Low pass filter on cb plus amplifer setup

[Elkhunter521]

This is a hypothetical question. On a system that has a cb radio and an antenna tuned to 1.2 vrs 1.3 channels 1 and 40, when you add a cheap 150 watt linear amp and the swr goes stupid high, where would you install a 30 mhz low pass filter and where would you measure swr after installing the filter???

[Lscott]

Hypothetically if one didn’t use the amplifier there wouldn’t be the necessity of asking the question.

[WRKC935]

Where are you measuring this increase in SWR?  At the radio or between the amp and antenna and what are you using for a meter to test it?

If you are seeing an increase at the radio that means the input impedance of the amplifier is off.  That's actually sort of common as there is little quality control in the manufacture of amplifiers for that band.

If you are using an SWR meter that is designed for CB at a 'legal' power level and doesn't have an adjustment for calibration to the power level then it may be fine.  You have to understand that if the meter is expecting 4 watts forward then a 1.5 SWR would be about a tenth of a watt.  If you connect a meter like that to a 100 watt source, 1 or 2 watts of reflect would be that same 1.5 SWR.  But 2 watts reflect on a 4 watt source is an SWR of like 10 or something like that. 

Short of that, you have a crap antenna or you are trying to drive a mag mount antenna and the capacitive linking to the car body for the counterpoise of the antenna is NOT coupling well enough and driving up the SWR.  A properly designed antenna system should NOT change SWR with a change in power level unless the power applied is beyond the breakdown voltage of the components of that antenna system.  And then it's actually failing and burning up, not changing resonance.  

 

Installation of a filter is NOT going to have any effect on the SWR if the filter is properly designed, and if poorly designed, it will INCREASE the SWR not lower it. 

I am gonna assume you are having issues with the RF interfering with other equipment and someone told you to use a filter?

If that's the case, you need to do two things.  First is quit driving the hell out of the amplifier and thinking that you need to get every last watt out of it by driving the circuit past saturation and linearity.  The second is buy a better amplifier.  RF gets amplified all the time.  Radio stations run tens of thousands of watts and interfere with no one.  So it's NOT a power issue.. it's a crap design issue.  There's not exactly a set of regulations for spectral purity for amplifiers in that band as they are illegal.  So stuff just gets slapped together with a cool name and flashy paint and it gets sold to unsuspecting guys that have no idea, they just want to get out better.

And DAMN few CB shops have real radio technicians.  Most of those guys got shown by someone it radio X that you cut this part, turn this adjustment and lie a lot to the radio owner.  Collect his money and wait on the next mark.  They get a bunch of old test equipment they have no clue on how to operate but it looks good to people that have no clue and put it on their work bench.  The really dishonest ones will have a modified watt meter that has a hidden switch elsewhere that they flip after 'tuning' your radio to show you your 4 watt radio that has a 6 watt final in it is somehow outputting 10 or 15 watts.  It's all bullshit.  I lived that life for a few years.  I watched co-workers screw people out of money putting 'kits' in radios that did NOTHING that were soldered into ground points and then after turning their mike gain back up and keying the radio as they would modulate and unkey as they stopped whistling get the watt meter to swing all over the place.  And they did NOTHING to the radio to make it better.  But it put money in their pocket.  And you somehow are led to believe that they would sell you a quality amplifier.  Sorry to burst your bubble there.... but thems the facts.

 

 

[SteveShannon]

34 minutes ago, WRKC935 said:

Where are you measuring this increase in SWR?  At the radio or between the amp and antenna and what are you using for a meter to test it?

If you are seeing an increase at the radio that means the input impedance of the amplifier is off.  That's actually sort of common as there is little quality control in the manufacture of amplifiers for that band.

If you are using an SWR meter that is designed for CB at a 'legal' power level and doesn't have an adjustment for calibration to the power level then it may be fine.  You have to understand that if the meter is expecting 4 watts forward then a 1.5 SWR would be about a tenth of a watt.  If you connect a meter like that to a 100 watt source, 1 or 2 watts of reflect would be that same 1.5 SWR.  But 2 watts reflect on a 4 watt source is an SWR of like 10 or something like that. 

Short of that, you have a crap antenna or you are trying to drive a mag mount antenna and the capacitive linking to the car body for the counterpoise of the antenna is NOT coupling well enough and driving up the SWR.  A properly designed antenna system should NOT change SWR with a change in power level unless the power applied is beyond the breakdown voltage of the components of that antenna system.  And then it's actually failing and burning up, not changing resonance.  

 

Installation of a filter is NOT going to have any effect on the SWR if the filter is properly designed, and if poorly designed, it will INCREASE the SWR not lower it. 

I am gonna assume you are having issues with the RF interfering with other equipment and someone told you to use a filter?

If that's the case, you need to do two things.  First is quit driving the hell out of the amplifier and thinking that you need to get every last watt out of it by driving the circuit past saturation and linearity.  The second is buy a better amplifier.  RF gets amplified all the time.  Radio stations run tens of thousands of watts and interfere with no one.  So it's NOT a power issue.. it's a crap design issue.  There's not exactly a set of regulations for spectral purity for amplifiers in that band as they are illegal.  So stuff just gets slapped together with a cool name and flashy paint and it gets sold to unsuspecting guys that have no idea, they just want to get out better.

And DAMN few CB shops have real radio technicians.  Most of those guys got shown by someone it radio X that you cut this part, turn this adjustment and lie a lot to the radio owner.  Collect his money and wait on the next mark.  They get a bunch of old test equipment they have no clue on how to operate but it looks good to people that have no clue and put it on their work bench.  The really dishonest ones will have a modified watt meter that has a hidden switch elsewhere that they flip after 'tuning' your radio to show you your 4 watt radio that has a 6 watt final in it is somehow outputting 10 or 15 watts.  It's all bullshit.  I lived that life for a few years.  I watched co-workers screw people out of money putting 'kits' in radios that did NOTHING that were soldered into ground points and then after turning their mike gain back up and keying the radio as they would modulate and unkey as they stopped whistling get the watt meter to swing all over the place.  And they did NOTHING to the radio to make it better.  But it put money in their pocket.  And you somehow are led to believe that they would sell you a quality amplifier.  Sorry to burst your bubble there.... but thems the facts.

 

 

Well, hypothetically speaking.