[Elecraft] Measuring SWR (Long)

[Fred Jensen]

Or 10:1 or even 20:1.  However ... there's always a "however" or a "but" 
... it doesn't matter.  The dielectric constant of air [the insulator 
between open wire conductors] is very close to 1 whereas it is much 
larger for various forms of coax.  Very high SWR creates very high 
voltages along the line.  The energy storage with high dielectric 
constants is much higher than with air, and dielectric losses go up 
dramatically.  In prehistoric times when we used vacuum tubes and 
resonant tank circuits, we coupled power to the line with a 2-3 turn 
link coil adjacent or in the middle of the resonant tank.

If the line exhibited reactance, it detuned the tank which we just 
retuned to resonance ["Dip the plate, increase the coupling, repeat"].  
The PA tank circuit became a thoroughly mis-named "antenna tuner."  It 
didn't matter what the SWR was on the open wire transmission line, and 
in fact no one paid any attention to it.  Then flexible coaxial cable 
was invented.  It was much more convenient than open wire line, however 
in reasonable physical sizes, it had very low characteristic [surge] 
impedances ... 50 and 75 ohm impedances were the result and now, SWR 
mattered.  The dielectric constant inside the cable was very much 
higher, and the higher voltages from high SWR resulted in much higher 
dielectric losses.

Then, someone invented the Pi-network [for all of you with fingers 
poised over the keyboard to pounce on my description of history, relax 
and breath deeply.  I'm making some of the non-technical stuff up to 
help hold your attention].  The Pi-network would transform the 50 or 75 
ohm impedance at the end of the coax to the several thousand ohm plate 
circuit impedance of the PA stage and power would flow to the antenna 
unimpeded.  It was at this point that sales of SWR indicators soared, 
SWR became a household abbreviation in the ham community, and an SWR of 
1.000:1 became the Nirvana of ham radio.  It has been thus ever since.

As electronics progressed [?] from the vacuous to the solid state, SWR 
took on an enhanced importance since the solid state was far less 
tolerant of overvoltage and energy dissipation than the 807's, 813's, 
and 250TH's of the previous era.  The resonant tank circuit faded, 
amplifiers now feed non-resonant filter networks and expect to see a 
50+j0 ohm load ... or else!  This made open wire transmission lines much 
more difficult to use, requiring some way to adapt a PL259 to connect 
them to the radio which is both mechanically and electrically hard, and 
sparked the incorporation of "SWR Alarms" into our radios.  It also 
spawned the "balun," a mysterious device that may be of alien origin 
since no one seems to know exactly what it does or how it does it.

I hope this helps. [:-)

73,
Fred ["Skip"] K6DGW
ex KN6DGW 1953
Sparks NV DM09dn
Washoe County

On 4/9/2019 7:21 PM, W2xj wrote:
> I agree except that even at 500KW a 2:1 or greater is the norm with open wire line.
>
> Sent from my iPhone
>
>> On Apr 9, 2019, at 19:10, Fred Jensen <k6dgw at foothill.net> wrote:
>>
>> Well, the reflected power is created by a "virtual transmitter" at the feedpoint of the antenna and heads down the coax which has a surge impedance of Z0 ohms.  It meets the SO-239 at the TX and sees an impedance of Z1, the impedance presented by the PA and output filters.  If Z1=Z0, the power is dissipated as heat in the PA and associated RF circuitry.  If Z1<>Z0, some is dissipated and some is reflected, where some is radiated, and some is reflected [virtual transmitter again].  Ad infinitum, and when forever is over, it is all gone and everyone lives happily ever after.
>>
>> SWR and all the associated measuring equipment and concern with it only became an issue when: 1) Coax replaced parallel lines and; 2) Resonant output circuits were replaced by solid state amplifiers with non-resonant filters.  When I sat for the Extra in early 1956, the only question that involved standing waves was one about how Lecher Lines could be used to measure transmitter frequency.
>>
>> It's important today but calculating it hasn't changed.  At 10 W, a 2:1 SWR will probably work ok.  At 1500 W, a solid state amplifier may not be happy with the voltages developed at its output connector.
>>
>> 73,
>> Fred ["Skip"] K6DGW
>> Sparks NV DM09dn
>> Washoe County
>>