What is my SWR?

[N4KVE]

I'm ashamed to ask this. In the past, I have always measured my SWR with ham specific SWR meters, where there were 2 swinging arms, & where they crossed was the exact SWR. Today, for the first time I am using my Bird watt meter I picked up at the Orlando Hamcation. It tells me my forward power is 30 watts, & my reflected power is 1 watt. But what is my SWR? I imagine it's pretty good, as its Comet SBB2 on a magnetic base mount just sitting on a couch. It will end up on a camera tripod which I don't have yet, but I wanted to test the SWR today. Results were the same from 447-467 MHz.

 

[n5rv]

This is handy.... Calculate VSWR from Forward and Reverse Power

 

[N4KVE]

Thanks. 1.44. Not bad considering no ground plane.

 

[Murphy625]

I hate those calculators that don't show you HOW the math is done.

Until you asked the question, I had no idea.. same as you.. Now I need to learn to do it with pencil and paper.

 

[n5rv]

I hate those calculators that don't show you HOW the math is done.

Until you asked the question, I had no idea.. same as you.. Now I need to learn to do it with pencil and paper.

I used to know the answer, but being 40 years of never using it, I forgot how.... LOL!

I tend to use rule of thumb values of "that looks good to me"....

 

[w2xq]

FWIW... https://www.kb6nu.com/use-forward-reflected-power-calculate-swr/

HTH.

 

[N4KVE]

I used to know the answer, but being 40 years of never using it, I forgot how.... LOL!

I tend to use rule of thumb values of "that looks good to me"....

Me too. LOL.

 

[ipfd320]

Not Really Sure What kind of Watt meter you Have--this may help you out

This is a PDF Users Manual of a Bird 43 Watt Meter

http://www.repeater-builder.com/test-equipment/bird/pdf/bird-43-wattmeter-old.pdf

 

[N4KVE]

Not Really Sure What kind of Watt meter you Have--this may help you out

This is a PDF Users Manual of a Bird 43 Watt Meter

http://www.repeater-builder.com/test-equipment/bird/pdf/bird-43-wattmeter-old.pdf

I saw that, but it was very slow to download. Gave up after waiting 5 minuted for page 10.

 

[ipfd320]

OK Here is the SWR Chart for the 43 meter

http://w4zt.com/testeqdocs/bird43/bird20.pdf

 

[Lauri-CoyoteFrostbite]

Hey 4KVE
.
.
You just got some really great pointers and references.... and you can't do much better as a ham owning a Bird wattmeter.
.
I'm going to will throw in a little extra spice to this mix- its probably heresy as a physicist to say this- but in my experiences, there are the "hard numbers" and the "soft" ones. As a scientist is so important to not get caught up in some of the numbers- while at other times its a matter of (literally) life or death.
.
So much of my career have been projects where we would 'walk the numbers' into compliance. Like your inital 30 to 1 Watt antenna reading- these were guideposts as to what a system was doing- but taken as absolute readings from certain instruments, they were only that.
Am I getting confusing here?....sorry. Let me put it another way. In your example, how do you know for certain that 1 Watt on the Bird IS truly 1 Watt, that 30 is 30 ? I am not being a Smart A** here- run those numbers thru another instrument-- and be surprised (ie; disappoint'd, shocked- pick the term )
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Variables- lots of them.................
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I can not tell you how many times in a field trial a contractor has assured me that XYZ radar met ABC parameters-- that is; on their lab equipment. We, unfortunately usually did not agree.
.
But still, I would pass the unit as perfectly acceptable.
.
Okay, Coyote-- what is the point ?
.
Selling a phased array and you need approval?... Pay attention to the numbers- but understand there is wiggle room in them. As a ham, get a "Feeling" for what your instruments are saying. If I got a "1 Watt reflected to 30 Watts forward" I might conclude the system was well within tolerance and the contractor's numbers (since he was very close to our's)-- were all +/- correct. (of course, unfortunately it is not all that simple, but Hey-- this is for illustrative purposes... )
.
Don't fly, especially as a ham, by the absolutes unless you really have too. When THOSE times occur, Cowboy, you have arrived-- you are now a true scientist !
(Welcome to the club - and all the headaches; for perfection does not come psychically cheap.)
Learn to read and *interpret* your instruments - Smiles
.
"Don't sweat the petty stuff... and don't petty the sweaty stuff"
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End of lecture... and congratulations on that wise choice to buy a Bird !
Love her; she's going to be your life long friend
.
.
Lauri
.

 

[N4KVE]

Hey 4KVE
.and you can't do much better as a ham owning a Bird wattmeter.
. In your example, how do you know for certain that 1 Watt on the Bird IS truly 1 Watt, that 30 is 30 ?
.

I thought that was why people buy the Bird, & why I junked my POS MFJ meter. LOL.

 

[RFI-EMI-GUY]

Determining absolute power with a Bird wattmeter is impossible. Especially so if you lack NIST traceable standards.

I have an interest in setting some GMRS radios as close to factory 5 watt power as possible. I have a Bird 43, a General Microwave TFT power meter, an HP8920B system analyser. They all disagree. Not surprisingly.

So I am putting together a simple diode power meter using a 50 ohm load, 1N5711 diode and bypass capacitor. Doing so removes a lot of variables and reduces the problem to the accuracy of my DVM. I measure the exact value of 50 ohm terminating resistor, apply a voltage equivalent to 5W, measure the diode drop Vf. Then apply RF, measure peak voltage, subtract Vf and calculate RMS power relative to 50 ohms.

Sent from my SM-T350 using Tapatalk

 

[prcguy]

Your method could have more error than a Bird 43. Everything is based on the premise that what you are getting off the diode and and bypass cap at 460MHz is what you measured at DC for Vf. It could be close or it could be 10% or more off. Do you know the accuracy of your DVM? Is it in cal?

If that method was accurate, companies could simply bench measure some diodes and other parts, assemble little power meters with scales calculated from milivolts and component loss, then ship them without any testing or cal.

An RF power meter that's somewhat in cal can have less than 1% error at the thermistor head. Using a known good RF power meter and calibrated attenuator will get you about as close as you can to a calibrated lab standard calrometer.

Determining absolute power with a Bird wattmeter is impossible. Especially so if you lack NIST traceable standards.

I have an interest in setting some GMRS radios as close to factory 5 watt power as possible. I have a Bird 43, a General Microwave TFT power meter, an HP8920B system analyser. They all disagree. Not surprisingly.

So I am putting together a simple diode power meter using a 50 ohm load, 1N5711 diode and bypass capacitor. Doing so removes a lot of variables and reduces the problem to the accuracy of my DVM. I measure the exact value of 50 ohm terminating resistor, apply a voltage equivalent to 5W, measure the diode drop Vf. Then apply RF, measure peak voltage, subtract Vf and calculate RMS power relative to 50 ohms.

Sent from my SM-T350 using Tapatalk

 

[RFI-EMI-GUY]

Your method could have more error than a Bird 43. Everything is based on the premise that what you are getting off the diode and and bypass cap at 460MHz is what you measured at DC for Vf. It could be close or it could be 10% or more off. Do you know the accuracy of your DVM? Is it in cal?

An RF power meter that's somewhat in cal can have less than 1% error at the thermistor head. Using a known good RF power meter and calibrated attenuator will get you about as close as you can to a calibrated lab standard calrometer.

Pretty much everything I have read on the subject has indicated that the Vf is largely dependent upon the meter load so I am not sure what differential would exist between peak (CW) RF voltage and peak DC voltage. I can vary Vf in my spreadsheet from 0.40 to 0.80 and I get only a 0.33 dB difference. Please point me to any recommendations as to DC Vf drop versus RF rectification.

I have a good but ancient power meter. But I can measure a full 1.5 dB difference between my signal generator and the power meter which is a big difference, but is consistent with the additive errors of both instruments.

Adding an attenuator to make it work at 5 watts requires careful calibration. I have measured insertion loss of many devices over the years. I would guess a .25 dB error range with my technique. However "known good" and NIST calibrated power meter are miles apart.

The diode power meter removes a lot of variables and I can calibrate a DVM from a voltage standard.

 

[majoco]

SWR = (Vf + Vr)/(Vf - Vr)

 

[K5MPH]

FWIW... https://www.kb6nu.com/use-forward-reflected-power-calculate-swr/

HTH.

Thomas im getting a Error 523 on that link.....

 

[majoco]

oops - my bad. That formula is only for Voltage.

bit more complicated for power....

SWR = (1+sqrt (Rev/Fwd))/(1-sqrt(Rev/Fwd))

sqrt = square root - my ¹ doesn't show very well.

so your 30 fwd and 1 rev goes like..

swr = (1+sqrt(1/30))/(1-sqrt(1/30)) = 1+sqrt(0.333)/1-sqrt(0.333) = 1+0.182/1- 0.182 = 1.182/0.818 = 1.44

as per the earlier calculation.

 

[w2xq]

Thomas im getting a Error 523 on that link.....

Hmm. The link works for me on my Samsung Galaxy Tab S3 tablet. I had to look up a 523 error code -- http://www.wiki-errors.com/err.php?wiki=523z -- as I don't recall encountering that code before.