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Accessories/Communications/CBs/ Measuring SWR:

> >I just got a radio shack SWR (Standing Wave Ratio) meter to check out some 
> >systems.
>
> In gereral, on some (most?) SWR meters with calibration, the
> calibration is done *without* depressing the PTT switch.  The
> calibration meaure is to simply 'zero-out' the needle prior to taking
> a reading.

I don't know about current Radio Shack SWR meters, but on mine (about 10 years old), that is not the case.

First, some background for those folks who don't already know: SWR is basically a measurement of the ratio between forward power (power sent from the transmitter towards the antenna) and reflected power (the power that gets bounced back down the line from the antenna due to imperfect impedance matching). To determine SWR, you need to measure both forward and reflected power. Excessively high SWR is Bad. It means that much of the power from your transmitter never makes it out of your antenna and just gets wasted. That wasted power needs to go somewhere, and unfortunately it gets turned into heat inside the transmitter. Excessively high SWR can destroy your transmitter. Many modern radios have circuitry which shuts down the transmitter or turns down the power when it sees highs SWR, but some radios don't.

An SWR reading of 1.0 is perfect, anything under 2.0 is good, between 2.0 and 3.0 is probably good enough, and over 3.0 is Bad.

There are basically 3 common ways that I know of to measure SWR:

  1. Use a gizmo called a "directional power meter" to measure both the forward and reflected power, then do the math to calculate SWR. Some have a forward/reflected switch, some require you to rotate the sensing unit 180 degrees, and some really simple ones require you to swap the input and output cables.

  2. Use a slightly fancier power meter that can do the math for you, with some help: with these units, you first need to adjust the meter so that the needle lands on a calibration mark when measuring forward power, and then you can read SWR directly on the meter when you switch to reverse power. The Radio Shack meter works like this. This is the most common type I have seen.

  3. Use a really fancy meter that reads our SWR automatically. I've seen two varieties: One kind has two needles in the meter, for forward and reflected power, and the intersection of the two needles tells you the SWR. I've also seen fancy digital ones that measure both forward and reverse power, do the math internally, and display the SWR on the display.

To use the Radio Shack meter (assuming that your is like mine):
  1. Plug it in between the radio and antenna, making sure the radio goes to the "trans" or "in" plug, and the antenna goes to the "ant" or "out" plug.

  2. Switch the meter to "forward", hold PTT, and adjust the meter until the needle is on the "cal" mark (full scale).

  3. Switch the meter to "reflected" and read the SWR.

If your radio has a switch to change output power, use the lowest setting that delivers enough power to let you calibrate the meter, and don't hold PTT very long until you get the SWR down under 3.

> Set the CB to channel 19 (half way between spectrum -40)  I calibrated the 
> SVR to the mark on depressing the mic switch (transmit-no voice).  Then 
> switched to SVR.  Repeated process.  The needle keep going to above 
> 3-(inefficent).  Ideal is 1.5-2.  Tried to adjust antenna length, no go.

Did adjusting the antenna length affect the SWR at all? If you start with antenna that is too long, the SWR should drop as you decrease the antenna length until it reaches some minimum value, and then rise again as you continue to decrease the length.

Once you minimize the SWR at one channel, the SWR will rise as you change channels in either direction. You should try to minimize the SWR at the middle of the radio's band (channel 19), then check the SWR at both extremes (channels 1 and 40) to make sure it isn't too high there.

> Any helpful to measure Field Strength==small regions of radio frequency 
> power??

The field strength mode in that meter can be used to measure antenna radiation patterns (i.e., how the antenna's performance varies in different directions). I think that's one of those things where if you don't already know what it means, then you probably don't care.

--
Mark J. Blair KE6MYK


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