high swr when more power is added

[76Q]

There are so many variables when it comes to complex impedance matching multiple components from driver to amp to coax to antennas, that all the basic SWR meter is good for is initial setup to identify if your install is safe to use for a certain amount of time. You know your system and when meters such as a basic SWR bridge shows something out of the ordinary then it prompts the operator to check their system especially when no change has been made. As far as amps putting out harmonics or running unstably, picket fencing, etc. the solution is to run them within their clean operating (linear) specs. This may mean that you have to spend a lot more money to achieve the intended power level if you want to maintain clean signal. The cost of one component is never the total cost, as all components in a radio system must keep up with the highest specified component in order for the system to be stable, especially at higher operating levels. To understand the math is one thing, but to be able to use the right equipment to analyse your output is a whole other challenge in terms of cost. A Spec An will tell a lot about your signal and in all honesty, any amp that is overdriven will not look good spectrum-wise, especially when you modulate. Hope this helps.

 

[MrSuburban]

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.

If I am not mistaken this simple test was made aware to this forum a few years ago by Concrete Man who told Ken White to do this simple test, the arguement was that if the swr changed with amp on you are somehow magically checking the output impedance of said amp.

 

[MrSuburban]

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 !!!!

Try searching about 5 years ago for a thread where Concrete Man schooled Ken White who had more EE credentials after his name.

 

[Shockwave]

The only thing that should effect VSWR at the transmitter end of the coax is the frequency of operation. The only exception to this rule would be increasing power beyond the rating of a particular antenna. If changing coax length effects your VSWR, you already had standing waves on your transmission line. If adding an amp makes your VSWR a 3:1, don't buy a cheap solid state amp. In fact some of these junkers run even less stable and clean when driven within the transistors specs. Drive them with low power and a self oscillation has a much better chance of becoming the dominant signal. Sometimes keying one on SSB will produce nothing but a full power signal that looks like a carrier on the watt meter but it's far from it.

Poor or missing negative feedback, lack of shielding between stages, poor decoupling from the DC line, use of single sided boards without a full back side ground plane, high inductance emitter connections, and wide band input and output stages with no tuned circuits all combine to make a wonderful free running oscillator. If the situation is bad enough you can even unkey the transmitter and the RF sensing relay in the amp will stay locked on. The amp has become an oscillator and now generates it's own RF that keeps the relay keyed. This RF is the dirtiest stuff you'll ever see on the spectrum analyzer. If the analyzer could talk it would say "are you kidding"?

 

[Shockwave]

If I am not mistaken this simple test was made aware to this forum a few years ago by Concrete Man who told Ken White to do this simple test, the arguement was that if the swr changed with amp on you are somehow magically checking the output impedance of said amp.

Years ago our local amp builder / pill stacker was making a similar argument to his customers that had radios with variable power. He said there was no need to reduce the carrier in an attempt to stop the Bird from flying backwards. He immediately made the connection that since the output impedance was being changed at the radio through the variable carrier, this was causing the reflected power on the output of his amps to sky rocket. That was a lot easier then admitting his amps were junk.

He had no desire to understand he was partly correct in that the output impedance would change but incorrect that alone would have any bearing on reflected power from the load. Reflected power is a function of the load impedance, not the source impedance. The only time the source impedance becomes an important factor is when it's operated at it's full rated power. Then it's critical due to heat dissipation and efficiency that the source match the 50 ohm load. For the device to produce less then full rated power it must increase it's output impedance and that's A-OK.

 

[MrSuburban]

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

You cannot read the Output impedance of a amplifier.

Maxwell described this with his conjugate match theory

 

[Se7en]

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

my texas star did that. no amp swr was flat high power swr was wack the amp was shit

 

[freecell]

given 100 watts of output from a fixed 50 ohm transmitter into a 50 ohm load, how can we confirm the output impedance of the transmitter? since Z = E / I, we measure the voltage and current present at the transmitter output. in the meantime we have already been given P and Z. now we derive I and E.

I = sqrt of P / Z
100W / 50Z = 2
sqrt/2 = 1.414A

given P, Z and I
E = I X Z
1.414A X 50Z = 70.7V

E = 70.7V and I = 1.414A. dividing the current into the voltage we get 50 ohms. i just measured the output impedance of the transmitter under non-reflective load conditions. you decide, is that the same thing as "reading" the impedance?

is the power output really 100W? 70.7V X 1.414A = 99.9698W.
is the current really 1.414A? 100W / 70.7V = 1.414A
is the voltage really 70.7 volts? 100W / 1.414A = 70.7V
is the impedance really 50Z? 70.7V / 1.414A = 50Z

there's no impedance without current and voltage
there's no SWR without impedance.

in this scenario, the only conditions under which the swr can possibly increase with an increase in power is when the combination of voltage and current deviates from a 50:1 ratio, caused by a change in the antenna load impedance, a change in the tuning and loading of the transmitter or a feedline (short or open) fault

actual measurements and calculations based on 100W CW, "brick on the key."