HFU HF Underground
Loggings => Longwave Loggings => Topic started by: Matt285 on January 30, 2020, 0031 UTC
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I normally do not tune into longwave, but I was messing around this evening a bit. I'm hearing stations in that range (non clearly), but I'm not sure if it's not am broadcast bleeding over. Only using a 135' OCF dipole
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Most likely you are hearing some IM mixing products caused by strong local MW broadcasters. If these are there during the daytime hours, that is most likely the case.
Just about every LW DXer use a LPF prior to their receiver. I DX with mainly an 8 foot amplified vertical whip, and I use a 550 Hz LPF before the RX.
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An 8' amplified whip. That surprises me as a newbie. I tend to think of long wires. However I'm sure you know your stuff based on postings. Pardon my ignorance, whats an LPF. Thanks
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Low Pass Filter
Does exactly what it says... passes everything below the stop band.
PAR Electronics makes a nice one... BCST-LPF.
The filter stop band begins at 540 kc, so it passes everything
below that frequency and stops signals above that frequency.
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Yes. Low pass filter. Now that I remember I use to own one at some point. I got it with a box of misc stuff years back. I'll look around. Thanks
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If you can’t find your LPF there are a number of designs available on the Internet. They are very simple to build. I agree with JFarley and Pinto - if you are serious about LF you need one.
I use one from rescueelectronics.com (though I am not sure if they are still in business - they aren’t answering email.)
One other thing I have discovered to be essential (at least for my installation) are common mode filters on the feedline, at both the antenna and at the receiver. I was surprised at how much local EMI they cleaned up. They have to be rated to work at LF - your typical ham CM filter stops at 160M. (Palomar Engineering has models rated to go to 100 kHz, but they are very slow on shipping.)
You can build your own LPF and CMF for little cost...a fraction of what buying commercial products will cost.
If you build your own, make sure you use Type 75 or Type 77 ferrite for the CM filter.
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If you've any old pc power supplies to tear down, the toroids used in them should be good down around vlf.
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If you have about 100 bucks, and dont feel like building a filter, I have heard good things about these filters from Kiwa Electronics
https://kiwa-electronics.com/500-khz-low-pass-filter-3.html
To clarify, I don't work for this guy, nor do i have one, just passing along stuff I heard about.
Best of luck in radios' basement -EC
edit-(whoops, looks like its about 140 bucks)
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Would it be this filter? Or is this an old model?
https://www.universal-radio.com/catalog/filters/1143.html
A lot cheaper.
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It looks like the same thing with a lower stopband attenuation. it probably will work just as well as long as your receiver has a reasonably robust front end. I don't even have a filter on mine, I just rely on the fact that my antenna matching transformer works poorly above 1 mhz. My receiver is actually a demodulator, and can handle a pretty strong input.
another trick that works really well if the strong local interference is on a single frequency is to shunt that one signal to ground with a series resonant circuit, this is effectively a notch filter and will provide vary good rejection on a single frequency.
Long story short, any filter is better than none, and you don't need a filter with super high stopband attenuation to get decent results.
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Would it be this filter? Or is this an old model?
https://www.universal-radio.com/catalog/filters/1143.html
A lot cheaper.
I have one of those. It works, but the one I have from www.rescueelectronics.com works a little better.
I would recommend caution with the bypass switch that the universal-radio filter has. The capacitance across the switch can degrade the overall rejection of the filter by allowing a little bit of signal to bypass the filter itself.
Also be sure to use a good quality jumper cable between the filter and the radio. Last year I noticed detectable signals in the stop band of a LF LPF I was using. It turned out that the jumper I was using was the problem. I replaced it with a different jumper and all was well again. Checking the jumper out on a Spectrum Analyzer I found that the poor jumper degraded the filter stop band by more than 20 dB. When I cut the jumper open I found very poor braid coverage.
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If you have about 100 bucks, and dont feel like building a filter, I have heard good things about these filters from Kiwa Electronics
https://kiwa-electronics.com/500-khz-low-pass-filter-3.html
To clarify, I don't work for this guy, nor do i have one, just passing along stuff I heard about.
Best of luck in radios' basement -EC
edit-(whoops, looks like its about 140 bucks)
I've had the Kiwa LPF for years - it works great - without it I am overwhelmed by local MW when hunting NDBs - with it, things are quiet as a mouse and NDBs are still plentiful - but they are slowly being decommissioned.
FPE
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Thank you for the heads up NJQA, I just ordered the RF Low Pass Filter from Rescue Electronics Surplus.
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Please let me know if you get a response from Rescue Electronics. They didn’t answer an email I sent them a few months back (as I noted earlier). You should be safe with a Paypal purchase as you can get your money back if he doesn’t respond. The filter does work well.
The owner is W1VLF, who has several useful Youtube videos available.
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NJQA I ordered the filter and about 45 minutes later I received an email from Rescue Electronics asking what type of connectors I wanted, I forgot to mention that on the order. Anyway I responded with the type that I wanted and received another email verifying the type and stating that he would start building the filter. So far excellent communication with Paul.
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Thanks. I will try contacting him again.
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NJQA I hope that you were able to contact Paul by now. I did receive my filter today, so just about one week from the order to my house. Excellent service and now to test it this weekend.
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Thanks. At least I know he is still alive. I still didn’t get a response to my latest email. I will calling him next.
I hope you like the filter.