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So this may be a terribly stupid question but since my RF knowledge is fairly limited I'm asking it anyway. Let's say you have an AM signal that's 5Vp-p at some frequency. To amplify this can you simply scale up the voltage and send it to the same antenna? Also, are antenna's rated for a certain power input/output? I'll try to clear up my question somewhat. I am only asking in regard to transmission, assuming the system works at low power levels and all matching is at acceptable limits. |
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Interesting and very valid question. If you have 5V signal source with zero internal resistance loaded to antenna seen as 50 \$\Omega\$ resistor, then you pass power of 0.5Watt. Say if you double the voltage using transformer 1:2, the power will be 2Watt. And so on. In this case you do not need amplifier. The practical problem is that the real signal source has non zero impedance. Typically 50 \$\Omega\$. So transformer 1:2 will make increase not in voltage terms but in impedance terms. The load will see the change as the source is now 100\$\Omega\$ of internal resistance. So original voltage was 10V in the unloaded 50\$\Omega\$ source to reach 5V on 50\$\Omega\$ load. Same 10V with transformed 1:2 load will see 50\$\Omega\$ load as if it was 25\$\Omega\$ load without transformer. The voltage will drop to 10*(25/75)=3.333. So power will be 0.44.. Watt instead of 0.5 watt. The loss of power as "reflected back" loss is caused by impedance mismatch. |
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An RF amplifier is similar in concept to an audio amplifier but built with components suitable for RF frequencies. Because of the higher frequencies involved, the impedance levels of RF amplifiers are generally much lower than for audio amplifiers in order to avoid the effects of parasitic capacitors and inductors. Often 50 ohms is chosen as a common impedance level. Thus RF amplifiers are usually specified as providing a given output power level into 50 ohms. RF antennas are designed to present a 50 ohm load at their input over their operating frequency band. When you say "same antenna", I assume you mean the use of one antenna for receiving and transmitting. This is usually the case in mobile transmitters and requires some form of transmit/receive switching so that the transmitter signal does not get into the receiver. Power is not a concern when the antenna is used for reception but it is important for transmission. The conductors used in the antenna must be sized to handle the transmitter power just like the conductors in an audio speaker must be sized to handle the output of an audio power amplifier. Because of the skin effect, in which high frequency currents tend to flow towards the outside of a conductor, high power antennas can be made out of tubing rather than solid material. |
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A HF antenna has a specific impedance, for instance 50 \$\Omega\$ or 75 \$\Omega\$. That's important if you want optimal power output. The amplifier has an impedance too, and you can prove that the antenna gets the highest power when both impedances are equal.
This graph shows the antenna power as a function of impedance for a 50 \$\Omega\$ output impedance amplifier. You can see that the power output is indeed at a maximum for a 50 \$\Omega\$ antenna impedance. |
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