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Monday, October 3, 2011

Antenna Question

We received a question about how the antenna used in our recent DRM tests affected the signal.  Now we get to tell you about an important part of our station that seems to be the most unknown to most of our readers.
The antennas at KTWR are all curtain antennas made of dipole arrays.  The one we used in the test is nearly identical to the one shown at the right.  It is configured as a 4x4x1.0.  This means it has 4 columns of 4 dipoles each with the bottom dipole being one wavelength above the ground.  There is a reflecter screen hanging behind the dipoles to force the signal to have one main beam instead of two.  The gain of this antenna can be as high as 22dB, depending on frequency and slew angle.  That makes the 75KW signal coming from the transmitter seem like 11MW heading to desired coverage area.
The direction of the main lobe of the broadcast signal is controlled by the slew box like the one shown to the left.  In the case of the antenna used for the DRM tests, the azimuth can vary from 290 to 345 degrees.  We used 290 degrees for the tests beamed toward India.  Some of the signal went to other places outside the main coverage area.  That is why people in Japan, Australia, and Brazil were able to hear the test broadcasts.
One problem with the tests was that signal propagation conditions in the ionosphere did not allow the signals to be received in India, as far as we can determine.  Had we performed these tests at night, the signal would probably have been very strong there.  A great deal of planning goes into the frequency choice and the timing of our broadcasts, so that they will be reliable for the entire broadcast season.

3 comments:

  1. crake.tony1@virgin.netDecember 20, 2011 at 10:31 AM

    That is a great picture of the aerial masts.. is this system for one band or say multi band? The switch box shown changes the direction of the beam.. ok but how does the aerial change bands.. does it have to be switched or is it just very wide band ? You have a most interesting website! It must be fairly unique for someone to build a transmitting station like that with volunteers! Well done..... !

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  2. These antennas cover multiple bands. Most of our antennas cover 9MHz through 18MHz. The design of the dipoles with the shorting bars allows the power to find the best path that is most resonant at the particular operating frequency. The power essentially takes the easiest path through the wires and bars with no switches needed.

    The fact that this station exists and is operated by volunteers is a testament to God's power to work in this world. We could not do this in our own strength for very long. However, KTWR has been on the air for 34 years, so far. The way things are going lately, it is obvious that the Lord is not done with us yet.

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  3. antenna tuning is one of the complex portion of RF feild.
    thanks for sharing knowledge regarding them through this article.
    keep blogging

    ReplyDelete