- May 17, 2016
freecell, you are right, we agree with that. The longer the cable and the higher the frequency, the more attenuation will be present, but that’s another story.
"VSWR should not be affected by length of coax cable." Fortunately it is, anytime the line is not terminated in a load = to the characteristic impedance (Zo) of the feedline or in the case of a feedline with excessive line attenuation losses.
A 500' line of RG-213 will lose 70% of the power produced by the transmitter before it is delivered to the antenna. If the load impedance is anything but a perfect match, additional power will be "displaced" due to reflection from the load back down the line.
"posts above suggest that a loss in gain of 70%...."
i was speaking of 70% power loss over 500 ft. of RG-213 due to line attenuation, nothing to do with any antenna.
Q) At what frequency?A 500' line of RG-213 will lose 70% of the power produced by the transmitter before it is delivered to the antenna
Q) Displaced?If the load impedance is anything but a perfect match, additional power will be "displaced" due to reflection from the load back down the line.
~MaxwellIn our efforts to obtain low feed-line SWR values of 1.1, 1.2, or even 1.5 to 1, we have gone far past the diminishing-returns point with respect to efficient power transfer, even for single-frequency operation. It is like installing no. 4 or no. 6 wire in a house-wiring run where no. 12 wire is sufficient.
Reference to the basic transmission-line equations, which have always been readily available in engineering texts and handbooks verify this analogy. In addition, such references make it clearly apparent that authors who simply insist on low SWR, or find 1.5:1 or 2:1 objectionably high, have failed to comprehend the true relationship between reflected and dissipated power. From the viewpoint of amateur communications, it can be shown mathematically, and easily verified in practice, that the difference in power transferred through any coaxial line with an SWR of 2:1 is imperceptible compared to having a perfectly matched 1:1 termination. This is true no matter what the length or attenuation of the line. Further, it can be shown that many typical coaxial feed lines we use on the HF bands with an SWR of 3 or 4, and often as high as 5 to 1, have an equally imperceptible difference at the receiving end. When feed-line attenuation is low, allowing such higher values of SWR permits operating over reasonably wide-frequency excursions from the self-resonant frequency of the antenna with the imperceptible power loss just described,
A 500' line of RG-213 will lose 70% of the power produced by the transmitter before it is delivered to the antenna.
Q) At what frequency?
the ability of the electrical half wave line to mirror and repeat the complex load impedance seen at the antenna feedpoint to the transmitter or analyzer (opposite) end of the line is indisputable.
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|May 24, 2023 @ 11:20 UTC|
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