High SWR when amp is on

This forum is for general discussions, and you are welcome to post in this forum if you are not able to find the right place.
User avatar
NoCorn
NEW DUCK
NEW DUCK
Posts: 0
Joined: January 2nd, 2022, 9:45 pm
Age: 24

High SWR when amp is on

#408554

Post by NoCorn »

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?







Donate to CBRT
User avatar
MN785
Donor
Donor
Posts: 33
Joined: October 12th, 2021, 7:18 pm
Age: 41

#408556

Post by MN785 »

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.
Last edited by MN785 on January 5th, 2022, 4:24 pm, edited 1 time in total.







Donate to CBRT
User avatar
Verified
Owner
Owner
Posts: 9,047
Joined: August 31st, 2004, 1:06 pm
Age: 48
17

#408557

Post by »

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?







Donate to CBRT
User avatar
MN785
Donor
Donor
Posts: 33
Joined: October 12th, 2021, 7:18 pm
Age: 41

#408558

Post by MN785 »

What kind of amp is it?







Donate to CBRT
User avatar
NoCorn
NEW DUCK
NEW DUCK
Posts: 0
Joined: January 2nd, 2022, 9:45 pm
Age: 24

#408559

Post by NoCorn »

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.







Donate to CBRT
User avatar
MN785
Donor
Donor
Posts: 33
Joined: October 12th, 2021, 7:18 pm
Age: 41

#408560

Post by MN785 »

For some reason they are becoming harder to find, but I saw this one on eBay
[Please login or register to view this link]







Donate to CBRT
User avatar
NoCorn
NEW DUCK
NEW DUCK
Posts: 0
Joined: January 2nd, 2022, 9:45 pm
Age: 24

#408561

Post by NoCorn »

I wonder if it works…







Donate to CBRT
User avatar
MN785
Donor
Donor
Posts: 33
Joined: October 12th, 2021, 7:18 pm
Age: 41

#408562

Post by MN785 »

heres one I think is better suited for that amp

[Please login or register to view this link]




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.







Donate to CBRT
User avatar
arcone
6 PILL USER
6 PILL USER
Posts: 57
Joined: October 30th, 2018, 4:43 pm
Age: 44
3

#408783

Post by arcone »

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.







Donate to CBRT
User avatar
jessejamesdallas
Verified
Wordwide & Qualified
Wordwide & Qualified
Posts: 6,211
Joined: October 10th, 2004, 7:02 pm
Age: 100
17

#408784

Post by jessejamesdallas »

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






Image
Image
Do not argue with an idiot. He will drag you down to his level and beat you with experience.

Donate to CBRT
User avatar
MDYoungblood
Verified
Site Admin
Site Admin
Posts: 10,398
Joined: June 12th, 2010, 8:05 pm
Age: 70
11

#408847

Post by MDYoungblood »

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






"321, NW South Carolina"

Official Moderator - CBRT Complaint Department

Donate to CBRT
User avatar
arcone
6 PILL USER
6 PILL USER
Posts: 57
Joined: October 30th, 2018, 4:43 pm
Age: 44
3

#408850

Post by arcone »

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.







Donate to CBRT
Post Reply Previous topicNext topic