Quick question about SWR/Power meter

NC1

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#1
I have a neighbor that thinks his CB is not putting out the full 4 watts and asked if I can check it for him. All I have is a MFJ-844 144/440 SWR / Wattmeter for my dual band radios.

My question is: can I use the MFJ to check CB power output (only) even though the unit is designed for VHF/UHF? I am inclined to think it should work, I know the SWR will not be at all accurate - which he is not concerned about at this time.

I know a meter for CB is cheap and easy to get (NOT looking to buy one), but would like to know if the MFJ can be used to check the output power.
 

NC1

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#3
So the frequency is going to give me an erroneous output power level? I am at a loss as to how that works. Could you please explain?

I may be a bit naive here, but I would think a 12v car battery would measure the same as a 12v lantern battery.
It is RF energy that I am measuring, which I believe is independent of frequency and does not know if it is HF or UHF, am I correct?
It's not like using a Volt meter set for DC when trying to measure AC line voltage.

I'm just having a little trouble understanding your answer.
 
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#4
Measuring RF power is not the same as measuring DC voltage and current. SWR meters and wattmeters are designed with RF components that only work well for specific frequency ranges.
 
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#5
Ideally you should be using a decent 50Ω load.

And I agree, the 144MHz SWR meter is going to have an error at 27MHz. You could test with a known good CB and get a reasonable idea of how much off it is.
 

NC1

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#6
Measuring RF power is not the same as measuring DC voltage and current. SWR meters and wattmeters are designed with RF components that only work well for specific frequency ranges.
I am more than aware that RF is not the same as DC voltage, and I am not checking for SWR. I know the SWR is frequency dependent, but was unsure if RF power was.
 

NC1

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#7
Ideally you should be using a decent 50Ω load.

And I agree, the 144MHz SWR meter is going to have an error at 27MHz. You could test with a known good CB and get a reasonable idea of how much off it is.
The only way I can measure is under a 50Ω load, whether it be an antenna or dummy. Again, I am NOT checking for SWR, but only the RF power output of the CB radio. It is the CB radio that needs to be tested for 4w output, so checking with a different CB would kind of defeat the purpose.
 
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#8
You need to check the forward power, which is basically the RF power output from the radio into a 50Ω load.
If your antenna is a perfect 50Ω match, then it will work fine. If it's not, then a 50Ω dummy load will give you more accuracy.

The reason I suggested testing with another known good radio is to determine what a 4 watt transmitter at 27MHz will look like on a 144MHz rated SWR meter. There will be an error, as in a true 4 watts will not show as 4 watts on your SWR meter. Knowing what that error is will be useful.

Once you know what the error is, test the suspect radio the same way and compare the readings. If it shows the same as your known good radio, then you can be reasonably sure it's putting out something pretty close to 4 watts.

If you are concerned about accuracy, then using an SWR meter that is not designed for the frequency you are testing is not going to give you good results.

Trying to be helpful here, but maybe I'm not explaining it in a way that's coming across right.
 
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#10
Most real wattmeters have a directional coupler as part of the RF pickup and the internal dimensions of the coupler along with some other parameters determine its usable frequency range. A directional coupler designed for VHF through UHF would generally have less coupling at 27Mhz and give a lower wattmeter reading.

You can be certain if a piece of equipment could operate satisfactorily from 27MHz to 450MHz they would advertise that. The fact its only rated for 144-440MHz tells me it probably barely works across that range.

So the frequency is going to give me an erroneous output power level? I am at a loss as to how that works. Could you please explain?

I may be a bit naive here, but I would think a 12v car battery would measure the same as a 12v lantern battery.
It is RF energy that I am measuring, which I believe is independent of frequency and does not know if it is HF or UHF, am I correct?
It's not like using a Volt meter set for DC when trying to measure AC line voltage.

I'm just having a little trouble understanding your answer.
 
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