However, an optimum RF receiver preamplifier is not "power matched" on its input but is matched for best signal to internal (thermal) noise performance.Īnd for reception, the whole idea of "grounding" goes astray. Because of that, signal power matching of receiver and antenna becomes a significant issue. You can use a high input impedance amplifier (or possibly a NIC ) to buffer the high impedance RF signal on the rod/wire to the receiver's input impedance.Īt frequencies above 75 mhz, antennas are much smaller and do not capture much signal power (as well as ambient "noise") as 1/(f^2). A radio signal of 10 microvolts per meter would average out as half the potential at the center of a 1M conductor. Voltage probe antenna depend on a very high input impedance (think a few pfd in shunt with a couple of meg ohms) and the inherent capacitance of the small conductive rod/wire. In the example video, it is HF (under 30 mhz) where voltage probe antennas work. 03 uV signal to open the squelch (ancient Motrac with a perma-K filter). For VHF communications receiver, I have measured, a. A good AM receiver will have a sensitivity of 50 micro/volts per meter while a good FM receiver (entertainment) will work with 2 uV. At AM frequencies, the background noise (even in a rural environment) is very high compared to frequencies above 30 mhz. So the answer to your question is: Yes it WILL work but have to be aware of its limitations and not knowing the strength of other signals in your vicinity we can't predict what may happen.ĭon't try to equate AM-HF antennas with FM (above 88 mhz) antennas. The typical effect is the receiver picks up lots of interference and stations at once, regardless of where you tune to. An antenna alone will reject signals well away from their resonant frequency but being able to cover a much wider frequency range, the active antenna circuit may still pick them up and cause overloading, either in the amplifier itself or at the receiver. Radio station signals typically produce a few microvolts at the antenna (1uV = one millionth of a volt) but other sources of signals may be hundreds or thousands of times stronger. The drawback to the telescopic antenna and active antenna circuit is it is poor at distinguishing the signal you want from all the other signals around it. It may also have some amplification which can in some instances help. The amplifier has a high input impedance which to some degree lessens the need for the telescopic bit to be the right length and has a low output impedance so the signal is better coupled to the receiver. The idea behind the 'active antenna' is that instead of making the antenna match the receiver, an intermediate stage is added. They convert EM waves into a voltage that the receiver can use.Īn optimal antenna has to be the correct length for the EM waves to resonate along its length and be the correct impedance it its feed point to efficiently convert the signal so the receiver can use it.Ī telescopic antenna can work efficiently but only at the frequency it's extended length resonates at. The answers overlook the fundamental point of how antennas work. I appreciate that this question could be answered. In the video which I have mentioned above the quality of the audio and reception doesn't seem to be of much improvement.Īnyways should it be okay to leave GND unconnected from J1? and just connect to the antenna the other end of the 470 pF?, will it be safe?. I could try to accomodate some thin wire inside my room but I don't think this would help much as it seems that the intended purpose of such antennas is to be placed outdoors. I'm not near to dimmers which I can tell, but there can be EM noise. My room has a window but I can't hang a wire outside as it is not practical, thus I though that I could use this circuit to improve reception. I wish I could use a traditional long wire antenna in the outdoors, but I don't have access to it. Or is it necessary to connect this to the GND of the receiver?Ĭan somebody please answer this question?. Therefore, is it okay to leave GND as no connection or floating?. Around second 37 he shows that, when tied to the telescopic antenna of a radio, the GND from the circuit is left floating, but because he used such circuit in a receiver which has an output with a GND access he could plug both connections in the BNC plug of the receiver.
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