[Elecraft] Measuring SWR (Long)

Wed Apr 10 16:16:48 EDT 2019

I believe I run the highest SWR of anyone here. It is close to 100:1 in 
normal operation according to EZNEC (I have never tried to measure it).

The line is open-wire line made with no. 12 (2mm) wire, only about 10m 
long. The highest SWR is on 40 meters, and I calculate the loss as 
around 1 dB. I calculated the peak voltage at around 7 kV. You know how 
the specs on vacuum relays have to be derated for RF? Believe them.

After using various more or less complicated matching arrangements, I 
settled on an old Johnson KW Matchbox. It does the job on all bands from 
40 through 10m, with just a little external help on 30m. The antenna is 
a simple rotary dipole whose total length is 10m.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
On 10/04/2019 20:23, Fred Jensen wrote:
> 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
>>>
> 
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