To be an efficient radiator, it must be resonant at operating frequency?

Discussion in 'General Technical Questions and Answers' started by KB7UXE, Aug 6, 2009.

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  1. N4CD

    N4CD Ham Member QRZ Page

    Well, yes and no.

    I just went back to my books...reading Reflections by Walt, W2DU.

    Your matching network, by the Conjugate Matching Theorum.....provides an impedance match - and 'tunes' the antennna system to resonance.

    Now, the there are two factors here. First, you have to look at question. The antenna does not have to be 'resonant' itself to be efficient, but that is not the only condition in a practical situation.

    If you have a lossless transmatch and lossless feedline, and lossless radiating elements, then a non-resonant antenna will radiate all of the power. (it might not have the pattern you want).

    So, if your 80 m dipole is 5:1 at 3.95 MHz, using a transmatch on a short feedline of heliax will make your non-resonant antenna efficient.

    Your transmatch, and the feedline, and the antenna will be in resonance when you are 'matched'. The 'system' is in resonance. That is the only way that the network composed of the transmatch, feedline, and antenna will take maximum power from the transmitter.

    Of course, the 'antenna' itself is not in resonance in the above example.
  2. N4CD

    N4CD Ham Member QRZ Page


    " OK, that last one was pretty long, but I forgot to say that in most cases, a tuner gets "the best compromise". Usually it does not allow all of the power to flow into the antenna, but maximizes the amount of power that does.

    As an example, without a tuner in the system, an antenna at a specific frequency may only receive 10 percent of a 100 watt output. The tuner may increase this to only 70 percent, but that is such a great increase, it appears to the operator to be working very well. Each doubling of power is the same as 3 decibles. So from 10 to 20 watts is 3, from 20 to 40 is another 3 and from 40 to 70 watts is somewhere around 1.8 decibles or so. Total increase with the tuner would be approximately 7.8 decibles. That is a lot of increase.
    Reply With Quote"


    A transmatch actually tunes the 'system' of the transmatch, the feedline and the antenna to resonance.

    It makes no effort to 'regulate' power.

    The loss in the feedline will be proportional to the length/type of the feedline, and you will suffer more loss if the SWR is high. Period. If you have a lossless feedline, say 100 feet of 3 1/2 in heliax at 160 m, you have esentially zero feedline loss.

    The loss in the tuner can be very small if the parts are very big - minimal losses in coils, etc. Again, it depends upon what transformation ratio is required.

    The transmatch makes no decision about 'how much power' to send to the antenna. That is simply determined by losses in the transmatch and in the feedline itself.

    if you run 600 ohm open line or heliax at HF, it is essentially zero loss regardless of the SWR.

    The only reason 'part' of the power makes it to the antenna is some loss in practical transmatch parts and some loss in the feedline most hams use - small coax. With lossless match and feedline, all of the power makes it to the antenna.

    And the system is 'resonant' if you find a 'match'.
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