Measuring SWR (Standing Wave Ratio) on your antennas with a Daiwa CN-801

SWR is one of those things that most people don't need to worry about.  However, when you start building your own antennas or need to get every mw of power out of your existing antennas, you might need to check/tune your SWR.

The Wikipedia article does a good job explaining what SWR is, but in a nutshell, it's the efficiency of your antennas coupled with your transmitter's frequency. An antenna that is a poor match for a specific frequency will be less efficient at transmitting that frequency and reflect some of that energy back to its source.  A SWR meter measures two things, forward, and reflected power; these two things together tell you what your SWR is.  SWR is measured on a exponential scale with 1 being the best, and anything under 2.5 being consider good.  Most store bought FPV antennas will be around 1.2.  I use they highly recommended DAIWA  CN-801 SII, which can measure 0.9 to 2.5 GHz.

To get started I'm going to connect my ~300 mw 1258 MHz video TX and an Ibcrazy 4 lobe antenna (which cannot be tuned).  Because of the scale on my meter, I put a 10 db attenuation on which puts the needles in a better place to read, however this hides my true forward power.  I'll remove it later to see how much power my TX is actually putting out.


When I turn on the video TX (no video signal required), I can read my SWR for this antenna by looking at the point the two needles cross.  This antenna is great with a SWR of ~1.2.

As I said before, this type of antennas can't be tuned (unlike say a dipole you could change the length on), however we can place an object in the antenna to see a similar change.


Now when we measure the SWR is worse.  Looks to be ~2.15.  Notice how the reflected power has gone up.  That means with some bulky object causing a short, it's reflecting more power back and not transmitting the full power of the TX.


Finally, we remove the 10 db attenuation to check the power output of my video TX.

And it's actually a little bit better than 300 mw it was sold as.  Not bad.


So even if you're not going to be tuning your own antennas anytime soon, it's important to remember that you should always try to improve your antennas before you start throwing power at a problem.  That 1W video TX might only be outputting 400 mw.  Also, if you're using something that is frequency hopping, generally you would tune your antennas for the middle of the band.  A good SWR meter also tells you how much power you're actually transmitting which is always helpful.  

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Comment by Gary Mortimer on February 13, 2013 at 2:04am

Brilliant its about time people started taking this seriously. A miss matched antenna is like using a wet piece of string, and worse it can damage your TX. How many times have I seen people say, "oh I just added more wire for a better signal"

You adding this makes me even more convinced its time to have a hill to hill contest to show just what a decent antenna setup can achieve. 

Comment by Jethro Hazelhurst on February 13, 2013 at 2:34am

That is so cool! I see the DAIWA  CN-801 SII is about $200 bucks

Comment by R. D. Starwalt on February 13, 2013 at 5:20am

Nice demonstration. A+

Another point - a good load (low SWR) on the TX is not always a good radiator of the RF energy.

Afterall, a dummy load (at 50 ohms) is a perfect load but a terrible radiator of energy.

-=Doug

Comment by Josh Potter on February 13, 2013 at 9:40am

Yeah it's still ~$200, but well worth it if you're an antenna building junkie like me.   I hear the gear required to do 5GHz stuff is even more expensive.  I bought this model on the recommendation of Alec (ibcrazy)


Gary, I think that's a good point, and goes back to people say "well I have bad range, i'll just change out this 100mw TX with a 2W model and this little crap antenna I found in the closet"

R.D. I didn't know that; Good to know!



Oh and if you're looking for a small pocket spectrum analyzer, check out this guy for under $300. 

Comment by Joe Frazier on February 13, 2013 at 3:56pm

High SWR can also cause the transmitter to go spurious - that is, to produce interference on other frequencies. Decidedly not goo for radio control vehicles!

Comment by Cool Dude on February 13, 2013 at 6:57pm

Can you please point to the pdf manual of Daiwa CN-801.

Thanks

Comment by Joe Frazier on February 13, 2013 at 7:12pm

Google is your friend: www.cometantenna.com/pdfs/CN-801_series_manual.pdf

took a whole 30 seconds. ;-)

Comment by Joe Frazier on February 13, 2013 at 7:15pm

Lets try that again in a form that's clickable:

www.cometantenna.com/pdfs/CN-801_series_manual.pdf

Comment by bruno on May 7, 2014 at 10:50pm

Hello

You ca also built your homebrew swr meter with very good precision 

http://nobru54.blogspot.fr/2014/04/swr-metre-wi-fi-24-ghz.html

https://www.youtube.com/watch?v=vWmQVbWbtbA

Comment by Jeff on July 29, 2014 at 9:24pm

Hi Josh,

Can you please explain to me how the 10db attenuator makes the forward power needle go up on the meter?  I actually purchased this same meter and a 10db attenuator and when I add the attenuator it makes the forward power needle go down when testing a RFD900 radio.

Thanks,

Jeff

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