Novel Compact Antenna for VLF

Discussion in 'Amateur Radio News' started by AE4G, Apr 12, 2019.

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  1. KQ6XA

    KQ6XA Ham Member QRZ Page

    Hi Mark,

    Congratulations again, to you and your colleagues for the impressive and ground-breaking work.
    Increasing power, 10x or 100x, wow!
    For power up-scaling, you may want to take a look at some of the early designs for power ultrasonic piezolith multi-element arrays for some engineering inspiration.
    They were much larger piezo systems than the modern lightweight piezolith parabolics.
    But, you probably have already looked at those :)

    In the 1 metre diameter LF loop antennas I utilize for 185 kHz "through-the-rock cave radio", adequate bandwidth is a major design problem.
    I had to intentionally design a lower Q into the loop antennas, to achieve a 1.5 kHz bandwidth (-3dB) for a voice SSB signal.
    This lower Q also lowered the efficiency and limited the distance for communications with the low power 5W SSB/CW transmitters utilized.
    Also, the frequency, 185 kHz was chosen so that a loop would have adequate Q for voice SSB, but a lower frequency loop antenna such as 35 kHz might be even better (but narrower bandwidth) for going through rock.

    The DAM (direct antenna modulation) technique is an excellent system to achieve more bandwidth for transmit.
    Pushing piezo DAM to a higher symbol rate or applying 8-ary FSK (or n-ary FSK) would be needed for real time voice.
    Obviously, this presents challenges in the method of modulation switching, but I have some ideas on how that can be achieved.

    The use of FSK and DAM with the efficient piezo methods you demonstrate, when leveraged for a cave radio application would be quite an advantage in efficiency, size, and weight.

    In the cave radio application, in addition to communication, the null of the loop antenna pattern is utilized in cave radio survey mapping (RDF the underground transmitter).
    Have any plots of the antenna pattern for the typical rod piezo antenna been produced?

    -Bonnie Crystal, KQ6XA
    (alumna, Stanford Amateur Radio Club W6YX)
    Last edited: Apr 15, 2019
  2. K8ERV

    K8ERV Ham Member QRZ Page

    If you do I'll build a better mouse. You can't win.

    TOM K8ERV Montrose Colo
  3. MKEMP

    MKEMP QRZ Member

    The advantages of the approach begins to dwindle as one goes higher in frequency. There, the tank or tuning circuit becomes easier to embody with more conventional lumped elements. We have an amateur radio club here at the lab and someone suggested for the sake of demonstration to try out something at the 2200m band. He mentioned it is a fairly new allocation.

    We are scaling up in power, primarily by making the device larger. The dipole moment scales with volume in addition to other factors. Recall, our efficiency will always be very low because we are constraining ourselves to such a short dipole. We only hope to be less horrible than conventional approaches.
    KK4HPY and KQ6XA like this.
  4. KQ6XA

    KQ6XA Ham Member QRZ Page

    Both piezoelectric , mechanical, and electro-pneumatic ultrasound lithotripter devices are utilized in medical applications.
    These may have application in electromagnetic antennas.

    Yes, perhaps we do need to revisit the Alexanderson-style techniques, but miniaturized with neo magnets, for modern LF magnetic DAMs :)
    W1YW likes this.
  5. MKEMP

    MKEMP QRZ Member

    With a single element, we should be approximating a Hertzian dipole. However, as you allude to, one of the ways we will look to increase power output is to array elements. As that development matures, we can pursue shaping our pattern as well.

    The pop articles have focused a good amount on the through-earth and underwater applications of VLF/LF, but we are really focusing on long-distance communication. The radiation resistance from an electric dipole rather than a magnetic dipole is much better, however the near field applications will favor a magnetic dipole.

    I'd love to stay in contact as we continue development. I'm sure I could learn a lot regarding implementation of this in a more practical system. Also, if you are in the Bay area, if you are interested, you are welcome to one of our field tests. Last one we had was up at the Stanford dish area. Quite nice area to get away from the 120Hz harmonics.
    KQ6XA likes this.
  6. KW1NG

    KW1NG Ham Member QRZ Page

    I built an amplifiers in the 90's to drive a piezoelectric wafer stack at frequencies up to 100kHz; it was used for a machining transducer. It took something like 2kW to move the thing a few thousandths of an inch, so not sure of this thing's efficiency or portability. Sonar guys have been doing this for years, this is nothing new.
  7. AA5CT

    AA5CT Ham Member QRZ Page

    re: "Yes, perhaps we do need to revisit the Alexanderson-style techniques,"

    The Alexanderson alternator was a crude form of signal-source (generator), not a radiator/antenna; solid-state techniques don't experience the wear that mechanical means of signal generation encounter ...

    Added: A Dremel tool might offer a cheap, easy way to implement something today in the way of an experiment.
  8. KQ6XA

    KQ6XA Ham Member QRZ Page

    Wafers are good for accentuating the acoustic/mechanical action of piezos.
    Other shaped structures may be better for the use of piezos for "radio wave" antennas.
    Driving the other shaped structures for the desired radio wave effect may be quite a different level of efficiency.
    At currently available materials, there may be a frequency sweet spot for the radio wave piezo antenna techniques, depending on the miniaturization of application and power levels.
  9. AE4G

    AE4G Ham Member QRZ Page

    Thanks Mark. Please keep us posted.

    I think we would especially be interested in your results if you do the 2200m band demo.

    Last edited: Apr 16, 2019
  10. KV4PC

    KV4PC Ham Member QRZ Page

    I wonder if an alternative choice of piezo materials might apply for optimizing this technique in different frequency ranges? For instance, selecting a material that would inherently result in a longer rod at higher frequencies. Thinking of the 2200m band, of course...

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