How long does my AM radio receiver antenna wire need to be?

Discussion in 'Wireless & RF Design' started by Chris Gravel, Sep 28, 2015.

  1. Chris Gravel

    Thread Starter New Member

    Jul 2, 2015
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    I have looked through many resources and calculators to find out how long my AM radio receiver antenna wire needs to be and I am confused at the conflicting information I am getting. Notably, I have used the following calculators (see below), on top of my own calculations and they do not match.
    1. http://www.qsl.net/w4sat/velfact.htm
    2. http://www.angelfire.com/mb/amandx/loop.html

    My goal is to build an antenna for receiving in and around the 570kHz frequency. Now I've read that 1/4 wavelength will give me a good gain. So I calculated that I need (c/570kHz = 526m * 1/4 = 131.5m) 131.5m of wire. Though I see a lot of examples on the internet where they definitely do not have that much wire. Am I missing something? Is there a better fraction that will still give me a good gain but a much smaller antenna size?
     
  2. MikeML

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    Oct 2, 2009
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    Hi Chris,

    Receiving antennas (especially for BC band) do not have to be 1/2λ, nor matched to coax cable. Try just a random length about 3m long, connected directly to the receiver terminal. If the receiver has a ground connection, try adding a short wire to a cold water pipe if it is metallic (PVC in my house), or to a driven ground rod.

    More importantly, turn off/unplug anything that has a switch mode power supply. Often, it is locally-generated RFI from switching supplies, CFL lights, LED lights, florescent lamp ballasts that limit what you can receive.

    I have some experience with using the 160m ham band, and if I could shoot all my neighbors SMPSs, I could hear a lot more signals. (I have shot [eliminated] most of mine...).

    The ultimate receiving antenna for BC and 160m is a magnetically-shielded loop. Frequently, you can eliminate local interference by orienting the loop to null the interference source, leaving the desired signal in the clear.
     
  3. Chris Gravel

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    Jul 2, 2015
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    Hi Mike, thanks so much for the reply! If that's the case then what is the significance of using special fractions of a wavelength?
     
  4. nsaspook

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    Aug 27, 2009
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    Special fractions of a wavelength use resonance to reinforce received field strength in a given size range of antennas. Every receive antenna is also a transmit antenna so some of the energy is recoupled back into space around the conductors of the antenna. The properties of resonance store EM energy 'near' the antenna as reactive fields in a way (at the proper phase) that some of that energy can be recoupled back in to the conductor to increase field strength at the reciver via the receivers RF input.

    When you have several elements in the antenna you can easily modify directivity by phasing.
     
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  5. MikeML

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    The reality is that at AM broadcast frequencies, the sensitivity (and noise figure) of most receivers is so good that the signal-to-noise ratio of the entire receiving system is determined by local noise sources. You do not need the antenna to drag in every last uV of signal, because of the mV of noise produced locally... Making the antenna bigger does nothing to improve the signal-to-noise ratio...

    Not true for transmitting. Note that your local AM broadcast stations use full-size (1/4λ to 1/2λ) antennas. Here is an example where reciprocity still works because your receiving antenna would be a terrible transmitting antenna, but still good enough...

    If I was a serious DXer that wanted to hear distant AM broadcast stations, I would be looking at the magnetically shielded loop receive-only antenna I mentioned earlier. It not only is less sensitive to local noise, it has a very deep null in one plane, so can be rotated to null stations on a given frequency so you can receive a more distant station transmitting on the same frequency (provided that its signals are coming from a different direction).
     
    Last edited: Sep 29, 2015
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  6. KL7AJ

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    Nov 4, 2008
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    The correct answer depends on how well you need it to work! A whip antenna on a car AM radio is a SEVERELY compromised antenna, but it works fabulously because you have so much extra reserve signal to work with. If you're down in the noise floor, it's a whole different matter.

    At H.F. frequencies and below, the noise is limited by the thermal agitation noise floor...this is the noise generated by a piece of wire at room temperature. Any modern receiver has FAR more gain than necessary to hear thermal agitation noise...the true limiting factor of sensitivity.

    The 1/2 wvelength rule is for a SELF RESONANT antenna...but for reception only, you have lots of other options. A multi-turn tuned loop can be nearly as effective as a full length wire, plus it has very sharp nulls which can be used to eliminate local noise and interference quite effectively.

    For 500 KC, a four foot diameter multi-turn frame loop is a killer receive antenna.

    Eric
     
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  7. KL7AJ

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    P.S. I'm currently building some 470-500kc antennas in anticipation of a new amateur radio band there...hopefully in the VERY near future.

    Eric
     
  8. KL7AJ

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    Hi Mike:
    I highly recommend reading W8JI's writings about shielded loops. Contrary to a lot of folklore, electrostatic shielding does little if anything to reduce noise directly. What the shielding DOES do is maintains the radiation pattern of the loop in the presence of nearby object, so that the nulls remain well-defined. It is these well-defined nulls that give the loop its great noise reduction properties. Incidentally, this aspect of shielded loops was well documented in the literature of the 1930s

    73!
    Eric
     
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  9. alfacliff

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    Dec 13, 2013
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    for a transmitting antenna, 564/f (in mhz) will give you a half wave in feet. for recieving, most modern radios are so sensative that anything over 30 feet will cause overload of the reciever.
     
  10. MikeML

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    To get a "resonant" antenna, the wires have an "end-effect" that makes the wire electrically longer that it is physically, so the formula that I use for a resonant half-wave dipole is 468/fMhz (answer in feet)

    That is derived from (0.5* 0.95 * 299,792,458m/s * 3.28084ft/m)/f = 468/fMhz, where 0.5 comes from 1/2λ dipole and the 0.95 comes from the 5% shortening due to end effects...
     
    Last edited: Sep 30, 2015
  11. KL7AJ

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    Indeed....that's been the rule of thumb for ages, and it works 99% of the time. However the current understanding of end effect is a lot better than in days of yore. End effect is NOT caused by the end attachment insulators, but is a DISTRIBUTED capacitance along the whole length of the wire. This is easily shown by operating a dipole at high order harmonics....each voltage null point is foreshortened by the 5% over the theoretical wavelength! This would not be the case if end effect were really all at the ends! :)

    Eric
     
  12. alfacliff

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    Dec 13, 2013
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    if you read the instructions that come with modern radios, like sony and such, they recomend an external antenna of no longer than 30 feet to prevent front end overload. recieve antennas do not have to be resonant.
     
  13. Chris Gravel

    Thread Starter New Member

    Jul 2, 2015
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    This has cleared up a lot of my antenna related confusion. I tested a small antenna today--the loop is the size of a fist--and I was able to receive from a transmitting antenna I had set up to be 50 centimeters away. However, I am unable to pick up AM bands. I'm not sure if I'm doing this right.

    I'm using thin wire (the kind of wire that will be used for building brushless motors). I've made about 30 loops of wire the size of my fist and got an inductance of about 160 mH. I then attached a variable capacitor to the antenna so I can tune the circuit--this definitely works since when I could tune to the frequency I was transmitting on by adjusting the capacitor (within the AM band range).

    Why can't I pick up anything in the AM band range?
     
  14. MikeML

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    According to this calculator, a 5" dia., a 30 Turn coil, bundled 0.5"x0.5" should be about 184uH. To resonate that at say 1e6Hz, it would take 138pF. Is this close to what you think you were using?
     
  15. Chris Gravel

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    Jul 2, 2015
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    Yes, actually that's quite accurate to what I what using. I have a variable capacitor (I don't remember the specific range) and I was able to tune it from 570kHz to 1Mhz; I tested this by using a transmitter nearby in a lab environment. But I couldn't pick up any AM radio stations.
     
  16. MikeML

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    What is the receiver?
    What happens if you replace your loop with a 5ft piece of wire?
    What is the building/environment you are in? industrial, lab, house?
    How far are you from the nearest BC transmitter?
     
  17. Chris Gravel

    Thread Starter New Member

    Jul 2, 2015
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    After further investigation and a conversation with a lab instructor who has run a lab similar to this, it seems that the reason I couldn't detect a signal is because the signal is too small to register on the oscilloscope (it only goes down to mV). I was hoping to be able to view it on the oscilloscope, but this was only possible when I transmitted directly beside it.

    The lab instructor told me to just use a single monopole antenna. Though I'm not sure if I can do it with a small loop antenna, it's enough to go off of after seeing his demonstration. I just need to properly handle and filter the signal. The previous design used a diode for filtering, but the voltage from the antenna was so small that the diode was eating it.
     
  18. MikeML

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    A standard AM receiver is about a factor of ten-thousand more sensitive than your oscilloscope...
     
  19. Chris Gravel

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    Jul 2, 2015
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    I did some more research and experiments and I found that I CAN detect a signal with my oscilloscope. Though it is very small, 0.2 to 0.1 mV. The problem was that I wasn't using a proper antenna and (I was only using an inductor) and I was not grounding it well enough (I had to attach it to a large metal object in order to get rid of the bad signals so I could actually see the signal I wanted.
     
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