high swr when more power is added

[troyota]

ok i built another antenna......scary huh? anyway, its a 5/8 wave vertical with 3 ground radials, i have tried it with 4 also. i used the tuning ring method for matching. the swr is a perfect 1.0.1 using my mfj analyser, rs=52 xs=1 or 2. when i add power it jumps up over 2. i have tried different coax, air wound choke, nothing has made a change. i have even moved the antenna location. i was thinking amybe rf in the shack but the air wound choke made no difference at all. i still think this is my problem. cause im getting some rfi in the tv, and phone...(well my wife says so anyway) LOL.
i took my cantenna out there to the antenna location to see if it was feedline. hit it some power and it was flat. maybe came up to 1.1.1 but still very good.
any ideas will be appreciated. also looking into making a 1.1 balun. was hoping the air choke would work but....
thanks guys

 

[swanny]

At the risk of asking a silly question, is there something close to the antenna? How high is it mounted? Is it possible that when you're adding power it's reflecting off something in the vicinity of the antenna causing your swr to go up? It happened to me while I was testing my swr after adding an amp in my mobile. Once I went to an open parking lot, my swr was the same within a couple tenths from barefoot to full power. Maybe the antenna needs to be higher?

 

[Shockwave]

As long as the antenna can handle the power being applied to it, the SWR should not change significantly with added power. The fact you are using a tuning ring on the 5/8 wave rather then a more fragile coil leads me to believe it may not be related to the antenna at all. If you are using a solid state (transistorized) amplifier, it's much more likely that the amplifier is producing output on a frequency other then the fundamental. This also contributes to the TVI issue. The most likely cause would then be an oscillation occurring within the amp. Very common problem with 11 meter transistor amps. It looks good on the dummy load because that represents a 50 ohm load at all frequencies and does not have a resonant frequency like the antenna.

 

[swanny]

the mobile isn't high enough to get over the house, and of course the base antenna would be (smacking self on forehead). I'll try and think about it a little longer before I post next time

 

[troyota]

hey swanny dont hit yourself too hard....LOL your mobile antenna is probably higher than my antenna is now. it is on a 8' mast now for testing. BTW any response is better than none my friend. as i said before i had moved the antenna to a different location...but the height was the same. there are some things that could be interfering around and causing reflection. my tower is a tilt over type...kinda. anyhoo it is not too much trouble to put it up to see....
shockwave i am using a solid state amplifier. i can use a tube style for testing as i have a few around here. as a matter of fact i have one i can try quickly. on another note i put it on my drake wattmeter. it shows about 200w forward carrier with about 10w reflected. when i modulate it will go up to about 450 forward and reflected will drop down to almost nothing???????? Huh?? maybe more going out than coming in forces it the other way...LOL wait.... before you guys toast me on that comment, i really dont think thats the reason ok..just trying to keep my spirits up..
i will try the other tube style amp to see what it does. will let you know.
thanks guys

 

[troyota]

DUDE!!!!!! shockwave you are the man!!! i cant @#%$^ believe it. the reflected is flat as a pancake with a tube amp. 0 watts reflected. checked with a different swr meter and it comes up about a needle width 1.1.1 maybe at most. BEAUTIFUL...... dang i wish i had posted here a month ago. been havin trouble with my antenna forever. i have built several different ones to try. i even gave up on my imax too. it was doing the same thing.
KEWL man, now to go weatherproof and secure the thing to go up!!!! this forum is great. i have had good luck with the people here and lots of help too. hopefully i can return the favor. again guys thank you shockwave. hey swanny thank you too. as i said before any response is a good one.
thanks guys
troyota

 

[swanny]

that's awesome and that also tells me to search for tube power when I decide to upgrade my base.

 

[Shockwave]

It's my pleasure to share knowledge with others interested in the hobby. It's even more satisfying when people use the suggestions effectively rather then argue over nonsense. I sure don't know it all but usually won't make a comment unless I have a firm grasp of the technical issues surrounding the situation.

On another note you may be able to stabilize that solid state amplifier by adding negative feedback if the circuit is not already in use in your amp. It's very simple and involves adding a resistor and capacitor in series across each transistors base and collector terminals. Typical values are 100 ohm 2 watt resistors and .01 MFD capacitors.

On the other hand if your tube amp is working well, I would just continue to use that in the base. Cathode driven tube amplifiers almost never oscillate and their output PI network offers harmonic attenuation. Personally, I avoid the use of solid state amplifiers except when the only available power source is 12 volts DC.

This is not to say that solid state amplifiers can't be made to work as well as tube amps. If you've ever looked inside something like the Icom PW-1 as compared to an 11 meter solid state amp, you'd be shocked how many more components are used in a well designed amp. About the only flaw with that amp is it's switching supply does not like spikes on the AC line.

 

[W5LZ]

The cause of that change in SWR isn't that trasnsistor amplifier putting out anything other than the desired signal, forget the harmonics/parasitics thing. The SWR changed because the output impedance of that transistor amplifier isn't close to 50 ohms (R=50, X=0). Simple as that, and quite common with "no tune" amplifiers.
- 'Doc

 

[Shockwave]

The cause of that change in SWR isn't that trasnsistor amplifier putting out anything other than the desired signal, forget the harmonics/parasitics thing. The SWR changed because the output impedance of that transistor amplifier isn't close to 50 ohms (R=50, X=0). Simple as that, and quite common with "no tune" amplifiers.
- 'Doc

With all due respect that answer is completely false. What I explained earlier is exactly what is going on with that transistor amp. The SWR meter does not react to the output impedance of the amplifier. The meter simply assumes you've connected it to a 50 ohm transmitter. Even if the output impedance of the amplifier is different, the 50 ohm line section in the SWR meter is still only looking at the load impedance it's connected to and not the output impedance of the amp.

For those who still question my response here. Try this simple test. All tube amplifiers can have their output impedance varied from about 25 ohms to 75 ohms depending on how you load and tune it. Feel free to adjust your load and retune the plate tune and observe your SWR meter after recalibrating the meter. I don't advise keying the amp for long here because it's not properly tuned but it will become very clear you can't change the SWR meter by varying the output impedance of the amp. Of course the meter will need to be recalibrated because of the different power level.

 

[loosecannon]

im with shockwave on this one.

while i can see where W5LZ's comments have merit, and are possible; more often than not, in practical everyday CB radio experience; the amplifier is putting out harmonics from being overdriven or just from poor design and no filtering on the output.

troyota, you can tell them who was right if you have a low pass filter.
put your low pass filter after the amplifier and before the SWR meter.
see if the SWR goes down when the low pass filter is in line as opposed to it not being there.


i have tried this myself and confirmed, as far as i can tell, that my DX400 puts out harmonics on its highest power setting.
is this because of an impedance mismatch having to do with the drive level of my radio?
i dont have the technical equipment or know how to confirm or deny that.
i only have the low pass filter test to determine what is going on in my system.

why do i think its harmonics?
because when i screw a dummy load to the antenna connector i get 1.1 to 1 readings on all power levels.
when i use the wilson 5000 antenna, the SWR will vary a bit on different power levels, and makes a big jump up when i get to the highest power level.

i put my drake low pass filter inline after the amp and before the SWR meter and lo and behold, the SWR held at around 1.5 to 1.6.

take the filter out of line and it goes right back up to 2.5+ on that same power level.

to the best of my current knowledge i have interpreted the results correctly, but i have never claimed to know it all. (well once maybe LOL)

LC

 

[bob85]

i don't doubt your response at all,
higher vswr is nothing to do with the amplifier output impedance not been 50ohms, thats copycat cb amp builder lingo/nonesense,

amplifiers can only ever be 50ohms at one drive level/supply voltage, you having calculated output tank circuit requirements would realise that,

you don't get high vswr when you change drive level or supply voltage on a stable amplifier,

you can alter transformer turns ratios and not get high vswr,

mistuning your tube amp away from a conjugate match won't cause high vswr,

not having the transistors matched to 50ohms through the correct transformation ratio for the running conditions of the transistor does not cause high vswr in an otherwise sound system with near perfect match between antenna and feedline,

we can get into the possibility of the rereflected wave from a mismatched load not seeing 50ohms looking backwards into the amp causing a further shift in the current/voltage standingwave interference pattern within the coax but first you need half an idea how amps work,

the conjugate match idea is still hotly debated in some circles, only in cb forums do some people blame it on high vswr,

on long runs of coax the meters detector diode none linearity can create a illusion of a small increase in vswr with increased transmitter power,

the large increase in vswr is likely to be as shockwave stated, an unstable amplifier

 

[Shockwave]

It's important to note that the symptoms described concerning high SWR on the solid state amp are not related to 2nd and 3rd order harmonics where you would need a spectrum analyzer to confirm their level of amplitude or need to filter them out to correct this problem. When an unstable amplifier breaks into oscillation as is the case here, the undesired frequency component is usually not very far from the fundamental. Often within a few MHz. You can buy a cheap scope on eBay for $75 that will spot this problem.

For example, when an amplifier is oscillating look at the carrier wave on the scope. You will see at least two signals mixed together. Lets say the main carrier is at 4 divisions peak to peak on the screen. Underneath that carrier you will see another at perhaps 2 divisions on the screen. The upper and lower envelope of the second undesired signal will often be lower in amplitude and full of distortion within it's envelope. Under conditions of low drive or SSB use, the oscillation may be the dominant signal.

So one may be asking, "Why does adding a TVI filter sometimes reduce the SWR in a case like this? The answer is simple. When you are dealing with an unstable amplifier, any slight change in the inductive or capacitive reactance of the load can cause changes in it's operation. Sometimes even on the input side. You can often find a situation where just adding or subtracting some coax will find a condition where the amp runs stable for now. Adding the TVI filter with all of it's inductors and capacitors can certainly shift these conditions.

 

[BOOTY MONSTER]

ok ..... my question is why in the 4 years that ive been coming to this forum has this never been explained this way befor ? LOL
very interesting thread !!!!

 

[Shockwave]

Well, I've only been here for less then a year