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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. AH6K

    AH6K Ham Member QRZ Page

    That is an excellent analogy. The transmatch operates much like the transmission in your car. The objective is to match the source to the load and convey as much power to getting the job done as possible. Matching the most efficient engine speed to the resistance of the road wheels involves nearly the same objective as matching the impedance of the transmitter to the impedance of the antenna.

    The dummy load works much like a dynamometer in simulating ideal road conditions much as the dummy load simulates an ideal antenna system. The analogy also extends to the fact that in both cases you are going nowhere, just testing.

    I find it useful to use the term “transmatch” to refer to a tuner located at the output of the transmitter and “antenna tuner” to refer to a network located at the feedpoint of the antenna. But, in real world language, “antenna tuner” can refer to the network placed at either end of the feedline. I use “antenna tuner” as the general term, while “transmatch “ refers to the network placed at the transmitter output. In this case it could be called an “antenna system tuner”.
     
    Last edited: Aug 7, 2009
  2. W5DXP

    W5DXP Ham Member QRZ Page

    In general, that is true. However, there are exceptions.

    If the autotuner installed at the antenna is less efficient than the tuner in the shack and the transmission line is low-loss, then it may not be "better". Here are a couple of facts to consider.

    Many shack tuners use a very high Q single coil in the tuner, i.e. low loss. An SG-230 autotuner, for instance, uses switched powered iron toroidal inductances in series to achieve a match. That method has considerably higher coil losses than a lot of shack tuners. In addition, an unbalanced autotuner may present problems when used on a balanced antenna.

    When one puts an autotuner at the antenna, one usually uses coax between the transmitter and the autotuner. Quite often, the matched line losses of the coax are actually higher than the total losses in a 600 ohm open-wire transmission line that is not matched.

    So it is possible that installing an autotuner at the antenna will actually result in less power being radiated than a well-designed conventional system.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::You might think so; but remember when you're receiving, the source is the antenna (not the rig) and the load is the rig (not the antenna), so "impedance matching" for transmission may be quite different from impedance matching for receiving. Even the transmission line SWR is different for transmission and reception, unless it happens to be 1.0:1 already.

    Thankfully, receivers don't require a power match to work well; a noise match is sufficient, and most receivers, at least for HF, are plenty sensitive enough even with a fairly poor match. If you have a manual (not automatic) antenna tuner, it is possible, and even likely, that if you re-adjust the tuning controls for "maximum received signal strength," you'll find those to be a different set of adjustments than the ones you used to peak for best transmission match, or maximum power transfer from the transmitter.

    However, even when doing so, the S/N of the received signal may not change at all. That's why "S" meter readings are unimportant. If a station is S1 and you can understand him, that's a lot better than "S9" if you can't.

    WB2WIK/6
     
  4. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    Ha ha, you have to cut your coax 11 meters long. That will fix that rascal.:D
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    Referring to the conjugate matching theorem, in a lossless system, when the 50 ohm Z0-match is established between the transmitter and the transmatch, a conjugate match is automatically achieved everywhere else in the system - including at the antenna feedpoint. That cancels the effect of the system reactance, i.e. it resonates the entire system which includes the antenna.

    In a low-loss system, common in ham radio, a near-conjugate match is achieved at the antenna feedpoint. Assume the antenna feedpoint impedance is 100-j100 ohms. When the transmatch is tuned for a match, if we disconnect the antenna and measure the impedance looking back down the transmission line, it will be pretty close to 100+j100 ohms thus neutralizing the reactance which ensures (almost) maximum transfer of available power to the antenna.

    http://www.w2du.com/Appendix09.pdf

    If this were true, we could measure the temperature rise in the "very short antenna" - but we cannot. The heat losses occur in the matching network which is not connected to the air.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::Who'da thunk?

    Now, what if I don't have 11 meters? Looking around here I could only find four. Can I make little sketches of seven more and just splice them in, somehow?
     
  7. W4JFA

    W4JFA Ham Member QRZ Page

    I think the writer of the question meant to ask this...
    All factors the same, same power input at the antenna feedpoint, will the antenna radiate better at the freq it was "cut" for than another freq. Too many variables. I don't think the question should be written that way. Bob
     
  8. W5DXP

    W5DXP Ham Member QRZ Page

    Would you believe that a 1/2WL dipole fed with 1/2WL of coax is not resonant if the coax is open-circuited at the transmitter end?:)
     
  9. WA4OTD

    WA4OTD Ham Member QRZ Page

    There willl some Q losses in the transmatch but with air inductors and capacitors that will also be small. After I thought about it I think small heat losses in the antenna also. I think the power is less because the antenna loads less. Since it is physically shorter it doesn't radiate as well as resonant antenna. It is matched, but is not as good radiator. I could not find anything describing this.

     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    Heat loss from an antenna depends upon the magnitude of the current in the element. A very short antenna will have a much higher reactive impedance and therefore lower current and lower heat losses than a resonant antenna. The problem with a short antenna is not that it won't radiate - the problem is that we are unable to shove much current into it at the feedpoint in an efficient manner.

    Assume a particular short antenna has a feedpoint impedance of 1-j500 ohms on 40m. The antenna tuner loss calculator at

    http://fermi.la.asu.edu/w9cf/tuner/tuner.html

    says the losses in the tuner will be around 10 dB.
     
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