Re: Does a low SWR mean your antenna is a great performer?


WK9M
 

Yes I think that a presentation would be a great idea--TLARC is looking for new presentations.  I myself put in memory the 4-11-25% rule.  i.e. 1.5:1 is 4% loss, 2:1 is 11% loss, and 3:1 is 25% loss.  I found a document online that confirms my memory is correct:
http://www.packetradio.com/pdfzips/SWRvsPowerNwatts.pdf

And as you point out, 2:1 is a .5 dB loss.  Even 3:1 is only a 1/5 of an S-unit (1.25dB loss); I'd say that a computer can hear that difference but probably not a human.

Many modern radios will start cutting power back at even moderate VSWR levels.  So as you say, watch the power out.  Both of my Kenwood HF radios are picky on 6m for example.

VSWR can have a bad effect on surge arrestors if you happen to buy a low power one and operate it with high VSWR.  (3:1 is literally 3x the voltage).  But I doubt any of us would do that, at least on purpose.  I went over that in my surge presentation about a year ago.

'73,
Randy

On 9/21/2020 12:48 PM, John Haskell via groups.io wrote:
What George quoted from ham.stackexchange is correct.

To take it a bit further, SWR can be pretty much ignored.  If your solid state transmitter functions with normal output power versus shutting down, then SWR really does not matter much for most installations.  This is particularly true at HF where cable loss is minimal.  At UHF and with a long transmission line SWR can cause unacceptable losses.  

Here are a couple of examples of SWR versus loss using inexpensive RG-58.

Assume 7MHz and 75 feet of RG-58.  The cable loss is .9dB with a SWR of 1.

But what if the SWR is 2:1?  The extra loss due to this SWR is .2dB, e.g., negligible.


For a mobile set up on 442.1MHz..   Many people go nuts with low loss cables.  Not me. I use RG-58 in the car.  Here is why.  For 442MHz, 10feet of cable, and a 2:1 SWR, the additional loss due to the 2:1 SWR versus a perfect match is .2dB.  I do not believe you will notice that extra loss.


I have been considering putting a transmission line/SWR presentation together for TLARC in hopes of dispelling some of the common myths.  If there is interest let me know.  Maybe that will get me off my butt.

73, K1AT

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