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receive antennas only

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB9MZ, Mar 8, 2011.

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  1. W5DXP

    W5DXP Ham Member QRZ Page

    As long as the received signal to noise ratio is not compromised, the efficiency of a receiving antenna doesn't much matter because of the 70+ dB dynamic range of the receiver AGC circuitry. However, the efficiency of a transmitting antenna certainly does matter and 70+ dB of gain at the transmitting antenna, even if it were legal, would probably melt any low-efficiency antenna. How efficient is your antenna-in-a-box? How much power can it dissipate without overheating or melting?

    Or looking at it another way - Assuming you are listening to an S9 signal, how much power do you have to deliver to your antenna-in-a-box for your signal to be S9 at the other end?
     
  2. W0BTU

    W0BTU Ham Member QRZ Page

    Beverages (or other receiving antennas) are not magical cures for lightning QRN, because it's difficult to null out. If they were, more people would operate 160 meters year-round.

    But they're a heck of a lot better than a vertical!
     
    Last edited: Mar 8, 2011
  3. W0BTU

    W0BTU Ham Member QRZ Page

    What do you have, a Slinky antenna?
     
  4. W5DXP

    W5DXP Ham Member QRZ Page

    I have essentially the same problem here in East Texas and my IC-756PRO filters plus ClearSpeech noise filter do a reasonable job on static. If I remember correctly, most frequencies of a human voice are correlated while most random static is uncorrelated so it should be possible to separate the two. Dang, I should have continued that research when I retired in 1998.
     
  5. K4SAV

    K4SAV Ham Member QRZ Page

    It's really very simple. A receive-only antenna is one that has good signal to noise ratio but poor gain. Good signal-to-noise ratio is obtained by an antenna that has maximum gain in the direction of the received station and low as possible gain in all other directions.

    If you have a local noise source from only one direction, an antenna with a null in that direction can help.

    These antennas could have reciprocal gain on both transmit and receive, but who wants to transmit on an antenna whose gain is -10 to -40 dBi in it's best direction, and much lower than that in other directions? Receive antennas like Beverages, K9AY, EWEs, etc are not resonant on the band they are used.

    A Yagi is not a receive-only antenna. It is good for both transmit and receive because it has good directivity AND good gain.

    Jerry, K4SAV
     
  6. K4SAV

    K4SAV Ham Member QRZ Page

    Well it's not that rare. All you have to do is wind the antenna in the shape of a coil and use very small wire such that the resistive losses are close to 50 ohms. With the right shape, stray capacitance can cause the antenna (coil) to be resonant wherever you like. Of course if you use reasonable size wire, the feedpoint impedance is going to be extremely low.

    Jerry, K4SAV
     
  7. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    Right, but you have to actually design it that way and it will be a different length than a straight line resonant length. So like I said, call the Vatican.
     
  8. N1CZZ

    N1CZZ XML Subscriber QRZ Page

    Thanks Mike, I didn't look at it that way before.

    Cecil, I hadn't heard of ClearSpeech noise filter or DSP. I do have a Timewave DSP-9+ I use with my IC-735. The Timewave works really good, I use it most of the time, however it didn't seem to do nearly enough if anything this last weekend for the static crashes. I will have to check again.
    It looks like both the Timewave and Clearspeech are no longer made. Is there anything else/new on the market?
     
  9. KB9MZ

    KB9MZ Ham Member QRZ Page

    With reference to the statement saying "the feed point impedance is going to be extremely low.", This is not a truism unless the arrangement is fully explained.
    If one creates a Meander. Low impedance is often a function of fractional wavelengths being used form array, depending on the amount of wire used ,it will start off with a high cyclic impedeance say 100 omhs and quickly dampen out to a damped cyclic impedance close to 52 ohms dependent on the number of wave lengths used. If one winds a high density pancake antenna of a unit
    ( small length solenoid) you will find that the current immediately increases to a maximum and then take on on near flat current response until the circuit is closed
     
  10. KB9MZ

    KB9MZ Ham Member QRZ Page

    That would work if you used enough wavelengths for the frequency used. Unfortunately the slinky has changed over the years. The original design is that coils on one side are wound opposite to the other side but now manufacturers wind them all one way. Your question as an attempt to be funny didn't work out all that well did it? The slinky is not a closed circuit form arangement so it cannot be in equilibrium as defined by Maxwell and in all mathematics.
     
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