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Old 10th Jan 2018, 9:14 pm   #1
Boom
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Default Ultra VLF signals?

Using Barneys SDR I am picking up what appears to be RTTY at 18 kHz, and at 36 and 45 kHz what seems to be garbled voice audio. There are more strange noises at 72kHz.

If these are genuine signals rather than oddball mixtures what are they likely to be from at such low frequencies?

http://remoteradio.changeip.org:8073/
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Old 10th Jan 2018, 9:44 pm   #2
MrBungle
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Default Re: Ultra VLF signals?

Interesting. Maybe submarine comms?
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Old 10th Jan 2018, 9:52 pm   #3
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Default Re: Ultra VLF signals?

Curious- could they be intermod products? I'd be surprised if there was much speech transmission below 153kHz (the bottom edge of LW broadcast) as that would seem to be anti-socially greedy of bandwidth, if nothing else. RTTY of one form or another is entirely understandable, though.

This brings us to definition pedantry- formally, VLF is 3-30kHz, though there's probably a fair amount of sloppiness in usage, I've even seen US references to the aeronautical beacon band (between LW and MW broadcasting allocation) as VLF!
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Old 10th Jan 2018, 9:56 pm   #4
Station X
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Default Re: Ultra VLF signals?

My 2006 edition of "Guide to Utility Stations" show 18kHz as being used by a Russian CW station.
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Old 10th Jan 2018, 10:03 pm   #5
hannahs radios
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Default Re: Ultra VLF signals?

The RTTY around 18 kc/s is submarine comma at from various country's the strongest one is from the big BBC station in Cumbria the name of which escapes me for now
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Old 10th Jan 2018, 11:15 pm   #6
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Default Re: Ultra VLF signals?

Anthorne ... cue russell w-b at this point.
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Old 10th Jan 2018, 11:52 pm   #7
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Default Re: Ultra VLF signals?

At 36kHz I heard music and some Spanish chat so I reckon it some sort of imaging or whatever it's called?
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Old 11th Jan 2018, 12:05 am   #8
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Default Re: Ultra VLF signals?

Was there not a VLF transmitter that ran for a short while over the Christmas period each year?
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Old 11th Jan 2018, 12:36 am   #9
Mr Moose
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Default Re: Ultra VLF signals?

Hello,
I think Refugee is refering to the Grimeton vintage transmitter that transmits on 17,2 kHz CW using an Alexanderson Alternator.

http://alexander.n.se/the-radio-stat...sions/?lang=en

They do transmit a few days a year, usually including one at Christmas although it would appear that the broadcast this Christmas was cancelled due to maintainance work.

Yours Richard
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Old 11th Jan 2018, 12:37 am   #10
turretslug
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Default Re: Ultra VLF signals?

SAQ Grimeton on 17.2kHz- apparently the last on-air Alexanderson alternator. They normally transmit for a short while on Christmas Eve and around the end of June but neither transmission happened last year. I think there may have been a fire in the aerial system, and the knock-ons from that have meant prolonged work. Or something like that.
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Old 11th Jan 2018, 12:17 pm   #11
Andrew2
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Default Re: Ultra VLF signals?

There are no speech signals below the LW AM band. I suspect what you are hearing are images or intermod products. There are a number of data and 'time & frequency' transmissions in the sub 150 KHz band though, and many of them sound like RTTY. I've attached a 0 to 200KHz grab from my SDR. There's a group of data sigs around 20 KHz (not sure where they originate) and 60 KHz and 81 KHz from Anthorn are clearly visible, as is DCF77. There are plenty of others, including the tele-switching EFR signals around 140 KHz which when decoded simply send time & date info. The big hump at 100 KHz is LORAN.
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Old 11th Jan 2018, 12:40 pm   #12
Julesomega
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Default Re: Ultra VLF signals?

There is an up to date list of VLF tx at http://www.mwlist.org/vlf.php

A small group of pioneering radio amateurs are sending narrow bandwidth data on 8.27, 5.17 and others, EIRPs are estimated in W and transatlantic reception has been reported several times in the past few months, but Chris Wilson is the man to ask!

Last edited by Julesomega; 11th Jan 2018 at 12:51 pm.
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Old 13th Jan 2018, 11:40 am   #13
G8HQP Dave
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Default Re: Ultra VLF signals?

36kHz and 45kHz are both multiples of 9kHz, the spacing used on MW and LW in Europe. They are very likely to be intermodulation.
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Old 13th Jan 2018, 12:30 pm   #14
turretslug
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Default Re: Ultra VLF signals?

Maybe the 72kHz burble, too. I suspect that a credible VLF receiver (of any topology) needs an effective LPF on its input to keep the powerful higher frequency stuff at bay.
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Old 13th Jan 2018, 10:19 pm   #15
Radio Wrangler
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Default Re: Ultra VLF signals?

Very much so.

Also it needs a good clean local oscillator with low sideband energy 72 kHz out from its carrier, because that will leak through the mixer straight into the IF.

The Racal RA1772 went so far as having a crystal notch filter in its LO feed which switched in when low frequencies were tuned, to avoid the noise floor lifting at LF. One HP instrument also did this.

Oddly, the Racal RA1792, which is very much a cost-cut box, is considered useful at these low frequencies, but it has very basic RF filtering and no notch in the LO feed.

A lot of the usual Japanese amateur radio receivers limit their tuning to 30kHz and higher, which could be interpreted an an indication of oscillator phase noise, but it's more likely imposed by transformer bandwidth and the mixer architecture.

However, radios with 'simple' single loop fractional-N synthesisers can have noise pedestals extending 1MHz out.

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Old 14th Jan 2018, 11:48 am   #16
turretslug
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Default Re: Ultra VLF signals?

I recall noticing that one of the "SWL"-type ATUs that was on the market a couple of decades back (Mizuho/Global/MFJ?- that sort of thing) had a sub-500kHz switch that brought in a LPF, I assume that this was to suit the then current range of consumer/amateur type receivers that may have offered LF coverage, but didn't necessarily budget for front-end filtering/sufficiently butch front-end and mixer devices to give good results.

The Racal LF converter made to go with the RA17 makes for an interesting circuit browse, with its care over balancing and filtering.
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Old 17th Jan 2018, 4:33 pm   #17
unixmanuk
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Default Re: Ultra VLF signals?

You may be referring to the Mizuho "Sky Coupler KX-3" like the one I have. Has a LF switch on it, that introduces a LPF giving a sharp cutoff above IIRC 500kHz.

Years ago, I used this and a comms receiver to decode a press fax transmission service on 139kHz, but that's long gone.
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