The AM Forum

..tune up the dx100b in cw, switch to phone and swr is through the roof..plate current is good, grid current etc in cw...it acts weird on AM only...sometimes the rig is werking fb, on AM, THEN swr jumps up during a transmission....ideas?

..sk..

What is between the transmitter and the antenna and where in that lineup are you measuring SWR? Sounds like something is overheating, had a similiar problem once, turned out to be the SWR meter itself was heating up and shorting out.

You are putting out a lot of power off frequency, or something is changing in the antenna/tuner/feedline during AM.

might be something going wild in the modulator to generate a lot of different frequencies in the output.
Can you monitor the RF output of the DX100 with a receiver to listen for any strange sounds when PTT is pressed and AM mode?
Does the DX100 monitor anything in the modulator circuit? Grid current of the modulator tubes?
Something is making a very different RF output in the final to make the SWR increase.
Fred

all appears ok, freq counter shows no odd transmit freq, etc...swr bridge has been changed, no difference...not been inside the rig as yet...

Does it do it into a dummy load?

Tuned up on a harmonic.

He says no odd frequency shows up on a counter. What about the antenna? I once knew a tri-band beam with a trap problem, sometimes it resonated on 10M, sometimes on 11M. Is there a resonant circuit up there? Do you use a tuner? We need more info, a transmitter change from CW to AM affecting SWR sounds mighty weird. Could be since you only apply RF briefly during tune up on CW the problem is far less likely to show up then.

I had the same thing on a 4CX1500B final. Tuned up perfectly and no signs of weirdness on the meters.
Something made me tune the RX for the second harmonic and there it was.
Freq counter is useless. A spectrum analyzer or an SDR will show it pretty fast.
Fred

On CW or tune up there is 100 watts +/-. On a modulation peak the peak power goes up 4X or so. An arc or flashover somewhere in the tuner / transmission line / antenna will cause the SWR to spike during an audio peak. I have an SO-238 coupling outside, and I have lost three of these in the last few years. They get wet inside, and then I blow them up. The reflected power hits the ceiling during the arc.

Jim
Wd5JKO

Good observation of the OP and how he described the problem. Modulation causes spike in SWR.
Forget the second harmonic
Fred

Are you using a peak reading wattmeter?  VSWR may appear to rise on audio peaks

I did a quick video to illustrate what I'm asking but, perhaps you are taking about VSWR rising without
any change in peak forward power?????

Here's a link

https://www.youtube.com/watch?v=GHkXQ56RojM

nice video....as you said though, the VSWR was constant at 1.5;1....This is normal. With the wattmeter on the higher sensitivity scale, the VSWR was also constant, but off the chart.

Back to my original post, if modulation causes VSWR to spike (like 1.5:1 to > 10:1), then something is either arcing (after the swr/wattmeter), OR (I am adding this now) the RF final is going into a modulation induced parasitic.

Jim
Wd5JKO

I agree that a significant change in the transmitter's output frequency, or the introduction of a much larger percentage of harmonic power, would cause the VSWR on the coaxial cable connected to the output port to jump up. For example, having most of the total output power of the transmitter switch from the fundamental to the 2nd harmonic of the nominal carrier frequency.

However, what is puzzling is that this transmitter has a pi network at its output... which should present a low impedance to any component of the plate current flowing through the output tubes whose frequency is the 2nd harmonic or a higher harmonic of the frequency that the pi network is tuned to (and also to high frequency parasitic components of the output tubes' plate current).* In fact, a class C plate current waveform contains lots of 2nd and higher harmonics... and the pi network's low impedance at harmonic frequencies prevents significant harmonic voltage from appearing across the output port of the transmitter.

*The component of the plate current at the resonant frequency of the pi network produces a voltage across the load impedance (the impedance of the loading capacitor in parallel with the output load) that is approximately: the amplitude of that component of the plate current x the load impedance at that frequency x the Q of pi network. The component of the plate current at the 2nd harmonic of the resonant frequency of the pi network produces a voltage across load impedance that is only: the amplitude of that component of the plate current x the load impedance at that frequency x 1/3.

Therefore, if the pi network is functioning properly, I would think that the forward (from the transmitter toward the antenna) output power of the transmitter would decrease dramatically (in addition to the VSWR on the coaxial cable connected to the output port going up) under most scenarios that are associated with such things as a significant increase in harmonic content in the output tubes' plate current or a parasitic oscillation of the plate current.

I suspect that something is arcing or opening on the antenna side of the SWR meter.
Examples: A) Switching to AM causes a marginal (high resistance) connection on the antenna side of the SWR meter to heat up... and, after a few thermal time constants, the connection opens up. B) An arc forms across the feedline to the antenna on an AM positive modulation peak; and the arc is sustained once it forms... until the carrier is dropped for at least a few milliseconds. C) A conducting path forms within a trap in a multiband antenna due to the breakdown of an insulator.

Stu  

..appears something in the driver stage...less drive and retuning the driver lessens the issue...still there...5763 or 12by7, or clamp adjustment...no time to get inside as yet..
..sk..

Keep in mind that reduced drive means reduced peak power on AM. Reduced peak power could keep the arc from happening, hence stable SWR.

Going off on a tangent...see me Gonset story below.

There was never a mention of which band this is happening. On my Gonset G-76 with the Knight V44 VFO, I multiply 12X to get to 15 meters. That VFO always runs on Top Band. If I detune my driver stage, (15m now) I can see harmonic or wrong multiplication factor content on the scope. With a slow sweep, I get horizontal lines just within the carrier horizontal edges. Playing with the drive tuning, I can make this worse, or better. Neutralization comes into play hera as well, since if perfectly neutralized, the grid current peak in TUNE is correct in TRANSMIT. If the neutralization is off, then I have to peak the drive tuning in TRANSMIT at reduced drive setting in order to peak out the modulation to fullest value, then increase the drive to specifications.

Jim
Wd5JKO

Also over-driving the 6146 finals can be worse.
3 ma is the max ????

What is the antenna?

One fond memory of my dx-100 was zorching traps in various antenna's with it.

Tims antenna is a 780 ft loop fed with OWL. Its happening on 80m. Don't think he has a dummy load.

I'm guessing that you're having an issue with an intermittent/bad connection at the antenna feed point. You've had this issue at least once before this winter as I recall, with all the high winds, ice storms and the snow with an antenna of that size, and with issues supporting it adequately. Things did seem to clear up when you tied the OWL feeders together, and fed the antenna as a long wire.

Looks like you may finally have to do some troubleshooting with that DX-100 Tim. You do have a few parts/non-working DX-100s there, so you have plenty of spare parts if needed. :)

Al VE3AJM

This is a classic problem of course, but nothing can be ascertained until you load directly into your dummy load and assess performance with a resistive load rated beyond your peak power.

daniel

Well, that is the problem, the antenna is too short...
780 feet, might as well use a hand held antenna.

there never was any arcing..into the dummy load it does the same thing...tunes up fine in cw, but in am it has a high swr...the rig has not been hot switched, etc...

..sk..

..should add, this happens even without any audio...mic not connected, and gain turned down...
..sk..

It is very strange that the VSWR reading with a 50 ohm dummy load attached to the antenna-side connector of the SWR/power meter would be anything except 1:1.

If the SWR/power meter is working properly, if the link between the meter's antenna-side connector and the dummy load (which could be a single coaxial jumper cable, or multiple coaxial jumper cables, in series, interconnected by barrels) is, in fact, a 50 ohm impedance link; and if the dummy load is, in fact, a 50 ohm dummy load.... then the meter should register an SWR of 1:1 when the transmitter produces output power, regardless of the details of what is happening on the transmitter side of the meter.

I am assuming that there is nothing between the meter and the dummy load that is frequency selective (e.g. no antenna tuner between the meter and the dummy load).

Have you checked to see if the cable between the SWR/power meter and the dummy load has a loose shield at either connector, or a tendency to form a short circuit at one of the connectors? Is the center conductor making good contact to the connector center pin at each end, and is the connector center pin making good contact to the socket at each end?

Separately, with the dummy load attached, does the VSWR reading jump up if you tune up on CW, and then run the transmitter on CW, key down, for 2 minutes?

Stu

I agree with Stu.....A dummy load is a poor indicator to show reflected power.
You need a frequency sensitive load like an antenna known to be good. An MFJ259 is a good start to get an idea what's going on with your antenna.
Got it a little clearer now. Switch to AM and NO MODULATION.
Try this........pull the modulator tubes ( TX off ,,, the tubes are cold) and see what happens when you TX in the AM mode. Go through the scenario of CW first and then AM mode with the mod tubes pulled out of their sockets. TX off and allowed to cool down OR you will burn your fingers
Fred

Wa hoppin Lucy, you fogeh you bestdos  ohbin mit again? (;->) I may be a bit of a dummy load myself, but I don't see the difference between the AM position with the tubes removed and the CW position. In the latter the mod iron secondary is shorted, do you suspect the transformer? What about another test with the mutilator tubes in and the driver out? Could be something freaky going on with them, ah dunno.

If it was something in the modulator, you should see modulator plate current, no?
And the antenna tuna would have to be very high Q and tight to show high swr from something the modulator was putting out....

You mean to say that into a dummy load in CW mode an SWR meter shows no VSWR, but in AM mode the same meter on the same dummy load does show SWR? How good is your meter and cabling?

Phil - AC0OB

I agree Phil...this thread is getting really confusing....I asked the OP to remove the mod tubes to see if the SWR problem stops in AM mode. I assumed the SWR is there when connected to the antenna. Now a dummy load is in the mix.
There can be no SWR into a dummy load. Freq range is from MF to light.......almost.
Someone else will have to sort this out......I'll be in the background reading.
Fred