Frequency Sweeps? What? Who?

[NoelC]

Before having an SDR with this wonderful waterfall model I used to hear the occasional sweep of a beat frequency and just figured it was someone from a nearby band tuning up or something.

Now I realize that there are these frequency sweeps going on all the time across the HF band, usually ascending in frequency over time...



I presume these are some kind of coordinated effort to check propagation or something... Anyone know more?

-Noel

[NK7Z]

https://en.wikipedia.org/wiki/Ionosonde

[NoelC]

So either those things are incredibly common (to where I and the various WebSDR sites are always near some) or they shoot signals with a lot of power up into the sky.

I'm presuming the latter...

Why do they need a lot of power to characterize the ionosphere?

And isn't it illegal to blast QRM all across the band? You can't listen to a given station for more than a minute or two without one of these sweeps crossing your frequency and upsetting your AGC / Squelch and wrecking part of what you're listening to.

-Noel

[mikael]

[...] or they shoot signals with a lot of power up into the sky.

Why is that? HF propagation is magic, you know. You can bounce around the globe using milliwatts if the conditions are right.

[NK7Z]

They typically use around 100 watts, not much power, and if you look at the scans, you will see some of them bypass the ham bands. The info is so useful, that no one really worries about the QRM.

[NoelC]

Why is that? HF propagation is magic, you know. You can bounce around the globe using milliwatts if the conditions are right.

Conditions are only that right occasionally. Since I see so many sweeps, even when conditions are only marginal, I can only assume it's either that there are a lot of these atmospheric probes or they're shooting significant power up there. Okay, 100 watts is not insignificant, though I admit it's not monster wattage either.

Right now, as an example, I'm watching WWV on 10 MHz. The signal is about S6 to S7 (-87 dB with a noise floor of about -110) here in Florida at the moment (roughly 2,000 miles away from Boulder, CO). WWV radiates at 10 kW with probably some of the best antennae out there. I see two sweeps running up across the waterfall with peaks on the display higher than WWV's. One attenuated itself just as it crossed 10 MHz, but the other, a stronger one, crossed 10 MHz at such a high level it blanked out WWV momentarily and even affected the AGC as it was traversing the waterfall display. Either that's someone pretty close by with just 100 watts or something pumping out much more wattage within a few thousand miles.

I don't mean to make a big deal out of this, because I realize in the grand scheme of things it's not, but I would not have guessed this kind of frequency sweeping was being done on such a wide scale based on those occasional chirps on my traditional receiver. But being able to SEE these linear sweeps run across a waterfall display, then hear them as they cross whatever station I'm tuned to, shows clearly what's being done.

SDR has certainly breathed some new life into this hobby for me.

-Noel

[NK7Z]

Here is a thought experiment...

Imagine you are listening to WWV, now imagine I live next door and am running an ionosonde at 1 watt. From your viewpoint, you would see the same behaviour, sweep swamping WWV. Unless you have all data, you can not infer anything about output power of one station from another...

I love these things, and this is an interesting discussion... Not thinking you are making a big thing at all, you are just curious, as I am... I have thought of building one, but I suspect my ham neighbors would burn down my home if I did that...

[NoelC]

I certainly understand the possibilities.

I have noted that all of the sweeping signals fade in and out as they cross different frequencies, implying they're not being generated locally, but are subject to propagation effects. I'm no expert, but I seem to recall a receiving station has to be some distance away from a transmitter before hearing a bounced signal.

I've also seen these sweeps on webSDRs, where the receivers are in quite different parts of the world. They're strong and/or numerous. I started SDRuno just now to look, and a frequency sweep crossed 10 MHz literally about 5 seconds after I hit the [ PLAY ] button, so they're certainly not rare.

Other observations:

Right now I see 3 sweeps happening while I'm watching 5 - 14 MHz... I believe I've seen as many as 4 at once when the bands have been more open. Presuming that any one of them will finish an entire sweep before starting over, the implication is that a fair number of them are running simultaneously. In fact, it looks as though the time period before one of the strong sweeps I see repeats is 5 minutes even.

A strong one that I have been watching just now crosses 1 MHz of bandwidth in about 12 seconds, and it repeatedly skips several frequencies. Others don't skip the same frequencies, implying different designs or regulatory concerns.

Summary:

• The sweeping signals I see vary in strength at different frequencies, implying propagation effects.
• Some are quite strong.
• I've seen as many as 4 sweeps simultaneously on the same waterfall display just today, and possibly more in the past.
• Some sweep rates are different from others, with a common rate being 1 MHz every 12 seconds.
• I've seen sweeps on some webSDRs while simultaneously NOT seen on others, implying the transmitters all over the world.
• I've seen a sweep on the W7RNA webSDR (San Francisco) while also detecting it on my own receiver (south Florida).
• Some briefly attenuate and thus skip certain frequencies: Some skips noted: 10.0 MHz, 13.375, 20.0 MHz.
• Some incur small delays in continuing the sweep at certain points, e.g., 8.3 MHz, 12.5 MHz. Antenna switching/tuning?
• Some are smooth, continuous sweeps and some are segments of about a second each at progressively higher frequencies separated by less than 100 kHz between test transmissions.

-Noel

[13dka]

There are ways to identify...

http://jcoppens.com/radio/prop/g3plx/index.en.php

...and utilize ionosondes for your own entertainment:

http://www.sgo.fi/~j/gnu_chirp_sounder/

Just identifying the ionosondes by their timing would give you a nice worldwide set of powerful beacons to assess the condx, following them across the bands (easy with a wideband spectrum display of several MHz) until they disappear may add some 1st hand information on the best band and the MUF for that path. Instead, I just check the qsl.net propagation page tho.

[NoelC]

I am but an egg...

-Noel