I apply the stimulus. I think you misunderstand the circuit. L1 is the antenna coil. This is a large loop of wire which
            Message 5 of 27 , Feb 26, 2013
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              --- In loopantennas@yahoogroups.com, John Popelish <jpopelish@...> wrote:
              >
              > On 02/26/2013 07:15 PM, redsp wrote:
              >
              > > I uploaded the simulation files to Antenna_LTspice.zip
              > > One is just the basic antenna circuit with the parasitic
              > > capacitance on the IC inputs. The other includes some
              > > circuitry I am adding to make the digital input work for
              > > low level AC signals.
              >
              > I took a look at your tuned transformer circuit and have
              > altered it to illustrate a few concepts. I changed out the
              > voltage source in the primary loop with a current fed, field
              > coil, as a signal source. I also added an inductor in the
              > primary loop to represent the antenna coil. The field coil
              > is very loosely coupled to the antenna coil (K = 0.001), to
              > represent the magnetic field the antenna will pick up from
              > space, without the field generator coil having any
              > significant effect on the tuning of the antenna inductor.
              >
              > I also rearranged the current transformer, so that the
              > secondary is a fixed inductance (as a prefab CT will have)
              > and made the primary inductance a function of the primary
              > turns count (that you will add).

              I have wondered about the way I apply the stimulus. I think you misunderstand the circuit. L1 is the antenna coil. This is a large loop of wire which will make N turns around the coil core. I am unclear if the stimulus signal should be in series with the primary loop or if it should be applied across the cap and coil. In one case the primary will be a series resonant circuit in the other a parallel resonant circuit. The manner of stimulus is the only distinction that I can see between the two.

              I'm not sure why the transformer should be changed the way you did. The antenna loop (L1) has a defined inductance. I actually had to research pretty hard to find a decent formula for it. Turns out there are *many* inductance formula and it is not so easy to figure out which one applies. How does the inductance of the two coils interact? I haven't found any good info on this. Most references assume both coils are wound in a similar way on a common core. I suppose the N turns around the core will affect the inductance of the primary. I don't have a number for the inductance of the secondary, the data sheet gives a very wide range, 180 - 335mH @1V, 10kHz.


              > I am working with a very clunky Windows simulator in a Linux
              > system, so I did not have time to clean up everything and
              > make sure all your measure commands still made sense. But I
              > hope the concepts I introduced are clear enough to be useful
              > for you.
              >
              > I uploaded the file to the Popelish folder as
              > Antenna_transformer_w_loop.asc
              >
              > If I get the chance, I may work on it a bit more.
              >
              > Let me know if this makes any sense to you.

              I appreciate the time you are spending on this. One thing that still doesn't work perfectly is the center frequency measurement. It finds the max output value ok, but finding the frequency of that max value is not so easy. The method I use sometimes seems to "skip" over the match to the max value and gives an error. No information in the error message of course. LTspice is pretty great, but ease of use is not at the top of the list. BTW, they say LTspice runs great under wine. Have you tried that?

              Rick
            • redsp
              Just after I sent the prior message I was looking at the schematic and I get what you are doing with the separate coils in the primary. One is the antenna
              Message 6 of 27 , Feb 26, 2013
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                Just after I sent the prior message I was looking at the schematic and I get what you are doing with the separate coils in the primary. One is the antenna loop itself and the other is the turns around the secondary coil core. That makes sense. I'll play with this new configuration a bit.

                Thanks,

                Rick
              • John Popelish
                ... There are probably several equivalent ways to put energy into the antenna, but I like the field coil approach, because I actually made a field coil to
                Message 7 of 27 , Feb 26, 2013
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                  On 02/26/2013 09:40 PM, redsp wrote:

                  > I have wondered about the way I apply the stimulus. I
                  > think you misunderstand the circuit. L1 is the antenna
                  > coil. This is a large loop of wire which will make N
                  > turns around the coil core. I am unclear if the stimulus
                  > signal should be in series with the primary loop or if it
                  > should be applied across the cap and coil. In one case
                  > the primary will be a series resonant circuit in the
                  > other a parallel resonant circuit. The manner of
                  > stimulus is the only distinction that I can see between
                  > the two.

                  There are probably several equivalent ways to put energy
                  into the antenna, but I like the field coil approach,
                  because I actually made a field coil to stimulate antennas.

                  The problem with your schematic is that the antenna coil
                  inductance is a separate inductive component than the
                  transformer primary, because one receives energy from the
                  external field and the other does not.

                  > I'm not sure why the transformer should be changed the
                  > way you did. The antenna loop (L1) has a defined
                  > inductance. I actually had to research pretty hard to
                  > find a decent formula for it. Turns out there are *many*
                  > inductance formula and it is not so easy to figure out
                  > which one applies. How does the inductance of the two
                  > coils interact? I haven't found any good info on this.
                  > Most references assume both coils are wound in a similar
                  > way on a common core. I suppose the N turns around the
                  > core will affect the inductance of the primary. I don't
                  > have a number for the inductance of the secondary, the
                  > data sheet gives a very wide range, 180 - 335mH @1V,
                  > 10kHz.

                  And the secondary inductance is not very high Q (it includes
                  significant core loss that shows up as something like a
                  parallel resistance across the secondary, because the high
                  permeability core is quite conductive). That is why the
                  entire system should not be tuned at the output of the
                  transformer. You don't want all the resonant stored energy
                  to have to slosh back and forth through that core. Most of
                  the inductive energy should be stored only in the high Q
                  antenna coil.

                  >> I am working with a very clunky Windows simulator in a
                  >> Linux system, so I did not have time to clean up
                  >> everything and make sure all your measure commands
                  >> still made sense.

                  (snip)

                  > BTW, they say LTspice runs great under wine. Have
                  > you tried that?

                  I tried, once, to install Wine, and hit a roadblock. My son
                  is the Linux expert in the family, and one of these days, I
                  want to try again, with his help.

                  --
                  Regards,

                  John Popelish
                • jpopelish
                  One other point. There are two completely complementary ways to use a coupling transformer, in an attempt to increase the signal output of your loop. The
                  Message 8 of 27 , Feb 27, 2013
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                    One other point. There are two completely complementary ways to use a coupling transformer, in an attempt to increase the signal output of your loop. The transformer primary can be in series with both the loop and the tuning capacitor, when the primary inductance is low (relative to the loop inductance), or in parallel with both the loop inductor and tuning capacitor, when the primary inductance is high (relative to the loop inductance). If the turns ratio is not large, you might also find a small improvement by putting the secondary in series with the primary (correctly phased to have their voltages add), since you do not need electrical isolation from the transformer, That way, both windings contribute to the output voltage.

                    In your simulations, I think you should compare the possibilities for both these approaches with connecting your amplifier input directly across the parallel combination of loop and tuning capacitor.

                    One of the three might be a clear winner.

                    --
                    Regards,

                    John Popelish
                  • redsp
                    ... ll And the secondary inductance is not very high Q (it includes ... But is this core loss included in the simulation? I don t see a term for that
                    Message 9 of 27 , Feb 27, 2013
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                      --- In loopantennas@yahoogroups.com, John Popelish <jpopelish@...> wrote:
                      >
                      > On 02/26/2013 09:40 PM, redsp wrote:
                      >
                      ll> And the secondary inductance is not very high Q (it includes
                      > significant core loss that shows up as something like a
                      > parallel resistance across the secondary, because the high
                      > permeability core is quite conductive). That is why the
                      > entire system should not be tuned at the output of the
                      > transformer. You don't want all the resonant stored energy
                      > to have to slosh back and forth through that core. Most of
                      > the inductive energy should be stored only in the high Q
                      > antenna coil.

                      But is this core loss included in the simulation? I don't see a term for that anywhere.

                      Actually I get a higher Q by adding some capacitance to the secondary which increases the secondary currrent, but requires a lowering of the primary capacitance. I assume this is because the reduced primary current raises the Q of the primary circuit. So in the simulation I think the Q of the secondary is higher than the Q of the primary.


                      > >> I am working with a very clunky Windows simulator in a
                      > >> Linux system, so I did not have time to clean up
                      > >> everything and make sure all your measure commands
                      > >> still made sense.
                      >
                      > (snip)
                      >
                      > > BTW, they say LTspice runs great under wine. Have
                      > > you tried that?
                      >
                      > I tried, once, to install Wine, and hit a roadblock. My son
                      > is the Linux expert in the family, and one of these days, I
                      > want to try again, with his help.

                      Some folks use a dual boot arrangement. Others run one OS and then run another in a sandbox under the first. I expect this takes a pretty good machine with lots of memory. Not something I have.

                      I worked on the measurement scripts a bit today. I am trying to measure all three features I see. I can calculate the frequency of the Vcap null exactly. That is the resonance of secondary inductance and capacitance. I can get within 5% of the first peak which is due to the total capacitance and the sum of the two inductors in the primary. But it is always low. I can't come up with any explanation of the higher frequency peak. No combination of inductance and capacitance in the circuit seems to produce this. I'm not comfortable not understanding what the simulation is telling me. That is rather the idea I think, to see what it does and make sure it fits with the model. Otherwise the model may not be right. Obviously I don't understand the model.

                      I don't fully understand the field coil circuit. Why did you pick 1 mH for the radiator L4? I have no idea how to include that into the calculations and I don't know if this changes the signal strength value, not that it necessarily is a meaningful number anyway. But in my original circuit I assume the generator is the "untuned" voltage the antenna would receive and so the signal strength at Vout would be a realistic estimate of the level at the receiver input. I will try the other two methods of stimulating the circuit.

                      I also have some trouble getting LTspice to display "measured" data in meaningful ways. If it displays some data in dB, then *all* data is measured in dB. So if you want to see a straight numbmer it has to be reversed to anti-dB before being displayed. That is what some of my measurement statements were doing. lol

                      If you are interested I've uploaded a new file to your directory. Antenna_trans_LTspice.zip This contains the schematic with the radiator inductance set to match the antenna coil inductance along with the measurement file that can be run from the menu, File - Execute MEAS. Script. It is also in the schematic as commands, but the measure script can be run subsequent to the simulation.

                      I'm learning a few things about LTspice, but I still find the .MEAS command to be very cryptic.

                      Rick
                    • John Popelish
                      ... If you put a resistor across the secondary, you have a rough approximation of it. I think something in the decade of 1meg is close. You might also add a
                      Message 10 of 27 , Feb 27, 2013
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                        On 02/27/2013 06:11 PM, redsp wrote:
                        > jpopelish@...> wrote:
                        >>... includes
                        >> significant core loss that shows up as something like
                        >> a parallel resistance across the secondary, because the
                        >> high permeability core is quite conductive). That is
                        >> why the entire system should not be tuned at the output
                        >> of the transformer. You don't want all the resonant
                        >> stored energy to have to slosh back and forth through
                        >> that core. Most of the inductive energy should be
                        >> stored only in the high Q antenna coil.
                        >
                        > But is this core loss included in the simulation? I
                        > don't see a term for that anywhere.

                        If you put a resistor across the secondary, you have a rough
                        approximation of it. I think something in the decade of
                        1meg is close. You might also add a series resistance to
                        that 300 turn secondary, since it is wound of pretty fine
                        wire. Something about two times the specified DC resistance
                        (to account for skin effect) might be about right. Of
                        course, you can use the alternate Chan model of a core and
                        get a lot closer for the distortion across the waveform, but
                        that adds little to an AC analysis.

                        > Actually I get a higher Q by adding some capacitance to
                        > the secondary which increases the secondary currrent, but
                        > requires a lowering of the primary capacitance. I assume
                        > this is because the reduced primary current raises the Q
                        > of the primary circuit. So in the simulation I think the
                        > Q of the secondary is higher than the Q of the primary.

                        This is especially true if the transformer primary is
                        connected in parallel to the loop and tuning capacitor
                        (voltage transformer, rather than current transformer),
                        since resonating the transformer will raise its overall
                        impedance at the signal frequency.

                        (snip)
                        > Some folks use a dual boot arrangement. Others run one
                        > OS and then run another in a sandbox under the first. I
                        > expect this takes a pretty good machine with lots of
                        > memory. Not something I have.

                        I have a pretty spiffy machine. So I think it will handle
                        Wine okay, once I get it installed, correctly.

                        > I worked on the measurement scripts a bit today. I am
                        > trying to measure all three features I see. I can
                        > calculate the frequency of the Vcap null exactly. That
                        > is the resonance of secondary inductance and capacitance.
                        > I can get within 5% of the first peak which is due to the
                        > total capacitance and the sum of the two inductors in the
                        > primary. But it is always low. I can't come up with any
                        > explanation of the higher frequency peak. No combination
                        > of inductance and capacitance in the circuit seems to
                        > produce this. I'm not comfortable not understanding what
                        > the simulation is telling me. That is rather the idea I
                        > think, to see what it does and make sure it fits with the
                        > model. Otherwise the model may not be right. Obviously
                        > I don't understand the model.

                        I am not sure I am following you, here. We may have to get
                        together via email and work on the sane schematic, at the
                        same time.
                        >
                        > I don't fully understand the field coil circuit. Why did
                        > you pick 1 mH for the radiator L4? I have no idea how to
                        > include that into the calculations and I don't know if
                        > this changes the signal strength value, not that it
                        > necessarily is a meaningful number anyway. But in my
                        > original circuit I assume the generator is the "untuned"
                        > voltage the antenna would receive and so the signal
                        > strength at Vout would be a realistic estimate of the
                        > level at the receiver input. I will try the other two
                        > methods of stimulating the circuit.

                        The field coil inductance and driving current source are
                        completely arbitrary. They both function only as a signal
                        scale. It would probably be a good idea to pick a field
                        coil inductance and current so that a plain loop inductance,
                        possibly terminated with a meg ohm resistor or some such,
                        produced a 0db output at 60kHz. Then all other signal
                        strengths would be equivalent to gains.

                        > I also have some trouble getting LTspice to display
                        > "measured" data in meaningful ways. If it displays some
                        > data in dB, then *all* data is measured in dB. So if you
                        > want to see a straight numbmer it has to be reversed to
                        > anti-dB before being displayed. That is what some of my
                        > measurement statements were doing. lol
                        >
                        > If you are interested I've uploaded a new file to your
                        > directory. Antenna_trans_LTspice.zip This contains the
                        > schematic with the radiator inductance set to match the
                        > antenna coil inductance along with the measurement file
                        > that can be run from the menu, File - Execute MEAS.
                        > Script. It is also in the schematic as commands, but the
                        > measure script can be run subsequent to the simulation.
                        >
                        > I'm learning a few things about LTspice, but I still find
                        > the .MEAS command to be very cryptic.

                        I'll take a look. I have more problems with measure
                        commands than all other features of LTspice, put together.

                        I'm hoping for an expanded help section on this feature,
                        eventually.

                        --
                        Regards,

                        John Popelish
                      • jpopelish
                        ... (snip) ... I added a second loop inductor to the schematic, loaded only with 1meg and also coupled to both the field coil and the antenna coil by the same
                        Message 11 of 27 , Feb 27, 2013
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                          <jpopelish@...> wrote:
                          > redsp wrote:
                          (snip)
                          > > I don't fully understand the field coil circuit. Why did
                          > > you pick 1 mH for the radiator L4? I have no idea how to
                          > > include that into the calculations and I don't know if
                          > > this changes the signal strength value, not that it
                          > > necessarily is a meaningful number anyway. But in my
                          > > original circuit I assume the generator is the "untuned"
                          > > voltage the antenna would receive and so the signal
                          > > strength at Vout would be a realistic estimate of the
                          > > level at the receiver input. I will try the other two
                          > > methods of stimulating the circuit.
                          >
                          > The field coil inductance and driving current source are
                          > completely arbitrary. They both function only as a signal
                          > scale. It would probably be a good idea to pick a field
                          > coil inductance and current so that a plain loop inductance,
                          > possibly terminated with a meg ohm resistor or some such,
                          > produced a 0db output at 60kHz. Then all other signal
                          > strengths would be equivalent to gains.

                          I added a second loop inductor to the schematic, loaded only with 1meg and also coupled to both the field coil and the antenna coil by the same 0.001 K factor, and it looks like a field coil inductance of about 80mHy produces a 0db signal across that calibration inductor at 60 kHz. It also shows about a +-0.2 db signal swing as the antenna circuit passes through resonance, so this gives you an idea of how approximate the field coil is at representing a far field. Perhaps it would be a bit better to lower the K factor to 0.0001 and raise the field coil inductance to 800mHy. (a 0.02db coupling of the antenna back into the field coil.)

                          --
                          Regards,

                          John Popelish
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