[TowerTalk] SWR and voltage

[Dan Maguire]

G3TXQ wrote:

>>> Once the SWR is no longer 1:1, there is an infinite range of impedances for any particular SWR, and therefore an infinite range of voltages and currents for a dissipation of 1500W. However, here are the upper and lower limits:

>>> 3:1 SWR: 474.3V / 3.2A thru 158.1V / 9.5A

For those folks who like to have pictures to go along with words here are a couple of ways to illustrate the voltage and current (both rms) along a transmission line.  For example, at 14 MHz with a load of 150+j0 (nominal 3:1 SWR) at the far end of 50 ft of RG-213 and with 1500 W input at the station end, you can use the "Generate Data - Transmission Line at Fixed Frequency (TLFF)" feature of Zplots:

http://s26.postimg.org/hy08a7h1l/TLFF1.png

http://s26.postimg.org/5i3iggnpl/TLFF2.png

Or you can use the TLW program by N6BV:

http://s26.postimg.org/f1d79xb7t/TLFF3.png

http://s26.postimg.org/9oocvsnbd/TLFF4.png

In both cases the max voltage/current levels are slightly less than what Steve quoted due to line attenuation.  And the small difference between Zplots and TLW is because of a small difference in the parameters used for RG-213.

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Zplots is free but requires that you have some version of Excel from the last 20 years or so.  More info and download here:

http://ac6la.com/zplots1.html

Low-cost back-level versions of Excel (Office) can frequently be found on eBay:

http://www.ebay.com/bhp/microsoft-office-2003

The TLW program is bundled with _The ARRL Antenna Book_.  If you have an older book you can get an updated version of TLW (v3.24) here:

http://www.arrl.org/files/file/QST%20Binaries/June2014/TLW3.zip

Dan, AC6LA