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Episode 13 -- May 9th, 2016 -- HF Antennas and the Ionosphere. Radio Dave guest stars.
Listen to the episode 53.8 MB MP3
More about Dipole Antennas
You can add a reflector and one or more directors around a dipole antenna.
Reflector -- If you put a metal (conducting) stick or pole or wire 1 quarter wavelength away next do a dipole and make it 0.05 wavelengths longer, that is called a Reflector. A Reflector will make the antenna hear less and talk less on the opposite side of the reflector from the dipole (which is also called, in this case, a "driven element") and towards the side away from the reflector, thus making the dipole work better than it did before in that direction.
Directors -- If put put one or more metal (conducting) sticks or poles or wires 1 quarter wavelength away on the other side of the dipole, these are called, "Directors" and they help direct the signal in one direction (in the direction of the director(s).)
If you have at least one reflector and at least one directors, your antenna is now called a "Yagi" or "Beam" antenna.
If you ad 7 more directors, the beamwidth gets very small and sharp so that the antenna performs amazingly well (like 15db better in 1 direction than a normal dipole).
An Inverted-V is a type of dipole in which the middle is higher than the ends but it is extremely important that the height of the middle is low enough so that the triangle made by the two sides with the ground remains obtuce and is NOT less than 90 degrees. Inverted-V antennas radiate in all directions (better than a dipole) and you can still put the two ends relatively close to the ground whereas a regular dipole won't work unless you mount it AT LEAST a quarter wavelength away from the ground.
An NVIS antenna is one which is so close to the ground that it won't get out more than around 400 miles. When your antenna is less than a quarter wavelength away from the ground, you'll not get your signal out beyond 400 miles or so. This is generally considered to be a bad thing.
Light is just a higher frequency radio wave.
You can use the idea of a sharp flashlight to imagine what yagi antennas do.
Such a highly directional antenna is good if you know exactly with whom you'd like to talk. But radio waves bounce off of rocks and mountains so you can also think of it like playing billairds. You wouldn't want a receiving antenna this directional if you didn't already know with whom you'd like to communicate.
A broadside array is a whole bunch of yagis facing the same way. Ham's generally don't make such gigantic, complicated antennas. But we do make 10 element yagis and you can put 2 or 4 (or 16 or 128) side by side to make a really dramatic sharply directional antenna. Such antennas are used for communicating with other hams by bouncing a signal off of celestial bodies such as the moon or the planet Venus. Yes, hams do that.
An antenna is supposed to appear to the transmitter (through the feedline) as though it is exactly 50 ohms of impedance with no capacitive or inductive reactance at the specific frequency upon which you're attempting to transmit. In reality, this rarely happens. Usually, you strive for an antenna which is tuned so well that, depending on what part of the band you're operating, the VSWR goes no higher than 2:1 (1.1:1 is best; 1.3:1 is good enough).
If your antenna doesn't do that, you can still operate with much reduced efficiency by using an antenna tuner.
An Antenna Tuner or, as we called them in the olden days, a Transmatch is a box with some big variable capacitors and inductors and the thing you do with it is turn the knobs (variable capacitors and/or inductor(s) so that the capacitive and inductive reactance of the antenna is balanced out and the impedance of the antenna (that the transmitter sees) is adjusted to better match what the transmitter is expecting and hoping for.
Generally, a transmatch requires no power (unless it is an autotuner, see below) it just requires a connection to the transmitter and a connection to the feedline ot the antenna. Often they come with a switch for multiple antennas and multiple connectors on the back to switch between antennas.
Occasionally they also come with a dummy load built in. A dummy load is a big resistor which looks to the transmitter like a nice big juicy antenna with exactly 50 ohms of impedance and no appreciable capacitive or inductive reactance but it does NOT put any signal out into the air so you can test or tune your transmitter without worrying that your putting out a signal. In the olden days, before transistorized radios, all tube type radios needed to have their output final amplifier manually tuned by you, the operator, every time you turned it on and chose a frequency upon which to transmit. This usually involved turning one big knob until the meter is peaked and then flipping a switch so the meter looks at something else and tuning another big knob so the meter dips. Wash, rinse, repeat. You keep doing this until you've reached an optimum peak and dip. You wanna do this into a dummy load so you don't transmit a long tone on the frequency everybody else is listening to before you talk, hence, a dummy load.
All tuners will have a range of frequencies or bands upon which they can operate (such as 160m - 10m or 80m - 6m, etc.) and also the maximum amount of power you can use through them. There is no advantage of any kind buying a tuner which can handle a lot of power if you're not actually going to send that much power through it, unless you're planning to buy an amplifier next year and you want to prepare.
Automatic Antenna Tuners, or autotuners have a microcomputer which flips a bunch of latching relays on and off really fast until it finds the correct combination of fixed capacitors and inductors to balance out your antenna without you having to do anything except push the button. They're spose' to make lots of clacking noises; that's them doing their job. They usually come with built-in SWR meters so you can see how good a job they did.
Most of them sit on the desk but a few are designed to sit in a waterproof box outside close to the antenna. Some have remote control operation.
The very best autotuner (in my not so humble opinion) is a Palstar AT-AUTO which, instead of a bunch of clacking relays and fixed capacitors and inductors actually has a big variable capacitor and a big variable inductor which are turned with servo motors controlled by the computer. This makes a better match every time. Cost approx. $1500. Most people don't need that, however.
Frequencies below UHF, the radio waves follows the curviture of the earth a little bit.
The knife-edge effect pulls radio waves down into a valley for some reason.
Propigation is the word we use to express how radio waves bounce (or not) off different layers of the ionosphere (HF) or different clouds or mountains (VHF and UHF).
The ionosphere is a layer of the atmosphere that bouces certain radio waves back to earth at certain times of the day or night, depending on what frequency they are at. Dave says it's like a magic mirror in the sky.
The Ionosphere is composed of 4 layers labelled, D, E, F1 and F2. Different layers reflect different frequency radio waves and different times of the day and night and different typs of the 11 year sunspot cycle.
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