"SPY" Antenna
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Practical Antenna Knowlege
As everyone knows radio waves are electromagnetic waves travelling through the air. As these waves pass over metal, the waves are conducted. We use antennas to grab these waves out of the air, and we use cases and shields to prevent RF energy from passing where we dont want it to go.
In this example we see a number of RF sources, and we are going to see how they affect different parts of the environment. We see a case with a cpu (internal RF source) and a shielded coaxial cable leading to an antenna. There is an external RF source radiating on the entire assembly. We will start with the antenna.
As the waves pass over the antenna, the magnetic energy causes differences in voltage along the antenna. If the antenna is too short for the wave then the voltage difference in the wave is smaller than it would be with a normally. If the antenna is too long there will be opposing voltages cancelling each other out. The antenna must be a certain lenth to get the most reactance from the right frequency from the outside RF source. The same is true for transmitting. The antenna lenth starts where it leaves the sheilding of the case.
The case acts similarly to the antenna. It allows RF energy to conduct, but instead of allowing voltage to charge it allows electrons to pass to and from ground to neutralize any voltages. The case physically surrounds the device and keeps RF interferance from getting into the enclosure. A case can be grounded to a rod buried deep in the earth (like the third prong in an wall outlet) or to a negative battery terminal (like in a walkietalkie or a cell phone). Any place with enough electrons to dissapate RF waves can act as a ground. Similarly case's can keep RF energy from getting out. This can keep RF energy from affecting other devices, like sensitive radios and processors. Our PDA phones have both, and they are shielded from each other.
Coaxial cables are an extension of the case, that is they bring the case and its shieldling properties out to the antenna. The shield of the cable is attached to the case, effectively extending the case along the lenth of the cable. This can be any lenth and and allows us to put the antenna anywhere we want. Now what antenna do we want to put out there?
Coil and Whip Antenna
This is a diagram showing 2 types of antenna. The first is a coil antenna and the second is a whip antenna. Also shown is a ground plane, and important part of how these phones function. If you think of a truck with a big whip antenna, the truck itself serves as the ground plane. The ground plane functions as a reference for the antenna.
The whip antenna is what most folks think of as an antenna. It extends a certian fraction if the wavelenth and converts electro magnetic energy into voltages which are brough into the radio's receiver.
The coil antenna is a remarkably compact and efficient antenna. It is the same as the whip antenna, just coiled up. They are the same lenth electrically, using the principal of induction to convert electro magnetic energy into voltage. As the magnetic field changes around the coil, energy is released. This is a hi-gain antenna that doesnt take up a lot of space, so its perfect for cell phones and mobile devices. The only drawback is that it only can sample a small portion of the wave. The hi-gain makes up for that drawback, but it isnt really much more efficent than the whip antenna.
The diagram shows a half wavelenth coil and a half wavelenth whip antenna, and the resulting sine wave is shown. At any point in time the antenna is measuring 1/2 the wave. We will use these as a referance for the dipole antenna.
Dipole Antenna
The dipole gets its name from the 2 elements in the antenna. In truth all antennas use the same basic elements, a radiating element and a ground element. The dipole measures signals 180 degrees seperated from each other, making it a high gain antenna. Because the same signal is measured 180 degrees apart the signal looks twice as big. The radiation pattern is horizontal, making a big donut shape, making it ideal for ground to ground comminication.
Each element is 1/4 wavelenth, 180 degrees rotated from each other. The antenna sits with each element end to end, making a full 1/2 wavelenth long. The leads should not be seperated from each other as much as possible, the more the wire shiled is stripped the more electrically seperated they become (meaning less total gain), untill at 1/8 wavelenth seperated (1/4 wavelenth total) There is no gain at all over a normal antenna. Strip them even further and the signal starts going into phase, subtracting the total signal.
The 180 degrees seperation comes from one lead being at the bottom of one pole and the top of the other. If you had both poles sitting side by side, both pointing up, they would measure the same signal. By rotating one physically 180 degress releative to the other one, you get 1 signal 180 degrees out of phaze of the other signal.
The diagram uses the letter h for wavelenth, when really the greek letter lamda should be used. Lamda looks like an h to me, so thats what I use as a substitute.
Impedance matching
Aftermarket antennas for the treo are made to match a 50 ohm impedeance. That seems to be only hard numbers we have to go on for the Treo and other cell phone transmitters. The dipole antenna is nominally 73 ohms. I performed an experiment to see if the Treo would want a lower impedance antenna. I found that by placing a 1/2 wavelenth rod across the lines of flux of a dipole antenna I would get a folded dipole antenna with the ends cut off, which is nominally 25 ohms. By adjusting the distance of the dipole to the rod I could adjust the impedance of the antenna from 75 ohms to 25 ohms. I tested several variables under many conditions, and came to the conclusion that the 75 ohm antenna behaved identically to a 50 ohm or 25 ohm antenna. My reason for thinking this is that an impedance mismatch would cause a standing wave in the transmission line, causing noise. With the maximum transmission power of 0.6 MW the standing wave is so small as to not be a factor.
