Hello, Frank here.
Hi everybody, anybody get any RF burns this week? If you did, hopefully it was on your hand and not inside your head. I'm going to start with safety and end with safety.
Today we're going to talk about signals and emissions and some other interesting things. Right now we're going to look at the word emissions. You want emissions to go from your radio, out your coax and ultimately out of your antenna. If you have things set up right they will. But if you have a piece of equipment that might be getting a little old and it's just wearing out, then you might get some emissions where you don't want them. Same thing can go if you have your antenna system hooked up incorrectly.
You need to have your equipment grounded. This will take these unwanted emissions that are bouncing around inside your metal radio and send them to ground, or Earth. If your piece of equipment is too hot, and it shouldn't be, this can be caused by many things. One of the causes is RF escaping from your radio at the wrong place, at the wrong time. It's kind of like if your car is running hot, and it's not a summer day and you're not pulling a trailer. So, things to think about and pay attention to. Ground your equipment and know how it works. Some pieces of equipment just operate warm all of the time, it's their basic nature. That's part of the reason that you need a good mentor from ARRL.
Okay, AM and FM. Most of you are familiar with AM and FM radio in your car, or home. These are the two primary ways that signals are sent through the air. But for listening purposes, in your car AM is amplitude
modulation and FM is frequency modulation. AM is older, FM is newer. Okay, let's say you are listening to an AM radio station on your dial, and you cross an old metal type bridge. There is a chance you will lose part of your signal because the bridge is acting like a Faraday cage. If you're on FM you probably won't notice a difference in the signal at all. Because FM is normally a higher frequency, or newer frequency. For the most part, 30 MHz and down are AM, and in most cases, but not all, 30 MHz and up are FM, which is what you're using right now if you're playing around on VHF and UHF. Why the change? Newer discoveries and newer technology.
So far we've talked about AM and FM as bands. But AM is a type of modulation, as is FM. By the way, on a side note. Most shortwave listening, SWL, is AM. Now for both of these, AM and FM, you have a carrier frequency. Let's say my voice is at a certain frequency. It is added to the carrier frequency. Remember what a sine wave looks like? The sine wave is the carrier frequency. Let's take 900 KHz and my voice at 8000 hertz, you add that to the 900. But you add it and make the amplitude of the sine wave get taller on the top side and the
same on the bottom side. The frequency really doesn't change, just the amplitude of the signal. That's why AM radio is more affected by weather. I'll explain more why in just a second.
Now with FM, which means frequency modulation, you still have a carrier frequency, which for radio purposes, let's say 100 MHz. You add my 8000 hertz voice, and it's added to the frequency. It goes through the air and the radio receiver separates the two and you hear my 8000 hertz voice again. But the reason weather, lightening and things, do not affect FM is because the top and bottom of the carrier frequencies are filtered and removed, wherein, AM is not. That's why FM has a cleaner nicer signal and less static. That's a whole lot for just the first two questions here, but that's basically how AM and FM work. Very basically.
So, skimming down page 22, you see talk about upper and lower side bands. These are used almost all the time in HF transmission. Ironically, the lower HF bands, 160, 80, and 40, use the lower side band, or LSB, and above 40, most of the time, upper side band, USB, is used. Okay.
What are side bands? If you have a regular AM signal, like you listen to on your car radio, or your SW radio, which is intermingled in between all of the ham bands, then this signal includes the carrier and upper and lower sidebands. When you are transmitting on 40 meter LSB, your radio removes the carrier frequency and the USB. This makes the bandwidth much smaller, takes less room on the bandwidth and your power can be used more efficiently. This may sound a little complicated, but if you let it soak in for a while, it will make sense. Just for knowledge purposes there is some SSB on VHF FM, but it's not used a great deal.
Okay, let's continue. Modulation is very important. If you're listening to HF and some guy sounds like a duck, then either you or he are probably off frequency just a little bit, or the sender could be over-driving the frequency. This boils down to too much power going into the microphone, often referred to as microphone gain, or he could have the microphone too close to his mouth. Okay, go ahead and read these pages and don't forget the links at the bottom of each section.
Page 24 breaks down some of the terminology of the parts inside of a radio. You will see these on the test. Go ahead and go through page 25 and we're coming up to antennas.
I need to give you a couple of formulas to start with. We're going to use the 'T' formula. It is: 300 = frequency x meter. The next one is: 468 = length in feet x frequency in MHz. So, build you a 'T', put the 300 on top, and on each side of the vertical underneath the 300, put frequency on one side and meter on the other. Now this is just used for estimations, but it will come in real handy on the test. And remember, once you start the test, ask to use scratch paper and write down these two formulas and use them all through the test.
Example: Your frequency is 150 MHz. You divide that into 300 and you get 2, which would be 2 meter. Or, if you know the meter and you want to try to find the frequency, let's say you know it's 10 meter. Take 300 divide it by 10 and you get 30. Is this exact? No. But 10 meter is 28 and 29 MHz. So, this is just for estimation, but it will get you in the ballpark. And if you have forgotten the frequency and meter relationship, this will get you real close.
The other one. Build you a 'T', put 468 on top. Put length in feet on one side of the vertical bar, and right beside it, on the other side of the bar, put frequency in MHz. This is to find the length of an antenna for 1/2 wave. This is also an approximation, but it's very close.
Example: You want to find 1/2 wavelength for 10 meter. So, you know that the frequency is, let's say 28.7 MHz. You take 468 divided by 28.7 and you get 16.3. So for a half wavelength antenna at 28.7 MHz, you need approximately a 16 foot antenna. Okay? Now, let's say you want a 160 meter antenna, which is 3.9 MHz. You take 468 divided by 3.9 and you get 120 feet for a half wave signal. Now at this stage you can tell that this is not going to be a vertical antenna. This will obviously be a horizontal antenna.
Let's try one more for fun. Your 2 meter VHF radio, which is, let's say 147 MHz. Take 468 divided by 147 and you get about 3.2 feet. Remember, the higher the frequency, the shorter the antenna. It's a whole lot easier to have a mobile VHF radio, than a mobile HF radio. Want to know why your cell phone has such a small antenna? Remember, this if for half wave. You can use a quarter wave or a one eighth wave antenna. Back to your cell phone. Take 468 divided by 900 MHz equals 0.5 foot. Which is about 6 inches. So a quarter wave antenna would be about 3 inches, and a one eighth wave antenna would be about an inch and a half.
So, obviously the higher the frequency, the easier it is for mobile communications. And, remember, almost all VHF and UHF are line of sight communications. And for all practical purposes, they're all vertical antennas. And for all practical purposes, all HF frequencies use a horizontal antenna. There are some exceptions for 10 meter and 12 meter, and there are still some 10 meter repeaters around the country. And don't forget 11 meter. You say, "What is 11 meter?" It is where the CB frequencies are located, which an HF radio. CB also has single side band, SSB, but they operate on LSB instead of USB. Don't forget the humble CB, it is a great radio. And some CB antennas are also good for 10 meter and 12 meter.
Go ahead and read along about impedance matching. But you have to understand that SWR is critical. You may have a tuner for your HF radio and it may tell you that you're tuned, but that doesn't mean that your SWR has decreased. It means that through the miracle of electronics your
radio is matched with your antenna, but your SWR will still be high and your power will be reduced significantly. An antenna tuner may allow you to transmit safely, but if your SWR is high, your power will be sharply reduced. You hear people talk about 1:1 SWR ratio. Well, that is in a perfect world and we do not live in a perfect world. Now, for test purposes you will see 1:1, 4:1, 6:1. You will notice that the larger number is always first. But on the test, you will find that sometimes the largest number is second. You can eliminate those as wrong answers. You're going to need SWR meters for HF and VHF/UHF radios. If you are mobile, you can check your SWR, get it set right and remove your meter. And if you want to, you can do the same thing for HF, but many leave the meter attached permanently.
Next time. Read ahead about antennas, starting on page 28, and please read through page 32. Most people will agree that your antenna is the most important part of your radio system. Always remember safety. If you don't know what you're doing, check with your mentor, or more commonly called, your Elmer. I hope this helps just a little bit.
We'll talk more later. 73, Frank