Looking at charts of cable loss due to SWR I note that any swr less than 8
or 9 to 1 results in hardly more than 1 dB of loss due to SWR using even
1/4" coax. So, really, why the FUD on this subject?

Lee

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While I agree that many hams pay far too much attention to VSWR, there are
several other issues beyond loss in the transmission line.

Modern "barefoot" rigs, especially those without tuners, will throttle back
their output power (to minimize heating finals) when they see excess SWR.

Many amplifiers, designed for nominal 50-ohm loads, will not operate
efficiently into severely mismatched antenna systems.

We could go on and on.

So it all depends on what you consider "a little" SWR. I'm not concerned
with 2:1 or even 3:1 in some cases, but above that I break out the antenna
analyzer.

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"Rockinghorse Winner" wrote in message
...

Looking at charts of cable loss due to SWR I note that any swr less than 8
or 9 to 1 results in hardly more than 1 dB of loss due to SWR using even
1/4" coax. So, really, why the FUD on this subject?





--- news://freenews.netfront.net/ - complaints: ---

On 9/17/2010 10:59 PM, Rockinghorse Winner wrote:
Looking at charts of cable loss due to SWR I note that any swr less than 8
or 9 to 1 results in hardly more than 1 dB of loss due to SWR using even
1/4" coax. So, really, why the FUD on this subject?

Lee


With rigs of old.. About the worst a HIGH SWR would do is arc over the
tank tuning capicator.

But with modern Solid State rigs you run them full power into a high swr
and it's off to the land of the soldering gun for a brand new set of finals.

High SWR means excessive voltage on the finals...

I had a VHF rig once where the T/R relay failed, THE SWR don't get much
worse than that.. Took me quite a while to clean the carbon off the
capacitor plates... But the tube survived.


Thankfully my TS-2000 starts with a low power for just long enough to
verify good match then it kicks up.. Bad match it beeps at me ... .==
..=. and remains in RECEIVE





--
Nothing adds Excitement like something that is none of your business.

I suppose you are right, SWR doesn't matter that much in the grand
scheme of things with short coax runs and a "tuner" near the rig.

As others have pointed out, SWR contributes to line loss and on long
runs of coax at high frequency this can be quite a bit. 3 dB takes
your 100W down to 50W delivered to the antenna but even worse it takes
half the signal level on the receive side too. When you get up into
the VHF and UHF world, it's very easy to get more than 3dB loss, even
on really good coax (LMR400 is just under 3 dB/100'). Higher SWR will
just make that 3 dB even worse. Then if you add that most solid state
rigs will "fold back" and reduce power output if faced with SWR's
above 3 or less, it can be quite disappointing to figure out that your
100W rig only delivers a few watts to the antenna. SWR is an good
thing to watch.

"Good" antennas usually have a lower SWR when properly "tuned" but
that doesn't mean that a lower SWR makes a better antenna than
another. The issue is how much energy does the antenna radiate in the
desired direction which may include SWR as a factor, but it doesn't
give you the whole picture. A 10 Meter beam 40' up that has a 2.1 SWR
and 12 dB gain would be a much better long distance performer than a
loaded dipole 2' off the ground that showed 1.8 to 1. There are a
number of common antennas that don't show low SWR when in operation,
yet they work just fine. Usually they are fed with ladder line and
not coax to avoid the losses and requre a tuner to couple your
transmitter to the feed line.

However, all that aside, SWR "changes" can be a good thing to watch.
Keeping a record of SWR at specific frequencies can be helpful in
alerting you that something is wrong or something has changed. If all
of a sudden your SWR jumps up and stays high, it might be time to take
a look at things and find out why.

-= bob =-

On Sep 17, 9:59 pm, (Rockinghorse Winner) wrote:
Looking at charts of cable loss due to SWR I note that any swr less than
8
or 9 to 1 results in hardly more than 1 dB of loss due to SWR using even
1/4" coax. So, really, why the FUD on this subject?

Lee

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2.6.32.14-127 Fedora 12
In rotation:http://www.jamendo.com 2.6.31.12-0.2 OpenSU
SE 11.2
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24-16 Mint Elyssa

On 9/17/2010 10:59 PM, Rockinghorse Winner wrote:

Looking at charts of cable loss due to SWR I note that any swr less than 8
or 9 to 1 results in hardly more than 1 dB of loss due to SWR using even
1/4" coax. So, really, why the FUD on this subject?

"FUD" isn't what I would choose to call the things other hams say about
SWR, since _that_ term refers to deliberate deception in pursuit of
commercial profit, and is usually applied to the marketing tactics used
by a well known software company in order to dissuade corporate
purchasing managers from buying competing products.

Let's use "folklore". Most SWR folklore comes from people who have
learned the hard way that SWR can bite: either from finding a hole
punched in their coaxial cable, or watching a pair of finals clearing
the trees on their way to electronic heaven.

There's nothing "wrong" with a high SWR when the system is designed to
account for the high voltages that accompany it. However, hams who have
seen what high SWR can do to consumer-grade components and cables have a
right to be leery, and their advice to others is simply a reflection
(pun intended) of their past losses.

It's not about signal loss: it's about money and time lost.

73,

Bill, W1AC
(Filter QRM for direct replies)

Thanks for the replies, guys it was a real education.

I have an indoor dipole cut for 20M that is 4:1 SWR at it's most resonant
part of the band. If I drop one leg to the ground, the swr drops to near
1:1, but the rf is everywhere, including the lcd display of the rig.

I run it with the high swr and a tuner, but the indoor location and hilly
country are not kind. Even so, I do get a weak signal onto the band.

Are the dangers to the coax cable independent of how well you match the
antenna/feedline to the rig?


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On 9/19/2010 8:28 AM, Rockinghorse Winner wrote:
Thanks for the replies, guys it was a real education.

I have an indoor dipole cut for 20M that is 4:1 SWR at it's most resonant
part of the band. If I drop one leg to the ground, the swr drops to near
1:1, but the rf is everywhere, including the lcd display of the rig.

I run it with the high swr and a tuner, but the indoor location and hilly
country are not kind. Even so, I do get a weak signal onto the band.

Are the dangers to the coax cable independent of how well you match the
antenna/feedline to the rig?



It has been a while since I did voltage calcualtions but a lot depends
on your cable.. You use coax designed for 2000 or 5000 watts, and feed a
4:1 antenna with a 100 watt transmitter.. NO PROBLEM coax wise.

You use coax with a 250 watt power rating.. You gonna have problems with
a 4:1 on a 100 watt transmitter. Just what the cut off is.. I don't
know.


--
Nothing adds Excitement like something that is none of your business.

"Rockinghorse Winner" wrote

I have an indoor dipole cut for 20M that is 4:1 SWR at it's most resonant
part of the band. If I drop one leg to the ground, the swr drops to near
1:1, but the rf is everywhere, including the lcd display of the rig.

I run it with the high swr and a tuner, but the indoor location and hilly
country are not kind. Even so, I do get a weak signal onto the band.

Are the dangers to the coax cable independent of how well you match the
antenna/feedline to the rig?

Assuming you're running no more than 100w and your coax is RG58 or larger,
there is no danger to the coax at 4:1. (Actually, you shouldn't be running
even 100w when using an indoor antenna.) I would try a different
orientation for the antenna and get it further away from nearby objects. To
me, the fact that you've got no better than 4:1 in its, um, "current"
configuration, tells me something's wrong with either its placement, its
dimensions, or its construction. Is the coax hanging straight down from the
feedpoint? If not, that could be part of your problem.

Howard N7SO