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Stacking Winegard HD-6065P antennas
On 1/12/2014 11:14 AM, Jeff wrote:
Try again. I had it back in college in the early 70's (as an EE major), and I don't think the laws of physics have changed. And the best impedance for a coax is that which matches the input and output impedance of the system, or if the input and output are of different impedances, acts as a matching stub between the two. And yes, I read the article. But you obviously don't understand it. The 30 ohms they are talking about was for ONE SPECIFIC REQUIREMENT. That does NOT mean it is true in different situations. I suggest you learn what you're talking about before you make yourself look even sillier. Jerry, let me put you out of your misery. The peak power capability of coax is not set by the ohmic losses but by the breakdown voltage, that is where the ~30 ohms for best max power handling comes from. Which has absolutely nothing to do with what we were discussing. And breakdown voltage of the coax is dependent on the dielectric only (material and thickness). While these will affect impedance, saying 30 ohm impedance is the "best" impedance is not true. The max *average* power capability is governed by heating of the cable due to the losses per until length (both resistive and reactive). The losses being proportional not the the cross sectional area of the conductors but to *surface* area (due to skin depth considerations). Which is directly related to the square of the current through the coax. The lower the impedance of the coax, the higher the current for the same power. Now whilst it is true that 75ohm cable will have marginally lower loss than 50 ohm cable, this is only true when the inner conductor diameter is kept constant and the 75 ohm cable hase a larger shield diameter. For cables with the same outside diameter the losses for 75ohm are greater due to the smaller diameter (and hence lower surface area) of the inner. So in practical terms for the same size cable 50 ohm has lower loss, greater average power capability, and greater peak power capability. If you don't believe me these figures can be easily verified by looking at the specs of 50 & 75 ohm cable of the same shield diameter. Jeff But as I said - you're the man. You are able to interpret this chart and contradict every physics textbook, engineer and professor in the world. You are the expert! NOT! -- ================== Remove the "x" from my email address Jerry Stuckle ================== |
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