circular modulation scope

[Opcom]

I found this circular pattern modulation scope project in an old 73.

It is interesting but I am curious as to how the idea (with or without deflection voltage amplifiers) could be made to cover all bands without a turret.

There are audio phase shift networks for SSB suposedly covering 10 octaves, but HF covers 20 octaves and it would be an RF phase shift network at that point.

It also seems to run off the antenna transmission line current, yet the author says that a reactance in the antenna will cause the display to be oval, so there has got to be some kind of phase reference to the transmission line voltage there. That does not seem to be explained.

Anyway I thought the idea would be appreciated.

[KA8WTK]

That's interesting. Thanks for posting it.

Bill

[Steve - WB3HUZ]

The reactance will cause a phase shift between the voltage and current on the transmission line. Many hams get fooled by this fact when building phased arrays. They cut the phasing coax to what they think is the correct number of degrees (based on the coax VF), but if there is mutual coupling involved the Z at the input of the line is likely not 50 Ohms with no reactance. So the phasing shift is something other than what they thought.

It also seems to run off the antenna transmission line current, yet the author says that a reactance in the antenna will cause the display to be oval, so there has got to be some kind of phase reference to the transmission line voltage there. That does not seem to be explained.

Anyway I thought the idea would be appreciated.

[WA1GFZ]

Yup been burned by that one. Once you drive the second antenna at a different phase the VSWR takes off.
Life is so much easier on 40 meters with your loop design.

[Opcom]

I am confused, seeing only that the instrument detects current via the 1:8 RF transformer. What is the mechanism for deflection according to voltage? The 2AP1 takes 150-250 volts per inch so to make that pattern one would need as much as +/-250V peak to the CRT.

That looks simple with the transformer acting as a 1:8 current trans. with a nearly unloaded secondary, and I don't understand how the voltage on the transmission line would affect the display. In the schematic L2 the input coupling turn is shown offset from center. In the picture, which is not very clear in the original either, the 'turn' is as long almost as the secondary L1. Maybe this has something to do with it.

[Steve - WB3HUZ]

There is DC from the PS on the deflection plates. The DC is modulated by the AC from the feedline sampling transformer. The sampling transformer steps up Voltage too. It will be developed across the 51k resistors.

[Opcom]

resurrection.. did ever anyone try the circular scheme?

[w1vtp]

No.  

I think that scope monitoring using this method can be misleading (probably better than no modulation monitoring at all).  It is, at best, an approximate idea of what one's modulation level is.

With the abundance of modern scopes on the market at <$100, a good, modern scope is a much better choice.  I think observing one's microphone phase, for example, would be hard to catch using this method.  However, of one wishes to experiment  - - - -

Al

[Opcom]

Yes it was just for fun. Some things can't be presented properly on a digital display that cost less than a used car. REA has done a very good job with their scope displays.

They look as good as they do because we do not usually want to see the intensity variations in the envelope that would be seen on a CRT, but rather most people want to see it solidly filled with a uniform brightness. I do not believe any CRT modulation scope ever had its intensity modulated by the RF envelope amplitude to achieve that.

Personally the brightening of the zero line on a CRT helps me catch the 100% negative peaks if they occur, and it would have been nice if the same would have had an intensification just at the peaks of the envelope.

None of this is necessary with the REA product as it indicates when those events take place. I don't have one so I don't know all the features.

[w1vtp]

Patrick

This scope would be a perfect unit for this circular AM modulation monitor.  Hmmm.  Perhaps you have something there.  Would be good as a display to watch out for the little pearl in the middle of the scope.  Now, if I could only find a place for it at the op table.

Al

[Opcom]

It would be perfect. Any scope with an astigmatism adjustment is a keeper! The 5" tube is much better than a 3" because it can be seen farther away.

How about moving that rack to a diagonal position in the right corner? The lower section seems unused so there would be no waste of rack space and those receivers and stuff would be closer to hand plus the scope looks fine there on top..

In the bottoms of a couple of racks where they are obscured by the desk, there are things like a mains filter/regulator, isolation transformer, AGM batteries, DC supplies, etc., and I gained a lot of space by doing that although some cables had to be longer.

I see you have an SE-3 too. I love mine especially when the band is difficult or an incoming AM signal is a bit weak. I like to talk to those 20 Watt carrier folks and it helps pull them out when they are far away.

[w1vtp]

Pat

Yup, the station is coming together.  Perfect application - direct deflection plate connection will eliminate the problem of the limited bandwidth of the scope's def... amps....