** SAFETY TIP ** SAFETY TIP ** SAFETY TIP **
TRANSMITTERS UTILIZE LETHAL VOLTAGES! NEVER OPERATE A
TRANSMITTER WITH THE SAFETY DEVICES BYPASSED OR THE CHASSIS
REMOVED! YOU COULD BE KILLED! RF ENERGY ALSO WILL LEAVE A
NASTY BURN IF YOU MAKE "CONTACT"! TREAT YOUR TRANSMITTER WITH
RESPECT AND CAUTION!
** SAFETY TIP ** SAFETY TIP ** SAFETY TIP **
"What is a transmitter?" Good question. A transmitter is a device that converts AC or DC
energy to RF energy. By itself, RF energy doesn't do us much good, so a transmitter also
requires audio information to "Modulate" the RF energy it generates. The modulated RF
energy is coupled into an antenna to be dispersed into the ionosphere. Now that you know
what a transmitter does, we're going to discuss variations and capabilities that can be found
when looking for your dream rig.
Variable Frequency Oscillator - Sounds like something right out of a Star Trek episode. This is the next step up from "Crystal Control". A transmitter that comes equipped with a VFO or can use an external VFO gives you freedom to operate on any frequency that it covers and where the transmitter is able to tune up. Most VFO's will cover the ham bands in 500 Khz segments and this can be utilized by the pirate to get outside the ham bands. It also allows for moving your frequency at a moments notice. The ONLY disadvantage to using a VFO is that they are more prone to drift. If you let your gear "warm up" drift is usually not a problem. With the newer synthesized types of VFOs drift is practically nonexistent.
We touched briefly on the basics of modulation earlier and now we are going to dive right into the good stuff! So put on your thinking caps and let me guide you into the mysteries of radio. Modulation of a RF signal is impressing audio information on a radio frequency waveform. Sounds exotic doesn't it?
AM modulation, a carrier wave determines your frequency and 66% of your transmitter power is used here! The modulating signal, audio information, is used to vary the amplitude of the carrier wave by means of upper and lower side bands. This is where the remaining 33% of the transmitters power goes. See Figure 1.
The range of audible frequencies to most people is 20 to 20,000 Hertz. In most amateur gear the audio bandwidth is restricted to 300 to 3,000 Hertz. Music withstanding, this is the audio range that affords the greatest intelligibilitly to a voice signal. Now in AM mode combine 2.7 KHz for both upper and lower sidebands and you have an AM signal almost 6 KHz wide. If the frequency response of the transmitter was increased to 10 KHz, the resulting AM signal would be 20 KHz wide! Now you begin to see why it helps to give broadcasting stations a W-I-D-E berth!
AM is also uses what is known as a 100% "Duty Cycle". This means that your transmitter is working 100% of the time. While this is not a problem in normal amateur communications, trying to run a amateur transmitter for 30 to 60 minutes continuous in AM mode is asking for trouble. The components in the transmitter will tend to overheat and you may need to cool them while broadcasting. An exhaust fan can be used to pull hot air out of the transmitter. Never blow cold air onto a hot tube! If you have managed to get a "broadcast quality" transmitter, don't sweat this as they usually built for continous duty.
SSB's duty cycle is based on the audio informations peaks, and will typically be around 50% which means 30 to 60 minute broadcasts are easily achievable with amateur gear. You still might have to provide cooling to your transmitter but SSB is far less brutal than AM.
The biggest complaint when using SSB for the transmission of music is that it "messes up" the music. Well, AM signals are the most easiest tuned but a properly tuned SSB signal is indistinguishable from AM! The key word to that statement is "properly"! Tuning is critical on SSB, 100 to 200 hertz mis-tuning will give unpleasant results! With time and good tuning skills a fair strength SSB signal can be tuned in to the proper pitch and will sound every bit as good as AM! As a pirate, you should remember to try and broadcast something that most people recognize to serve as a "Tuning Guide". It is up to the listeners to improve their tuning skills! For practice, try tuning the ham bands or RFPI/HCJB's SSB transmissions. It takes a while to get used to it, but look folks: SSB is here to stay!
My final pitch for the use of SSB over AM is that most shortwave receivers are MORE sensitive to SSB signals than AM. Typically 1 to 2 microvolts. Since we are dealing with low power stuff here, any edge, no matter how slight, should be taken advantage of!
For the more advanced pirate, it is quite possible to modify your transmitters tuned circuits and get your transmitter to operate in places it was never designed to go. Remember this, A MODIFIED TRANSMITTER HAS VERY LITTLE RESALE VALUE TO THE LARGEST PART OF YOUR POTENTIAL MARKET! If you think there is even a slight chance that your are ever going to sell off your transmitter, you are better off NOT modifying it in any way! If you are going to keep it forever, dive right in ....
Be careful because some ham tranmsitters were specifically manufactured for QRP or "Low Power" operation and will have around 10 watts out! Older novice type ham transmitters will have a maximum power output of 75 watts or so. Some transmitters were also made that had output powers of 500 watts. Just about anything is posible to find so make sure and ask!
I would like to tell you a little bit about the myths of power. For example, say I use 100 watts and I'm heard S7 at Joe DXer's. I can increase or reduce my power 50% without any noticable difference! Sounds incredible doesn't it? The only time Joe DXer will notice a difference in my transmitted signal will be by doubling or halfing my power! 200 watts is 3dB louder than 100 watts BUT most people cannot detect a 3 dB difference in signal strength! This means for people to even detect a louder signal you would have to go up to 400 watts for a 6 dB increase to actually be noticed! If you use 400 watts now, you would have to increase your power to a whopping 1600 watts to be louder than 400 watts! If I am running 100 watts and I'm heard right at the noise level at Jane DXer's, then almost ANY increase in power will improve my signal. If you are ever going to try "crossing the pond" with your station, a fair amount of power is typically required. If your transmitter is supposed to put out 100 watts but you can only get 75 watts out, the only thing that will be noticed is the slight decrease in your electric bill.
Tube type transmitters are perhaps more easily found, can usually be purchased quite cheaply and are more "forgiving" when handling mistuning. The downside is that they can be drifty, difficult and expensive to replace bad tubes, are very dangerous because of high voltages used with tubes, and are generally quite large in physical size and weight. They don't call them "Boat Anchors" for no reason!
Solid State Transmitters are smaller, some can be simply modified to transmit anywhere in the shortwave spectrum, are more easily used because most of the tuning is computer assisted. The downside is, they are much less forgiving to impedance mismatches and can be difficult if not impossible to repair yourself.
So you can see, both have advantages and disadvantages. It is up to you to decide what you can or cannot deal with! I will not make any reccomendation other than use what you can get! ANY transmitter, usually, is better than NO transmitter!
"Is there anything that should be avoided?", Yes. Do not purchase military surplus equipment! Unless you have a lot of experience with radio equipment, military surplus stuff can often turn into more trouble than it's worth. Connectors will be diffiucult and expensive to find, power requirements are often non-standard and repair parts can be almost non-existent! Beginners should stick to amateur gear.
OK, now that you know enough about transmitters to hopefully make a wise purchasing decision, where can you find them? If money is not a issue, then there are a plethora of busnesses that sell New and Used amateur equipment. A good starting place is to pick up a copy of any amateur magazine and start looking at the adds and calling for catalogs. When it comes to new equipment, the sky is the limit! On the other hand, if you're like me, money is an issue and generally I don't have enough of the stuff! There are many ways to find used gear, start by keeping a look-out in your local newspaper's classified section. Occasionally, you will find Amateur gear listed there and the advantage is it will be a local call to find out more information and possibly see the rig in action before purchasing it. Most equipment that you find listed here will be from Bootleg CBers, and you can expect just about anything. Another alternative is if you live in a larger city with a Amateur Radio store, they usually will sell used gear along with new stuff and again you will probably get to see the rig in action before purchase. Also you might get some kind of token warranty with it, but not always.
Another potential source of equipment are Ham Fests. These electronic swapmeets take place all over the country and generally during the summer months. This is where you can find just about anything and prices are generally negotiable. There is also the increased risk of buying JUNK and, while not common, it does happen that people do get burned, so be careful! The best way to find out about Ham Fests is to make contact with someone in your local ham community. If you know of no local hams, drive around till you spot the tell-tale antenna farm and introduce yourself as a radio enthusiast, potential ham, etc. and ask him or her about ham fests.
Most Hams attend these functions and it is not difficult to ferret out information about them. They are open to all and just because you're not a ham will have little impact. So you have found out when and where the next one is, the most important rule of hamfests is to show up EARLY! That way you can browse over all the gear that is available and compare prices on similar equipment. If something is unmarked, don't be afraid to ask how much. If the seller says "Make me an Offer.", you might not have any idea what to offer. It could be wise to move on or ask what the "rock bottom" price is. Sellers generally love to talk about their wares and this is a good way just find out general information on items you have no intentions of purchasing, remember KNOWLEDGE IS POWER! The more you know, the better off you are!
Another area to capitilize on is the END of the Hamfest. Some sellers may not relish the idea of lugging all their stuff home and will be more likely to cut a better deal! Ham fests are also great sources of general station supplies like connectors, wire, components, etc.
There are also several publications that deal with used amateur equipment. The only down side is, you really have no idea what you are buying unless you know what the radio is. Caveat Emptor!