SWR considerations.

Just getting back into Radio since 1980s, and like everyone else antenna and swr is a subject that can baffle! spent many an hour reading on the subject from best swr number to matchers / tuners ect and the different views people have. Came across this yesterday and though I am no tech and bad at math, I could understand the concept being discussed. thought I would post and see others views. Interesting article.
http://hamuniverse.com/wc7iswr.html

This article is mostly correct.  There are a lot of misunderstandings out there of what SWR is and isn't.  Before coax existed it was seen as an oddity that had no real effect on how well the antenna worked,  Now many people see SWR as the end all and be all of an antenna system...

I call it the coaxification of antenna knowledge, or ideas that have come about since we started using coax on a wide (almost universal) scale.  A lot of other things that people think they know also fall into this category...


The DB

With these modern radios it's important for them to see a decent-ish match otherwise the protection circuits foldback the output power. With my old valve amps and early transceivers it really wasn't a huge problem.

SWR is meaningless as a measure of performance. One of the best antennas Ive had was an inverted L which I used multiband and modelling showed the SWR on some bands as high as 80:1. As it was fed directly at the feedpoint with a SGC230 antenna coupler it didn't matter.

Northern Crusader wrote:

SWR is meaningless as a measure of performance. One of the best antennas Ive had was an inverted L which I used multiband and modelling showed the SWR on some bands as high as 80:1. As it was fed directly at the feedpoint with a SGC230 antenna coupler it didn't matter.

And so your transmitter saw a 'good match' and did not foldback power.

Great subject material there Phil - PJD1 and some great input from knowledgeable members here. 


There's a lot of myths, pet theories and current accepted understanding surrounding antenna systems and radio so it can all be difficult to wade through. Just when I thought I was getting to grips with it all I discovered 'common mode current' which threw a right spanner in the works.

SWR definitely isn't the 'be all and end all' to antenna systems and as mentioned above rotates around coax use age. If you've ever used open or ladder line feeders you'd soon realise as such as well as if you'd utilised valve tuning systems of old.

If you have a field strength meter as well as SWR meter you'd quickly find out that a 'perfect' or low SWR doesn't equate to the highest reading on the field strength. Conversely a higher field strength meter reading usually results in a higher SWR reading! 
(Worth experimenting tuning an antenna to see these results and once upon a time even CB users realised this when setting up for mobile operations.)

You'll find similar difficulties such as antennas having to be 'resonant' and yet some of the best antennas, (and commonly used), usually aren't resonant on the frequencies they'll be used for. A typical 5/8th wave or even the Extended Double Zepp are such examples. (My homebrew 2m vertical is actually resonant on 6m!)


It can become quite difficult understanding all the concepts but is usually part of the fun for some when it comes to antennas. I'm still learning and enjoying it thoroughly as well as reaping the benefits of homemade antennas.

For those not wishing to venture further into the quagmire we use what is available to us which normally means a manufactured antenna and keeping the SWR reading low. Usually this is just to keep our semiconductor radios happy so if we get a 'reading' less than two we're good to go and three or more the radio as mentioned will drop it's output.

There's the proverbial 'quest for the holy grail' of the perfect antenna system and some of just simply want to get on air. 
I still firmly believe any antenna is better than no antenna....but I've had a lot of fun finding out more.


Great posting and well done to everyone for adding their own expertise and understanding. Awesome stuff.  

All the best,
Victor

TrunkY wrote:

And so your transmitter saw a 'good match' and did not foldback power.

That's only part of it though.  Whilst you obviously want to deliver all your power to the load (antenna), you also want the antenna to radiate that power effectively.

SWR tells you nothing of the antenna system's effectiveness.  Remember, a dummy load has a perfect SWR, but isn't a great antenna.

43CT016 wrote:

TrunkY wrote:

And so your transmitter saw a 'good match' and did not foldback power.

That's only part of it though.  Whilst you obviously want to deliver all your power to the load (antenna), you also want the antenna to radiate that power effectively.

SWR tells you nothing of the antenna system's effectiveness.  Remember, a dummy load has a perfect SWR, but isn't a great antenna.

I totally agree, just saying that it is important to some transmitter finals to see a low SWR.

When I ran a 3CX1500A7 amp on 144MHz I was relatively unworried about the SWR (although as I was using a commercial beam I knew it would be in the right ballpark)  and more about whether the antenna could take the power