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   Home   Help Search  
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Author Topic: CB SWR / Watt Meter  (Read 1447 times)
KD9FRQ
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Posts: 174




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« on: September 29, 2017, 06:36:26 AM »

The meter I was given quit working.

Would a CB SWR / Watt meter that I can buy at the local Pilot work for 40/20m?
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K5WLR
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Posts: 226




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« Reply #1 on: September 29, 2017, 06:53:14 AM »

The meter I was given quit working.

Would a CB SWR / Watt meter that I can buy at the local Pilot work for 40/20m?

Well, a qualified maybe...

You can check the SWR portion as follows....

Hook up the meter as per normal instructions into a 50 ohm dummy load. Then, set the forward reading to the maximum position in the SWR forward setting. Then, without changing any settings, reverse the cabling (the input cable to the output socket and the output cable to the input socket). Now, switch the meter to reverse, and, if the meter reads the same as it did in the previous setup, the diodes that read the SWR are matched, and the SWR portion of the meter is usable.

The only way to verify the accuracy of the power portion of the meter is to check it against a properly calibrated power meter. Otherwise, who knows?

Hope this helps!  Cheesy

73
Will Rogers
K5WLR
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K5DH
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Posts: 3


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« Reply #2 on: September 29, 2017, 07:39:05 AM »

I've found that many cheap CB-type VSWR meters work fine across the HF spectrum, and some actually pretty well at 2m also.  Don't worry about the calibration and accuracy of the meter.  What difference does it really make if your VSWR is 1.1 or 1.2 or 1.3?  You're going to tune your antenna for the lowest reflected power, period.  Whatever that number is, well, it is what it is. 

I've also found that many cheap CB-type wattmeters work well across the HF spectrum.  Again, does it really matter if your transmitter puts out 100 Watts, 95 Watts, or 105 Watts?  It puts out however much power it can muster, period. 

73,
Dean K5DH
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WA3SKN
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Posts: 6492




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« Reply #3 on: September 29, 2017, 07:49:22 AM »

Maybe.
There are three basic SWR designs, all with plus and minus's.
But what is wrong with the present meter?  It is quite possible you could fix it... they are fairly simple devices.  Usually a diode just opens.

-Mike.
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K8AXW
Member

Posts: 6307




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« Reply #4 on: September 29, 2017, 09:09:44 AM »

WLR presents the method of calibrating the SWR meter.... which will confirm that the meter is working properly.

Just remember that CB SWR meters are QRP meters....5-10w so sneak up on the meter if you're using a 100w transceiver!

They are also calibrated for the 11m band and the sensitivity drops as the frequency is lowered.

 
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WB6BYU
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Posts: 17046




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« Reply #5 on: September 29, 2017, 09:54:24 AM »

I've used cheap CB SWR meters quite a bit over the years, from 80m through 220 MHz.
No, they're not perfect, but adequate for a lot of tasks, such as adjusting a tuner.

Some points to keep in mind:

Sensitivity varies with frequency.  On 160m I couldn't get enough power out of my rig
to get a useful indication on the meter to adjust the tuner except when the tuner was
already properly adjusted for low SWR.  On 2m I could drive the meter with just a
signal generator.  However, the internal wiring is not always designed for VHF/UHF
use, and it was utterly useless to try to use it on 440.  With some meters, even 2m
may be questionable.

On the meter I've used the most, full scale is about 25W on 80m, 10W on 40m, and
3W on 20m at maximum sensitivity.  It was OK for adjusting my tuner with QRP on
40m and 20m, but marginal on 80m.  These numbers will vary a lot with specific
designs.

Reflected power reading varies with power level.  If you can barely drive the meter
to full output at maximum sensitivity, the reflected power reading will typically be low,
so your SWR measurement will be also.  All of them have handled 100W without
any issues, assuming the sensitivity control is turned down first.

If there is a calibrated power meter, it may either come from the forward power
circuit (which is frequency sensitive) or be a simple RF voltmeter across the feedline
(which shouldn't change much with frequency, but is only accurate at low SWR.)
If the power meter is separate from forward / reflected power, then it probably is
the latter.


Besides the checks already suggested, I also find it useful to put two dummy loads
on a T connector and see how close the meter reads to 2 : 1.  This is also handy for
checking how much the reading changes at different power levels.


On the other hand, the most common failure mode I've seen is a blown diode, and
those often are easy to replace (if you can get to them.)
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KD9FRQ
Member

Posts: 174




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« Reply #6 on: September 30, 2017, 05:30:23 AM »

Thanks for all the replies.

I do not know what happened to the meter I had.  It is built in to the MFJ-969 tuner.  The meter on the radio seems to be working. I have power set at 30w for PSK and it says I am outputting 30w into the sidecar tuner. I am using an FC-30 with my FT-450 (not D).

I had the CB meter sitting around and thought I would ask.  Funds are tight so I will need to wait for Santa on the next meter.

73s, everyone.
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KD7RDZI2
Member

Posts: 216




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« Reply #7 on: September 30, 2017, 09:04:50 AM »

I have a CB SWR meter (no PWR meter) and it works fine from 160 to 6m band Grin
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