High SWR when amp is on

#408554

So my SWR is at 1.1 when the amp is off, and when I turn it on it jumps way up to 2.5ish. The meter is between the amp and the antenna. What could be causing this and how do I fix it?

MN785 · #408556

Could be a bad output tune in the amp, but I'd bet the farm that's not the case. Many CB amplifiers have absolutely no output filter, so with the amp on, you are not only transmitting on 27MHz, but also 54MHz and many more harmonics. The antenna, being tuned to 27MHz, is not going to take the harmonics, so those get reflected back to your SWR meter. The SWR meter only sees forward and reflected power, it does not differentiate between the fundamental and any harmonics present. This also means that the watt meter reading is misleading as it too counts everything, not just whats actually on frequency. What you likely need is a low pass filter at the amplifier output to remove these harmonics.

#408557

NoCorn wrote: ↑January 5th, 2022, 12:26 pm So my SWR is at 1.1 when the amp is off, and when I turn it on it jumps way up to 2.5ish. The meter is between the amp and the antenna. What could be causing this and how do I fix it?

Is this mobile or base?

MN785 · #408558

What kind of amp is it?

#408559

I just got a cheap rom kl203p, just for testing it out I am new to cb and Ssb so I bought a cheap one till my buddy’s extra amp. Also do you guys have a low pass filter in mind?

This is a base.

MN785 · #408560

For some reason they are becoming harder to find, but I saw this one on ebay
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#408561

I wonder if it works…

MN785 · #408562

heres one I think is better suited for that amp

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Post Merge Complete

Added 12 minutes 7 seconds after previous.
And it's worth noting, those particular amps start to output garbage if you drive it with more than 4w so don't expect a full 100w RMS out of it (peak, sure, but not RMS). Remember, it takes 6dB to gain one S-unit on the other end, and you need to double the watts twice to get that 6dB increase, so don't overdrive it thinking a few extra watts will make a difference on the other end. Trying to drive it harder to go from 65w to 100w RMS output on dead key will only cause you serious problems. With a filter, if you see 60w dead key coming out, be happy and stop there.

#408783

Particularly if that amp is second hand, I'd suspect damaged MOSFETs or even a damaged output transformer. Especially if it has been over driven or the antenna had a high SWR, or both. A high heat cycle will kill these, always run a fan on it.
Personally, I'd replace the transistors for IRF520s and see what that does. That is, if you're in a position where you want to repair the amp. Another key component that I know can fail is C6, even from new. The other one I've personally experienced from new is either one of the two relays are faulty.

#408784

NoCorn wrote: ↑January 5th, 2022, 5:31 pm I wonder if it works…

I've tried several of those low-pass filters with amps before over the years...never seen one that worked running a cb type amp...

best just to get your SWR set as low as possible with the amp OFF, and not worry about what your SWR Meter reads when amp is turned ON...
FYI, those KL 203's like a radio with a low DK setting and not allot of swing...2w's or less, swinging 10w or less is ideal

#408847

There could be so many things causing a high SWR, you never said the type of antenna you are using. CMC (common mode current) into the meter is the most possible problem, bad coax jumpers, insufficient power supply, all these were never mentioned. Gives us a little more info to help you sort it out.
BTW, I see you never posted here, [Please login or register to view this link] , this will let you see more of the forum after making a post.

3's

Greg

#408850

MDYoungblood wrote: ↑March 31st, 2022, 8:26 am There could be so many things causing a high SWR, you never said the type of antenna you are using. CMC (common mode current) into the meter is the most possible problem, bad coax jumpers, insufficient power supply, all these were never mentioned. Gives us a little more info to help you sort it out.
BTW, I see you never posted here, [Please login or register to view this link] , this will let you see more of the forum after making a post.

3's

Greg

Greg's right.
The most common problem I've seen with this are coaxial connectors, namely, patch leads.
So many use these cheap twist on types here from Jaycar. Personally I want them banned. And there often installed incorrectly anyway.
The higher power will often uncover these weak electrical connections, usually over some time. And if it's a jumper that's often being unplugged etc. adds to the likelihood of it becoming faulty.

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