959B SSB SWR

[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."

 

[144inBama]

caused by a change in the antenna load impedance

I'm reading this as a piss poor magnetic ground can more than likely be the cause?

 

[144inBama]

I just had an epiphany.... When I wash my truck, I remove the antenna base to get the lime dust (biproduct in stainless steel production) from under/around it. I have a permanent "ring" where the mount sits so I can always put it back exactly where it was, within a mil or two. Sooooo, the only thing that changes is removing then sticking it back on disturbing the magnetic bond. So the only sensible explanation (to me) is that the magnetic bond is disrupted then has to "find itself" again. I could be way off in left field, in another city but it's all I have....

 

[Handy Andy]

IF this helps,

The "Auto-range" function the SWR meter uses, remember it has to "measure" against an average...PEP or RMS - anything really - like a governor on a lawn mower - senses power against a given RPM - as the load changes, so does the RPM - so that makes the Governor spring work like it does trying to keep the engine running at the same speed under various loads. It' isn't perfect but then it works better than trying to adjust by hand all the time you run over an obstacle.

So the peak you see - well, take another SWR meter and put it in line and if you can run the amp with that one, well it could help answer the questions you have.

I'm sure you've tried to use SWR CAL on any typical CB Radio - and works great for AM modes but not so much with SSB - why? Because of the level of power SSB is, it's not very level - it's really your voice on one specific frequency and there's no carrier with it.

But did you ever try to see by keeping the Meter in SWR mode, but adjust SWR CAL? Many people raise the sensitivity by making the SWR CAL knob act like a fine tune range adjust - helps someone find a low SWR by turning up the sensitivity to RF Reflected, by gathering more REF power detected at the bridge - but you don't use the CAL side - you'd damage the meter - you keep in on SWR side to find the best dip settings (Lowest SWR)

So how does it your MFJ sense? Well, that relies on the circuit ability to detect and average.

So if you're seeing 1:1.5 don't worry.

You can check with a SEPARATE Manual adjust SWR meter. Capable of the same wattage capacity. You do the work of setting the Calibration for max peak of power - but on SSB this would be difficult - but you can always set the CAL knob to be more sensitive and review the SWR in REF mode. Keeping it in REF mode would let you see the SSB SWR reflected as you modulate - but once you stop or pause, the meter settles down to ZERO.

If you understand the above, then that may help you know how the Autorange function in a meter works. In MFJ's case, SSB peaks tend to be overly strong in Detection so it doesn't display REF power in the best way that other meters more dedicated to sensing, detection and display can do.

So if you don't trust the meter - get one with a Manual knob and trust it with knowing you can set the sensitivity as you see fit.

A good way to look at this is why some CB radios use Analog - while others use a segmented digital bar-graph display.

So the differences are one uses a physical method (D'arsonval) while the other uses a transistorized method to check that same electrical signal.

One has mass to work against, the other doesn't - so one can check at "light speed" while the other uses a trusted means of inertia and momentum against a spring.

That is part of the problem, both methods are used - and the way to display and drive them vary - but MFJ uses a method that requires the needles to jump at a moments notice to show work, but the digital side of the PEAK and autorange function is forcing these needles to act like they do like they are too sensitive - and in a way, you want them to be for if there was something wrong - some of todays electronics would fry before you were able to know and turn off the power.

So to validate the MFJ is doing it's job, use another standard reference of known values - in this case the manual adjust CAL knob. You have to test both against each other, then you can also see by making the Standard Manual CAL knob's REF side more sensitive to changes - you might be able to see why the MFJ is acting the way it does - it's too sensitive at the start making the needle jump.

It sees a strong SWR reflection but then too it's also working against it's own internal Attack and Decay latent effects of trying to show meter action properly calibrated in a sea of RF that thrashing the needles with Audio dynamics - not a steady state flat-sea; flat-earth; ocean of current called carrier.