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#9
November 6th 14, 06:27 PM
posted to rec.radio.amateur.antenna
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A short 160M antenna
rickman wrote:
On 11/6/2014 11:08 AM, John S wrote: On 11/5/2014 7:16 PM, rickman wrote: On 11/5/2014 7:28 PM, wrote: I started to do some modeling on a short antenna for 160M and got what I think are interesting results. I will post those as soon as I get a chance to write up all the data. All this stuff for short antenna is in the context of transmissions, right? For receiving a short antenna is at a disadvantage, no? I seem to recall a parameter called "effective height". For loop antenna it pertains to the signal collected irrespective of the actual dimensions of the loop. For other types of antenna I assume this is not the same and does relate directly to the length of the antenna. Is that correct? I ran a simulation to confirm that the received signal is some function of the length of a wire antenna. My model was a 6 foot zero-loss wire 10 miles from the source with a load of 1000 ohms. The frequency is 1MHz. Wire length Volts received 6' 0.001499 12' 0.005408 So, it appears that doubling the length of a short antenna captures about 3.6 times the signal. Is this what you wanted to know? That is a nice experimental verification. I guess I figured this is the sort of thing that there would be an equation for. A loop antenna has a simple equation defining its effective height (ability to convert the field to a voltage). I expect there is a similar equation for each antenna type. I guess the point is that for receiving it is important to match the size of the antenna to the signal to receive the maximum power. Or is there something equivalent to the matching network that would equalize the power received? In your example you said you used a 1000 ohm load. Is there a way to improve the signal from the shorter antenna? All antennas are reciprocal. One result of that is that if a given voltage at input produces a particular field, the same field will produce the same voltage upon receiving and the terminal voltage and field are related by the effective height as discussed at length in the third link I gave you. In the second link I gave you it says: "For an antenna with a symmetrical current distribution, the center of radiation is the center of the distribution." -- Jim Pennino |
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