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I just got a ham radio license, and I am trying to figure out the mystery of RF.

From the output on your transceiver to an antenna or an RF amp, there is a 2 lead coax cable. If antennas only have one wire sticking up, what is the point of a 2-lead system to carry the RF.

What happens to the outer wire when at the antenna, is it just a pointless wire, which gets discarded, or does it have other uses? I don't see 2 wires on the antenna, just a thin piece of metal going to the air, which only 1 wire can be connected to.

The only thing I can think about is the 2-wire system prevents the wire acting as an antenna itself and maybe the 2-conductor system makes it more like a transmission wire.

skyler
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    I'm surprised that the process of getting a HAM license didn't cover things like ground planes and antenna theory. – JYelton Aug 29 '13 at 18:04
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    @JYelton It depends on the license. On the entry license, there's not much more than Ohm's Law and "Don't stand in front of the microwave dish". – W5VO Aug 29 '13 at 18:24
  • @W5VO It's different from when I did it, then! :) – JYelton Aug 29 '13 at 18:40
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    "I don't see 2 wires on the antenna" you should try look harder. The antenna is one conductor and the ground plane or the cable's copper cladding is the other. – jippie Aug 29 '13 at 19:07
  • http://en.wikipedia.org/wiki/Counterpoise_%28ground_system%29 – Kaz Aug 29 '13 at 19:19
  • @JYelton Yeah, i think it has gotten a lot easier over time, before you had to know all this morse stuff, and now its just like "memorize 400 questions and u got it" – skyler Sep 03 '13 at 23:39
  • @Skyler Looks like. No offense, just an observation. Hopefully EE.SE can help where the ARRL isn't! :P – JYelton Sep 03 '13 at 23:48
  • Just curious, do a lot of EE's get their Radio Amateur/Ham because of all of the focus on electronics? – skyler Sep 04 '13 at 23:21

1 Answers1

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All antennas have two connections, although sometimes one of the connections may not be obvious. Take a look at a basic dipole antenna. That clearly has two connections, which should be symmetrically driven.

Some "whip" antennas and others may appear to have only one connection, but that is because they use a ground plane. Typical commercial AM transmitters are common examples of that. The antenna is a single conductor (usually some kind of tower) sticking up from the ground. The tower is insulated from the ground. One side of the RF feed connects to the base of the tower, and the other to ground right under the tower.

A ground plane is basically a mirror for radio waves. The AM tower is really only half of a antenna. If you could see at radio wavelengths, from a distance you would see a whole antenna because the top half would be reflected by the ground plane to make a apparent complete antenna. Half of this apparent antenna would be above ground and the other half below ground. The whole acts pretty much like a dipole. One difference between a real dipole and a apparent one made with a mirror is that the second type only works in the half-space above the mirror. However, if you're in that half space, there is little to distinguish the mirrored antenna from the real thing at a distance.

Another way to look at this is that the laws of physic don't go on vacation just because the frequencies get higher than what you are used to. To have a circuit with current flowing thru it still requires a closed path somehow. If you only connected one side of the transmitter output to a antenna, there would be no current coming out of the transmitter, hence no power and no signal gets sent anywhere.

A properly tuned antenna looks like a resistor electrically. That means it absorbs real power from the circuit. However, unlike a resistor that turns the power into heat, the antenna sends it into space as radio waves. If everything is tuned right, your transmitter can't tell the difference between a common dipole at the end of the RF feed line and a 75 Ω resistor.

Olin Lathrop
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  • @DrFriedParts: No, I meant the resistance as written. A dipole doesn't just have a arbitrary resistance. It has a resistance of 75 Ohms at the right frequency. A folded dipole, for example, has 300 Ohms resistance (again, at the particular frequency it is tuned for). Other antennas have other characteristic resistances. They are not arbitrary or unpredictable. – Olin Lathrop Aug 29 '13 at 19:57
  • I said your value was arbitrarily chosen (or rounded), not that the antenna's impedance/radiation resistance is arbitrary. Your answer is fine up to the point that you include a specific number without explanation. For example, the half-wave dipole is 73 Ohms, not 75 Ohms... – DrFriedParts Aug 29 '13 at 20:37
  • So would a Dummy load be putting a 75 ohm resistor across the 2 connections on the transceiver? – skyler Aug 29 '13 at 23:15
  • @skyler: Yes, if your transmitter is intended to drive a 75 Ohm load. That is a common value, but is not universal. You have to check its specs. You also have to know the antennas impedance at the frequency you want to use, with possibly some impedance matching circuit in between the transmitter and antenna. Some transmitters are tolerant of a wide load range, so if you're willing to give up some power you may not need to match impedances. However, some transmitters can get damaged if the load impedance is significantly different from what it was designed for. As always, read the manual. – Olin Lathrop Aug 30 '13 at 13:19