Eric's Weekly Post #3 (Warning, science enclosed. Do not fold or bend.)

Just a thought Eric...

To illustrate my point, I once experienced the same effect in London, England, while listening to a radio station transmitting on 1.214 MHz. I could receive a weaker, but perfectly clear signal on 310 KHz, a fourth harmonic of the true signal...

Errrr... I didn't know that "sub"-harmonics existed!... [edit] (Note: They don't! However, you can see an interferometric effect whereby the amplitude of a signal gets modulated at the difference ferquency from the modulating/interfering signal. This is different from generating a (higher frequency) harmonic from non-perfectly-sinusoidal signals or from FFT decomposition of one signal. Corrections welcomed!) [/edit]

Errrr... I didn't know that "sub"-harmonics existed!... [edit] (Note: They don't! However, you can see an interferometric effect whereby the amplitude of a signal gets modulated at the difference ferquency from the modulating/interfering signal. This is different from generating a (higher frequency) harmonic from non-perfectly-sinusoidal signals or from FFT decomposition of one signal. Corrections welcomed!) [/edit]

More seriously, you may well have had that (strong) signal breaking through on the mixer stage alternate frequency of one of the IF stages in your receiver. Was it a non-superhetrodyn receiver with only one (or no) IF stages even?

"Ghosting" as commonly seen on TV pictures where you get multipath reflection is from the same frequency signal arriving via multiple paths. You get the same signal but displaced in time. Hence, for an analogue modulated TV picture that is scanned across the screen horizontally, you get multiple identical pictures overlayed but slightly displaced horizontally.

If what you notice are indeed "ghosts" but time delayed, then that would suggest multipath pickup. The "multipath" part could be ANY different route, whether external or just different routes through the pickup path in the affected equipment.

It could just be that the interference source produces that repeating pattern and that it "just looks like that".

Who knows?

Until Eric tracks it down.

It would be interesting to see what more of the WUs show and how consistent/repeatable it is, and so on...

Much better to cure the source rather than to try to remove the symptoms.

(Hey, we could make a reality-TV show out of "What next!"... :-) )

There are all kinds of chirps and birdies, undesirable mixes (like fundamental + if when you wanted fundamental - if, or mixing with a harmonic of the LO, or a whole bunch of other ugly things), or just plain so much signal from a strong nearby source that it just plain comes in.

The signal source could be leaking from another instrument on the telescope.

Maybe someone with more radioastronomy background can comment, but I'd think that multipath distortion is a terrestrial phenomenon -- you need some hills to refract around, or a nice big reflective building to make something other than the direct path viable. I don't think a relatively small nearby object like Jupiter is big enough to cause enough multipath distortion to matter.

Hafta agree about the Jupiter thing, but Arecibo does have some hills around, and 1.4 GHz is well within the band used for UHF TV transmissions round the world. I wonder: does the interference show up on BOTH polarisations, or only one? (Horizontal or vertical?) I take it the Alfa receiver doesn't check for helical.

This example isn't a sub-harmonic, it's a harmonic. 4 times 310khz is 1240khz.

Hafta agree about the Jupiter thing, but Arecibo does have some hills around, and 1.4 GHz is well within the band used for UHF TV transmissions round the world. I wonder: does the interference show up on BOTH polarisations, or only one? (Horizontal or vertical?) I take it the Alfa receiver doesn't check for helical.

It shows up in both polarizations (and the polarizations are linear, but it would be possible to derive circular polarizations from the data stream.)

What is polarization as it applies to Seti signals and the way Alfa receives them? I understand that with light it allows the waves to move in only one direction. So if polarized glasses are oriented correctly they will reduce glare while one is driving. Apparently glare is polarized in a direction 90 degrees from that of the sunglasses. Maybe glare is polarized horizontally(?).

Hafta agree about the Jupiter thing, but Arecibo does have some hills around, and 1.4 GHz is well within the band used for UHF TV transmissions round the world. I wonder: does the interference show up on BOTH polarisations, or only one? (Horizontal or vertical?) I take it the Alfa receiver doesn't check for helical.

It shows up in both polarizations (and the polarizations are linear, but it would be possible to derive circular polarizations from the data stream.)

That's interesting, but doesn't make your job easier. Most transmitters radiate in ONE polarity. I'm not sure if helical (circular) polarisation would turn up in a linear-polarised receiver. If so, it would be pretty weak. Beginning to look like the local equipment theory may be right.

Incidentally, does the receiver distinguish between Amplitude and Frequency modulation?

Here's potentially a new and unexpected interference source:

"Alarming microwave"

Anyone nearby got a new car (and car alarm)?...

Or even bluetooth/WiFi devices in general?

And then there could be the old problem of an electrical power switch acting as a spark-gap transmitter...


Unless the source can be RF sniffed, it's down to looking for what is active when the interference is seen.

Eric: Any further clues?? Are we warm, cold, or completely off the planet?

Regards,
Martin

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Most transmitters don't radiate -- it's the antenna that does that.

For our sun, a very interesting artical is "The Viking Sunstone". That suggests that the light from our sun is indeed preferentially polarised. Anyone know if this is so? Determined by the sun's magnetic field??

snipped...

Are there any natural phenomena that produce circular polarisation?

and a bit more snipped...
Cheers,
Martin

Hafta agree about the Jupiter thing, but Arecibo does have some hills around, and 1.4 GHz is well within the band used for UHF TV transmissions round the world. I wonder: does the interference show up on BOTH polarisations, or only one? (Horizontal or vertical?) I take it the Alfa receiver doesn't check for helical.

It shows up in both polarizations (and the polarizations are linear, but it would be possible to derive circular polarizations from the data stream.)

That's interesting, but doesn't make your job easier. Most transmitters radiate in ONE polarity. I'm not sure if helical (circular) polarisation would turn up in a linear-polarised receiver. If so, it would be pretty weak. Beginning to look like the local equipment theory may be right.

Incidentally, does the receiver distinguish between Amplitude and Frequency modulation?

Most transmitters don't radiate -- it's the antenna that does that.

What is polarization as it applies to Seti signals and the way Alfa receives them? I understand that with light it allows the waves to move in only one direction. So if polarized glasses are oriented correctly they will reduce glare while one is driving. Apparently glare is polarized in a direction 90 degrees from that of the sunglasses. Maybe glare is polarized horizontally(?).

No worries, Clyde

What you got was pretty typical. Polaroid lenses are like the rest of us: imperfect. Therefore, although the theory says they will blank out the light, in practice, a little does get through. Obviously, as always, it depends on quality.

Radio waves are polarised by the orientation of the antenna. A standard whip antenna stands vertically, producing vertically polarised radio waves. The same whip placed horizontally will produce horizontal polarization. TV signals are almost universally horizontally polarised, by the way, which limits their range. Helical polarisation is achieved by a coiled antenna, which can be clockwise or anti-clockwise. Many "outside broadcast" units have Line-of-Sight (LOS) antenna, composed of coiled antenna enclosed in a plastic or glass fibre tube. You may have seen a news van or similar, prowling round sporting one on it's roof, often on a telescopic mast.
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Thanks again, LittleGreenMan. I'll be more observant next time I walk or drive and notice all types of antenna I see. Sometimes I see parabolic antennas pointing horizontally, on towers, perhaps at others a mile or so away, but I don't know whether those signals are polarized or not. Maybe they're relays.