Question about Cobra 19 DX IV CB Radio

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Can a digital volt meter be used to estimate swr?

Since swr is a voltage standing wave ratio i was wondering if there was a way to convert voltage readings at the output into swr.

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  • tdill49 Jan 30, 2009

    Thanks, I am an industrial engineer and can't help but experiment. I generally program PLC's in ladder logic or assembly lang.

    I used to be big into the CB biz when I was a teen and was just playing around with a little mobile.

    Thanks for the come back.

    Terry Dill

    Tupelo MS.

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  • 31 Answers

No, Well. not easily that I am aware of. besides the other function of the SWR meter is to measure RF power.

Plus, the Meters are Cheap. it's not worth the bother.

Posted on Jan 29, 2009

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1 Answer

What are swr's


The SWR meter or VSWR meter measures the standing wave ratio in a transmission line. The meter can be used to indicate the degree of mismatch between a transmission line and its load, or evaluate the effectiveness of impedance matching efforts.

It is a way of tuning the aerial length to be correct to the radio.

SWR meter Wikipedia the free encyclopedia

Apr 03, 2016 | Radio Communications

1 Answer

I have a Cobra CB Radio and was wondering if I have it on SRF or SWR or CAL


S/RF/SWR/CAL SWITCH. When in the S/RF position, the meter swings
proportionally to the strength of the received signal. When transmitting,
the meter indicates relative RF output power.
When in the "CAL" position, the SWR meter can be calibrated by adjust-
ing the "SWR CAL" control to the "CAL" mark on the meter face.
When in the "SWR" position, the standing wave ratio is measured.

You will want to operate on S/RF position.

Jul 14, 2011 | Cobra 29 LTD CB Radio

2 Answers

Can this SWR be accurately be used for a boat VHF radio antenna? I have tied it and it seem to work but do not know if the reading are good.


Standing wave ratio is standing wave ratio. This meter is not reading based on frequency. It is reading based on the measurement of electrical energy. I see no reason why it wouldn't work.

Jun 22, 2011 | Astatic (PDC7) CB Radio

1 Answer

Swr meter not moving


follow these steps to adjust the
standing wave ratio(SWR).
1. Turn on the CB.
2. Set S/RF/SWR/CAL to CAL.
3. Key the Mic and rotate SWR CAL so the meter points
to CAL.
4. Unkey Mic.
5. Set S/RF/SWR/CAL to SWR.
6. Key Mic again and note the actual measurement on
the SWR scale. Refer to the following table to interpret
the reading.

1:1 - 1.5 Great
1.5 - 2:1 OK
2:1 - 3:1 Not So Good
Higher than 3.1:1 Terrible

Jan 14, 2011 | Cobra 29 LTD CB Radio

1 Answer

I want to calibrate my cobra 29 NW Ltd. Classic


Calibrate For SWR (Standing Wave Ratio) http://www.cbtricks.com/radios/cobra/29ltd_bt/graphics/cobra_29ltd_bt_om.pdf
The CAL is only to calibrate meter for a swr check of you antenna system it does not calibrate radio.

Jan 11, 2011 | Cobra 29 LTD CB Radio

1 Answer

This radio has swr meter


S.W.R == Standing Wave Ratio, this is a Relative measurement of the Amount of Power Going UP the Antenna, Versus the Amount of power being "Reflected" Down, the Antenna. This ratio, is displayed, as a reading on the meter. Also Often this SWR meter is dual used as an "S" Meter, this is used ti display relative signal levels from other transmitters. and is shown, as an "S" Scale. 0 - 9 on the meter.

Jun 07, 2010 | Galaxy Radios DX- 88HL Base CB Radio

2 Answers

Tunning the antenna on radio shack cb trc-447


After installing the antenna, follow these steps to adjust the
standing wave ratio(SWR).
1. Turn on the CB.
2. Set S/RF/SWR/CAL to CAL.
3. Key the Mic and rotate SWR CAL so the meter points
to CAL.
4. Unkey Mic.
5. Set S/RF/SWR/CAL to SWR.
6. Key Mic again and note the actual measurement on
the SWR scale. Refer to the following table to interpret
the reading.

1:1 - 1.5 Great
1.5 - 2:1 OK
2:1 - 3:1 Not So Good
Higher than 3.1:1 Terrible

The ideal standing wave ratio (SWR) is 1:1, or a meter
reading of 1 on the SWR meter's top scale. A SWR ratio
of 1.5:1 to 2:1 is excellent for most mobile CB antenna
applications.

Check the SWR on Channel 1 and Channel 40
If the SWR is higher on 1, make the antenna longer
If the SWR is higher on 40, make the antenna shorter
Most antenna's are adjustable. Make small adjustments at a time.

Oct 10, 2009 | Radio Shack TRC-447 CB Radio

2 Answers

I got a output voltage of 100.1 on a trace dr3624


The rms voltage is what counts, because it tells how much power the output will deliver to a resistive load. Inexpensive multimeters on their AC ranges are usually average-responding rms-calibrated meters. This means they measure the average of the absolute value of the AC component of the signal, and display that average multiplied by about 1.11 (actually, pi over sqrt(8)), the ratio of rms to average value for a pure sine wave. That way, the meter will give the right rms reading for a sine wave.

If the signal is a square wave, where the average and rms values are equal, the average-responding meter will read 11% too high.

Many inverters put out a modified sine wave (MSW), which sits at zero for a while, goes to a constant positive level for a while, goes back to zero for a while, and goes to a constant negative level for a while to complete the cycle. The positive and negative parts of the signal have the same magnitude and duration.

The rms and average values of an MSW depend on its duty cycle D, the fraction of a cycle for which the signal is not at zero. In a well-designed inverter, the duty cycle will be adjusted when the DC input voltage goes up and down to maintain the nominal rms output voltage. If we use peak voltage Vp to mean the magnitude of the positive and negative voltages the signal goes to, then Vavg for an MSW is equal to Vp times D, and Vrms is equal to Vp times the square root of D.

The duty cycle for which an MSW will have the same rms to average ratio as a sine wave is 8 over pi squared, or 81%. For any duty cycle less than this, an average-responding meter will read a lower voltage than the inverter rms output, and for a duty cycle higher than this, the meter will read too high.

If your MSW inverter is putting out 120 volts rms and its duty cycle varies from 50% to 75%, the meter reading will vary from 94 volts to 115 volts. I avoid the problem by using a Radio Shack 22-174B true rms digital multimeter.

Aug 27, 2009 | Xantrex Technology DR2412 Inverter /...

1 Answer

John Q president cb


Hi,

If by calibration, you are referring to the check if your antenna is tuned, below information may be of help.
S/RF/SWR/CAL SWITCH. When in the S/RF position, the meter swings
proportionally to the strength of the received signal. When transmitting,
the meter indicates relative RF output power.
When in the "CAL" position, the SWR meter can be calibrated by adjusting
the "SWR CAL" control to the "CAL" mark on the meter face.
When in the "SWR" position, the standing wave ratio is measured.

SWR METER. Measures standing wave ratio of the antenna system.
Used to properly adjust the length of the antenna and to monitor the
quality of the coaxial cable and all RF electrical connections. If there is any
degradation whatsoever in any of the foregoing, due to humidity, salt
spray, vibration or corrosion, the SWR meter reading will rise, thereby
indicating that a problem exists.
To calibrate, switch to the "CAL" position, transmit by pressing the mike
switch, and adjust the SWR control to the "CAL" mark on the meter; then
switch to "SWR" position for the SWR measurement.

If however you are referring to the internal calibration or tuning of the transmit and receive sections, you would need several instruments/equipment such as RF generator (0-30Mhz with 1Khx modulation), Spectrum Analyzer, Modulation Scope, Dummy Loaded Wattmeter, Frequency Meter and some others. These of course are not readily available to the average user.

Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.

Good luck and kind regards.

Thank you for using FixYa.

Jun 12, 2008 | Cobra 29 LTD CB Radio

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