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1.5-volt active loop antenna?

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  • qrp.gaijin
    Greetings list members, I have built the M0AYF active broadband loop antenna described at the link below. It works well (I use 2N3904 transistors).
    Message 1 of 14 , Nov 4, 2013
      Greetings list members,


      I have built the M0AYF active broadband loop antenna described at the link below. It works well (I use 2N3904 transistors).

      http://www.qsl.net/m0ayf/active-loop-receiving-antenna.html

      I am wondering if it is possible to run this antenna off of very low voltage, 1.5 to 3 volts. What would be the recommended changes to the circuit, and what would be the likely impact on performance?

      The reason for wanting to run it off of low voltage is that I want to incorporate this antenna as part of a low-voltage homebrew transistor shortwave receiver. The receiver itself runs off of 2 AAA rechargeable batteries. It would be nice if I could run the antenna off of the same power source.

      Any comments appreciated.
    • K2TL
      Even if you could, the current drain between the amp and the receiver would kill 2 AAA batteries real fast. The gain of the amp would be reduced to almost
      Message 2 of 14 , Nov 5, 2013
        Even if you could, the current drain between the amp and the receiver would kill 2 AAA batteries real fast.  The gain of the amp would be reduced to almost nothing with 1.5 volts anyway.  
        I use the same amp almost everyday, but I supply it with 13.5 volts via the feedline .
        K2TL

        Monday, November 04, 2013 10:32 PM
         

        Greetings list members,


        I have built the M0AYF active broadband loop antenna described at the link below. It works well (I use 2N3904 transistors).


        I am wondering if it is possible to run this antenna off of very low voltage, 1.5 to 3 volts. What would be the recommended changes to the circuit, and what would be the likely impact on performance?

        The reason for wanting to run it off of low voltage is that I want to incorporate this antenna as part of a low-voltage homebrew transistor shortwave receiver. The receiver itself runs off of 2 AAA rechargeable batteries. It would be nice if I could run the antenna off of the same power source.

        Any comments appreciated.
    • Chris Trask
      ... You could, but the IMD (distortion) performance would be severfely impacted. You could approach that problem to some degree by replacing transformer T1
      Message 3 of 14 , Nov 5, 2013
        >
        > I have built the M0AYF active broadband loop antenna described at the link below. It works well (I use
        >2N3904 transistors).
        >
        > http://www.qsl.net/m0ayf/active-loop-receiving-antenna.html
        >
        > I am wondering if it is possible to run this antenna off of very low voltage, 1.5 to 3 volts. What
        >would be the recommended changes to the circuit, and what would be the likely impact on performance?
        >
        > The reason for wanting to run it off of low voltage is that I want to incorporate this antenna as part
        >of a low-voltage homebrew transistor shortwave receiver. The receiver itself runs off of 2 AAA rechargeable
        >batteries. It would be nice if I could run the antenna off of the same power source.
        >

        You could, but the IMD (distortion) performance would be severfely impacted. You could approach that problem to some degree by replacing transformer T1 with high turns ratio, perhaps 1:9, so that the transistor collectors will see a very low load impedance. This will greatly reduce the signal voltage so that there will be a lesser tendency for the transistors to approach saturation or cutoff.

        Also, the biasing arrangement will need some serious modification. I have a schematic in mind if you're interested.




        Chris
      • qrp.gaijin
        ... Thanks for the suggestions. Yes, I would be interested in any further schematic details you might provide, even if only tentative. As an experiment I tried
        Message 4 of 14 , Nov 5, 2013

          ---In loopantennas@yahoogroups.com, <christrask@...> wrote:

          >  I have a schematic in mind if you're interested.



          Thanks for the suggestions. Yes, I would be interested in any further schematic details you might provide, even if only tentative.


          As an experiment I tried running the antenna off of about 2.4 volts, with no circuit changes. The loop element is about 2.5 meters of copper foil shaped into a diamond. Though signal levels were lower than normal, the antenna did work reasonably well indoors for SWBC listening, and I could hear a few hams on 40m as well. I noticed no IMD, probably because the concrete building kept signal levels low enough to avoid problems.


        • Chris Trask
          ... I just added the file Active Loop Amplifier for 1p5V Operation.pdf to the group files section. I should point out that for loop antennas the input
          Message 5 of 14 , Nov 6, 2013
            -----Original Message-----
            >From: qrp.gaijin@...
            >Sent: Nov 5, 2013 9:34 AM
            >To: loopantennas@yahoogroups.com
            >Subject: RE: Re: [loopantennas] 1.5-volt active loop antenna?
            >
            >---In loopantennas@yahoogroups.com, <christrask@...> wrote:
            >> I have a schematic in mind if you're interested.
            >
            >
            >
            >
            >
            > Thanks for the suggestions. Yes, I would be interested in any further schematic details you might provide,
            >even if only tentative.
            >

            I just added the file "Active Loop Amplifier for 1p5V Operation.pdf" to the group files section. I should point out that for loop antennas the input impedance of the amplifier should be very low, so I will later add a schematic for a more suitable design later.

            >
            > As an experiment I tried running the antenna off of about 2.4 volts, with no circuit changes. The loop
            >element is about 2.5 meters of copper foil shaped into a diamond. Though signal levels were lower than
            >normal, the antenna did work reasonably well indoors for SWBC listening, and I could hear a few hams on
            >40m as well. I noticed no IMD, probably because the concrete building kept signal levels low enough to
            >avoid problems.
            >

            Good IMD performance at low voltages will be tricky. You'll notice in the schematic that the biasing is accomplished by way of current mirrors so as to maximise the emitter-collector voltage.


            Chris
          • Chris Trask
            ... You don t want to run low-voltage amplifiers at high currents as the saturation voltage rises with current, and you soon find yourself with a limited
            Message 6 of 14 , Nov 6, 2013
              >
              >Even if you could, the current drain between the amp and the receiver
              >would kill 2 AAA batteries real fast. The gain of the amp would be
              >reduced to almost nothing with 1.5 volts anyway.
              >

              You don't want to run low-voltage amplifiers at high currents as the saturation voltage rises with current, and you soon find yourself with a limited dynamic range. Take a look at the curve family photos in my paper on transistor selection at:

              http://www.home.earthlink.net/~christrask/Bipolar%20Transistor%20Evaluation.pdf


              Chris
            • qrp.gaijin
              ... Thanks for the ideas. A couple of questions, if I may: 1. The emitter resistance has been replaced with an inductance. Will its reactance variation with
              Message 7 of 14 , Nov 6, 2013

                ---In loopantennas@yahoogroups.com, <christrask@...> wrote:


                > I just added the file "Active Loop Amplifier for 1p5V Operation.pdf" to the group files section.

                Thanks for the ideas. A couple of questions, if I may:

                1. The emitter resistance has been replaced with an inductance. Will its reactance variation with frequency affect the wideband nature of the loop? A similar question apples to T1, which seems to replace the original base bias resistors going to ground (R6 and R7 in the original M0AYF schematic).

                2. How should the values for R1, T1, L1 be computed?

                It will be interesting to build the new circuit and see how it compares to the original amplifier circuit (run off of both high voltage and low voltage). As I mentioned, with the original circuit, 2.4V was, somewhat surprisingly, enough to deliver audible reception of ham and SWBC signals with the antenna indoors; 1.2V operation was, however, impossible and yielded no audible stations.

                > I should point out that for loop antennas the input impedance of the amplifier should be very low, so I will later add a schematic for a more suitable design later.

                I look forward to seeing this design when it is ready. Differential pairs are still a bit of a mystery for me, but I'm learning that they have a lot of uses in low-voltage applications: as oscillators, regenerative detectors, or, as we see here, loop antenna amplifiers.
              • Chris Trask
                ... The purpose of the inductance is to retain the differential nature of the amplifier. At low frequencies, the amplifier will become a pair of
                Message 8 of 14 , Nov 6, 2013
                  >
                  >---In loopantennas@yahoogroups.com, <christrask@...> wrote:
                  >
                  >
                  > > I just added the file "Active Loop Amplifier for 1p5V Operation.pdf" to
                  > > the group files section.
                  >
                  >
                  > Thanks for the ideas. A couple of questions, if I may:
                  >
                  >
                  > 1. The emitter resistance has been replaced with an inductance. Will its
                  > reactance variation with frequency affect the wideband nature of the loop?
                  >

                  The purpose of the inductance is to retain the differential nature of the amplifier. At low frequencies, the amplifier will become a pair of common-emitter amplifiers. The differential amplifier provides immunity from common-môde distortion.

                  >
                  > A similar question apples to T1, which seems to replace the original base
                  > bias resistors going to ground (R6 and R7 in the original M0AYF schematic).
                  >

                  Transformer T1 acts as a biasing choke, and it provides immunity from common-môde interference, such as from strong nearby broadcast signals.

                  >
                  > 2. How should the values for R1, T1, L1 be computed?
                  >

                  The values for T1 and L1 will depend on the lowest frequency of interest. The value for R1 is chosen so as to provide the best IMD performance.

                  >
                  > It will be interesting to build the new circuit and see how it compares to
                  > the original amplifier circuit (run off of both high voltage and low voltage).
                  > As I mentioned, with the original circuit, 2.4V was, somewhat surprisingly,
                  > enough to deliver audible reception of ham and SWBC signals with the antenna
                  > indoors; 1.2V operation was, however, impossible and yielded no audible stations.
                  >
                  >
                  > > I should point out that for loop antennas the input impedance of the amplifier
                  > > should be very low, so I will later add a schematic for a more suitable design
                  > > later.
                  >
                  >
                  >
                  > I look forward to seeing this design when it is ready. Differential pairs are
                  > still a bit of a mystery for me, but I'm learning that they have a lot of uses
                  > in low-voltage applications: as oscillators, regenerative detectors, or, as we
                  > see here, loop antenna amplifiers.
                  >


                  Chris
                • hmaxim2000us
                  I would sure like to see values for the components of this circuit,,maybe for about 400khz at the low end up through at least 4 Mhz . Thanks! Jim ... Thanks
                  Message 9 of 14 , Nov 7, 2013

                    I would sure like to see values for the components of this circuit,,maybe for about 400khz at the low end up through at least 4 Mhz .

                    Thanks!

                    Jim



                    ---In loopantennas@yahoogroups.com, <qrp.gaijin@...> wrote:

                    ---In loopantennas@yahoogroups.com, <christrask@...> wrote:


                    > I just added the file "Active Loop Amplifier for 1p5V Operation.pdf" to the group files section.

                    Thanks for the ideas. A couple of questions, if I may:

                    1. The emitter resistance has been replaced with an inductance. Will its reactance variation with frequency affect the wideband nature of the loop? A similar question apples to T1, which seems to replace the original base bias resistors going to ground (R6 and R7 in the original M0AYF schematic).

                    2. How should the values for R1, T1, L1 be computed?

                    It will be interesting to build the new circuit and see how it compares to the original amplifier circuit (run off of both high voltage and low voltage). As I mentioned, with the original circuit, 2.4V was, somewhat surprisingly, enough to deliver audible reception of ham and SWBC signals with the antenna indoors; 1.2V operation was, however, impossible and yielded no audible stations.

                    > I should point out that for loop antennas the input impedance of the amplifier should be very low, so I will later add a schematic for a more suitable design later.

                    I look forward to seeing this design when it is ready. Differential pairs are still a bit of a mystery for me, but I'm learning that they have a lot of uses in low-voltage applications: as oscillators, regenerative detectors, or, as we see here, loop antenna amplifiers.
                  • Chris Trask
                    ... Let me work on that. I finished off the schematic for the low input impedance amplifier, so I ll incorporate that as well. It will use the HFA3046 NPN
                    Message 10 of 14 , Nov 7, 2013
                      >
                      >I would sure like to see values for the components of this circuit,,maybe for about 400khz at the low
                      >end up through at least 4 Mhz .
                      >

                      Let me work on that. I finished off the schematic for the low input impedance amplifier, so I'll incorporate that as well. It will use the HFA3046 NPN transistor array, which is a good device for low-voltage RF applications such as this.


                      Chris
                    • K2TL
                      Thank you! What kind of gain do you expect with this amp? And can it be used with a higher supply voltage? Jim
                      Message 11 of 14 , Nov 7, 2013
                        Thank you!  What kind of gain do you expect with this amp?  And can it be used with a higher supply voltage?
                        Jim

                        Thursday, November 07, 2013 9:29 AM
                         

                        >
                        >I would sure like to see values for the components of this circuit,,maybe for about 400khz at the low
                        >end up through at least 4 Mhz .
                        >

                        Let me work on that. I finished off the schematic for the low input impedance amplifier, so I'll incorporate that as well. It will use the HFA3046 NPN transistor array, which is a good device for low-voltage RF applications such as this.

                        Chris

                      • Chris Trask
                        ... I just deleted that earlier PDF file and replaced it with Active Antenna Amplifiers for Low Voltage Operation.pdf that has the low input impedance
                        Message 12 of 14 , Nov 7, 2013
                          >
                          >>
                          >>>
                          >>>I would sure like to see values for the components of this
                          >>>circuit,,maybe for about 400khz at the low
                          >>>end up through at least 4 Mhz .
                          >>>
                          >>
                          >> Let me work on that. I finished off the schematic for the low input
                          >> impedance amplifier, so I'll incorporate that as well. It will use the
                          >> HFA3046 NPN transistor array, which is a good device for low-voltage
                          >> RF applications such as this.
                          >>
                          >
                          >Thank you! What kind of gain do you expect with this amp? And can it
                          >be used with a higher supply voltage?
                          >
                          >

                          I just deleted that earlier PDF file and replaced it with "Active Antenna Amplifiers for Low Voltage Operation.pdf" that has the low input impedance amplifier on the second page. This is still a work in progress.

                          Not certain what sort of gain to expect with either of these. The maximum supply voltage will depend on the collector-emitter breakdown voltage of the devices. For the low impedance amplifier using the HFA3046, the maximum supply voltage would be 5.0V.


                          Chris
                        • qrp.gaijin
                          Greetings, I was again pondering some possible low-voltage RF amplifier designs and I stumbled upon a possible idea for a low-voltage (1.2 volts) wideband
                          Message 13 of 14 , Dec 2, 2013
                            Greetings,

                            I was again pondering some possible low-voltage RF amplifier designs and I stumbled upon a possible idea for a low-voltage (1.2 volts) wideband (untuned) active loop antenna amplifier. I'm wondering if the idea could work.

                            Consider a common-base NPN BJT amplifier configured as follows. At RF, the base is grounded by a 10 nF capacitor. At DC, the base is connected directly to the collector. The collector in turn goes through a resistor to +Vcc. The resistor value is adjusted for the desired current. The emitter is grounded, but not through a resistor as is usual: instead, the emitter is grounded through the loop antenna conductor itself. Thus the loop current (which we want to sense and amplify) is flowing directly through the emitter.

                            Amplifier output is taken from the collector through a coupling capacitor.

                            Could such an idea work as well as a low-voltage BJT differential-pair amp? The appeal of the above design is its simplicity.

                            >
                            >---In loopantennas@yahoogroups.com, <christrask@...> wrote:
                            >
                            >
                            > > I just added the file "Active Loop Amplifier for 1p5V Operation.pdf" to
                            > > the group files section.

                          • qrp.gaijin
                            ... I m thinking about building the low-input-impedance amplifier on the second page, using 2N3904 transistors and running off of 1.2 volts. Intended use is
                            Message 14 of 14 , Jan 1, 2014

                              In loopantennas@yahoogroups.com, <christrask@...> wrote:


                              > I just deleted that earlier PDF file and replaced it with "Active Antenna Amplifiers for Low Voltage Operation.pdf" that has the low input impedance amplifier on the second page. This is still a work in progress.


                              I'm thinking about building the low-input-impedance amplifier on the second page, using 2N3904 transistors and running off of 1.2 volts. Intended use is for shortwave listening and maybe MW listening. A couple of questions:


                              1. Are the ferrite beads necessary?


                              2. I'm thinking about winding the inductors and transformers as follows. All inductors will be wound on FT50-43 cores. T1: 20 turns, center-tapped. T2: 20 turns primary, 20 turns secondary, each center-tapped. L1: 20 turns. T3: 20 turns primary (center-tapped), 20 turns secondary (output). Do these values sound reasonable?


                              3. For Q1-Q4, what base and collector voltages should I aim for?


                              4. Is symmetrical physical construction important? 


                              5. Why can't we take the antenna output directly off of the T2 secondary, using Q3 and Q4 only and eliminating Q1 and Q2?


                              6. I'm planning on using the coaxial cable to supply power to the antenna, using a bias tee and running +Vcc through the T2 secondary (similar to the original M0AYF design). Any problems with this approach?


                              Thanks for any advice.

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