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Would relay-coil feeds interfere if cable-tied to dipole elements?

Discussion in 'Radio and Communications' started by Sinedup, Sep 25, 2015.

  1. Sinedup

    Sinedup New Member

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    Hi. Would 2x (or more) small SPST 12V relays work for extending home-built 1/2 wave dipole from (HW) 1.111m (135MHz AM) to 3.0>3.4m (50>~48MHz AM)?
    Would relay-coil feeds interfere if cable-tied to dipole elements? (remotely operated resonant-length tuned switching idea).
     
  2. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    Its not polite to hijack other threads... I move it here...
     
  3. JimB

    JimB Super Moderator Most Helpful Member

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    Yes.
    The idea is a complete non-starter.

    The usual way to make an antenna resonant on two widely spaced frequencies it to put a tuned "trap" at the end of the inner higher frequency section.

    So in your case, start with a 135MHz dipole, and add a parallel tuned circuit which is resonant at 135MHz to each end of the dipole. Then connect wire/rod to the free ends of the tuned circuits to extend the length of the dipole elements to resonate at the require frequency. 50 MHz in your case.

    The next problem is that there is usually a fair amount of interaction between the two parts of the dipole and the traps, and it is necessary to do some measurements and experimentation to get it right.

    Having said all that, what is it that you are really trying to do?
    There may be an easier way.

    JimB
     
  4. dave

    Dave New Member

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  5. Sinedup

    Sinedup New Member

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    To JimB: Absolutely what I needed, thanks. Ian Rogers: My sincere apologies, I thought I had it right - I am new to the art of blogging, so taking baby-steps, stumbling now and then. If others can read this post, such as the curt reply about "talk less think more", please exercise tolerance with new members. Trying to navigate my way around. Thank you.
     
  6. Sinedup

    Sinedup New Member

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    I found a table of radio frequencies (UK-based) including the amateur range around 50MHz and Air/Civil bands above 110MHz. Only for temporary DX'ing. Recently bought an old book "Art of Electronics" with hundreds of AM/SSB etc. receiver circuits (want to construct some). Aviation enthusiast so 135MHz is also our "air" band.
     
  7. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Here are some things to consider:

    If receiving only through an antenna, resonance is not required. I would build a dipole, or a ground-plane antenna for the lowest frequency, and then just use it as-is at a frequency up to 3X higher...

    Antenna polarization is much more important than resonance. In the US, most of the signals on six meters (50MHz) will be horizontally polarized (horizontal antenna elements), while the aircraft band (118 to 137 MHz) and the ham and commercial FM radio signals above 144 MHz will be vertically polarized (vertical antennas). Dipoles work best for horizontal polarization. Sounds like two antennas in your future...

    For transmitting at two or more frequencies, you can build two dipoles, one resonant at say 120MHZ, another resonant at say 150Mhz, connect their feed points in parallel, put the two dipoles in divergent orientations ( a few degrees apart in space), and the two antennas will not interact much. (fan dipole). This saves running two coax feedlines.

    The best all-around vertically-polarized broad-band VHF antenna is a Discone. I am using one here as a test antenna. It can be used for transmitting or receiving with a VSWR less than 1.5:1 over the range of 100MHz to 1.5GHz.
     
    Last edited: Sep 26, 2015

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