Register Account

Login Help

Forum Home - Rules - Help - Login - Forgot Password
Members can access, post and reply to the forums below. Before you do, please first read the RULES.

Displacement Modulation - I Was Just . . . Wrong

Jul 31st 2023, 11:52


Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
Well, after four years, I have The Final Answer. The Radio Amateur's Handbook, 43rd Edition 1966 was NOT wrong . . . I was. And, I hereby apologize to the editors of the Handbook (ancient and modern), and to everyone who has spent some of their valuable time following along through these topics.

Displacement Modulation, as I have described it, CAN be created in the ways I have discussed AS a current wave and voltage wave in the plate circuit of a simple transmitter - BUT it cannot propagate through the ether, just as the Handbook article warned.

Until yesterday, there has always been a Great Mistake in my test methods: I assumed that if I was intercepting the 'ether wave' from a distance of 15 or 20 feet from a fairly low-power transmitter (30 to 60 watts plate power) that I was seeing the pure wave accurately. But until yesterday, 30 July 2023, the only 'distance testing' (about 100 ft out) was with receivers that appeared to handle the modulation perfectly.

Yesterday, for the first time, I lugged my 'wave sensing' oscilloscope (EICO 460 equipped with a 40M tuned tank at the Vertical Input terminals and with antenna and earth ground) outside over the 100 ft or so to our old corrugated tin barn. Once set up, I was able to see the signal thrown over a reasonable distance, for the first time. What was detected was nothing but a weakly modulated AM signal. There was no evidence whatsoever of my carefully nurtured modulation.

This is exactly the result predicted by some engineer friends of mine at the forums. They explained the theoretical basis for the impossibility of propagating both RF and audio frequency concurrently (which is exactly what I was trying to do, when you think clearly about it) and so I finally realized I had to do such a test. I really expected to be vindicated by the observed physical reality - but, it was not to be. As an old farmer finally admitted as he was struggling to give travel directions to a passing stranger, "Well ... ya just can't get there from here!"

So, I apologize to everyone who wasted any time over the last couple of years in following along with this. Perhaps a few useful things were learned along the way - there were for me, for sure. The only way what I did could be used for anything would be a case were we needed hard-wired RF with AF modulation; but, in such a situation, AM would work just as well and is just as easy to produce.

One thing my engineer friends tried to point out (though their equations were as incomprehensible to me as Swahili) was that my method would ALWAYS produce some AM (and thus, some sidebands, of course), even if perfectly executed. This is why my 'distance' experiments with three different receivers appeared to validate the method - what I was tuning was really just AM after all.

Using the equipment I designed and built, as well as purchased and modified, I will now turn to learning all I can and perfecting (as much as possible) AM done at the control grid of the RF amplifier. This method, though somewhat inefficient, allows modulation with a minimum of expense at the audio end, because the AF power requirement is incredibly low even when excellent results are achieved.

I am, as always:

73 to all,
Larry Cottrill K0WUQ

Back to Top


Instragram     Facebook     Twitter     YouTube     LinkedIn