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zenrandom

cross meter swr meters

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How the heck do I read these things? I just got an mfj-862 and I'm just confused at how to interpret what it tells me.

 

Thanks,

ZR

 

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If it ain't broke, I can *fix* that...

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There are three scales. One is forward power, one is reflected power and the other is swr. Most likely, your wondering how to read the swr scale. The swr is where the two needles cross while transmitting. Be sure to look directly at it in front of the meter to eliminate parallax error.

 

Poindexter

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Just last week I purchaced an MFJ-941 tuner (with integrated SWR/power meter) & I absolutely love it. As Poindexter said... the point at which the Forward Power and Reflected Power meters intersect vis-a-vis the SWR lines that are screened on the meter's face indicate your actual SWR. I really appreciate this arrangent since the user isn't required to recalibrate the needle every time there's been a change in parameters (antenna length, power level, etc.). As I'm sure you already know, you want the reading to be as low as possible (i.e., the lower-righthand area of the meter of an MFJ product)

 

Good luck,

Brian (KA6WSR)

Riverside, CA

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quote:
Originally posted by zenrandom:

How the heck do I read these things


 

As has been said, the VSWR (Voltage Standing Wave Ratio) is shown where the needles cross.

 

Basically, you want the Forward power to be as near to the output of the transmitter as possible and the Reflected power to be as low as possible.

 

If you then look in the middle, the VSWR reading is shown as a set of curves with details of what the measured reading is on it.

 

A few warnings about VSWR meters now...

 

1. They are not always very accurate. Check the specifications, but plus or minus 10% would be a good one! So, 100 watts on the meter might be anywhere between 90 Watts and 110 Watts.

 

2. VSWR is only half the story. A good, non-inductive 50 ohm resistor (or Dummy Load if you prefer to call it that) gives almost 1:1 across a wide frequency range, depending on how well it's been made. Would you want a dummy load on the end of your coax. cable instead of a good radiator of RF?

 

3. The VSWR meter can be fooled by a particular length of cable! The match of the antenna to the feeder will still be as good, or bad, as it ever was but making the coax. a certain length will fool the VSWR meter.

 

4. Radio Amateurs did very well before the VSWR meter became the 'law'. They only really help when you have a Rig that cannot stand a bad match to its output. So, solid state PAs have increased the 'law' of the "1:1" VSWR.

 

In a nutshell, don't trust the VSWR meter, unless all you want to do is deliver full power into a system (including the feeder and the radiating element) and hope for the best!

 

Consider getting a grid dip meter as well, or an antenna analyser. The accuracy may only be as good as the better VSWR meters, but it will give you a reading of the resonance of the system, which should be at 1:1 VSWR but often isn't on even expensive VSWR meters!

 

Also, design the antenna properly, feed it with as low loss feeder as you can and then fool the rig into delivering power, either by using a tuner of some sort or by arranging for the coax. to show 50 ohms to the PA.

 

--... ...--

Morseman

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