Loading ...
Sorry, an error occurred while loading the content.

Remote tuning

Expand Messages
  • tenorman1952
    So, it occurred to me that it would be good to come up with a cheap and easy way to turn a tuning capacitor on a loop that is mounted somewhere outside. Now
    Message 1 of 6 , Jul 31, 2008
    • 0 Attachment
      So, it occurred to me that it would be good to come up with a cheap
      and easy way to turn a tuning capacitor on a loop that is mounted
      somewhere outside.

      Now this is just an idea... I have flow radio control (aka R/C) model
      aircraft for many years.

      The receiver and servos that operate the control surfaces in the
      plane run off 4.8 v nicad packs. Actually, they will run on anything
      from 4.8 v - 6.0 v. So, if power can be sent out to the antenna this
      could be made to work.

      Tuning capacitors rotate 180*. Most servos only rotate 90*, but can
      be modified to rotate further.

      Inside the servo is a feedback potentiometer driven by the output
      gear of the servo. These pots are usually 10k ohms. By attaching a
      4.7 k 1/8 w resistor on each outer terminal of the pot, and
      connecting those leads to the resistors, the servo will now rotate
      180*.

      Now a direct connection can be made from the servo's output shaft to
      the tuner shaft with a piece of rubber or vinyl tubing.

      A cheap two channel R/C car/boat radio system can be used for this,
      under $100.

      Just a thought...

      Paul C.
    • cmdrcosmac
      ... Paul, That s basiclly a good idea and it s probably workable if the 180 degree mod can be done with a servo powerful enough to turn the cap. The small
      Message 2 of 6 , Jul 31, 2008
      • 0 Attachment
        --- In loopantennas@yahoogroups.com, "tenorman1952" <tenorman1952@...>
        wrote:
        >
        > So, it occurred to me that it would be good to come up with a cheap
        > and easy way to turn a tuning capacitor <snip>


        Paul,
        That's basiclly a good idea and it's probably workable if the 180
        degree mod can be done
        with a servo powerful enough to turn the cap. The small tuning caps
        from cheap radios
        are usable in loops and turn easily. But these days remote tuning is
        usually done with
        a varactor diode. This replaces the cap and is tuned by a DC bias
        across it. This variable
        voltage is usually sent up the signal feedline, isolated by
        capacitors. Other posters on
        this forum have given links to examples of this.
        The servo would, however, be a good way to rotate the loop. You can
        find circuits on the
        web to operate the servo directly without the R/C stuff.

        Chuck.

        >
      • Chris Trask
        ... I remotely tune my receiving loops with varactors. The tuning voltage is also the amplifier supply voltage, and the amplifier is designed to work with the
        Message 3 of 6 , Aug 1, 2008
        • 0 Attachment
          >
          > > So, it occurred to me that it would be good to come up with a cheap
          > > and easy way to turn a tuning capacitor <snip>
          >
          >
          > Paul,
          > That's basiclly a good idea and it's probably workable if the 180
          > degree mod can be done with a servo powerful enough to turn the cap.
          > The small tuning caps from cheap radios are usable in loops and turn
          > easily. But these days remote tuning is usually done with a varactor
          > diode. This replaces the cap and is tuned by a DC bias across it.
          > This variable voltage is usually sent up the signal feedline, isolated
          > by capacitors. Other posters on this forum have given links to examples
          > of this. The servo would, however, be a good way to rotate the loop.
          > You can find circuits on the web to operate the servo directly without
          > the R/C stuff.
          >

          I remotely tune my receiving loops with varactors. The tuning voltage is also the amplifier supply voltage, and the amplifier is designed to work with the varying supply voltage.

          I'm going to be working on a remotely tuned transmitting loop where the motor is controlled with the DC voltage going up the coax. Using a variable capacitor without a stop pin on the shaft relieves me of the trouble of having to add stop switches and diodes.





          Chris

          ,----------------------. High Performance Mixers and
          / What's all this \ Amplifiers for RF Communications
          / extinct stuff, anyhow? /
          \ _______,--------------' Chris Trask / N7ZWY
          _ |/ Principal Engineer
          oo\ Sonoran Radio Research
          (__)\ _ P.O. Box 25240
          \ \ .' `. Tempe, Arizona 85285-5240
          \ \ / \
          \ '" \ IEEE Senior Member #40274515
          . ( ) \
          '-| )__| :. \ Email: christrask@...
          | | | | \ '. http://www.home.earthlink.net/~christrask
          c__; c__; '-..'>.__

          Graphics by Loek Frederiks
        • tenorman1952
          I received a good follow up question from one of the members. Do servos need a continuous pulse stream to maintain a position? No, but if you just turn off
          Message 4 of 6 , Aug 1, 2008
          • 0 Attachment
            I received a good follow up question from one of the members.

            "Do servos need a continuous pulse stream to maintain a position?"

            No, but if you just turn off the transmitter, the momentary drop in
            voltage distorts the pulses just enough to make the servos jump a
            bit, often as much as 15*-20* off their original position. (I'm
            using * for degree)

            With a wheel type car/boat transmitter, the centering spring would
            have to be removed from the steering wheel. With a "two stick"
            transmitter, such as used with gliders, the centering spring would be
            removed from one stick, and set so it will stick in position, such as
            used for a throttle. This is easy to do.

            http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXARP0**&P=7

            If you have 12 v going out to the antenna, such as to operate an
            active antenna, you could come off the 12 v with a 7805 (5v) or 7806
            (6v) regulator chip (TO-220 type) with a capacitor accross in and out
            legs to ground. I've done this with R/C gear run with higher voltage
            battery packs used for electric boats and with an electric powered
            glider. A receiver only draws about 50-80 ma, and servos draw about
            50 ma at idle, and 100-200 ma moving, so this won't tax a 7805 at
            all, which will put out 1 amp easily. And a tuning cap has almost no
            resistance, so, moving the servo won't draw but about 80-100 ma.

            I may be wrong on the resistors added... if the pot is 5k, use 2.2 k
            resistors on each side. If 10 k, use 4.7 k on each side. The
            feedback pot is simply a voltage divider.

            The way a servo works, input pulse is stretched, and turned into a
            voltage. Longer the pulse, the longer the voltage. The servo
            circuit compares the pulse/voltage with the voltage on the feedback
            pot via an op amp. If one voltage is higher than the other, the op
            amp produces a + or - voltage, which drives the motor one way or the
            other until once again the two voltages are equal. So, doubling the
            resistance of that feedback pot will double the throw of the servo
            from 90* to 180*, which is the rotation of the tuning cap. Putting a
            resistor on each end of the feedback pot will keep it approximately
            centered.

            Stick movement to servo movement is very precise and repeatable,
            otherwise it wouldn't make a good aircraft or whatever control.

            Now, here's another possibility...

            Instead of the whole receiver/servo in the antenna... just a servo
            and supply it with 5v to operate. No radio, but control it with a
            servo tester, which is just a little box with a knob and a battery.
            You would probably have to use an optocoupler or op amp buffer due to
            the long line.

            http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXVK79&P=ML

            http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXUK84&P=ML

            So, no tx, no rx, just 5v for the servo tester, 5v for the servo.
            See the leads? Black is common or -, red is 5v, white is signal (or
            pulse). It would be easy to put an op amp buffer inline.

            In a second email he commented:

            "I asked the question as I was concerned in case the servo needed a
            continuous pulse stream. As you would be sending LF square waves out
            to control the servo I was thinking of any possible broadband
            interference that might result."

            I replied:

            That is valid a concern.

            If the pulses are cut off cleanly, that is, just turn off the white
            signal wire, the servo stays in the last position. If the
            transmitter is turned off, as the voltage falls, the servos jump.
          • tenorman1952
            ... You are correct... a sail control servo, as used in R/C sailboats would be more appropriate for rotating the loop. And as I just posted about a
            Message 5 of 6 , Aug 1, 2008
            • 0 Attachment
              > The servo would, however, be a good way to rotate the loop. You can
              > find circuits on the
              > web to operate the servo directly without the R/C stuff.
              >
              > Chuck.


              You are correct... a "sail control" servo, as used in R/C sailboats
              would be more appropriate for rotating the loop.

              And as I just posted about a controller:

              http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXVK79&P=ML

              Sail wench servo.

              http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXGZU3&P=7

              Paul
            • Jim Dunstan
              ... Hi Paul, Yes it can be done. Whether you use a wire connection or a wireless connection (R/C) ... the problems are mechanical in nature. I have seen some
              Message 6 of 6 , Aug 1, 2008
              • 0 Attachment
                At 11:18 PM 7/31/2008 +0000, you wrote:

                >So, it occurred to me that it would be good to come up with a cheap
                >and easy way to turn a tuning capacitor on a loop that is mounted
                >somewhere outside.
                >
                >Now this is just an idea... I have flow radio control (aka R/C) model
                >aircraft for many years.
                >
                >The receiver and servos that operate the control surfaces in the
                >plane run off 4.8 v nicad packs. Actually, they will run on anything
                >from 4.8 v - 6.0 v. So, if power can be sent out to the antenna this
                >could be made to work.
                >
                >Tuning capacitors rotate 180*. Most servos only rotate 90*, but can
                >be modified to rotate further.
                >
                >Inside the servo is a feedback potentiometer driven by the output
                >gear of the servo. These pots are usually 10k ohms. By attaching a
                >4.7 k 1/8 w resistor on each outer terminal of the pot, and
                >connecting those leads to the resistors, the servo will now rotate
                >180*.
                >
                >Now a direct connection can be made from the servo's output shaft to
                >the tuner shaft with a piece of rubber or vinyl tubing.
                >
                >A cheap two channel R/C car/boat radio system can be used for this,
                >under $100.
                >
                >Just a thought...
                >
                >Paul C.

                Hi Paul,

                Yes it can be done. Whether you use a wire connection or a wireless
                connection (R/C) ... the problems are mechanical in nature. I have seen
                some locals using remote control of the variable capacitor and the problem
                they had was slowing the servo motor sufficiently for smooth tuning. Often
                disabling switches are used at either end of the variable rotation. It is
                really the only way to go if you are using the loop to carry any
                significant power. A receive only loop is best tuned remotely with
                varactor diodes.

                Jim, VE3CI
              Your message has been successfully submitted and would be delivered to recipients shortly.