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Author Topic: Help me remember. . .  (Read 941 times)
AA4PB
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Posts: 12679




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« Reply #15 on: September 19, 2008, 03:27:35 PM »

Actaully the SWR as measured at the transmitter DOES change with length due to loss in the feed line. Lets say I have a line with 3dB loss and a 100W transmitter. I insert a reflected power meter at the antenna and measure 50W forward and 25W reflected. Next I move the meter to the transmitter and I read 100W forward and 12.5W reflected. That's because at the antenna the coax loss is reducing the forward power but not the reflected power. At the transmitter end it is reducing the reflected power but not the forward power.

System losses can mask a poor SWR so if you have a lot of cable loss you need to measure the SWR at the antenna in order to get a true reading. Of course if the antenna matches the coax then the reflected power is zero no matter where you measure it. The forward power will change but the reflected power can only be zero.
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N4NOO
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Posts: 106




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« Reply #16 on: September 19, 2008, 03:48:37 PM »

That's what I remember as well.
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VK1OD
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Posts: 1697




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« Reply #17 on: September 19, 2008, 04:07:00 PM »


There is obviously a gap between what some people 'learned' and what they understood then or understand now.

When you understand concepts, the understanding is more likely to stick with you than 'learned' snippets.

Much of the mythyology of ham radio would be dissipated if learners asked "why?" of their teachers. If the teacher cannot explain it to the learner's satisfaction, they should regard the information as suspect until explained adequately.

Elmer responses are often directives without explanation.

It is not lost on me that the new way of qualifying hams is to get to learn responses to a set of exam questions.

So whilst various people have offered support for the quarter wave ROT of the OP, can anyone explain why it would work? Not to construct cases where a quarter wave might work better, but to explain why a multiple of quarter waves is NECESSARY for maximum power transfer, and why maximum power transfer occurs WHENEVER you use a multiple of quarter waves.

There is no such explanation.


Owen
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N4CR
Member

Posts: 1652




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« Reply #18 on: September 19, 2008, 10:21:13 PM »

I'm probably partly to blame for this. A couple of weeks back, someone asked how long should shack jumpers be. I answered that I always make my jumpers 1/4 wavelength long.

Of course, the more astute here figured out that every length is a 1/4 wavelength of some frequency. So, I take it back. I didn't mean to confuse anyone. From now on, all of my shack jumpers are a FULL wavelength long.

Anyway, if you read this far in the thread you can blame it on me.

Phil, Resident Scoundrel and Chief Confuser.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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