83 lines
4.9 KiB
Plaintext
83 lines
4.9 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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"FEEDING" THE TRANSMITTER
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Feeding your program audio into your transmitter properly can be a difficult and frustrating
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challenge! To start with, a Microphone level signal is High in impedance, typically, and quite
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"small" electrically. This is what the input circuits of your transmitter are expecting to "see" when
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you operate it in a voice mode. Now, the typical output level of a tape deck is Medium in
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impedance and electrically much higher than a microphone signal. This is where the
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trouble starts.
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Ways to tell if you are experiencing problems is that your transmitted signal will be "Under
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Modulated" meaning you are not supplying a large enough signal ;or the reverse will be true, your
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transmitted signal will be "Over Modulated" meaning it will sound distorted, will be wide, and
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generally, unlistenable. Take heart that both can be cured and all you need is a little knowledge!
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What you need is a matching network , more commonly known as a PAD, between your
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program audio and Mic Input of your transmitter. I present you a simple circuit that should work
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about 99% of the time - Chauncy's Magik Matcher. See Figure 1.
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Before we jump into the circuit and talk about it's uses, I need to lay down a little ground work
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first. You should be aware that you have a "Mic Gain" or sometimes called "Audio Gain" control
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on your transmitter somewhere. What you would like to end up with is a Program Audio Level
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that will sound correct at about 3/4 of this controls range. This will ensure that you are supplying
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enough audio without Over or Under Modulating your transmitter. Another recommended item is
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a way to control the output level of your tape deck. If you are going to purchase a tape deck, it
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will help if you get one that has a output level control on it. If the one you use does not, you can
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use the audio part of a Radio Shack Video Processor box to control the line level audio up OR
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down. These are quite available and are typically $20.
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For those sharper readers, You may have noticed I haven't mentioned anything about mixing
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boards! Well, since this manual is geared to the neophyte, live broadcasting can bring on all sorts
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of problems with RF Feedback into equipment that the beginner doesn't need to deal with. Not
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that I am opposed to live broadcasting; it's just much easier to produce programs on cassette and
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then feed the transmitter with a tape deck. You still might experience RF Feedback problems with
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simpler setups but the cure will be much easier and less overwhelming to deal with. There is
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another chapter to this manual that discusses RF Feedback and how to cure it.
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OK, now that we have gotten that information out of the way, let's talk about our matching
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network or Pad. A Pad simply is a "buffer" that will bridge the impedance mismatch between two
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pieces of equipment. The circuit we are presenting does this, and also will "Knock Down" the
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signal level to an acceptable level your transmitter will like and will also "Roll Off" the Low Audio
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Frequencies that your transmitters audio section won't transmit anyway.
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You might have noticed there are no values given for the components. This has been done for a
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reason as it would be impossible to determine the correct values for every type of installation.
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What I am going to do is give you a "Starting Value" and then it's up to you to experiment with
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the values by increasing or decreasing them until you find the proper ones. R1 should start out at
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around 10,000 Ohms or 10K. R2 is a variable resistor with a value of 20K. C1 should start out
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around .05 uF and can be Ceramic Disc or Mylar. When you use this circuit to pad your
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transmitter, you can plug the components into a solderless breadboard. While transmitting, listen
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to yourself "Off the Air" and adjust the values of the components until you find the ones that
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sound the best to you.
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R3 is optional to better match the impedance of the transmitter. A possible starting value of
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around 20K would be a good place to start.
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Once you have determined the best values, You can solder the mess up and wrap it with
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electrical tape right into the cable feeding your transmitter! Or if you want to get fancy, you could
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mount everything into a box. Either way is fine and not critical.
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