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Dipole antenna theory: a few questions

  1. Jan 7, 2009 #1
    Hi guys,

    I'm in my final year of my physics degree and for my first semester project I had to make a dipole radio antenna, and it's proven to be quite tricky (at least in the theory side of things). I would seriously appreciate some help with a few things if anyone has anything to offer :)

    Firstly, I don't understand the beamwidth thing at all. How is it calculated? The best I've managed to find is images of the ideals but with no information as to how they're found, or an explanation. Here's the images I found of the patterns for the height I'm using:

    Single Dipole ~7m (lambda 1/2) above ground
    x ... ground plane - z ... zenit

    top view

    I used a ~25m length of coax as my feedline because I wanted to take readings for 48 hours to try and get some galactic drift readings so needed to keep the laptop and receiver indoors: will this have any major effects on my readings?

    Also, would I go about calculating the gain of my antenna, and what exactly IS gain?

    I set up a single half-wavelength, East-West dipole, ~7m off the ground and ~7m in total length (two ~3.5m monopoles, centre fed) to receive waves of ~22MHz. This was attached directly to the receiver, which was in turn connected to the laptop. Is that the right way of doing it?

    Sorry if these are newbie questions, I'm just really struggling to get to grips with it all!

    Thank-you so much in advance!

    PS If anyone's wondering, I modelled it closely on NASA's Radio Jove. I would have taken readings of Jupiter but unfortunately after 9 weeks they still hadn't sent the receiver so I had to make do with a ham radio receiver and only had time to set up the single dipole :(
  2. jcsd
  3. Jan 7, 2009 #2
    You need to quote height in terms of wavelength. What frequency are you using?

    Gain is focusing power in the crudest terms. A dipole is 2.16 dB ( I think) better than idealised isotropic radiator which does not exist. Peak gain is measured in the direction of peak radiation.

    Horizontal dipole near the ground will tend to fire at steepish angles. For low angle radiation you want a vertical dipole or ground plane antenna.
  4. Jan 7, 2009 #3


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    Staff: Mentor

    You didn't mention the balun -- what type are you using to match the dipole elements to the coax? What impedance coax are you using? What source are you receiving at 22MHz? Is it a licensed transmitter?

    Are you going to rotate the antenna and keep some transmitter fixed in order to map out your real patterns, versus the ones you've pasted in above?
  5. Jan 7, 2009 #4
    Ah ok, sorry, the height is 1/2 the wavelength

    Sorry, I didn't make that clear in my post: it was set up to pick up galactic background and to detect any solar flares (which I'm well aware is extremely unlikely at this time of year and, judging by recent activity, any time at all at the moment) or other noise from the universe.

    The impedance of the antenna is supposedly 50 ohms and the coax impedance 75 ohms. Our assigned lecturer told us that that difference wouldn't affect our experiment and so we took his word as gospel... I hope he was correct! Now that I think about it, could we have matched it with the balun?

    About the balun, we used a ferrite choke at the feedline with ratio 1:1. This part was down to my lab partner, was he right to do this? What would happen if it wasn't there? Would it make much of a difference?

    Thanks a lot for the help so far and for humouring me if this is easy stuff! I feel like I'm starting to get a foothold on this so I really appreciate it :)
  6. Jan 7, 2009 #5


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    Staff: Mentor

    The impedance of a half-wave dipole (two opposing quarter-wave elements) is about 75+j0 Ohms, balanced. If you are using a 1:1 balun, that matches to 75 Ohm unbalanced coax, which most likely is what your HAM receiver expects.

    An impedance mismatch between the antenna and coax in receive will mainly make the receive sensitivty less. An impedance mismatch between the antenna and coax, plus a mismatch at the receiver, will introduce distortion in the receive signal (from the multiple back-and-forth reflections), which is bad. Best to make the whole system 75 Ohms, or use a different balun to get down to 50 Ohm coax if the receiver is 50 Ohm input. And do not transmit with a mismatched radio/feedline/antenna system -- bad for the transmitter.
  7. Jan 8, 2009 #6
    I didn't see the 22 MHz bit. Half wave above ground.. No wonder there are some high angle lobes.

    You wont hear anything galactic on 22 MHz .. FAR too much noise on that frequency. I don't think solar flares are detectable by radio directly. They can cause ionospheric fade-outs and later aurora.

    I don't think Cosmic Background Radiation extends much below 100 MHz and is pretty weak at that freq. You have to go up a few GHz.
  8. Jan 8, 2009 #7
    I can't answer your questions but VHF radio's used in marine applications might provide you a source of information as all the discussion I recognize as pertaining to such whip antennas.
    It's also possible single side band (SSB) HAM radio antennas would also provide background.

    I googled DIPOLE RADIO ANTENNA DESIGN and all sorts of information popped up including a number of available on line computer programs for antenna design...
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