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Old 6th Nov 2020, 8:48 pm   #21
trh01uk
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Default Re: Advice on the LOW PASS Filters

You can quickly eliminate receivers as the problem. And that is to use the receivers that have an attenuator in the antenna circuit. Of the receivers you list, the IC-R75 has a 20dB attenuator, Tecsun s2000 has a 10db and 20dB attenuator, the FRG-7 has an attenuator but its unclear just how much attenuation it provides, and the 9R-59D has no attenuator.

Use of an attenuator will - in most case - quickly tell you whether an interfering/spurious signal such as you describe is due to a problem in the receiver or not. When you are listening to a MW transmitter appearing on the 4 - 6MHz band, what you do is to switch in the attenuator. If the signal disappears then its being caused by intermodulation somewhere in the receiver. And in that case, a filter such as we have been discussing will almost certainly eliminate the problem.

If the MW signal doesn't disappear when you switch in the attenuator, then its not a receiver problem, and fitting a filter will have no effect whatsoever. It may be the signal you are picking up is generated on the broadcast transmitter site. It may also be generated by something else in the vicinity. Either way, there is probably nothing you can do about it, apart from use directional antennas (e.g. a loop) or move house.

Loop antennas can be very effective at removing point sources of interference. You have to orient them so that their null is pointing at that point source - in this case Westerglen broadcast transmitting site.


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Old 7th Nov 2020, 11:34 am   #22
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Default Re: Advice on the LOW PASS Filters

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Originally Posted by G6Tanuki View Post

Have you considered a simple un-amplified ferrite-rod antenna tuned to the band-in-question? It'll give you some good directivity at least, and will probably deliver all the wanted-signal you need.
Great points, Tanuki.

I was thinking because the signals I have been trying to copy were all very weak due to the distance and their low power TXing often less than 1kW, I would need more gain from the amplified device. But it has the horrendous drawbacks, which are cross modulation and overloading.

And I was not aware of the fact that those powerful MW transmitters were not too far away from me. It does not help the situation.

I used to read about the tuned ferrite rod antennas with strong directivity and high Qs, but mostly they seem for MW and LW bands. Would it work well for on SW in DXing too? If they do, I would love to try it out of course.
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Old 7th Nov 2020, 11:40 am   #23
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Default Re: Advice on the LOW PASS Filters

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Originally Posted by trh01uk View Post
You can quickly eliminate receivers as the problem. And that is to use the receivers that have an attenuator in the antenna circuit. Of the receivers you list, the IC-R75 has a 20dB attenuator, Tecsun s2000 has a 10db and 20dB attenuator, the FRG-7 has an attenuator but its unclear just how much attenuation it provides, and the 9R-59D has no attenuator.

Use of an attenuator will - in most case - quickly tell you whether an interfering/spurious signal such as you describe is due to a problem in the receiver or not. When you are listening to a MW transmitter appearing on the 4 - 6MHz band, what you do is to switch in the attenuator. If the signal disappears then its being caused by intermodulation somewhere in the receiver. And in that case, a filter such as we have been discussing will almost certainly eliminate the problem.

If the MW signal doesn't disappear when you switch in the attenuator, then its not a receiver problem, and fitting a filter will have no effect whatsoever. It may be the signal you are picking up is generated on the broadcast transmitter site. It may also be generated by something else in the vicinity. Either way, there is probably nothing you can do about it, apart from use directional antennas (e.g. a loop) or move house.

Loop antennas can be very effective at removing point sources of interference. You have to orient them so that their null is pointing at that point source - in this case Westerglen broadcast transmitting site.


Richard
Great info, Richard.

Will try that. Yes, they have the attenuator, which I forget to use in most of the time.

And I will try to get hold of the filters also just to try out. Maybe they will be helpful in this situation of this QTH.

I was totally unaware of the the MW transmitters were not far away from this QTH either until you pointed out. Thanks.

With my MLA30 on the rotatable stand, I used to get swamped with the MW breakthroughs on East West direction, and I couldn't understand it at all.

On North South way, it clears it. But my RX direction is very limited, and hopefully filtering can make difference? Will give a try. Thanks 73s.
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Old 7th Nov 2020, 7:02 pm   #24
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Default Re: Advice on the LOW PASS Filters

Attenuators are great diagnostic tools.

Distortion and intermodulation products are non-linear artefacts. EVERYONE knows that.

What can be missed is that the level these problems appear at is a non-linear function of the signal levels driving your thing into non-linearity. (Sounds obvious now doesn't it?)

So if i stick in a 10dB attenuator, the wanted signal will drop by 10dB (a little less if there's some compression going on) but distortion and intermod products will drop by a lot more.

Second order products usually drop by 20dB (yippee!)
Third order by 30dB
Fourth order by 40dB

Begin to see the pattern? These ratios aren't guaranteed because they tend to go off a bit when the signal to distortion ratios get small. THey work fine where distortion products are becoming small, which is where you want to be anyway.

Good?

Now let's drop in 20dB attenuation

Wanted drops 20dB
2nd drops 40dB
3rd drops 60dB
and so on.

Wow, we're really motoring now.

And the aerial attenuator also drops band noise, SMPS crap and all by the same level as the wanted signal. So you don't lose any signal to noise ratio until you start to impinge on your receiver's noise floor.

However

Only the stuff AFTER the attenuator sees the improvement. Any distortion from rusty fences near the transmitter or near you, bad joints in your antenna and active antennae are unaffected.

So while not a fix in all cases, trying an attenuator gives a good indication of whether you have trouble in your receiver, or before the signal even gets to the receiver.

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Old 7th Nov 2020, 7:13 pm   #25
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Default Re: Advice on the LOW PASS Filters

By all means try the filters. However I have pointed you towards other solutions because the filters will only work if the spurious signals (i.e. MW stations showing up in the broadcast band 4 - 5MHz) are actually caused by intermodulation in your receiver. And I would hate you to spend a lot of money on filters only to be disappointed!

I think removing all active devices in antennas - like the MLA30 - is very likely to produce a better result. Intermodulation occurring in the amplifier in the MLA30 will not be cured by fitting a filter between the MLA30 and your receiver. The only cure for that is to remove the intermodulation where its occurring - or to fit a filter **in front** of the active device inside the MLA30. That may well be impractical - depending on how the MLA30 is designed.

Talking of loop antennas, a passive loop which is tuned to the signal you want may well be the most effective antenna you can use in the situation you are in. These can be built or purchased. An example of one you can build is here - there are others if you search for them. Locating it inside your shack can avoid the need to have a remote tuning capability which adds to the complexity considerably. Given you have a fixed interference source, you probably don't need to also have remote antenna position (angle) control, since you will need the loop's "null" permanently pointing towards Westerglen!

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Old 8th Nov 2020, 12:34 pm   #26
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Default Re: Advice on the LOW PASS Filters

Great advice and points, David and Richard. Thanks.I will remember using the attenuators when the problems arise in the future.
For the filters, they were the last resort. I also hate spending money on the filters which may have no effect, and plus they will just introduce some degree of signal loss in the chain from the antenna. But if they could sieve out the cross modulated signals, I thought it would be a good investment. But as you say, they may not work for this situation.
For the Active Antennas like MLA30, they work great in most cases, but in the early evenings when the skip changes, it gets overloaded with the cross modulation on certain portions of the band, mostly 4-6Mhz. But when it is rotated to some other direction, it gets better.
More overloading prone loop is the Tecsun AN-48X. It works very well above 5Mhz, but below 5Mhz, it gets 100% overloaded with the MW signals.
The reason I got these active antennas were because the DX signals I wanted to copy were so weak. They seldom get heard during normal band conditions and with the passive antennas.
But MLA30 heard the weak DX signals from Brazil a few times, and I thought the active device is the only way to hear the weak low powered DX signals.
I also have a YouLoop. This is a passive broad banded loop, and it is very quiet. It doesn't have any problems with cross modulation or overloading. But at the same time, it does not hear weak DX signals at all unless it is in very good band condition on the day.
I also have a longwire in the garden and wire dipole in the loft. Sometimes these antennas pull the signals better than the MLA30 or AN-40X.
But I will definitely do a resonant loop antenna project, like the one in the link you provided, and point to the WGlen for null. That sounds a great idea.
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Old 8th Nov 2020, 3:25 pm   #27
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Default Re: Advice on the LOW PASS Filters

Being able to amplify very small signals needs an amplifier to have plenty of gain and a low noise figure.

Being able to handle big signals means that the amplifier needs to be built like a small power amp, and every dB of gain makes it harder. This normally means using transistors with rather high quiescent current and dissipating a few watts.

If you want one circuit to do both of these things, it will have to be quite a sophisticated design capable of high dynamic range.

It them passes that dynamic range of signals along to your receiver... and the receiver had better handle that dynamic range all lifted by the gain of the amplifier.

So you could be lacking performance in the amplifier part of your active antenna and/or your receivers.

High dynamic range receivers that can handle this sort of this sort of thing are not common. Many classic communications receivers aren't up to it and need the operator to select RF attenuation fairly skillfuly. Often they need so much attenuation that the tiny wanted signal slips below the noise. The Racal RA17/RA117 were not very good in this respect and when they were transistorised (RA1217) they were worse! Eventually Racal understood the issue and developed a particularly good set, their RA1772. Be aware that the later RA1792 was quite poor, lacking any bank of RF band selecting filters.

RA17/RA117 had preselectors built in and still weren't good. RA1772 has a built in preselector and is good.

So even receivers built for government agencies have problems. On the amateur radio front the best receivers seem to get put in transceivers or else are expensive. The Icom IC-765 was good and that transceiver is renowned. The same HF receiver went in the Icom R-9000 receiver along with other circuitry covering up to 2000MHz. It looks like the scanner from hell. About £5000 new in the UK. My current Icom IC7700 transceiver is excellent.

All the receivers in the world won't help you if the active antenna is distorting.

An answer mentioned above would be to have a passive loop antenna resonated with a variable capacitor in a weatherproof box right at the loop, and maybe remote tune it with a stepper motor.

You've just discovered that unusually good receivers and ancillaries are actually needed some of the time in what seems to be quite ordinary situations.

David
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Old 11th Nov 2020, 12:35 pm   #28
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Default Re: Advice on the LOW PASS Filters

Great post David. Yes, I was quite surprised to learn that even high grade receivers can have problems with cross modulation and overloading from the strong MW break-in signals on HF.

I would have thought that the more expensive professional grade receivers would have solid front ends and circuitries that would be free from these problems.

I also took back on the theory that the problems could actually occur in the receivers themselves, rather than external causes. And in this situation, no filter device can clear the problem, no matter how good the filters were.

So, the problems and the causes of the problems seem very complicated in nature.

As you said, the best answer would be to use the passive tuned loop type antennas in order to prevent the problems in the receivers.

But as I have said earlier, I have tried those type antennas, and yes they are quiet and free from the problems of overloading, but at the same time they cannot hear the weak DX signals under normal band condition. And we have been having less than normal band condition for years now on HF.

The active antennas have all the problems of distortion, cross modulation and overloading, but sometime they can hear the weak DX signals when the passive antennas cannot, hence they are in demand and getting used with popularity.

Not only the active antennas but even passive longwire seem to give fair share of problems with the overloading and cross modulation problems especially if the QTH is near the strong MW LW transmitting site.

As usual I have learned a lot in this thread from the excellent posts with good info and advice. Thank you & 73s
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Old 12th Nov 2020, 9:55 pm   #29
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Default Re: Advice on the LOW PASS Filters

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I also took back on the theory that the problems could actually occur in the receivers themselves, rather than external causes. And in this situation, no filter device can clear the problem, no matter how good the filters were.
Its worth just correcting that statement (assuming its not a typo!). Its precisely when the intermodulation problem is *within a receiver* that filters can help (that's a filter between the Rx aerial input and the aerial). They help by reducing the level of interfering (unwanted) signals that reach the receiver - in your case strong MW signals.

In *all* other circumstances, where the intermodulation (or other spurious) is occurring somewhere else - filters will do absolutely no good whatsoever.

So an obvious example is if Westerglen site is radiating the 4th harmonic of Virgin Radio with its fundamental on 1215kHz. The 4th harmonic will be at 4860kHz. You can add all the MW rejecting filters you like - but that dratted 4th harmonic is going to come sailing through, right into your tropical BC band....


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Old 13th Nov 2020, 1:52 am   #30
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Default Re: Advice on the LOW PASS Filters

... Because that signal from Westerglen, if they were radiating some 4th harmonic would really be on 4860kHz. Anything you did to stop it would stop all signals on 4860, even ones you wanted.

Let's say Westerglen is innocent and scrupulously clean. What if there's a barbed wire fence round a field not too far away from Westerglen? It will receive volts of signal, and fences routinely are rusty, make bad connections etc. The connections can act like manky diodes and create harmonic currents, which the fence gleefully radiates.

Around transmitting sites out to some distance, you may find that the site operator has done some work on fixing (electrically at least) fences.

This problem really bites at multi-use sites. At Craigkelly, serving Edinburgh with TV etc, there are also many private and government agency 2-way services on the same mast. A mile or so away is the old Radio Forth mast blasting away with a VHF channel.

You don't just get harmonics being created, you also get the big signals and their harmonics intermodulating.... inter-mixing in other words. This can create unwanted spur emissions from fences, bad joints in the mast etc that surround and lie in and amongst the broadcast channels. If you're beginning to think this sounds like a right royal pain, it is. There are transmitters and receivers up there for RNLI, mines rescue and all the blue-light people and they all needed protecting.

Westerglen is relatively tame by comparison.
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Old 13th Nov 2020, 8:29 am   #31
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Default Re: Advice on the LOW PASS Filters

So I suppose, David, at Craigkelly all the local fences are stainless steel, there isn't a metal gutter left anywhere, and old cars are definitely unwelcome?

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Old 13th Nov 2020, 9:59 am   #32
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Default Re: Advice on the LOW PASS Filters

No, they just got cleaned and retightened every so often.

It's up on a bluff and far enough away from houses.

(I'm a bit paranoid about the mast at Craigkelly - it's following me around! It's the nearest broadcast site to me here, but it was moved. Before being at Craigkelly, it was at Emley Moor, near where I grew up. It was replaced at Emley by the famous mast, the one that fell down.)

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Old 19th Nov 2020, 3:13 am   #33
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Default Re: Advice on the LOW PASS Filters

I have been near strong broadcast band stations. These have never been a problem for
"boatanchor" tube vacuum tube radios. But they were for anything
transistor and especially SDR ones. With the modest price (12 bit) SDR ones I have
problems with strong signals in the 50 meter band, especially on exactly on 6MHz.

These were all solved with filters. I have highpass filters at 1.8, 5.6, 8.9, 13.1, 19.8,
and 27.1 MHZ, and low pass ones at 0.490, 2.6, 5.3, 10.7, 15.9, 19.7, and 27.9 MHz.
These vary from 7 to 11 poles and are (approximately because made with
stock inductors) Chebychev at 0.2 to 2 dB ripple. They work like a charm.
I designed, built and tuned them. They can be switched in in any combination.
I should add that the whole affair was not in the least cheap!

I also use an untuned antenna preamp before the filters , but it is just an emitter
follower with only current gain. Its reasonably linear up to five volts peak to peak fed
from 10 volts DC. At my current location the total input is typically 0.2 to 0.8 volt peak
to peak, from 15 kHz to 30 MHz. It causes no problems at all at 0.8 volt, even when the
battery drops to 6.5 volts.
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Old 23rd Nov 2020, 8:07 pm   #34
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Default Re: Advice on the LOW PASS Filters

Try Elsie http://tonnesoftware.com/elsiedownload.html if you want to make your own filters.
I've used it many times for filters to go in front of SDR dongles.

John G4IJD
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Old 24th Nov 2020, 2:49 pm   #35
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Default Re: Advice on the LOW PASS Filters

I had a cheap filter for blocking MW signals in HF from Amazon, and it was about £10 from earlier this year.

It seems work sometimes, but there are times, the strong MW signals are still heard on 4-6Mhz in HF even with the filter inline.

I left it inline just in case.
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Old 24th Nov 2020, 2:58 pm   #36
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Default Re: Advice on the LOW PASS Filters

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I had a cheap filter for blocking MW signals in HF from Amazon, and it was about £10 from earlier this year.

It seems work sometimes, but there are times, the strong MW signals are still heard on 4-6Mhz in HF even with the filter inline.

I left it inline just in case.

That suggests that the signals appearing on HF (4 - 5MHz) are being generated somewhere else - not in your receiver. Could be harmonics of the MW transmitters. Could be intermodulation in bits of metal around their site - "rusty bolt effect". Filter won't help with these problems.

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