[Amps] Input SWR revisited

[Roger D Johnson]

I think we're getting a couple of things confused here. The SWR on a 
transmission line and the
impedance along the line.

In Don's example, we have a 100 Ohm load fed thru a piece of 75 Ohm line. The 
SWR on the line
is determined by the load impedance and the line impedance. In this case 100/75= 
1.33. This
does NOT change with changes in the length of the line. However, the impedance 
looking into the
feed end of the line DOES change. If you use a Smith chart and draw a 2 to1 SWR 
circle, adding
transmission line is equivalent to moving around the circle. The SWR remains the 
same but the
impedance is different at every point. As you move around the circle, there are 
two points where the
impedance is resistive. The first is when the electrical length of the line is 
90 deg (1/4 wave) and the
other is when the line is 180 deg (1/2 wave). The first case is the well known 
quarter wave transformer.
In Don's example the impedance will be about 56 Ohms at the line input...a 
pretty good match to
50 Ohms. Since this magic only occurs when the line is electrically 90 deg (or 
odd multiples) it's
essentially a single band solution.

In the "old" days of linears with untuned inputs and drivers with Pi networks, 
there was often a magic
length of line that allowed them to work together on all bands. With solid state 
drivers, it's necessary
to have a reasonably good impedance match looking into the amplifier input.

73, Roger



On 1/14/2015 3:10 PM, donroden at hiwaay.net wrote:
> Explain the case where a 100 ohm load ( 2:1 ) can be a 1:1 swr at the 
> transmitter with a length of 75 ohm cable.  I've changed the cable length and 
> improved the SWR.
>
>
>
> Don W4DNR
>
>
>
> Quoting Gary Schafer <garyschafer at largeriver.net>:
>
>> It means that-- if you are looking at the SWR at your radio and there is a
>> tuner after the SWR bridge, any changes in cable length after the tuner will
>> show a change in SWR at the radio if you do not touch the tuners tuning.
>> This assumes that the load on the end of the cable is not 1:1.
>>
>> 2nd, the SWR will always remain the same at the radio no matter what length
>> of cable is used (except or cable loss) or what the load impedance is as
>> long as that load impedance remains the same.
>>
>> 73
>> Gary  K4FMX
>>
>>> -----Original Message-----
>>> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of
>>> donroden at hiwaay.net
>>> Sent: Tuesday, January 13, 2015 2:01 PM
>>> To: amps at contesting.com
>>> Subject: Re: [Amps] Input SWR revisited
>>>
>>> Clear as Mud .........
>>> What does it all mean ???
>>>
>>> Don W4DNR
>>> _______________________________________________
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>>> Amps at contesting.com
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>>
>>
>
>
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