Wednesday, January 28, 2009

Energy, Power, and Time by Lee Bond N7KC

January 28, 2009 Educational Radio Net, PSRG

For the 36th session of the Educational Radio Net, I have chosen to review basic and important concepts that cannot be avoided when dealing with electrical equipment including radios. There are many but the really important ideas wheel around voltage, current, resistance, energy, power, and time. Probably the least understood are the relationships of energy, power, and time so let's start with these three.

Energy and work are equivalent. If you perform any task that requires physical exertion then you have expended energy in some form. Move a heavy box from the floor to a table top and you have done 'work' and expended energy in the process. Another example might be your car on a level street. Think of a level, and straight, street 1/4 mile long. Simply starting the engine requires electrical energy to turn the starter which does work on the flywheel which then rotates the crank, etc., and the engine starts. Moving the car toward the street end requires energy expenditure and, finally, stopping the car requires energy expenditure as well. So, the starter uses electrical energy to start the car followed by burning fuel to run the heat engine followed by braking which produces heat in the brakes to stop the car. The car starts from a standstill and stops at a standstill on a level street so the net energy is zero. This means that all of the energy used to accelerate the car is exactly equal to the energy dissipated in stopping the car using the brakes.

Lets look at another example using an antenna tower. Jim and Joe weigh the same but Jim is in very good condition. Both men start from the ground and climb the tower to the 100 foot level. Jim makes the top in one minute but Joe requires two minutes to make the same climb. Both Jim and Joe perform exactly the same physical work climbing the tower but Joe takes longer. Jim is the more 'powerful' climber since he expended the same energy as Joe in one half the time. By definition, power is the rate that energy is expended. The word 'rate' always denotes something per time like miles per hour, pounds per second, furlongs per fortnight, feet per second, or joules per second.

Electrical energy is measured in joules so electrical power is expressed by joules per second. You will not be surprised to find out that one joule per second is one watt. A tiny bit of algebraic manipulation shows that one joule is one watt-second so power times time equals energy.

Enter the kilowatt-hour meter on your house. Notice that power given by kilowatts multiplied by time given by hours represents the energy consumed in the house. So you end up paying the power company for the total work performed to maintain your home over one billing cycle. KwH meters are average indicators in that they cannot register the rate that power is delivered to the home. Some industrial energy meters also measure 'demand' or how fast the energy is delivered to the plant in addition to total energy used over one billing cycle.

Common knowledge tells us that electrical power can be computed by multiplying voltage and current, or multiplying current squared and resistance, or by voltage squared divided by resistance. All three of these operations yield watts and now we know that there is a time connection since all three operations are really representing energy per second.

In summary, easy to understand examples in the physical world relating work, power, and time can be extended to the electrical world as well. Knowing definitions and being able to manipulate variables such as current, voltage, and resistance will certainly increase your enjoyment of radio phenomena many fold.

This concludes the set up discussion of energy, power, and time. Are there any questions with regard to tonight's discussion?

This is N7KC for the Wednesday night Educational Radio Net.

Wednesday, January 14, 2009

Extra Class Exam Grab Bag

Tonight we will cover a few questions from the Extra Class exam pool and discuss the reasoning behind the answers.

E1B08 (D) [97.121]
On what frequencies may the operation of an amateur station be restricted if its emissions cause interference to the reception of a domestic broadcast station on a receiver of good engineering design?
A. On the frequency used by the domestic broadcast station
B. On all frequencies below 30 MHz
C. On all frequencies above 30 MHz
D. On the interfering amateur service transmitting frequencies
This question makes it clear the the burden of avoiding interference to your neighbors is on you. Even if you are operating with a clean signal, completely within the amateur bands, if you are causing interference on someones well designed radio or TV, you can be restricted in your activity. Of course, there are other very good reasons to take it upon yourself to help resolve the situation.
  • You should have the knowledge to add a filter to your neighbor's antenna connection or do what else is necessary to fix the problem. The person being interfered with is not likely to know what to do.
  • It is a good idea to do what you can to keep your relations with your neighbors as positive as you reasonably can.
  • You will be helping the entire Ham community to keep a better opinion of our hobby. This will help ensure that we keep the privileges we enjoy today.

E1F09 (D) [97.113]
When may the control operator of a repeater accept payment for providing communication services to another party?
A. When the repeater is operating under portable power
B. When the repeater is operating under local control
C. During Red Cross or other emergency service drills
D. Under no circumstances
This one is probably pretty obvious, but I have it here just to reinforce that we can never receive payment in return for operating in the Ham bands.

E1F10 (D) [97.113]
When may an amateur station send a message to a business?
A. When the total money involved does not exceed $25
B. When the control operator is employed by the FCC or another government agency
C. When transmitting international third-party communications
D. When neither the amateur nor his or her employer has a pecuniary interest in the communications
This one is interesting in that there is no prohibition against communicating with a business over ham radio as long as you are not doing business on the radio. There has been a lot of discussion about just where to draw the line. It can be open to interpretation whether you are doing business or not. So, if possible, it's best just to avoid communicating with a business on the ham bands. One circumstance where one could find it necessary to communicate with a business on the ham bands is in emergency communications.

E1F11 (A) [97.113]
Which of the following types of amateur-operator-to-amateur-operator communications are prohibited?
A. Communications transmitted for hire or material compensation, except as otherwise provided in the rules
B. Communications that have a political content, except as allowed by the Fairness Doctrine
C. Communications that have a religious content
D. Communications in a language other than English
I don't know what the exceptions are.

An interesting part of this question is that communication in a language other than English is not prohibited. It is okay if others don't understand what you are saying as long as it is not your intent to obscure the meaning of your transmission.

Wednesday, January 7, 2009

Radio Waves and Polarization

So far we have discussed some aspects of radio wave creation and propagation and in those discussions we have touched on polarization but tonight we will go a bit deeper into what it is and why it's important to understand.

Most hams know that a vertical antenna creates a radio wave with vertical polarization and a horizontal antenna creates a horizontally polarized wave. This may seem pretty obvious but we need to take a closer look at this in order to describe another useful polarization, namely circular polarization.

Before we get into polarization, let's look at what makes up a radio wave, also known as an electromagnetic wave. There's a reason we call them electromagnetic waves and not just electric waves. In an electromagnetic wave there is an electric field and a magnetic field. These fields vary over time depending on the frequency of the signal. The interaction of the two varying fields is what causes an electromagnetic wave that can travel through empty space. As it happens the electric field, called the E field is parallel to the antenna's radiating element. The magnetic field is perpendicular to the E field and in the same phase. You don't really need to bother with magnetic fields any more, I just wanted you to be aware of them.

Linear Polarization is the usual kind and what it means is that the polarization of the radio wave as it is sent out doesn't change over time or space, it always points in the same direction, whether vertical, horizontal or otherwise. In HF where we use the ionosphere to bend our signals back to earth, the polarization is changed by the ionosphere in complex ways which I'm not prepared to go into now. I might try to tackle that in a future net. But for local communications in VHF and UHF the polarization stays vertical or horizontal and this can have quite an effect on how well you can communicate with another station. In theory, a horizontal antenna would not pick up a vertically polarized radio wave at all. In practice it can severely reduce the signal to make it hard to communicate even if your antenna does pick it up. And even in local communications radio wave polarization can be affected by reflections or other interfering bodies.

There is one kind of ham communication where linear polarization is not suitable and that is space communication in general and satellite communication in particular. In ham radio, we communicate with satellites using VHF and UHF signals where the polarization is not seriously affected by the ionosphere. What's more, it is not practical to try to always know the orientation of the satellite and match your antenna to it. For these reasons we use circularly polarized radio waves that don't depend on knowing the orientation of either party. So what is circular polarization and how do we make it?

Circularly polarized radio waves sound pretty strange. A circularly polarized wave is one with constant amplitude where the orientation of the wave turns through a circle according to the frequency of the wave rather than the amplitude changing and the orientation staying constant. As it turns out, this is pretty easy to create in theory and in practice as well. The way you do it is to create two sine waves that are at right angles to each other and 90 degrees out of phase.

Let's now follow it through part of a wave transmission. Assume that the first wave is vertical and the second horizontal pointing to the right. When the first one is at it's peak amplitude the second is zero resulting in a vertically polarized field. As the amplitude of the vertical signal is reduced, the amplitude of the second one is increased exactly in such a way that the total amplitude stays constant and the direction of the field rotates to the right. Finally, when the vertical wave has reduced all the way to zero, the horizontal wave has reached it's maximum and the resulting electric field is horizontal and pointing to the right. This works only because both waves are sine waves. But since our transmitted waves are sine waves with only minor variations, it works in practice for ham communications.

So how do you make such a wave? Clearly we don't have transmitters that create two waves that are 90 degrees out of phase. The answer is to have a T-connector splitting the signal into two paths and have one path be a quarter wavelength longer than the first. Now by connecting one side of the T-connector to one driven element of a yagi and the other side to a driven element of a yagi at right angles to the first, you get your circular wave.

There is another common circular wave antenna called a Helical antenna. Unfortunately I didn't have time to prepare a discussion on this antenna. Like the ionosphere effect on polarization, I may tackle this topic in a future net.

Good web page on polarization used with satellites

Informal Nets on Dec 24 and Dec 31, 2008

December 24th, 2008 and December 31st, 2008, being Christmas Eve and New Years Eve, were informal nets with no prepared material. This is why there are no blog entries for those dates.