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Consider these 3 thoughts..
(1) Zero Gravity
When NASA first started sending astronauts into space, they quickly discovered that ball-point pens would not work in zero gravity. To combat this problem, NASA scientists spent a decade and $12 billion developing a pen that writes in zero gravity, upside-down, on almost any surface including glass and at temperatures ranging from below freezing to over 300 C. The Russians used a pencil. Your taxes are due again - enjoy paying them.
(2) Our Constitution
Officials keep talking about drafting a Constitution for Iraq . Why don't we just give Iraq ours? It was written by a lot of really smart guys, and it's worked for over 200 years. And, we're not using it anymore.
(3) Ten Commandments
The real reason that we can't have the Ten Commandments in a courthouse is that you cannot post 'Thou Shalt Not Steal,' 'Thou Shalt Not Commit Adultery' and 'Thou Shall Not Lie' in a building full of lawyers, judges and politicians. It creates a hostile work environment.
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WE LOVE TO PLAY WITH ANTENNAS
(Do we know what we're doing?)
As we learned in preparing for our first amateur exam that a half wave dipole has a resistance of about 75 ohms. This isn't quite the whole truth! It really has a resistive impedance of 73 ohms and a small inductive impedance of about 11 ohms. The 11 ohms depends on the ratio of length to diameter of the wire or tubing that the antenna is made with. If we shorten the antenna from the half-wave length we will get additional capacitive reactance. If we could stretch it we would add inductive reactance.
Thus to make the antenna totally resistive we must cut the antenna a few percent shorter that a half-wave. The precise amount depends on the length diameter ratio of the wire. Typically an amateur cuts the antenna 5% shorter. That's why we use the number 468 divided by frequency as the length in feet of a half-wave antenna, the common dipole.
As the dipole is shortened, the radiation resistance falls sharply and it becomes a very inefficient radiator. That 5% that we cut off to get only resistive impedance results in a radiation impedance of about 63 ohms. The antenna now shows no capacitive or inductive reactance. Note that originally we had a radiation reactance of 73 ohms but now we have 63 ohms. But what we want is the greatest radiation reactance possible. What if we had left the dipole a half-wave in length and inserted 11 ohms of capacitive reactance in the
circuit? There would be no adverse reactance and the full 73 ohms would be working.
In a mobile situation, by the time the dipole length has reduced to 1/10 wave length the radiation resistance has decreased to about 2 ohms and the reactance has increased to between 1 and 5 kilo-ohms depending on the diameter of the tubes. Not the best thing to have happen, but in the mobile there's not a lot than can be done. Coils and various matching devices are your choices.
In that mobile situation we refuse to lose any length of the antenna and instead get rid of the reactances we don't want by countering the capacitive reactance by introducing an opposing inductive reactance (coils and tuners). Perhaps we can learn from that and not chop up our dipole but match it with a proper reactance. Notice that we'll get a lower SWR either way, chopping or matching, but there will be more radiation if we match.
Much of the preceding information came from papers written by D. Jefferies.
Marion WA8MFL
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