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Antenna Theory Question
On 31 mar, 13:57, "AndyS" wrote:
{text deleted} Andy writes: Thanks to all for the discussion....... I have used the results of these calculations for many years but wanted to see what explanations I would get from others who also have experience in these matters..... For capture area, I use " Gee lambda squared over four pi ", which is the standard definition for a well behaved antenna with a main lobe, and it has always worked well for me. I don't remember doing the derivation for this, but I'm sure I must have done it in the past (olden times)... Yet, I was not comfortable with what seemed like a discrepancy between "all the extracted power goes to the load" and the proposition that the cosmos and antenna acted like a generator and therefore had an internal impedance which must be Thevenin matched for max extracted power.... I resolved it, in my mind, by ignoring the latter explanation (grin)..... and apparently it was the correct thing to do... So, thanks again, guys........ Andy W4OAH Hi Andy, As you mentioned, Thevenin does apply, and your effective area formula is correct (gain w.r.t. isotropic radiator). So a thin halve wave dipole antenna has an internal resistance of about 70 Ohms, and half the power is lost, but not as heat in the antenna. The driving (incident) EM-field induces voltage with causes a current to flow in your 75 Ohms load (and in the antenna). The current that flows into the antenna structure (two bars) radiate. So part of the incident field is reradiated by the dipole. It doesn't matter whether the antenna current is caused by a transmitter or incident field. As your dipole delivers 1mW to the load, also 1mW is reradiated into space. Behind the antenna, the reradiated field cancels (partially) the incident field (causing a shadow effect). When you remove the load, the output voltage at the antenna doubles and the current in the dipole reduces to a low value, hence less power is reradiated. When you short circuit the dipole, the current will double (w.r.t. to the matched situation). This will also happen to the current in the bars forming the dipole. As the current has been doubled, the reradiated power is 4 times higher (w.r.t. the matched situation). The reradiated power can be detected. Radar and passive UHF RFID use the reradiated power. For example in UHF RFID, the chip on the RF tag changes the load to the dipole (that receives power from the interrogator), and controls the reradiated power. In this way the ID of the tag is transmitted from the tag to the interrogator (the tag has no battery present). The more gain you have in your antenna, the higher the reradiated EIRP in the direction of maximum gain. I hope this clarifies the antenna internal resistance. Best Regards, Wim PA3DJS |
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