SWR @ .75 Then Amp Increases It To 4+

[BCL1766]

SWR @ .75 with just my Galaxy 959 and K40 mount on a metal utility truck on roof. Purrs like a cat. I have bought a RM KV 400 Linear and installed it. Without the power hooked up, just the 3' jumper (US Made 3 ft PL-259 Jumper - Times Microwave LMR-240 Coaxial Cable UHF VHF HF LMR240) my SWR is uncontrollable, 4+. Am I using the wrong type of jumper cable or is it the amp? Sorry if this sounds rookie, but I am. I am getting back into CB & SSB after many years not doing it. All help would be appreciated. Thanks! Brett.

 

[kb2vxa]

Being SWR stands for standing wave RATIO expressed as 1 to x (1:x) I don't understand how you arrived at those figures. That aside, meters are fooled by the length of the cable so if you started with a good SWR I'm not surprised lengthening the cable by 3' upset the apple cart. Without getting all technical, adjusting the length of the jumper will bring it back in line not counting for the lumped impedance presented by the "leenyar" especially since when it's operating SWR measured at the exciter (CB rig) is dependent on the amp's input impedance.

Besides all the usual anti illegal amplifier comments long beaten to death there are numerous technical reasons that all come down to setting one up properly is one big hassle that easily confounds those not technically minded. Since you're installing one in a utility truck just ask yourself if you REALLY need high power to talk a mile down the road. If no the answer is obvious but if yes due to road alligators (all mouth, no ears) you don't need CB at all. Unless you're a glutton for punishment you don't need lumps from banging your head on the steering wheel while cussing out the joker who keeps stepping on the guy you're trying to hear.

Oh consider this, a bigger mouth won't help your ears, it just makes you another alligator. Now before the anti-hammie crap begins, I figured THAT out 30 years ago back in my CB daze so take it from an experienced CBer who cut his teeth on a 6LQ6.

 

[OCO]

Replace the amplifier with just a barrel connector and see if the SWR is still bad. If it's good, the wiring within the amp is suspect....all of this assuming you are not putting DC power to the amp. If you were and it's showing bad SWR on keydown, it's the input circuitry to the amp that needs looking at. Back in the stone ages, a good matched antenna didn't care about the length of coax, and the real way to measure SWR was with a real wattmeter that measures power (Like a good Bird), not voltage. Most consumer grade VSWR meters actually just measure voltage and putting in a different length of coax would let the meter see where transmitted RF voltage nulled reflected RF voltage at that specific point on the coax...

 

[LtDoc]

First, where is that 3 foot jumper being added? Between the radio and that amplifier? If so, then the same impedance presented to that radio is now being presented to the amplifier. So, there should be no changes in SWR unless the output impedance of that amplifier is not the same as the radio's. The length of the feed line hasn't changed.
Second question, where are you measuring that SWR, between the radio and amplifier or between the amplifier and the antenna? If between the amplifier and the antenna, then see question one's results, the impedance of the amplifier's output is wrong (no change in feed line length).
There's a 'catch' in there. If the amplifier isn't 'ON' then there should be a by-pass length of feed line in there that goes around the amplifier from it's input connection to it's output connection. That short length of feed line is -short-, and unless there's a problem with it, shouldn't make -that- much difference in SWR. Just something else to check, is it 50 ohms as it should be? (I'm assuming all the coax is 50 ohms!)
If the feed line length makes any significant changes in SWR, and if all the feed line is 50 ohms, and if the output impedance and the input and output impedances of the amplifier are 50 ohms, then there's only one place left that can cause a significant change in SWR, the antenna isn't 50 ohms.
[I'm using the term 'significant' changes in SWR because nothing is absolutely perfect. A little bit of change is absolutely normal because none of the things being connected is going to be absolutely 'right'. They are all 'man-made'. If you are driving a 'service' truck, then you know that anyway, right?]
It can start to get complicated pretty quickly unless you know what's going on. That's why the long way of trying to explain this stuff. I hope some of it helps.
- 'Doc

 

[OCO]

LtDoc..
I took him at his word that he wasn't applying power to the amp and by that I assumed he meant DC power. Leaving the jumper in place and just replacing the amp with a barrel connector would answer the question if the high SWR was due to issues within the amp or the antenna.

 

[LtDoc]

OCO,
I have to assume there was no power applied too. But, the same things still apply amplifier powered 'on' or not. The reasons for that are still basically the same too, an impedance mismatch. Right?
- 'Doc

 

[OCO]

Yes.....

 

[BCL1766]

Thanks for the info, as I know it’s hard to diagnose a problem without being hands on. Galaxy 959 with 3' jumper going to the amp which is mounted 8" from CB and then K40 hooked to output of amp. I originally had power hooked up to the amp and had the high SWR so I disconnected the power as a safe guard until I have the High SWR problem under control. I also noticed that if I raise the RF power it increases the SWR as well. Where I live there is always a hill (Mountains) and I talk on AM as well as SSB. Unfortunately between the mountains and the million other people around here with 1200 watt plus kickers, it does make it a little hard to talk to people. I did the VR18, Vr14, Vr16, Vr13 and Vr17 mods as per discussions on this site as well as others I have been reading. Thumbs up to you guys! Theses mods helped allot. I was and still am not crazy about installing another piece of equipment in my truck, but on some days it would be nice to have a little more punch to continue a conversation when conditions are not favorable. Now it is more of a challenge to me to get this thing working, plus being I already bought it. If it is an impedance mismatch, how and could I repair this? Should I try a 6' long jumper first? The one main problem I have is that there are no more "radio shops" around or close to me, they all went out of business. There is one in Lodi which is 1 1/2 hour drive one way, I went to the store and the store owner unfortunately does not know much about what he is even selling. I think I know more which is scary. I welcome your input on my little problem up here in the mountains. Thank you, Brett

 

[BCL1766]

It is raining so I had some time to play again. Long and short of it was the American "Made With Pride" jumper cable from CB to Amp I bought new, has a bad ground. I tested it and had weird readings, so I had a piece laying around and put male ends on it and my SWR after the hook up went from .75 to 1.2 with amp in line and on. 32 through 40 are a little higher @ 1.4. Thank you for your help and making it all come together. Smart with wood but learning all over again with CB & SSB. It's been too long. Brett.