PROPER WAY TO CHECK SWR WHILE USING AN AMP?

[Stellasstillarat]

Should one calibrate an swr meter and take a reading with there amplifier on, or should the swr reading be taken with the radio's output only (3watts)? if swr is checked with 3 watts is it normal for the swr to change once the ampllifters wattage is applied or should one recalabrate and take a separate reading with the amps output wattage?

 

[psycho]

Look through the forum history and you will find dozens of similar threads.

 

[midnight special]

check swr with amp out of line,, when the antenna is set it wont change when amp is in line,,, trying to check swr with amp in line and on will just give you false readings,,,, i dont even use a swr meter anymore since i got my mfj 259,,,you dont use the radio just the meter and the antenna it gives a lot of info on how the antenna is doing besides the swr,,,,

 

[RatsoW8]

check swr with amp out of line,, when the antenna is set it wont change when amp is in line,,, trying to check swr with amp in line and on will just give you false readings,,,, ,,,,

Why would an SWR meter give you false readings if the meter is placed after the amp? Assuming there's no issue between the radio and amp.

 

[binrat]

Why would an SWR meter give you false readings if the meter is placed after the amp? Assuming there's no issue between the radio and amp.

It won't but the "assuming" part is where most people get screwed up. Most CB style amps have issues shall we say. That's why many people just add longer coax between radio / amp.

 

[midnight special]

most amps produce harmonics that are not in the cb band,,, now i said most not all,,,but the harmonics is what causes the false readings so it is just better to set the antennas swr with just the radio and not the amp in line,,, after the antenna is set it shouldnt change the swr unless something is wrong with amp or the patch cable,,,,,,, i have done it this way for years,,,,,,,, never had no problem with my setups,,,,,

 

[RatsoW8]

Alrighty then. Some interesting theory of operation in play here.

 

[Stellasstillarat]

Thanks for that information. Much appreciated. [QTE="midnight special, post: 501447, member: 24773"]check swr with amp out of line,, when the antenna is set it wont change when amp is in line,,, trying to check swr with amp in line and on will just give you false readings,,,, i dont even use a swr meter anymore since i got my mfj 259,,,you dont use the radio just the meter and the antenna it gives a lot of info on how the antenna is doing besides the

check swr with amp out of line,, when the antenna is set it wont change when amp is in line,,, trying to check swr with amp in line and on will just give you false readings,,,, i dont even use a swr meter anymore since i got my mfj 259,,,you dont use the radio just the meter and the antenna it gives a lot of info on how the antenna is doing besides the swr,,,,

 

[vkrules]

Ok maybe I'm pedantic but I like to check swr with the lowest possible power.then take my readings again with the amp on . As stated earlier VSWR should remain the same, ( maybe a slight increase or decrease due to most meters not being linear in there reading s at all power levels) That way I can see any problems that may occur. When running my amp I monitor input VSWR output VSWR Volts and Amps.Any change indicates a problem.

 

[midnight special]

if the radio or amp is producing harmonics it will give false readings because the antenna is set to one specific band or freq... it might be 1 to 1.3 in the cb band but harmonics in the 13.5 mhz or 54 mhz band would show a high swr,,,my figures there are just examples because harmonics could be any frequency,,,, so when a swr meter is used and harmonics is being put out it will show up the other freqs too but you dont know that because you tuning with a cb band radio.. harmonics is produced from amps not filtered right or filtering not put in or disabled,, it also is produced by some radios that have been hacked to make them seem they can put out higher power when it is actually just a harmonic fooling the meter... spreading the tvi coils in some radios is example of this..... i know a lot of people think different of this idea but 35 years cbing i pretty much have seen it all,, just a few thoughts not wanting to upset any one,,

 

[vkrules]

I agree completely.But if the amp is doing that You shouldn't be using it.

 

[midnight special]

people use them every day..........and bleed over and have distortion, and think they get out good..........but never really know the truth,

 

[David Waits]

Radio to Amp to Meter to antenna, always check the SWR with the Amp off.

 

[Tallman]

I agree completely.But if the amp is doing that You shouldn't be using it.

Most people don't care if they are splattering all over the radio spectrum. The amps that are the worst are the "El Cheapo" class "C" biased amps that get overmodulated.
You said you might be pedantic, I disagree. Methodical is the way I like to look at it. I have worked in calibration labs and all of the procedures are there for a reason. As Midnightspecial mentioned do not tune an antenna with the amp in line. Forget turned on, Not even in the circuit. You are tuning a transmitting antenna. Add a box with all of the inductors and capacitors and you throw the whole thing off. If the antenna is tuned at four watts it will be tuned at four hundred. The difference is the harmonics. I have observed the output with a spectrum analyzer and the harmonics are being reflected since the antenna is not tuned to 54 MHz it is reflected back and will skew your readings. Sorry about the book.

Tallman