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 Saturday night technical puzzle 
Silent Key

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Post Saturday night technical puzzle
I just read an old posting on another forum encouraging folk not to be too concerned about VSWR. The poster quoted that a 2:1 VSWR only amounts to 10% "lost power".

So here's a Saturday night technical puzzle on the same topic - no prizes for the correct answer!

We connect a 100W transmitter with an output impedance of 50 ohms to a 50 ohm dummy load through an exact half-wavelength of loss-less 600 ohm ladder line. There's a high VSWR on the ladderline of 12:1 (600/50). This means that at the junction of ladderline and the dummy load about 71.5% of the forward power is being reflected.

Here's the question: How much power reaches the dummy load; and if it's not 100W, where does the "lost" power go?

Answers on a postcard please ......

Steve G3TXQ

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Sat Nov 14, 2009 5:08 pm
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 Saturday night technical puzzle 
Silent Key

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Post Re: Saturday night technical puzzle
Was that too tough? I'll make it "multiple choice":

1) 28.5W goes into the dummy load and the remaining 71.5W gets reflected and dissipated in the ladderline
2) 28.5W goes into the dummy load and the remaining 71.5W gets dissipated back in the transmitter
3) All 100W goes into the dummy load.

Which is true?

Steve G3TXQ

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Sat Nov 14, 2009 8:11 pm
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 Saturday night technical puzzle 
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Post Re: Saturday night technical puzzle
3, 50 ohm load ,halfwavelength of ladderline to 50 ohm input

the halfwave ladderline will repeat the z of load at its far end ( radio) regardless of characteristic z.. the dummy oad will absorb all power to it

do i win ?

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Sat Nov 14, 2009 8:20 pm
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 Saturday night technical puzzle 
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Post Re: Saturday night technical puzzle
if not im having another beer

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Sat Nov 14, 2009 8:26 pm
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 Saturday night technical puzzle 
Silent Key

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Post Re: Saturday night technical puzzle
m0jha wrote:
3, 50 ohm load ,halfwavelength of ladderline to 50 ohm input

the halfwave ladderline will repeat the z of load at its far end ( radio) regardless of characteristic z.. the dummy oad will absorb all power to it

do i win ?


Billy,

Congratulations - you win!

1) Can't be true because I said it was a loss-less line
2) Would be silly - the transmitter can't absorb 71.5W ..... well, not for long, anyway :)
3) Is correct - the transmitter "sees" a 50 ohm load - in fact it doesn't know there's any ladderline involved - so it delivers 100W into the ladderline. And because the ladderline is loss-less there's only one place the 100W can go .... into the dummy load.

Now, for extra points, here's a slightly harder Part 2:

If we were to measure the Forward and Reflected power at some point in the ladderline, what would the numbers be?

Steve G3TXQ

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"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;" (Lord Kelvin 1883)


Sat Nov 14, 2009 8:38 pm
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 Saturday night technical puzzle 
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Post Re: Saturday night technical puzzle
now if were talking maths im out on this one .. i know iv had 4 pints of real apple scrumpy and as im not normally a drinker im ready for my bed.. so i bid you goodnight ..

i have an idea but im not prepared to post in fear of making myself look a real plonker..

till the haze clears i bid you all 73..

billy

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Sat Nov 14, 2009 9:20 pm
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 Saturday night technical puzzle 
Silent Key

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Post Re: Saturday night technical puzzle
G3TXQ wrote:
Now, for extra points, here's a slightly harder Part 2:
If we were to measure the Forward and Reflected power at some point in the ladderline, what would the numbers be?


* We know that at the ladderline/load junction 71.5% of the forward power is being reflected; that means that 28.5% must be flowing into the load.
* We know that the power going into the load is 100W; therefore the forward power must be 100/0.285=351W
* If the forward power is 351W, and 100W is flowing into the load, the reflected power must be 251W

So the correct answer is: Pf=351W and Pr=251W

To summarise:
1) At the ladderline/load junction the forward power in the ladderline is 351W, 251W of it is being reflected and 100W is flowing into the load.
2) At the Tx/ladderline junction the 251W of reflected power is totally re-reflected and adds to the 100W from the transmitter to produce 351W in the forward direction.

It surprises some folk that with a transmitter providing only 100W there can be 351W forward power in the line, but it's a fact! If you have a directional wattmeter monitoring a line with a non-unity VSWR, when switched to Forward it will indicate more power than the Tx is delivering.

Just to tie up all the numbers:

* In the 600 ohm ladderline the 351W forward wave must be a voltage of SQRT(351*600)=459v
* In the 600 ohm ladderline the 251W reflected wave must be a voltage of SQRT(251*600)=388v
* Where the forward and reflected waves add (mid point of the line) we'll get a voltage of 459+388=847v
* Where the forward and reflected waves subtract (either end of the line) we'll get a voltage of 459-388=71v
* So the VSWR must be Vmax/Vmin=847/71=12 which is the VSWR we set out with - QED!

Why have I bothered with all this? Simply to enforce the point that the reflected power in the line is not "lost power"; it gets re-reflected and eventually finds its way to the load.

For anyone who's trying to follow the numbers, summary below:

Image

73,
Steve G3TXQ (now gone to lie down in a darkened room)

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"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;" (Lord Kelvin 1883)


Sun Nov 15, 2009 9:35 am
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 Saturday night technical puzzle 
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Post Re: Saturday night technical puzzle
good stuff steve, i know reflected power gets "bounced" back and forth along the line but i didnt know re the accumilated tx power when metering..

i wouldnt of got that bit anyway . i knew there was maths and volts but thats me confused then

god that scrumpy was strong... i know why its not a regular thing now :lol:

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Voice is for CBers, amateur radio operators, the average citizen, and the military. In other words, voice is for everyone with a mouth. CW is for those who choose this newer mode of communication. Newer? Why yes. Voice has been around for a million years.


Sun Nov 15, 2009 10:05 am
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 Saturday night technical puzzle 
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Post Re: Saturday night technical puzzle
my brain hurts !! :mrgreen:


Sun Nov 15, 2009 7:10 pm
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 Saturday night technical puzzle 
Silent Key

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Post Re: Technical puzzle?
G7IGB wrote:
Steve, perhaps you could pose a 'technical teaser' for members on a regular basis? :)


Rob,

I'd be happy to, provided it doesn't bore folk to death! I note only Billy responded to this one, but perhaps others were reading with interest?

As you probably gathered, any expertise I might have tends to be in the HF range, and with antennas/transmission lines/tuners etc

I'll pose another one in a few days :)

73,
Steve

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"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;" (Lord Kelvin 1883)


Sun Nov 15, 2009 7:13 pm
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 Saturday night technical puzzle 
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Post Re: Saturday night technical puzzle
I was reading with interest and i think its a great idea, it will certainly help a lot of people grasp some of the technical side of the hobby.

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Sun Nov 15, 2009 7:31 pm
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 Saturday night technical puzzle 
Silent Key

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Post Re: Saturday night technical puzzle
g0jky wrote:
my brain hurts !! :mrgreen:


Fortunately no-one asked me the really difficult question about this example - a question which requires some understanding of wave mechanics to answer; that really does make the brain hurt :)

But I'll leave you all in suspense ......

Steve G3TXQ

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"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;" (Lord Kelvin 1883)


Sun Nov 15, 2009 7:43 pm
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 Saturday night technical puzzle 
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Post Re: Saturday night technical puzzle
Hi all
i have been reading this thread and think its smart
steve plz carry on the info u give..
i for one am one of the numb nuts who knows nothing
but i do sit here and try to work things out
carry on steve

ian
g7pun

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Sun Nov 15, 2009 8:52 pm
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 Saturday night technical puzzle 
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Post Re: Saturday night technical puzzle
Steve,

I was also in the background watching with interest , Maybe we should all respond saying i dont know just to so you know there is interest :mrgreen:

I always follow your input with interest which ever forum i find you on :lol:

Sam


Sun Nov 15, 2009 9:08 pm
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 Saturday night technical puzzle 
Silent Key

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Post Re: Saturday night technical puzzle
OK folks, there seems to be some interest. I'll start a new thread for each "Technical Puzzle" to keep things tidy. I'll do one more this evening - answer tomorrow - and then probably wait until next weekend before doing another one.

I'm very happy with "I don't know" postings, or questions if there's something I haven't made clear. Don't be nervous about asking a question - there are probably others out there who would also like to know the answer but daren't ask. I'm also happy to hear that I'm talking rubbish if I make technical slip-ups - I'm here to learn, too :)

I'll try to concentrate on puzzles which address issues that I see as "Ham myths"!

73,
Steve G3TXQ

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"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;" (Lord Kelvin 1883)


Sun Nov 15, 2009 9:35 pm
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