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Shortwave random-wire antenna question

Discussion in 'Electronic Basics' started by Dave, Apr 3, 2004.

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  1. Dave

    Dave Guest

    I recently hooked up a thirty-six foot (plus or minus a couple of feet)
    piece of four-stranded wire with alligator clip to the internal "whip"
    antenna of my portable shortwave receiver, for the extra performance such a
    device offered. It works so well that I now cannot usually use my "DX"
    setting because of all the background noise (sounds like hundreds of other
    broadcasts vying for attention.) I don't know the frequency source of all
    this background noise, but would like to filter out as much of it as I can.
    One manufacturer of a similar "wind-up" antenna adds a capacitor to the wire
    in order to lower the resonance frequency of the wire. If I were going to
    try something similar (adding a capacitor, in series) in an attempt to bring
    the resonance of the wire down into the 30 MHz range, what size (roughly)
    capacitor should I use? Should I just try a few with different ranges, or
    does anyone here have any suggestions?

    TIA

    Cross-posted between sci.electronics.basics and rec.radio.shortwave


    Dave
     
  2. Do you have some filtering built in to your radio? Some of the better rigs
    have noise blanking, AGC, extra RF amplification (which can be bypassed),
    frequency shift/width, etc.
     
  3. Telamon

    Telamon Guest

    Since you have cross posted to sci.electronics.basics lets try to look
    at this logically and as non technically as possible.

    The are two things that you need to accomplish to hear a station on your
    radio in the way of signal strength.

    1. The signal must large enough for the radio to amplify it and
    reproduce it at the speaker.

    2. The signal must be stronger than the noise floor of the radio and any
    external noise the antenna picks up by some margin over the station you
    want to hear. Usually this is something like 10 dB.

    You can't do anything about the noise floor of the radio unless you want
    to modify it. The basic sensitivity of the radio is a decision you made
    when you bought it.

    That leaves the antenna. What you did was to put up the most basic type,
    which is called a Marconi or common mode antenna. For a simple antenna
    it is about as non-selective as you can make hence the noise level is
    high on the radio. Worse you might be over loading the radio and the
    radio itself may be generating some of the noise. Portables are designed
    to be sensitive and simple so they can't handle much signal. A strong
    signal out or in band could be causing you additional trouble.

    Whether 36 feet of wire is to much or not depends on where you live but
    for most radios generally won't cause the overload problem it's just
    that it is picking up everything well including lots of locally
    generated noise.

    That is the basically where you are at.

    What can you do about this. You want to pick up more of the signals
    (stations) you want to hear without hearing noise from other electrical
    appliances or stuff out of band.

    You need a more complex antenna design that will not pick up as much
    noise as the signal you want to hear. Noise is on all frequencies and
    comes from all directions.

    A more complex antenna design can do things like:

    1. Limit the direction it picks up signal or noise. You can benefit from
    this by pointing the antenna at the signal you want or conversely
    attenuating a noise source.

    2. Changing the type of energy the antenna picks up. The antenna type
    determines whether it picks up common mode or differential mode.

    3. The antenna type also determines whether it is sensitive to the
    electric, magnetic fields or both.

    4. The antenna type also determines the band or bands of frequencies it
    will pick up well.

    All the above will limit the total amount of noise energy it will
    present to the radio so it has less to deal with. Basically you use the
    antenna design to preselect the signals you want to pick up. The
    downside of this is short wave covers a wide range of frequencies so you
    will need more than one antenna. For some type of resonant antenna the
    smallest number of antennas you need are two and better would be three.

    To get started with a more complex antenna and to see if you are really
    improving your reception start with a weak signal using just the radios
    whip antenna. Use a station on a high band (smaller antenna) during the
    daytime.

    Make a simple resonant antenna like a dipole cut for that frequency
    connected to a coax and determine how to connect the coax to your radio.
    If it is a portable radio try operating on the batteries as some of the
    wall wart power supplies are noisy or noise on the house wiring is being
    conducted to your radio through the power cord.

    Now to test the antenna to see if it really helping you can disconnect
    it from the radio and extend the radios whip antenna and collapse it
    again reconnecting the external antenna to see which works the best.

    You can put the external antenna outside away from noise generating
    electrical equipment or switch them off.

    Once you have a dipole making an improvement on weak signals you can
    make other antenna types and antennas for other frequencies.

    There are plenty of antenna sites on the web and ideas on finding local
    noise sources.
     
  4. Antenna basics with formulas

    http://www.electronics-tutorials.com/antennas/antenna-basics.htm

    you may also want to do a search on "antenna tuners"
     
  5. Bob Myers

    Bob Myers Guest

    This isn't likely to help your basic problem, which appears
    to be a lack of selectivity in the radio (in other words, it
    is accepting signals over too wide a bandwidth, so you hear
    not only the station you're interested in, but those "to either
    side" as well. Having the passband too wide also makes
    for more noise in general. There are filters that can be
    added (within the receiver) to improve this, but your best
    bet may be to simply look for a better receiver.

    Bob M.
     
  6. Rich Grise

    Rich Grise Guest

    Not necessary at all. With a little studying of how antennas work,
    Dave could build an antenna tuner and preselector all rolled into
    one, with only a few parts.

    For specifics, that's a homework problem and I'm personally
    currently engaged in a project much like this; I'll post when
    I have something a little more concrete.

    Cheers!
    Rich
     
  7. Dave

    Dave Guest

    Thank you, Rich. That is just what I have decided I need. Where are you
    finding your information on building such a device?

    Dave
     
  8. -=jd=-

    -=jd=- Guest


    I've no interest re-inventing the wheel, so if you come up with a design
    or have links to the design(s) you've settled on - and if you are
    agreeable to it - please share.

    Thanks!


    -=jd=-
     
  9. Rich Grise

    Rich Grise Guest

    Well, I was just basically thinking of an ordinary antenna tuner,
    which can be as simple as one capacitor or one inductor, or various
    combinations, depending on what kind of impedance you're seeing
    at the feed point.

    The seat-of-the-pants way to do it is get a variable cap and
    variable inductor in a range appropriate for the freq, and just
    stick them in various arrangements and play with the tuning
    until you get the best signal.

    If you want to be scientific aboutg it, you might find something here:
    http://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q="antenna+tuner"

    Have Fun!
    Rich
     
  10. Private

    Private Guest

    Dave,

    I have been in a very similar situation. I used almost an identical
    antenna and had fantastic results. The radio likely does not have
    sufficent filtering in this case (Antenna is likely working great). I
    would start by ensuring the antenna has a good ground connection to
    the radio.

    An antenna tuner/preselector would defantely help. Purchasing a new
    or used amateur radio tuner (i.e. MFJ) 1.8Mhz-30Mhz would be the
    easiest option. Adding capacitors separetely could work, but from
    experience is a challenge to achive an workable solution for all
    frequencies.

    Homac
     
  11. -=jd=-

    -=jd=- Guest


    The variable inductor is my stumbling block. I want that continuously
    variable functionality, but without dropping $100 - $200 for a new/used
    inductor. I'm thinking about trying to build one - but that's all I'm
    doing... just thinking...

    -=jd=-
     
  12. Dave

    Dave Guest

    The antenna does indeed seem to be working great, and the radio is not
    operating too shabily either as it is only the "background noise" I am
    trying to reduce. I hooked a 100 mH RF choke up to it with good results,
    and am planning on adding another one or two similar devices in an effort to
    cut down on higher frequency interference. Question: how would I ground
    this antenna? I have a grounding rod right outside the window, but don't
    know what to hook it too. The negative battery terminal? This radio does
    have an external antenna input, but that has a plastic ring around the
    outside. Open to suggestions. And thanks for the input RE purchasing a
    new/used tuner.

    dave
     
  13. Dave

    Dave Guest

    Hello Bob,

    The selectivity doesn't actually seem to be too bad, as it is only the
    weaker signals that I have trouble digging out of the hash and trash. It
    does seem to be picking up some out-of-band signals, but they are very, very
    weak. As I posted in another message, a 100 microhenry RF choke cut that
    stuff out considerably, and I am planning on adding another 100 or 200
    microhenry RF choke to see if that helps make the slightly stronger signals
    a little more discernable. Next on the worksheet is an antenna tuner of
    some sort.

    Thanks all,

    Dave
     
  14. Yodar

    Yodar Guest

    DAVE : Wind yer coil of at least a hundred turns of magnet "war"
    around a 1" PVC pipe segment (when I was a KID I used a lacquered
    toilet paper core) and take taps off it every 2-5 turns (arbitrary)
    and use the selector switch to derive signal from the taps that give you
    the best performance (it will vary per band) The capacitors are from
    old garage sale radios and may not even be needed

    This accomlishes what you have already proven with your work, that
    certain frequencies can be canceled out by selection of signal
    inductor-load.in series with your antenna The capacitors merely
    help you tune it in more making a tank circuit that resonates with the
    frequency you're "working"

    I did perhaps the same thing by winding 100 turns of magnet war on a
    salvaice ring torid core from a ttransistor powwer supply off a dead
    computer...The antenna went to 1 end of this 100 turns and the other end
    was grounded. On top this hundred turns I wound 25-40 turns and ran on
    end of THAT winding to my radio's antenna IN connector and the other end
    to the radio's ground

    You have just made a 4:1 balun...which all in one almost does what the
    couple above does...no adjustment tho

    Or you can do the quick and dirty trick with a TV 300 ohm to 75 ohm
    coupler, connecting the 300 ohm end to the antenna and ground as above,
    and the 75 ohm connectors to your radio's antenna and ground

    You wont notice the difference

    Yodar




    ...
     
  15. Yodar

    Yodar Guest

    I just tried to send a post with a schematic for a simple antenna tuner
    and the diagram was cut out...how do I add it?

    Yodar
     
  16. Dave

    Dave Guest

    Maybe post it to alt.binaries.schematics.electronics. That is set up to
    take diagrams.

    Dave
     
  17. CW

    CW Guest

    For many years, it was common practice to use a lead and aligator clip in
    place of a roller inductor. Continuously variable and cheap.
     
  18. CW

    CW Guest

    You don't in this newsgroup (shortwave). This is a text only group.
     
  19. -=jd=-

    -=jd=- Guest

    I'm thinking a bead (say, 1/2") threaded on a rod (brass for both bead and
    rod?), positioned so that the bead can be lifted just slightly, then slid
    along the coil - but when you release the bead, there's a bit of tension
    pressing it against the coil. I thought about the gator clip method, but
    I'm
    thinking it might be easier to fool with if I had something I could adjust
    without taking my focus off the radio.

    Then again, alligator clips and leads are a more widely available than the
    pseudo roller inductor parts rattling around in my head...

    -=jd=-
     

  20. I'll suggest you try the alligator clip and wire first. It will be alot
    easier to do any modifications. Also, you might find the whole experiment
    isn't particularly helpful in your situation. If so, you might as well find
    out right away.

    Frank Dresser
     
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