Radio 4 heard on 396kHz

[saddlestone-man]

Hello All

I was just tuning my Sailor 66T (ex-trawler) radio across the NW band and I could hear BBC Radio 4 loud and clear on 396kHz. This looks like the second harmonic of the 198kHz broadcast.

Can anyone else hear this on 396kHz?

best regards ... Stef

[Paul_RK]

Nothing there on the Panasonic RF-9000, which has continuous coverage from 150kHz- 30MHz but for a gap from 420-520kHz.

Paul

[turretslug]

Strong clear 396kHz signal here in SW edge of London (Eddystone 670A, short vertical wire). I first noticed this signal a few days back at the extreme HF end of the LW band of this set and assumed it was a sprog of some sort but hadn't got round to doing the possible maths permutations. Yes, second harmonic of the transmitter sounds plausible, it's like having the pre-war "MLW" band back!

[Radio Wrangler]

So the possibilities, for a second harmonic are:

* Fault at transmitter likely in the output coupling/filtering section
* Rusty bolt effect somewhere the transmitter field strength is high, likely a long fence.
* Receiver overload.

The first action is to check that it's a real signal being received, and not due to a receiver problem. An easy way to do this is to insert an attenuator in the aerial feed. If you insert X dB, the fundamental should decrease by X dB, but a second harmonic generated in the receiver will fall by something more like 2X dB.

If you're happy with the receiver, then it's time to use a loop antenna or ferrite rod and try DF'ing the harmonic signal and the fundamental. If you get an angular difference, then you're close enough to the area around the transmitter to go hunting the thing exhibiting the rusty bolt effect. If you don't get an angular distance, you're either too far away to discriminate between the source and the rusty bolt, or the source is in the transmitter/transmitter site.

Another check would be to listen when R4LW switches to different programming. If the unexpected signal doesn't, you might have found someone with a pantry transmitter that's a bit too ebullient.

So, there's a bit of detective work that can get some more clues.

The best tool would be a metal-cased portable receiver needing an external antenna. This allows attenuation to be inserted in the aerial feed to prevent overload as you get into the area of interest.

David

[Keith956]

Nothing on my Sony ICF-SW100 at that frequency.

[Jon_G4MDC]

Fences and gutters can generate this but it is a very local effect.
Says one who lives close to Brookmans Park...there is always Radio5 in Top Band from somewhere. 1818 is a not a frequency for a CW sked.

[G6Tanuki]

Harmonic generation in bad joints can be a problem to track down particularly if they are on a third party property.

Fortunately back in the day when I had the third harmonic of Radio1 247M appearing on 80M I tracked it down to an oxidation on my antenna downlead.

[Andrew2]

The R4 fundamental is at -26dBm here, and the 396 kHz signal s at -80. I'm sure it wasn't there a couple of days ago when I was poking about the 400 kHz area looking for NDB's.

[paulsherwin]

I wouldn't rule out a transmitter fault. It's clear that Arqiva aren't putting much effort into maintaining it.

[G0HZU_JMR]

I'm located near Tewkesbury which isn't that far down the M5 from the Droitwich transmitter. I had an informal look at 396kHz last night using a 2.5m diameter passive loop antenna This antenna is made from coax with no ferrite so it should be distortion free.

I used my Agilent E4440A spectrum analyser as the receiver. The mixer in this analyser typically has a second harmonic intercept point of about +50dBm but this degrades at very low frequencies.

However, I received BBC R4 at -45dBm at 198kHz and when I tuned the analyser to 396kHz the harmonic was visible at -118dBm. The analyser had 10dB attenuation selected and increasing this in 2dB increments did not affect the measurement. So I don't think the harmonic distortion is in the analyser. The signal was definitely a harmonic of R4 because I could see the low frequency (AMDS) PSK modulation on the carrier.

[Graham G3ZVT]

The 3rd harmonic of the Holme Moss TV sound transmitter fell inside the 2m amateur band, where it became an unofficial, yet broadly welcomed propergation beacon.

[G0HZU_JMR]

I have another analyser here that can capture IQ data over a very long time and this means I can run an extremely big FFT on the data and measure frequency accuracy using a very small resolution bandwidth. This analyser has a decent and very stable frequency reference and I've used it in the past to measure the frequency of offair signals.

I should be able to get down to a resolution that is a really tiny fraction of 1Hz. The aim here would be to see if the frequency of the harmonic is exactly twice the frequency of the Droitwich transmitter. There is the remote possibility I'm actually receiving the second harmonic from another R4 transmitter. I think there are two other R4 transmitters located in the UK? The frequency of the Droitwich transmitter is very accurate but I'm not sure how accurate the time reference is on the other two transmitters. If they are all locked to a reference with similar accuracy then I'll be wasting my time...

I'll have another go at this stuff this evening.

[turretslug]

When I first heard this signal, I was sceptical about it being a straight transmitter harmonic, as I assumed that anything that fell in this NDB/maritime section of the spectrum would be quickly spotted and remedied, and that the 198kHz transmitter and its associated filtering and matching would have been specifically designed and set up to minimise the second harmonic possibility. I put it down to one of those sprogs that one occasionally finds in any superhet, and parked it in a quiet part of the grey matter. However, it seems that there's a degree of corroboration by others here, it was certainly clear and listenable late last night. Wonder how long it'll be around?

I'm reminded of an occasion, I think late '80s, when I heard a Radio 1 DJ (possibly DLT?) broadcasting an urgent appeal for someone out there to stop blocking 2182kHz with an apparently inadvertantly re-broadcast Radio 1, the supposition being that an emergency transmitter was being modulated by a nearby radio. A post here a while back revealed that a particular 1089kHz transmitter was prone to going faulty in some way and producing markedly increased second harmonic output every now and then, I wonder if that was at the root of this particular broadcast appeal.

[paulsherwin]

Originally Posted by G0HZU_JMR

There is the remote possibility I'm actually receiving the second harmonic from another R4 transmitter. I think there are two other R4 transmitters located in the UK? The frequency of the Droitwich transmitter is very accurate but I'm not sure how accurate the time reference is on the other two transmitters.

There are 50kW transmitters at Burghead and Westerglen, but they're both a very long way from Cheltenham.

[G0HZU_JMR]

Thanks. I looked those transmitter names up and it may be the case they are all synched up very closely in frequency. I agree they are a long way away from me so I'm unlikely to hear much of a signal from them.

I built up a 5th order high pass filter this evening using high quality film caps and T80-2 powdered iron toroids. It passes 396kHz with about 0.3dB insertion loss and has just over 30dB rejection at 198kHz. This filter should provide negligible distortion at these signal levels and it gives the spectrum analyser an easier time as the 198kHz signal reaching the analyser is very low now.

I got the same results as last night where the harmonic was still visible at 396kHz at about -118dBm. This is a very small signal but it is definitely a genuine signal and not an artefact of the antenna or the analyser. I got the same result with another spectrum analyser.

I'm not sure what level of harmonic suppression is normal for R4. I did notice that the 198kHz R4 transmitter and the 693kHz R5Live transmitter was shut down for a while a few days ago. Both were back on later the same day. I can't remember what day this was but I recall the transmitters were both off some time around 9:30am. Maybe this was for maintenance work?

[Aub]

Yes, R5 live on 693, was off the air for a while last Tuesday 2nd morning. I don't know about R4 longwave though.

Cheers
Aub

[peter_scott]

Here in Edinburgh (Westerglen) I pick up R4 on 198kHz strongly on my spectrum analyser but absolutely nothing on 396kHz.

Peter

[Pfraser]

Interesting topic. Some corroboration from online SDRs: a very noisy S7 in Weston-Super-Mare; even weaker in Stourbridge.

WRT this signal not having been eliminated yet - could it be that action will only follow a complaint?

[turretslug]

Just now, the usual rasping mush and rubbish around this part of the band- but definite speech modulation burble in the background identifiable as synchronous with 198kHz, but completely unreadable (at least to my ears). Maybe 396kHz behaves more nearly like the familiar MF tendency towards good night-time propogation than 198kHz and LF broadcastings' near round-the-clockness. Last time I heard it, at around midnight, it was very clear but after another 45 minutes or so was receding in strength and clarity.

Long ago, I recall that the otherwise quiet 30-32MHz region was an intriguing hunting-ground for the second harmonics of 19m broadcasters, invariably pretty low signal strength but often very clear and listenable. The strength of the fundamental had little correlation with any harmonic's presence, with location/propogation and individual installation competence presumably having a greater significance.

I wonder if the ground around the Droitwich aerial installation has become very dry and upset the system matching.....

[Radio Wrangler]

Anyone checked for other harmonics?

David