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## [TenTec] Too Low An SWR Can Kill You!

 To: [TenTec] Too Low An SWR Can Kill You! utahfolk@konnections.com (utahfolk@konnections.com) Wed, 24 Sep 1997 14:04:45 -0700
 ```Leading line stolen from Walt Maxwell ..... What AA4LR just presented is truth, what Wal Maxwell has written is the same truth. This subject is the most misunderstood in all of hamateur radio. Let me be foolish enough to try to explain it perhaps more clearly. A load (antenna) at the end of a feedline of characteristic impedance Zo has an impedance of Za = Ra + jX. We will assume that X is not zero, that is, the antenna is not resonant. Resonance is that condition in which net reactance X = 0. Here is the theorem in network theory underwhich all this falls. I state it in as simple terms as I can. Given a source (TX) and a load (ANT) connected by a lossless transmission line of impedance Zo (no matter what value), place a four port network consisting of only lossless L and C components anywhere along the transmission line (feedline) and adjust L and C such that a conjugate match is acheived. THEN, cut that feedline anywhere and measure the impedance looking into both sections at the point of the cut. If the impedance looking into one section is R + jX then the impedance looking into the other section is R - jX, which are conjugate impedances, which when combined produce net X = 0, or RESONACE at the point .... therefore the system is resonant everywhere. Therefore, cut the line at the point where it connects to the antenna and you have R + jX of the antenna matched to R - jX of the line and hence, the feedpoint has no net reactance, the ANTENNA IS TUNED! The four port network is the antenna "tuner" we all have in the shack! Introducing losses in the feedline affect the above condition only in the matter of efficiency, which is reduced only thru power loss to heat. The antenna is still tuned to resonance! After the conjugate impedance match is acheived at the antenna feedpoint we are left with a resulting impedance that is a pure resistance, consisting of the antenna resistance and the system loss resistances. We now have the feedline terminated in a net resistance of Ra which seldom, if ever, comes out to be Zo. If Ra=Zo, then the line SWR is 1:1, otherwise NOT! The only two parameters that set line SWR are the terminating impedance (here Ra) and Zo. Nothing else plays except line loss in practical lines which only tends to improve (lower) the SWR at the input point. The feedline transforms the termination Ra into an impedance Zi = Ri + jXi at the feedline input (in the shack) and now the tuner transforms (matches) Zi into the source (TX) impedance and all the power is transferred from the rig to the line. How much ends up radiating depends on how much line loss you have and the reflection coefficent determined by Ra and Zo. If you have a lossless feedline, all the power goes into the antenna and is radiated only if the antenna is 100% efficient! Antenna efficiency is the bear to determine ... it equates to knowing the true GAIN of the antenna .... which FEW (including most mfrs!) have little real knowledge of .... efficiency is NOT 100%, count on it! If you don't have a lossless feedline, efficiency is not 100%, and practically the most power robbing element in the antenna system is always the feedline loss itself. Tuners seldom exhibit significant loss! Thus, if you have lossline line (open wire comes close!!!) then SWR is of no concern whatsoever! SWR problems didn't exist way back when, blame the dude who invented coax (introduced lossy feedlines) ... (chuckle chuckle). Walt Maxwell has clearly stated time and time again that a transmatch/tuner/ATU, whatever, TUNES and MATCHES. TUNES means tunes the antenna and MATCHES means taking the resulting Ra and matching its transformation impedance Zi at the source, i.e., in the shack for us. Walt has explained the reflection physics very very well in the ARRL Ant Handbook ... it's real world stuff ... the way things ARE! Even in practice you can consider that all coils and capacitors are lossless, their actual losses are due to associated resistances which you can lump into a loss resistance. Hence, the above theorem holds. The smaller those losses, the higher the system efficiency ... which you CAN easily compute by the way. Walt has given you all you need to know in SIMPLE calculations in the ARRL antenna handbooks. Perchance an example is worth it: Assume a feedline of 50 ohms, and ideal with NO LOSS. Let the 100 Watt TX be 50 ohms also and let the antenna impedance be 300 + j200, clearly non-resonant. Without the antenna tuner the line SWR is 8.7 and 36.9 watts is transferred to the antenna (if the antenna is lossy less than 36.9 watts is radiated!) 63.1 watts is reflected. Total efficiency (antenna eff withstanding) is therefore 36.9, I call this transfer efficiency. Oops, forgot to mention the reflection coefficent at the antenna feedpoint in this case is 0.8. Now, put a tuner in the shack and adjust it properly. The line SWR is now 6 (300/50 because X=0!), the reflection coefficient is 0.7, a slight improvement. Forward power at the transmatch is 204.2 watts, 100 watts is transferred to the antenna, 104.2 watts is reflected, a 49% transfer and 104.2 watts is returned to the shack. Mismatch loss is 0 (line loss is zero who cares about SWR!), shack SWR = 1:1 and 100 watts is delivered to the antenna! Thus an improvement of 63.1 watts if no tuner was used! The important point to note here and in all other cases, with a tuner, is that the reflected power arriving back at the tuner is reflected IN PHASE with the transmitter output thus increasing the forward power by that amount. This power DOES NOT go back into the rig and "BURN UP DEM FINALS!" .... wisdumb on the airwaves! Now, same two cases except we put 3 db loss in the feedline. First case no tuner. Line SWR is still 8.7, reflection coeff is 0.8. 50.1 watts leaks out at the antenna end of the feedline, 49.1 watts heat up the coax (totally lost to heat) Efficienty of transfer is still 36.9 percent and 18.5 watts goes into the antenna, 31.6 is reflected back to the shack. Only 15.8 arrives at the shack (feedline loss). SWR of 8.7 at the antenna now reads only 2.3 in the shack and the total efficiency is a mere 18.5 watts out into the antenna. Now put the tuner in the shack and adjust it correctly. Line SWR is still 6.0, reflection coefficient is still 0.7. Forward power 114.7 watts, 57.5 watts arrives at the end of the feedline and 28.2 watts goes into the antenna, the transfer efficiency is still 49%. 29.3 watts is reflected back to the shack and only 14.7 watts arrive. Shack SWR 1:1, 100 watts is delivered into the system by the transmitter (this is what those of grate wisdumb call "fooling the transmitter"!) for a total efficiency of 28.2 percent. Loss due to mismatch at the feedpoint amounts to only 7.3 watts. Thus, if you were to replace the antenna with one whose net resistance after tuning to resonance was 50 ohms (matching the feedline) you would only gain 7.3 watts output. So the 300 ohm antenna match (SWR = 6) is only 80 percent as "efficient" as the perfect Ra=50 match. That amounts to a difference of a mere 1 db! An S-unit is 6 db! Like Walt said : Too low an SWR and KILL YOU!.... meaning we chase SWR and "resonance" with such zeal that if you climb the tree enuff times trimming the antenna for that last ounce of ummmp, you'll eventually fall out of the tree and kill yourself ... for NOTHING! :) You wanna make a difference in your signal, use the lowest loss feedline you can afford ... open wire reigns!!!! Perchance this long speel is of some value .... de Dave W0MHS (utahfolk) P.S. Why not bring up the debate on whether the antenna tuner needs to be put at the antenna feedpoint and that all feedlines have to be half wavelength multiples .... that'll keep things going for another hunnert years ... BTW I'll be found on the low end contesting while the debate rages .... -- FAQ on WWW: http://www.contesting.com/tentecfaq.htm Submissions: tentec@contesting.com Administrative requests: tentec-REQUEST@contesting.com Problems: owner-tentec@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
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