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A few Questions on Antennas

  1. Jun 27, 2016 #1
    Antenna Size

    So I was reading the above thread, and loved the explanation at the end. But my remaining question on Antenna Size is that..Okay I get why for low frequencies we *need* to have bigger antenna, so how it is that people like Hams transmit on lower frequency, long wavelength channels? Do they literally build these massive antenna?

    Also *why* is it that short wavelength, higher frequencies can get away with short antenna? Is it about the resistance? Can sw waves be "Pushed out" easier with the low resistance, unlike longer waves?

    My last question is related as well... *why* is it that smaller waves with high frequency are better at atmospheric and building penetration than the lowers? I know that frequencies in the VHF and ULF + range are much better at breaking through the ionosphere and making their way outward to space. What is it about the lower frequencies than makes this impossible for them?

    Pre-Thanks everyone for your time!
     
  2. jcsd
  3. Jun 27, 2016 #2
    This is a huge topic impossible to fully cover here. Generally speaking, and antenna needs to be at least half the wave length in size, otherwise its resistance absorbs too much of the power. That is why long wavelengths require large antennas.
    It was originally thought that the lower the frequency the better, and amateurs were pushed out of frequencies below 1.5 MHz. However, they found that these shorter waves sometimes went much further, even right round the World, contrary to the views of experts. This is caused by the ionosphere, a reflecting layer about 150 km above the Earth, which enables the signals to travel beyond the horizon. Frequencies above about 30 MHz pass through into space. All frequencies below this figure are returned to Earth, but sometimes with heavy losses unless the frequency is very low.
    Building penetration is sometimes better with UHF signals as they can pass through small windows etc and bounce around. But the general rule is, the lower the frequency, the better the propagation.
     
  4. Jun 27, 2016 #3

    berkeman

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    Staff: Mentor

    Yeah, many of the larger antennas are DIY based on published plans or just back-of-the-envelope calculations. That's part of the fun of HAM radio, regardless of what band you are communicating in.
    Think of the resonant frequency of a bell. The larger the bell, the lower the sound frequency you get when you strike or drive it. It's the same with antennas -- you get the most efficient performance (coupling to EM radiation) when the physical size of the antenna matches the wavelength of the EM you want to transmit or receive. As @tech99 said, the magic length is a half wavelength for a dipole antenna. :smile:
     
  5. Jun 28, 2016 #4

    sophiecentaur

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    If you try to make an antenna that is considerably shorter than a quarter wavelength in size then it will be very inefficient; the resistances in the feeder, matching network and the ground can become very significant and may dissipate more power than you actually radiate. This is even true for LF and MF Broadcasting antennae that have loads of money thrown at them and use vast copper 'Earth Mats' surrounding the mast base. Ham radio powers are strictly limited, which means that 200kW+ transmitter power is just not allowed. But, as an exercise in 'just doing it', it appeals to a certain sort of mind and people can transmit a long way using the old Morse Code which requires a very narrow bandwidth and which can creep through all the background shash that you find on the lower frequency bands.
    RF propagation is a very complex subject and it takes many years of experience (or large consultancy fees) to get the best out of any comms frequency channel. Local Ham Radio groups are very enthusiastic and helpful in these matters. If you are interested, then that is the road to go down.
     
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