Amp raising SWR reading !

[FlyBoy50]

Thought I would ask a general question of why does adding a amp to your radio system sometimes raise your SWR reading and what are some cures for it to bring your SWR back down to acceptable levels ? I've experienced this more than once but can't remember ever reading any posts on a solution.

 

[loosecannon]

well the first question i have is, which SWR are you talking about?

when you add an amp in line after your radio, you actually introduce an additional "load" that is different from the antenna "load".

let's say you have a cobra 29LTD that has a built in SWR meter.
you've checked it against a good external SWR meter and you know it to relatively accurate.

so, you connect your antenna to the 29LTD and you check your SWR.
let's just say you read a 1.3:1. great!
your antenna is tuned well.

now, you put an amp in between the 29LTD and your antenna.

When the amp is on, the radio is no longer reading the SWR of the antenna.
it is reading the SWR of the input stage of your amplifier.

so moving ahead with our scenario of a known 1.3 SWR without the amp in line.

now, with the amp on, you key the mic on the 29LTD and check the SWR again.
now it reads 2.5:1.

This is not a problem with your antenna, this is a problem with the input circuit of your amp.

some amps make this an adjustable setting, but most do not.

this is why you see a lot of people taking their radios and amps to techs asking them to "marry them" or "match them".

the tech is going to tune the input circuit of the amp to as close as they can get to a 1.1:1 SWR from the radio into the amp.

this way, most of the power coming out of the radio goes into the amp and is not reflected back into the radio.

to actually test your SWR at the antenna when using an amp, you have to add an SWR meter AFTER the amp.

if you tune your antenna to a 1.3 and then with an amp in line and on, the SWR measured after the amp rises by more than a few tenths, you either have a problem with the output stage of the amp or the antenna itself.

hope that helps.
LC

 

[FlyBoy50]

if you tune your antenna to a 1.3 and then with an amp in line and on, the SWR measured after the amp rises by more than a few tenths, you either have a problem with the output stage of the amp or the antenna itself.

hope that helps.
LC

Thanks all info is golden !

 

[9C1Driver]

Great explanation. Unfortunately quite a few “cb” amps are this way. You will find for the most part the custom amp builders pay closer attention to input/output tuning.

 

[FlyBoy50]

Great explanation. Unfortunately quite a few “cb” amps are this way. You will find for the most part the custom amp builders pay closer attention to input/output tuning.

You got that right, I've run into that more than once. I think I'm going to run my 2 Q6Pros without amps and just use the variable power, and settle for that !

 

[9C1Driver]

Back in the day I owned a Magna Force 100, 200, and 350 (built by X-Force). They all had a nice low SWR and worked great. They were class B for SSB use, should have kept them lol. Carl is back at it if you really need a well built amplifier. I also had a Joker amplifier with regulated AB bias which was probably the best amp I ever owned. Wish Peakaboo was still building amplifiers.

 

[dave457]

He is if you know how to get ahold of him

 

[nomadradio]

This is why we won't hook up an amplifier on the bench without two wattmeters. One between the radio and amp input, and the other one between the amp and the dummy load. The input circuit has to fool the radio into thinking it's hooked to an antenna with a low SWR. Some amp builders pay more attention to this performance issue than others do.

73

 

[Chris Millas]

This is why we won't hook up an amplifier on the bench without two wattmeters. One between the radio and amp input, and the other one between the amp and the dummy load. The input circuit has to fool the radio into thinking it's hooked to an antenna with a low SWR. Some amp builders pay more attention to this performance issue than others do.

73

In my opinion, if your running an amplifier, you should absolutely have a meter between the radio and amp and another meter after the amp and before the antenna.

 

[secret squirrel]

I am sure everyone can provide a better explanation than me. When I hook up my amp, KL 503HD, the harmonics on the output side are bad. An old Drake Low Pass filter helped a lot. I am sure its just a patch on a poorly set up amp, but it seems to make a huge difference. If I am understanding correctly this is why Nomad is saying the two meters are necessary. Please correct me if I explained poorly.

 

[9C1Driver]

The watt meter between the radio and amp lets you check the SWR on the input side of the amp. If the SWR is high then the amp was not properly tuned for 50 ohms and needs some work.

 

[FlyBoy50]

The watt meter between the radio and amp lets you check the SWR on the input side of the amp. If the SWR is high then the amp was not properly tuned for 50 ohms and needs some work.

What all is involved in tuning it for 50 ohms ? I'm not a electronic tech but hope it would be something simple like adding a resistor in the right place. But I'm probably a " Wishful Thinker " LOL

 

[Ranch55]

This is why we won't hook up an amplifier on the bench without two wattmeters. One between the radio and amp input, and the other one between the amp and the dummy load. The input circuit has to fool the radio into thinking it's hooked to an antenna with a low SWR. Some amp builders pay more attention to this performance issue than others do.

73

That is exactly right.....

 

[Ranch55]

What all is involved in tuning it for 50 ohms ? I'm not a electronic tech but hope it would be something simple like adding a resistor in the right place. But I'm probably a " Wishful Thinker " LOL

Yeah, wishful thinking....

 

[Dr_DX]

Usually, it is adjusting capacitance to ground - pf range. For example, the KL-703 has 120pf to ground on its input (Relay in TX, not bypass). This usually results in a 1.4~1.5 input SWR. By adding some additional capacitance to ground - 20~40pf (fixed or variable) - you can get it down to 1.1 or less. Just make sure your voltage rating on your capacitor/s is high enough to handle the input watts. Sometimes, if you have an inline inductor, you can change the value of it as well to help reduce your input SWR.

A well designed amp usually has a good SWR in bypass mode, but you may need to work on that too if you sometime run with the amp off.