Antennas

Discussion in 'The Projects Forum' started by tornado151, Apr 17, 2010.

  1. tornado151

    Thread Starter Member

    Apr 15, 2010
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    Hi,
    I have transmitter that recommends using a 1/4 whip antenna.

    I have had a look and 1/4 whip antenna is quite big, I noticed that there was antenna that worked on the same frequency but is smaller and was called a helical antenna.

    Will using a helical antenna instead of 1/4 whip antenna make any difference other than size?

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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  3. tornado151

    Thread Starter Member

    Apr 15, 2010
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    Hi,
    Thanks for your reply, I am using an RF transmitter & receiver at 433Mhz.

    The bit I am concerned about is this:

    They recommend using a 1/4 whip antenna, so I would assume the feed was 1/4 not 1/2 which according to above would cause adverse operation. I am just wondering what this means?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Well, a 1/4 wave antenna for 433MHz would only be about 6.8" long. Is too long for your application?

    If you use a helical antenna, you might have a higher VSWR than with a 1/4 wave antenna.
     
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  5. tornado151

    Thread Starter Member

    Apr 15, 2010
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    Hi,
    Thanks for your reply.

    From looking at the internet a high VSWR means it is less efficient at transmitting, is this correct?

    If so do you have any idea how much it would affect the range?

    The antenna needs to be as small as possible, but if using a smaller helical antenna decreases range considerably then I will go with the 1/4 whip antenna.

    Thanks
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    The lower the VSWR, the better the match between the transmitter and the output load (your antenna). Very generally, a VSWR of 1.2:1 (usually just called 1.2) or lower is a good match. 1:1 is perfect.

    The higher the VSWR, the more energy is being reflected back into the transmitter instead of being radiated from the load. This means lower range and higher power dissipation in the transmitter. If the VSWR becomes very high, permanent damage to the transmitter will result.
     
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  7. tornado151

    Thread Starter Member

    Apr 15, 2010
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    The helical antenna says less than 1.8 VSWR.

    Here is the two antennas I am considering:

    Helical ( Link )
    1/4 Whip Antenna ( Link )

    Can you tell me if there would be much difference in the range?

    Thanks
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, it'll make a difference in the range. The better (lower) the VSWR, the more power will leave the antenna.

    For 433MHz, a 1/4 wave antenna will measure 6.815 inches in length.

    A perfectly tuned 1/4 wave antenna will have a 1:1 VSWR.

    The helical antenna you are looking at has a 1.8:1 VSWR. That's not so wonderful. If my CB radio VSWR were that high, I would not transmit until I fixed the cause of the problem.
     
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  9. Wendy

    Moderator

    Mar 24, 2008
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    Impedance matching is a pretty old science with electronics and radio. A simple toroid transformer might be all you need to match impedance.
     
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  10. SgtWookie

    Expert

    Jul 17, 2007
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    Yeow, Bill! :eek:
    Now you're really opening a can of worms!

    Actually, a balun can be used for tuning antennas over a broad range of
    frequencies, but this would be rather difficult to explain to a newcomer, who likely does not have a VSWR meter, and probably is not inclined to spend $50 for a piece of equipment they will likely use just once.

    But more to the point, you'll be hard-pressed to find a toroid that will perform at 433MHz. More than likely you'd need a transformer wound on an air core, as 433MHz is beyond the range of just about any ferrite material that I know of. Even with materials designed for HF, Q drops off quite a bit after 200MHz.
     
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  11. tornado151

    Thread Starter Member

    Apr 15, 2010
    34
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    Thanks for all your advice, it has been very helpful!

    I have bought the 1/4 Whip antenna and will try and fit it into my design somehow.
     
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