98 lines
5.9 KiB
Plaintext
98 lines
5.9 KiB
Plaintext
PIRATE RADIO SURVIVAL GUIDE
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Note: this chapter is from the book "Pirate Radio Survival Guide" written by; Nemesis of
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Radio Doomsday, and Captain Eddy of The Radio Airplane. If you like this book and would
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like to support their efforts, you may send a donation of your choice to either Nemesis or
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Capt. Eddy at PO Box 452, Wellsville NY 14895.
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Please note that some chapters refer to illistrations or drawings, these could not be included in
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this BBS version of the book. If you would like the illistrations or have other questions you
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may inquire at the above adddress.
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SWR
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Standing Wave Ratio or SWR is probably another mis-understood concept in radio that has a
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tendency to mess up some people. While I'm not going to get extremely technical, I will try to
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explain what SWR means and how you can cure High SWR readings.
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SWR is a term used to measure the amount or ratio of RF Power that IS being radiated by your
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antenna to RF Power that is NOT being radiated and reflected back into your feedline/transmitter.
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The actual formula to determine this involves some complex math and I'm not going to get into it.
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Instead, I refer you to the diagram of a typical station layout showing the transmitter to antenna
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RF path. If you see where the SWR/Power Meter is placed in the line, you can understand why it's
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able to measure this.
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A perfect SWR is 1 to 1. This means that all RF power is being radiated by the load or antenna
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and none of it is being reflected back. "What causes RF Power to be reflected back?", Most
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typically the cause of this problem is by using a Non-Resonant or Random Length Antenna. You
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can quickly understand the importance of using resonant antennas is to MAXIMIZE your on air
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signal by actually radiating ALL the power you are transmitting!
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"What happens to the Reflected Power?", It returns back into your transmitter's tank circuit and
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finals to be dissipated as waste heat! This should be avoided if possible. In fact, some of the newer
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solid state transmitters will shut down if the SWR becomes too high, in order to protect the finals.
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As your SWR increases, your radiated power decreases. Typically, your SWR can be as high as
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2 or 3 to 1 before any real problems start. To maximize your signal you should try and keep the
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SWR below 2 to 1.
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SWR/Power meters can be purchased easily and even the cheaper ones tend to work just about
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as well as the more expensive models. There are several features available with these instruments.
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You must make sure that the meter can handle the power level you are running! Typically, most
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meters will measure up to 1KW but not always. I prefer the dual meter type where I can watch
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my FORWARD and REVERSE power at the same time. Any type of meter is fine as long as you
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obtain and use one!
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"How can I reduce the SWR of my dipole antenna?", In order to do this, you need to determine
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the RESONANT FREQUENCY of your existing antenna. Now, in the chapter on antennas we
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discussed how to "cut" a dipole for the 40 meter pirate band, However, depending on the
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antennas proximity to other structures and height from ground, the math used to determine the
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length of the antenna is only a close approximation.
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To find out just where your antenna IS resonant, tune up and check the SWR at 100 Khz points
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across the 40 meter band. For example:
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7000 Khz ......... SWR 3.5
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7100 Khz ......... SWR 2.4
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7200 Khz ......... SWR 1.8
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7300 Khz ......... SWR 1.0
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7400 Khz ......... SWR 2.0
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7500 Khz ......... SWR 3.2
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As you can see from this SWR plot, our antenna is RESONANT around 7300 Khz and becomes
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LESS resonant (i.e. SWR increases) the farther we move away from 7300 Khz. To correct this
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problem, we would need to CUT wire, about one or two inches at a time from each end of the
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dipole and repeat the measurements until we make the dipole resonant around 7400 Khz. If the
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antenna were resonant around 7500 Khz. you would need to ADD wire to each end of your
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dipole until you LOWERED the resonant frequency.
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Now I would like to talk about Antenna Tuners or Matchers. A ANTENNA TUNER WILL
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NOT MAKE YOUR ANTENNA A MORE EFFICIENT RADIATOR! "What?", you say! "With
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my antenna tuner I can make my SWR 1 to 1 where before it was 4 to 1.", that's true, but if you
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look at our station diagram, a antenna tuner only makes the standing wave 1 to 1 between itself
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and the transmitter! NOT between the tuner and Antenna! If you doubt this, move your SWR
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meter to the Antenna side of the matcher to see what is going on. The only advantage to using an
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antenna tuner is in the rare case you cannot erect a resonant antenna, a antenna tuner will let your
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transmitter load up into just about anything, but it is NOT and should NOT be used as a substitute
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for a RESONANT antenna!
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The last theory I would like to debunk is that of "Cutting Your Coax" to decrease the SWR. The
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length of your feedline IN NO WAY will affect your SWR! Cutting or Lengthening it will
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accomplish nothing but wasting your time! Purists will argue this point and I will tell you that for
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certain antenna applications, feedline length DOES play an important part. Even more so, Radio
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Buffs will tell you that for the most accurate SWR measurements, the instrument should be placed
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at even wavelength multiples in the feedline, or better yet,
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right at the antenna feedpoint. These are true and much more difficult to accomplish. Just placing
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the meter in line will give you a close enough reading to spot any troubles in your antenna or
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feedline.
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