help with navtex/medium wave receiver sensitivity and decoding

On a sunny day (12 Feb 2007 10:17:08 -0800) it happened "bigorangebus" wrote in :

It seems to me you could perhaps used a tuned circuit there, with sufficient bandwidth. In fact the active antenne you describe perhaps needs an LC to ground. Else indeed any spike will overload the FET, easy with a 'metal plate'.

In spite of the rich detail you've given us, this is still a problem with many interesting dimensions.

Atmospheric noise is very strong at MF, so even with a capacitive probe antenna such as yours it should still dominate. I would start by measuring your signal strength at IF or baseband with the antenna connected, then with the antenna replaced by an equivalent impedance. Ideally you'd see an increase in noise level from 3 to 6dB -- much less than that and your system's inherent noise is your limitation, much higher than that and you've got too much gain ahead of the mixer, which will make it susceptible to distortion.

If its practical I'd do a similar test with the antenna. I'd calculate the antenna's effective capacitance to ground, then replace it with a capacitor of that value from ground to the FET gate. You should see the same 3-6dB increase in noise with the antenna vs. the noise with just the cap connected -- if you don't, then your performance is being limited by the antenna.

After noise, the next thing you need to worry about is distortion. If your active antenna or your mixer are being overwhelmed by strong signals (such as AM broadcast signals) then your desired signal will either be blacked out, or the strong signals could combine in your electronics to generate a signal at your desired frequency. Building low distortion front ends is worth several pages if not a whole book, but the gist of it is that you want to limit your signal's bandwidth as much as you can, and you want to use intelligent design practices. Your active antenna can't do much before the signal hits the FET gate, but you can take Jan's suggestion and put a filter between the active antenna and the mixer. I'd put in an elliptical filter that had good attenuation in the AM broadcast band, to give your little Navtex signal a fighting chance.

Assuming that you're limited by atmospheric noise at the output of your IF, the only thing you can do is to improve your signal processing. I'm not familiar with Navtex signaling, so I can't give you more than general guidance, but here goes:

The nature of the atmospheric noise at MF is _not_ Gaussian. Noise in that band is predominantly from electrostatic discharges both near and far. It has a noise density that is more like a Cauer density, in that for all practical purposes it has an infinite variance. Any sort of signal processing scheme which is based on an assumption of Gaussian noise will be compromised by that assumption.

The best way to deal with MF atmospheric noise is to take advantage of the fact that it tends to occur as 'crackles' or 'hits' that have a high-amplitude, wide spectrum characteristic. In general what one does is to use 'side information' to detect when a discharge event has happened, then discount the data that's collected during that event. This is what noise blankers in communications radios do.

If the Communications Gods are smiling on you then there are clear channels adjacent to your signal, or Navtex uses forward error correction. In the case of the clear adjacent channels, you can use an IF that's wider than your signal and use a noise blanker. Your signal will go quiet during the crashes, but that non-information is demonstrably better than having huge signals get into your demodulator's filters to mess things up. In the case of FEC, you can demodulate the signal as usual, then look for unusually large filter outputs. When this happens you replace the demodulated bit with an erasure and forward the whole shebang to an error correction algorithm that can benefit from the erasures (and some do, to a significant degree).

I hope this helps.

In what sense too big? A ferrite rod is not that big.

I'm at a different location of course but a speccie analyser fed by a high impedance buffer with a 18" telescopic aerial plugged in, just shows noise and more noise. Biggest signal was a national radio station about 900kc at

The active antenna has back to back diodes to protect it in this case

This is brilliant...thanks for taking the time to put in this level of detail....I will spend this eve looking through your points..

A ferrite rod is small but very directional...in a boat you never know which direction you're going to be facing...and the receiver needs to work on a fixed antenna on its own!

On 12 Feb, 21:13, "john jardine" wrote:

Interesting... yes the incoming signal is 518k and I'm mixing it with

There are some fundamental differences between the way an E field antenna (like a probe) and an H field antenna (like a loop) respond to locally generated noise. Your may want to investigate the Ryan storm scope that can determine the distance of a lighting strike by comparing the relative amplitude of the e and H field. You may want to inquire over at rec.amateur.antenna. And you may want to consider using two perpendicular loops.

Mark

F Further to this...I think I will need to get my hands on a spectrum analyser. My scope is just not set up to read the small signals pre- mixer and is probably just showing its own interaction. Its on max sensitivity reading the output of the mixer, so pretty sure its not being overwealmed. I did once try amping pre-mixer as Ive heard the

What is interesting is that the noise as appearing on the scope in time domain at any rate looks the same but bigger when I hold the plate antenna. Of course that could be a red herring as I have no idea what other frequency components there are in there, but the general shape of the background noise in the time domain remains the same.

Ive heard of someone using a large hoop antenna to clear up the signal, though thats too directional for my application. Presumably with the dominance of the background noise making the antenna bigger wouldnt work, but narrowing the bandwidth could. However if I narrow it any more I wont be able to pick up the adjacent

I wonder if there is a better active antenna circuit than the simple fet circuit in the active antenna.

Really appreciate your input Andy

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filter

I will ask the question about the antenna in rec.amateur.antenna assuming Tim above doesnt get uppity about cross posting again! (I posted this question in the radio amateurs site also)

I found speccies to be unbelievably useful. If you enjoy the radio stuff (obviously!) should be No1 on the kit list. ('scope a near 2nd)

Seems you've covered most stuff worth doing on the Rx. Looks like an electrically less harsh environment is the order of the day, (something a distance from civilisation, something say like a boat trailing a kite flown long wire). Failing this, my parlance would say you're buggered :) john

Demodulating FSK using center crossing really isn't very optimal. If that's what you're doing you'll pick up quite a bit of range just using the two filters method or a discriminator.

ISTR sets which used a combination of a ferrite rod and a collapsible whip but maybe that wouldn't work in your case.

I also recall two loops set at right angles to each other. Remember that the highly directional point is the null signal not the peak signal.

On a sunny day (12 Feb 2007 13:40:08 -0800) it happened "bigorangebus" wrote in :

Would 2 90 degrees crossed ferrite rods work? Sum the signals from the coils?

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1. Go with a ferrite loopstick antenna. Try pointing it vertically so the null is at the zenith, and straight down into the ground. A lot of the noise at that band is electrostatic, which your active antenna seems to be designed to pick up.
2. Try for the narrowest bandwidth at the front end. Tune the antenna, or have a high Q LC filter, or even build a crystal or ceramic resonator filter at 518 (and 490) kHz. It only needs to be 500-1000 Hz wide. That applies to your IF bandwidth, too.

1. Look at "RTTY Terminal Unit"s, ie. fsk demodulators. The standard design was two filters as narrow as possible (determined by the bit rate) that were compared. One side could drop out and the other half could still compare signal at that shift with the "no signal" voltage.

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

Regarding using a vertical ferrite loopstick antenna (I presume this is a ferrite bar would with a coil as usual), if you put it vertically, and the transmitter is a vertical mast..does this cause a sensitivity issue? Normally ferrite rods are held horizontally. But my antenna theory is basic so I'm probably missing something...

My problem with using the RTTY model is that lo drift will take me right off the sweet spot (only have a 170Hz to play with between mark and space).

IThanks Mark, this is very useful

no, by just summing you'd still get two nulls. (at 135 and 215 degrees)

If you can phase them so that the signal from one is (ideally) 90 degrees different from the other signal you'd get 360 degrees range with no nulls

On a sunny day (15 Feb 2007 09:09:13 GMT) it happened jasen wrote in :

OK, should be no problem to make 90 degrees at _one_ frequency, either slightly detune it, or just a phase shift network before the add. I think the idea from the other poster to put the rod vertical is good too, I just tried it with the local radio station on MW 1007kHz, and the directional effect is gone (I can zero it horizontally), and the signal does not attenuate a lot.