﻿VK5AJL - Myths and Crazy ideas

MYTHS AND CRAZY IDEAS
There are some big misconceptions out there.

MYTH 1 - AN ANTENNA HAS TO BE RESONANT - TUNER'S DON'T HELP

I have read one story about tuners reflecting energy back and forth between the tuner and antenna/feeder point and all of this energy turning into heat. Only the resonant part radiates. Bollocks! Antenna tuners are all about matching impedances. This involves transforming voltages and currents. (You can't do one without the other.)

Without getting too complicated, think of it this way. There is a tuner, some feeder and an antenna. This antenna can be capacitive, inductive or purely resistive. It is only the purely resistive part that radiates (radiating resistance) BUT if you are reflecting energy back and forth, the voltage is higher and so more power is consumed (radiated) by this resistance. The power you radiate from the antenna is always V2/R where V is the voltage at the feed point and R is the radiating resistance of the antenna. Of course the power is also I2R but the same applies. Increasing the voltage into the same resistance increases the current.

Remember also the radiating resistance of a perfectly resonant half wave dipole is supposed to be 75Ω. If you are feeding it with anything else like 50Ω coax or 300Ω ribbon or 450Ω ladder line, there will be reflection and losses involved at the antenna feed point or 75Ω to ??Ω junction.

MYTH 2 - MAIN EXTENSIONS CORDS SHOULD NOT BE COILED.

Why not? In a Caravan and Camping book I read recently, it says a coiled mains cord creates an induction coil. Bollocks. It is almost right. It creates TWO induction coils each with currents in exactly opposite directions so they both cancel each other out and therefore create no magnetic field whatever. If there was a magnetic field created, the earth wire would end up with a voltage on it and the earth return safety switch would trip.

Actually, the earth return wire isn't connected to the tripping mechanism. This relies on no magnetic field being built up when two equal and opposite currents are passed through it viz. the active and neutral.

MYTH 3 - AN ANTENNA WITH GAIN INCREASES THE AMOUNT OF POWER YOU RADIATE

What comes out of the radio is all you can ever radiate. An antenna can't just get extra energy from nowhere. The gain of an antenna is only achieved by directionality. Instead of radiating in all directions, the energy radiated in a direction where it will not be received, is concentrated in the direction that it will. This inceases the effective radiation in the desired direction but decreases it in others.

On one website there is an antenna described which is supposed to have improvements in both gain and directionality. It might work but if the author doesn't know antenna gain and directionality are the same thing, I'm not going to bother building one.

Conclusion: Some people will never understand the basic laws of physics. Energy can not be created or destroyed. It can only converted from one form to another.

MYTH 4 - A CRAZY STORY ABOUT COAX

I was listening to a couple of hams the other day on a repeater. One of them was saying that, by using coax to feed a centre fed HF dipole, he had a SWR less than 2:1 on all HF bands without any tuner.

SWR is a measure of the ratio of forward power to reflected power. The radio was putting out plenty of forward power that was heating up the coax on the way out to the antenna. Any reflected power was also being absorbed by the coax on the way back so that very little of the original signal was making it back to the radio. OF COURSE THE SWR WAS LOW.

If you connect, say, 100 metres of RG58 to a radio, and short circuit the other end, you will have a SWR close to 1:1, do no damage to the radio but radiate nothing.

Conclusion: some people will never understand transmission lines and antennas or the jobs they do.

MYTH 5 - GROUNDING THE SHACK TO KEEP OUT RF

One often stated reason for grounding the radio shack is to keep RF out. This sounded like bollocks to me so I tried it. I have a truck with radios in it completely isolated from the ground by rubber tyres. The back of the truck is a metal container that can be completely closed up. There are metal overlapping shutters on all windows and doors.

The only RF connection to ground is a few picofarads between the truck floor and ground a metre away. The radios were operated from batteries with no other connection of any kind to the outside world.

With a radio connected to a dummy load completely inside the truck, I tested the ability of radio waves to penetrate the container by transmitting from a 2m handheld on 1 watt 6 metres away. There was, of course, a good signal from this short distance that was measured at about S1. (There was another person involved.) I then grounded the metal container and could not measure any difference in signal at all. I tried several grounding connections including fences (deeply set into the soil), water supply pipes (copper underground) mains power ground and various combinations.

This test by itself would not seem to prove anything much but I then opened one of the shutters on the opposite side to the radio source by only 5mm. I was going to open it 5mm at a time but it would have proved to be a complete waste of time. The signal level inside the truck went to full scale. Again, grounding and not grounding the metal container made no difference I could measure.

Conclusion: If you are worried about RF inside the shack, close it up completely with metal shutters and doors. A single glass window will let in thousands of times more RF than any possible effect grounding the shack might have.

MYTH 6 - FEEDER COAX MUST BE AN EXACT MULTIPLE OF ½λ

An antenna system usually includes two things, a transmission line and a radiator. The general idea is to have the radiator do all the radiating and the transmission line doing the conducting back and forth between the radio and radiator with as little influence to either as possible. The page on transmission lines describes the properties of just the transmission line part of the system. If it is terminated with other than a resistor of the same characteristic impedance it creates reactance. This reactance will vary with length. If it is terminated with a resistor exactly equal to its characteristic impedance, it will not make and difference at all how long it is.

Conclusion: If you manage to change the characteristics of an antenna system by changing the length of the coax (other than change coax losses), the antenna is working against the coax outer and is simply a lousy antenna.

MYTH 7 - SPEAKER LEADS - ONE FOR THE AUDIOPHILES

It is amazing what lengths some people will go to just so they can claim they have a better system than everyone else. One of these is to use really thick oxygen free copper power cable for speaker leads. The only reason one person gave me was to avoid RF getting into the amplifier outputs.

Sorry fellows but a resonant length is a resonant length and there isn't anything you can do about it. The only thing that can be done is to put an RF choke on the audio amplifier output. If you take a 1m long 25mm thick piece of pure silver (still a much better conductor than OF copper) and connect it to earth, I can still put kilovolts at the top of it without touching it and so could any other competent amateur operator.

The best speaker lead is normal 3 conductor 10 amp mains flex. With two conductors used for the return path and one for the signal, the transmission line impedance is closer to 8 ohms than anything else easily available. With the frequencies involved, transmission line impedance won't make the slightest bit of difference but neither will silver coated oxygen free copper.

In any case, welding cable is a lot cheaper if you want to simply keep the resistance down.

Conclusion: If you are worried about RF getting into audio amps, move away from RF sources or make a simple RF choke.

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