Part15

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Helical Antenna Alert

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Yes, there’s a really strong string about helical antennas somewhere, but I can’t find it. So here’s what I am excited about adding.

Yes, there’s a really strong string about helical antennas somewhere, but I can’t find it. So here’s what I am excited about adding.

In TV Technology (tvtechnology.com), Apr. 11 edition, Page 12, there’s a very informative article by Steve Harvey titled WIRELESS MICS ADJUST TO THE NEW RF ENVIRONMENT. The wireless mic aspect is, of course, interesting as a Part 15 subject, but right now I am talking about a photograph showing double-length helical antennas inside transparent tubes with the open end of the tubes pointed toward the wireless receivers, and the back end attached to mic-stands.

THE SIGNAL IS SHOOTING OUT OF THE END OF THE TUBE.

These antennas are described as highly directional helical antennas, which offer the most forward gain and the tightest polar pattern.

In discussions of helical antennae for Part 15, it has been assumed that radiation will be toward the horizon from all sides of a vertical spiral, but now I’m not so sure. If the example in the article is a firm indication, the signal shoots out the top or “far end” of the helix.

It doesn’t matter that we’re using lower frequencies because the antenna sizes we are forced to use are more suited for HF.

I wanted to link the photo, but on the tv tech website the electronic edition only allows up to page 10 and then requires subscription. As a veteran tv producer I have a complementary subscription, and so could you, by filling out their form. Just say “presto” and start a video division for your station.

My scanner is not up right now, so, no picture.

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  1. Helical Antenna Alert
    Hi Carl,

    I’ll be the first to admit I don’t know anything about end-fire helical antennas, but the Wikipedia article on “helical antenna” has a decent description:

    http://en.wikipedia.org/wiki/Helical_antenna

    —– quote:
    “Helical antennas can operate in one of two principal modes: normal mode or axial mode.

    In the normal mode or broadside helix, the dimensions of the helix (the diameter and the pitch) are small compared with the wavelength. The antenna acts similarly to an electrically short dipole or monopole, and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis. The radiation is linearly polarized parallel to the helix axis.

    In the axial mode or end-fire helix, the dimensions of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna’s axis. It radiates circularly polarized radio waves.”
    —- end quote

    To get the end-fire mode, the dimensions need to be “comparable to a wavelength”. That doesn’t sound very practical for AM, but might be practical for FM?

    • Helical Antennas
      The kind of “axial” helical antennas you describe were used in the early days for on-board satellite antennas. At 22,000 miles from earth, on-board sat antennas needed a ton of gain and lots of directionality. The earth station antenna systems had ganged arrays of helical antennas for extreme gain for picking up very weak signals. Sat system transmitters were in the 10 to 30 watt range and had to cover 22-thousand miles through space and atmosphere.

      Of course, these antennas are axial helicals because the signal propagation is end fire, off of one end of the antenna. The gain is produced in a similar fashion to the old Yagi-Uda directional antennas, used for TV reception. Frequencies for axial helicals are for vhf and uhf service (above 60 MHz.). Otherwise, they would be absolutely ungainly in size. Earth station axial arrays could easily, at uhf frequencies, be 100 feet high and 150 feet wide depending on configuration.

      The helical antennas used for low band use (AM BCB, shortwave, hf ham radio up to 30 MHz.) are (normal) parallel helicals with the signals propagating off the side of the antenna, either as a balanced dipole or an unbalanced vertical. Many early 70’s and 80’s ARRL antenna Handbooks contained plans for both types of helical antennas.

      This explanation is greatly over simplified.

      A normal helical could be designed for Part 15 use (3 meter length). A tuning network and ground radials would still be required to fully use the capabilities of the antenna.

      • Double Helix
        First, thanks for all the input about the Single Helix antenna. It might be fair to describe its various forms thusly: In the beginning is a coil consisting of closely wound wire. If the wire is stretched out putting greater space between windings and the coil-diameter made small, a helical antenna is achieved which will radiate omni-directionally with horizontal polarity. If stretched further, making the spacing greater, also the innercircle being made relative to wavelength, the RF begins to focus toward the top end and becomes directional with circular polarity.

        Moving on, the study of DNA has revealed a fundamental building block of life in the Double Helix strand, opening the new field of genetic engineering. What happens when a Double Helix pattern is applied to antenna design?