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Joined: Mon, Dec 3rd 2012, 11:13 Roles: N/A Moderates: N/A

Latest Topics

Topic Created Posts Views Last Activity
Run Plate Transformer at Half Voltage? Feb 28th 2022, 12:21 1 2,691 on 28/2/22
How To Do AM? Feb 25th 2022, 21:40 1 3,398 on 25/2/22
Flip Antenna Phase? Feb 14th 2022, 23:55 6 3,105 on 16/2/22
Heterodyne Rockets Feb 13th 2022, 12:52 3 3,087 on 15/2/22
Displacement Modulation Feb 5th 2022, 23:29 8 3,304 on 14/3/22
Appropriate 40M Crystal Frequencies Feb 3rd 2022, 10:58 5 3,139 on 7/2/22
Proper Grounding of Coaxial Feed? Feb 2nd 2022, 15:26 1 2,743 on 2/2/22
Monitoring Modulation Feb 2nd 2022, 15:01 1 3,031 on 2/2/22
Mixed Mode AM/SSB QSOs? Jan 31st 2022, 23:58 3 2,922 on 1/2/22

Latest Posts

Topic Author Posted On
New ARRL website / changes? PE1HZG on 14/6/22
This my latest attempt to post to the forums. I am working with Dennis Budd K3DGB at ARRL to resolve my problem. If you can see this, it means my problem is resolved and I can post again!

Please feel free to delete this test post.
Larry K0WUQ
Chimney mounted vertical antenna grounding KY4RQ on 14/3/22

Here's something I remember that I have no idea whether it will be helpful or not, but I thought at the time (1960s) that it was a neat and logical design detail.

I purchased a 40M vertical, Hy-Gain brand, I think. At the bottom end it had a big plastic mounting that brought the aluminum antenna and the mast together in such a way that a small lightning gap was formed between them. I don't remember how that was adjustable, but it seemed logical because it gave lightning a short, straight path to ground (assuming the mast was fully grounded, of course). One property of lightning is that it will try to take the straightest low-impedance path - one reason you should never have short, elbow-like bends in your grounding wire.

Probably an 'old hat' idea, but maybe something you haven't seen before if you're new to working with verticals.

Larry K0WUQ
Displacement Modulation K0WUQ on 14/3/22

I have now done a simple set of experiments clarifying the 'blocking capacitor' effect mentioned earlier. The results are interesting and useful.

My initial observation of the effect was that when Displacement Modulation was achieved, the oscilloscope showed the modulation clearly when the Vertical Attenuator was used in its DC mode, but the modulation disappeared in its AC mode (i.e. the signal was shown as nothing but the RF carrier, exactly like a CW transmission). Looking at the scope schematic, the ONLY difference is that in AC mode, the first thing the Vertical Input signal meets is a .1 mF blocking capacitor (basically, this allows you to measure a small AC ripple on a high-voltage DC power supply, for example).

Using the scope in its DC mode and scoping a good-quality low-level modulated RF signal, I tried a few values of capacitors between the top end of the tank circuit and the Vertical Input terminal on the scope. Results are as follows:

.001 mF old-fashioned tubular: Modulation blocked except for tiny residual ripple
260 pF disk: Modulation displayed at original amplitude, slightly distorted
520 pF (two disks in parallel): Reduced modulation amplitude, more distortion
780 pF (three disks in parallel): Reduced amplitude, severe distortion
1040 pF (four disks in parallel): Reduced amplitude, modulation clearly visible but no longer resembles original - note that this capacitance closely matches the small tubular tested first (.001 mF = 1000 pF)

So we see that small capacitors allow the modulation to get through in a distorted mode, UNLESS old-style tubular capacitors are used! This is useful because:

In the Displacement Modulation wave, ANY deviation from the upper RF envelope being identical to the lower envelope is in fact low-level AM. This is a basic type of distortion we could get by, for example, driving the modulation beyond what the amplifier will allow (basically, over-modulation). By using the blocking capacitor, the scope trace will show us ONLY the AM modulation component. So. IF AM exists, it will get past the blocking capacitor and will show up on the scope. This AM will be very low-level compared to the Displacement Modulation developed, and will produce correspondingly low-level sidebands. I assume the AM and sidebands would be distorted versions of the audio input.

Thus, we could use an ordinary oscilloscope with such a blocking capacitor as a simple 'over-modulation detector'.

Larry K0WUQ
Displacement Modulation K0WUQ on 9/3/22

A little while ago, while cleaning out one of the horse stalls, it occurred to me: What if I'm ALWAYS creating AM, and the large amount of distortion I get on demodulation IS the AM?

In Displacement Modulation, ANY error between the shape of the upper and lower RF envelopes will be, by definition, distortion. That is, to be UNdistorted DM, the moment-by-moment algebraic DIFFERENCE between the two envelopes must be CONSTANT. This will not be true if ANY AM is imposed on the RF signal along with the intended DM.

The same thing would be just as true the other way around - what if the demodulation I've been listening to all along has really been the AM part? The DM part would be heard as extreme distortion.

I will figure out a way to test this - probably involving a .1 mF capacitor ;-)

Larry K0WUQ
Displacement Modulation K0WUQ on 9/3/22

By now, I should be able to report full success at this every time I try it. But, I am forced to admit that I cannot. Much of the time I am getting unexpected results, as seen on my scope driven by my makeshift tunable tank circuit. In all cases, the 'failure' of my method is evidenced by getting low-level AM instead of the Displacement Modulation desired.

This doesn't mean I haven't proven anything; but to a scientifically skeptical reader, it throws doubt on either my claims, or at least, on my methods of achieving them. Obviously, it's a lot easier proving something to yourself than proving it to an 'unbiased' outside-world observer. But, here are the things I THINK my efforts have shown, so far:

1. It is easy to create a Displacement Modulated wave in an RF amplifier and it is provably possible to propagate such a wave across 'space' to a receiver that can detect and demodulate it.
2. However, our ability to propagate such a wave is not sure-fire - there are problems of unclear origin.
3. It has not been proven that the created wave is totally free of a low-level amplitude modulation component - this AM component has been observed (always at very low level) even when Displacement Modulation is STRONGLY propagated.
4. High modulation levels have never been established without noticeable audio distortion at the receiver; this distortion has not been visible at the scope trace, however.
5. Differences in setup, procedure, etc. that lead to success or failure of the modulation are not clear; a variance that seems to work one time fails when I try to duplicate it another day. This means there are unknown systematic errors.
6. The modulation, when done at 7 MHz RF, can be filtered out by a .1 mF 'blocking capacitor', although any AM present will make it through perfectly. This doesn't happen when a much smaller capacitor is tried (e.g. .001 mF). Thus, it is not simply low frequency blocking through a passive high-pass filter. The main reason this might be important is that any 'capacitance' effect in the atmosphere between transmitter and receiver could diminish propagation.

Sometime soon I will craft Yet Another Reply that will 'flesh out' these observations, just for those who enjoy slogging through the weeds.

Larry K0WUQ

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