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Author Topic: SWR vs reflected power at QRP levels.  (Read 6983 times)
KA0HVE
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Posts: 117




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« on: July 21, 2013, 05:51:37 AM »

At QRP levels is SWR really that much of a concern?

If I have a power meter that measures forward and reflected powers need I concern myself with a ratio or should I just try to minimize the reflected power back into my rig and make sure it doesn't exceed a threshold I've chosen?

I want antenna system efficiency and that's more what SWR represents but an SWR of 1.5:1 means totally different reflected powers when transmitting 5 or fewer watts vs transmitting 100 watts.
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AA4PB
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« Reply #1 on: July 21, 2013, 06:06:57 AM »

Regardless of the absolute power level, SWR is only important in the following two regards:
1) Can the transmitter match the resulting load impedance at its output so that it delivers full power into the load?
2) Is the increased feed line loss caused by the SWR unacceptable as far as total antenna system efficiency?

The absolute level of the reflected power doesn't matter, normally. It doesn't matter much whether you are reflecting 10% of 100W or 10% of 5W - it results in the same signal change on the other end. The only concern is if the power level is so high that you exceed the voltage rating of the feedline, causing it to arc over.
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KA0HVE
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« Reply #2 on: July 22, 2013, 12:11:51 PM »

Regardless of the absolute power level, SWR is only important in the following two regards:
1) Can the transmitter match the resulting load impedance at its output so that it delivers full power into the load?
2) Is the increased feed line loss caused by the SWR unacceptable as far as total antenna system efficiency?

The absolute level of the reflected power doesn't matter, normally. It doesn't matter much whether you are reflecting 10% of 100W or 10% of 5W - it results in the same signal change on the other end. The only concern is if the power level is so high that you exceed the voltage rating of the feedline, causing it to arc over.


I hear where you're coming from but maybe I need to clarify where I'm coming from.  As far as the health of my transceiver is concerned, if my transceiver is putting out 100 watts and 10 watts are reflected back that's much worse than putting out 5 watts and having 1/2 watt reflected back.  Same SWR but ... First I want to protect my transceiver.  Second I want to put out the most signal.

If I can keep the absolute level of reflected power below a certain threshold my transceiver should be fine although my antenna may be radiating poorly.

Now my next concern is where to establish that threshold.  What is a safe amount of power to reflect back at my transceiver?

Being only concerned about SWR seems to me to miss the mark.  My SWR when running 100 watts has to be much lower than when I'm running 5 watts to keep the reflected power below the threshold so I'm really not concerned about SWR, within reason.

Something else that may help is that I'm not only talking about QRP transcievers and power levels but also talking about portable operation with an antenna tuner so working to minimize SWR (other than the tuner) may not be much of an option.

Do the QRP indicator lights on the little tuners without a meter really indicate SWR or reflected power level?

Am I making my point or am I thinking all wrong?
« Last Edit: July 22, 2013, 12:37:00 PM by KA0HVE » Logged
WB6BYU
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Posts: 13231




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« Reply #3 on: July 22, 2013, 01:50:02 PM »

Quote from: KA0HVE

...As far as the health of my transceiver is concerned, if my transceiver is putting out 100 watts and 10 watts are reflected back that's much worse than putting out 5 watts and having 1/2 watt reflected back.  Same SWR but ... I want to protect my transceiver first and put out the most signal second.



No, it's not any different if we assume that the final in the QRP rig is designed for 5W output.
It still is subjected to the same potential over-voltage and/or over current compared to the
design values.  That is, the percentage change in voltage or current as seen by the output
device will be the same in both cases, so you have the same percentage increase in heat
dissipation, etc.

Now, if you are using a 100W rig turned down to 5W instead of a rig designed for 5W output,
then your comments start to make more sense.  But that isn't clear in your post.


In the end it comes down to the design margin of the specific design:  how close are
the final devices being run to their physical limits?  If you are running a 100W radio at 5W,
certainly it can tolerate a higher reflected power than at 100W.  But a radio designed for 5W
in a small package may already be pushing the safe power dissipation due to poor heat sinking,
and thus will be more susceptible to changes.


Quote

If I can keep the absolute level of reflected power below a certain threshold my transceiver should be fine although my antenna may be radiating poorly.

Now my next concern is where to establish that threshold.  What is a safe amount of power to reflect back at my transceiver?

Being only concerned about SWR seems to me to miss the mark.  My SWR when running 100 watts has to be much lower than when I'm running 5 watts to keep the reflected power below the threshold so I'm really not concerned about SWR, within reason.

Something else that may help is that I'm not only talking about QRP transcievers and power levels but also talking about portable operation with an antenna tuner so working to minimize SWR (other than the tuner) may not be much of an option.

Am I making my point or am I thinking all wrong?


Perhaps...

First, SWR doesn't damage transistors.  Transmitter finals may fail due to several factors,
the most common of which are:

1) overheating
2) maximum peak voltage
3) maximum peak current
4) loss of cathode emission (for tubes)

(Tubes and transistors have different responses to the latter two, as tubes can often handle
up to twice the rated plate voltage under momentary conditions, and cathode emission limits
the maximum tube current.)


The effect on the transmitter depends not really on the SWR or the amount of reflected power,
but on the load impedance that the rig is trying to deliver power to.  In fact, when we look
at it that way, we can ignore the meter reading for reflected power and just look at what happens
in the transmitter for a particular load impedance:  that is really what determines the conditions
for the final.

(As an example, I remember one of the old Johnson tube rigs was rated to match antenna
impedances from 40 to 600 ohms.  If it saw a 500 load at the end of a piece of coax it would
be perfectly happy delivering power into it, even though the SWR on that coax would be 10 : 1
and the reflected power meter would be jumping all over the place.  But a 25 ohm load, with
an SWR of only 2 : 1, was more of a problem, even though the number of reflected watts was
lower.)





Of the common causes of failure, overheating is by far the most common in my experience, other
than loss of emission in tube equipment (especially those using sweep tubes).  This can be due to
extended transmissions at high duty cycles, poor circulation of air to the heat sink, poor tuning of
the output that increases heat dissipation, misadjusted bias currents, etc.  It isn't immediately fatal,
in that heat has to build up over time until the temperature of the transistor junctions gets too hot
(though if the thermal resistance between the transistor case and the heat sink is too high, it can
happen pretty quickly.)  A small fan aimed at the heat sink on the back of the radio makes a big
difference in providing cooling:  damage may occur due to the failure of a built-in cooling fan or a
clogged dust filter.

If you run high duty-cycle modes such as AM, RTTY or SSTV (or even SSB with large amounts of
speech processing) then the output devices have to dissipate more heat.  With unprocessed SSB on
a normal voice, dissipation is less, so the rig will be tolerant of a higher SWR before overheating
becomes a problem.

Fortunately the heat sink temperature isn't difficult to monitor, even if it just means sticking
a "digital temperature sensor" on it - if you burn your finger, it clearly is too hot.  If you can
leave your finger on the heat sink it probably isn't a problem.


Transistors can be quickly destroyed by excessive voltage and current, but actual damage due
to that is not that common (except, perhaps, for transients from lightning strikes or power surges.)
Most commercial rigs use transistors that are capable of nearly double the rated output power
(at least for a very short time) if linearity is not a concern.  That would mean that there probably
is at least a 40% safety factor in normal use, and the self-protective circuitry in most commercial
rigs will kick in before that to protect the finals. 


So there isn't an exact answer to what I think your question is, because the answer really
depends on the specific design, both electrical (choice of transistors, impedance matching,
fuses, protection circuits) and thermal (heat sink area and effectiveness, thermal resistance
of connection between transistor and heat sink, provision for fans, air filters, over-temperature
sensors, etc.)  Generally for most commercial 100W solid state transmitters, at 10W output
there is virtually nothing you can do to damage the rig as you crank an antenna tuner.  The
same might be true at 25 watts output.  If your rig has a protection circuit you should be OK
with whatever it will permit.  As long as you have sufficient cooling on the finals, you might
even get by at 25W reflected power, at least for short periods of time.  All my rigs have
survived intermittent antenna connections at full drive more than once, so they really aren't
as fragile as some might make out to be.  The only times I've blown a final in a rig were
(1) an FM rig mounted under the dash in my car, right where the heater was blowing on it
in the winter (and that only when I used it for an extended period at full power); and (2) the
final of a QRP rig I was building when I turned up the power too high:  the transistor was
only rated for 150mA, and it blew somewhere under 200mA when I tried to get 2W from it.

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AA4PB
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« Reply #4 on: July 22, 2013, 02:10:56 PM »

In addition, transmitter mfgs usually rate their transmitters for maximum SWR (often 2:1). I've never seen one rated for maximum absolute reflected power. The protection circuits in most modern transceivers are triggered by high SWR rather than by some absolute amount of reflected power.
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KA0HVE
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Posts: 117




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« Reply #5 on: July 22, 2013, 02:26:54 PM »

Here's what brought up my thinking on all of this.

The OHR WM-2 power meter measure forward and reflected power up to 10 watts.  If I'm using a rig built for CW at QRP power levels and an antenna tuner do I need an SWR meter?

I was thinking that if I kept my reflected power below a given threshold as measured by the OHR WM-2 I'd be okay but it sounds like I don't really need to worry about the rig.

That leaves me with antenna efficiency, i.e., SWR, to concern myself with.

Back to my previous follow-up question; if I use one of the tuners with the SWR indicator light what is it really measuring, reflected power?
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WB6BYU
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Posts: 13231




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« Reply #6 on: July 22, 2013, 03:04:20 PM »

Quote from: KA0HVE

Here's what brought up my thinking on all of this.

The OHR WM-2 power meter measure forward and reflected power up to 10 watts.  If I'm using a rig built for CW at QRP power levels and an antenna tuner do I need an SWR meter?



So are you drawing a distinction between the OHR WM-2 directional wattmeter and an
SWR meter?  If so, that is a distinction without a difference:  the ratio between the
forward and reflected power readings gives you the SWR, so it is merely a matter of
dial calibration.

If you are adjusting the tuner for minimum reflected power, that comes close to
being equivalent to tuning for minimum SWR.  There still may be some quirks in how
the rig handles various impedances that can cause a lower reflected power at a higher
SWR because the total output power is less at that particular impedance.  But if the
forward power is relatively constant, then minimum reflected power corresponds to
minimum SWR and you don't need a separate meter calibrated in SWR.


Quote

I was thinking that if I kept my reflected power below a given threshold as measured by the OHR WM-2 I'd be okay but it sounds like I don't really need to worry about the rig.



Some rigs built for CW at QRP levels actually have less design margin than a typical
100W SSB rig.  That's because they can push the transistors harder without worrying about
linearity.  Is the final output transistor heatsinked to the chassis or an external heat sink, or
is it just sitting in the (still) air inside a small cabinet with no ventilation?

Basically it isn't good practice to drive the rig to full power into a bad load impedance, at
least not for any extended period of time.  Occasional use while you're adjusting your antenna
tuner is probably OK (since many QRP rigs lack a provision for reducing the output power.)
But it will vary a lot depending on the specific design.



Quote

That leaves me with antenna efficiency, i.e., SWR, to concern myself with.



At a high SWR the rig might not deliver full output power to the coax, which is not
the same as higher losses in the feedline due to SWR.  (And antenna efficiency
is an unrelated topic, actually, at least for those of us who consider the antenna to
be a separate entity from the feedline.)

As long as the transmitter can deliver sufficient power without overheating, you're
probably OK.  But remember that the total power delivered to the antenna is
Forward Power - Reflected Power:  with a 5W rig it is quite possible to read 20W of
forward power and 15W of reflected power, which is still just 5 watts output.
(You're more likely to measure such things when you put the wattmeter on the
output of the antenna tuner, of course.)



Quote

Back to my previous follow-up question; if I use one of the tuners with the SWR indicator light what is it really measuring, reflected power?



It should be measuring relative reflected power, though I haven't been too sure
with some of the designs I've seen.  And, because for a given output power, minimum reflected
power corresponds to minimum SWR, it also indicates relative SWR.
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KA0HVE
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Posts: 117




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« Reply #7 on: July 23, 2013, 06:38:15 AM »

Thanks everyone.
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KA0HVE
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Posts: 117




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« Reply #8 on: August 02, 2013, 04:09:46 PM »

I'm getting really tired of reading manufacturers' information on their SWR meters only to find that the reviews (eham, ARRL, et al) make them sound like junk.

Give me the make and model of some good, accurate enough SWR meters for QRP use that, hopefully, give a power reading.

I found one that sounded good only to find that it takes 5 watts to bump the SWR meter.  Not good for QRP.

Others have 30 watts or 200 watts as their lowest power ratings.

Come on, there has to be some good ones out there for QRP.  Cheesy

Thanks.
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AA4PB
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« Reply #9 on: August 02, 2013, 04:19:44 PM »

Oak Hills WM-2
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NO2A
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Posts: 779




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« Reply #10 on: August 02, 2013, 04:47:19 PM »

If your rig has a built in swr/wattmeter use it to determine the lowest swr,not your tuner`s meter. When I use my FT-857D with my manual tuner,I always go by the rig`s meter,even if I`m using my amp. I can always tell when the radio is happy,cause it runs cooler and the fan only stays on a short time.
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WB6BYU
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« Reply #11 on: August 02, 2013, 05:35:09 PM »

Quote from: KA0HVE

...Others have 30 watts or 200 watts as their lowest power ratings...



I've used the built-in dual-needle meter in my MFJ antenna tuner for QRP and it is quite
adequate.  While it does got to 30W full scale, the readings are compressed at the high
end, so the 5 to 10 watt range is about mid-scale.  I think such meters are available
as a stand-alone unit.

And if that isn't low enough, all the power levels are adjustable using the calibration pots.
So if you want to readjust it to read 10W full scale you probably can.  (The accuracy starts
to suffer at low levels - especially on reflected power - due to the finite forward bias
voltage required by the diodes.  But at the 3 - 5 watt level it seems to work well.)
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K0JEG
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« Reply #12 on: August 02, 2013, 07:32:13 PM »

Elecraft W1 wattmeter will autoscale 1.4, 14 and 140 Watts. Fairly easy to build in an evening or two.

http://www.elecraft.com/mini_module_kits/mini_modules.htm
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W1JKA
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« Reply #13 on: August 03, 2013, 02:52:09 AM »

Re: Reply #8

Check out an inexpensive MFJ 813 QRP swr/power meter, mine checks right in with A/B comps to my Bird watt meter and Drake MN7 matching network.
« Last Edit: August 03, 2013, 02:56:06 AM by W1JKA » Logged
W9WLS
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Posts: 4




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« Reply #14 on: August 04, 2013, 03:49:00 AM »

Below you will find a link to the Bird manual (.PDF) , scroll down through it and you will find both the high and low power chart's .

http://www.bird-technologies.com/~/media/Bird/Files/PDF/Products/manuals/920-43.ashx

The ratio is the same whether you are looking at HIGH or LOW power levels, the math is the same.
The big question is whether or not  your non-Bird or non-Collins (cheaper non-commercial grade) watt / swr meter is calibrated properly and / or whether or not you trust it at the power level you are dealing with.
Compare what you have to a unit of known quality and calibration, then you'll know.
 
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