Spi-Ro traps

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W0CMO, May 20, 2019.

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  1. W0CMO

    W0CMO Ham Member QRZ Page

    I have a set of T-40 Spi-Ro traps. They have a 1/8" black dot on ONE end of the trap. Does this dot have any significance? I have been unable to find a phone number or email address for Spi-Ro but I posted a question in the Help section of their website. They say to expect an answer within 48 hours. Yeah, right. It's been 10 days!


  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    This guy might know.:)


    It's just a coil of wire with a capacitor wired across it, so I don't know why it would have any "polarity."

    I'd stay away from "Spi-Ro Manufacturing" stuff. Evidently a 1-man part-time operation, no real way to contact them (they had a listed phone number in the past, but today it's disconnected). Last business listing shows them in Jonesborough, TN but no phone number that connects.
    NH7RO likes this.
  3. W0CMO

    W0CMO Ham Member QRZ Page

    Funny. I'll bet there aren't too many folks who remember him.

    I, too, have heard about the poor reports on Spi-Ro. I bought these traps YEARS - decades - ago and they have just been sitting in my storage closet. Getting ready to set up another station at my vacation house so I thought I might use them. I couldn't figure out the significance of the "dot" because, like you said, it's just a coil and a capacitator in parallel. However, one thing I have learned in life, things can always come up to bite you in the butt! Maybe one day I'll tell you about the time I connected some carbon resistors with the polarity backwards.....

  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I tried hammering a nail into a board backwards.

    It's fun, and the nail wins.:)
  5. W9GB

    W9GB Ham Member QRZ Page

    Last edited: May 20, 2019
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That was helpful!

    It seems Rick was WB4IRC -- don't know why that isn't mentioned in his obit.

    It also seems the only mechanical support for the wire-to-trap connection is the wire wrapped around a screw terminal. With the winds and tree sway we have here (and most places, I'd think) that wouldn't last very long -- the stress on the wire at the screw terminal would break the wire before too long:

    I'd never make an antenna wire connection also the mechanical support for something as long as a 40/80m dipole. Maybe for 2 meters.:)

    In any case, I still don't know what the "black dot" is for -- I didn't see it mentioned in the K4MSG writeup.
  7. AF9US

    AF9US Premium Subscriber QRZ Page


    Thirty+ years ago, I wrote to Spi-Ro regarding a trapped dipole they made. It was a multi band dipole. I assembled the dipole for a friend.

    At the time, the traps were made with clear plexiglass housing. If I remember correctly, the color dots indicated the band-pair of traps. Black dots indicated the 40 meter traps, and other colors were used to identify the traps for 20, 15 and 10 meter bands (pre-WARC band days).

    The address in the instructions was a Miami, Florida address.

    I'm still waiting for the reply to my question about the wire lengths between the traps in the multi band dipole. In their defense, I've moved at least a dozen times since 1978.

    That's my speculation for the meaning of the color dot on the Spi-Ro traps.
    Bernie, AF9US
    WB5YUZ, NH7RO and WQ4G like this.
  8. W9GB

    W9GB Ham Member QRZ Page

    This Spi-Ro approach would not be advisable with a trap dipole antenna, under tension.
    The electrical utilities do not use that approach for their electrical Service Drops.
    It is unclear whether this company is still in business, after founder’s death. The only web site update is a posting of the founder’s death. Lack of Support in providing electronic copies of the Data Sheets is a Bad Sign. This technology is NOT proprietary .... original Morgan US patents (1938) lapsed ... at same time Chester Buchanan, W3DZZ published in QST (1955).
    The Spi-Ro traps are housed inside in white 1.25-inch OD PVC pipe with PVC end caps (glued on). The overall length is 7.25-inches.

    Paul Bock notes:
    The dimensions given in the data sheet (shipped with T40 traps) for an 80/40 meter dipole seemed a bit short based on previous experience with trap dipoles.
    An e-mail by Paul to Rick at Spi-Ro confirmed that the dimensions provided in the T-40 data sheet were for the Center of the Phone bands on 80 and 40 meters.
    Paul Bock, K4MSG wanted his antenna to resonant in the CW band, so he factored extra wire into his design. His 80/40 antenna using the Spi-Ro T-40 traps had dimensions:
    Inner Section (Center Insulator to T-40) of 34 feet (each)
    Outer Section (T40 to End Insulator) of 23 feet (each)
    These dimensions are LOGICAL,
    since the Inner Wire Section of a 80/40 trap dipole is usually the same as a 40-meter dipole.
    The Outer Wire Section lengths determine the 80-meter resonance frequency.
    The 80/40-meter trap antenna and 40-meter dipole can be tuned to 15-meters (as a 3/2-wavelength antenna) with the usage of a Gimmick Capacitor (a hanging wire or wire twisted in a figure-8) at the correct 15-meter point (wavelength/frequency) from the center insulator.
    G3TXQ and W9GB discussed this application in QRZ Q&A (2015)
    The multimatch antenna system
    Chester L. Buchanan, W3DZZ
    QST magazine (ARRL), March, 1955

    Fig. 1. Sketch illustrating the three fundamental modes of the multimatch antenna.

    1. Buchanan, "Duo-band ham antenna," Radio & Television news, December, 1950.
    2. Morgan, "A multifrequency tuned antenna system," Electronics, August, 1940.
    The usage of traps in antennas can be traced to H. K. Morgan, US patent # 2229856 in 1938.
    Morgan then published the first article on a multi-band trap antenna in 1940.
    Morgan H. K., Multi-frequency Tuned Antenna System, Electronics, Vol. 13, August 1940, pp. 42-50.

    Unfortunately, the outbreak of World War 2 interrupted further experimentation by US radio amateurs. In the 1950s, as amateur radio returned, C. L. Buchanan, W3DZZ published his classic article, The Multi-match Antenna System in the March 1955 issue of QST magazine, pp. 22-23, 130.

    LeRoy Buchannn, W9AQW / W3DZZ graduated from Purdue University in 1939.
    He later became Chief of Ocean Engineering Branch of Naval Research Lab.

    The development of commercial and DIY home-made trapped HF wire, beams, and verticals for multi-band coverage followed in late 1950s to present day.
    Last edited: May 21, 2019
  9. W0CMO

    W0CMO Ham Member QRZ Page

    Thanks for all of your comments...…...

    Mystery solved. As it turns out, I was rummaging through some stuff yesterday and found the instruction sheet for the Spi-Ro traps. Is says to position the traps so that the "dot" end of the trap is nearest the center feed point. Didn't say why.

  10. NH7RO

    NH7RO Ham Member QRZ Page

    The Black Dot Mystery deepens...

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