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Technical Topics => Equipment => Topic started by: ChrisSmolinski on June 25, 2019, 1745 UTC

Title: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: ChrisSmolinski on June 25, 2019, 1745 UTC
The new 550 kW 9330 kHz WBCQ transmitter is already causing overloading issues for many SWLs. One solution is to built a notch filter. This can easily be done using a Tee adapter and a piece of coax cut to 1/4 of the wavelength (taking into account the velocity factor of the coax). Put the Tee adapter in series with your coax run from antenna to radio. Then from the other part of the Tee connect the 1/4 section.

9330 kHz is 32.15 meters, 1/4 of that is 8.04 meters, or 26.13 feet. The velocity factor of coax varies depending on the type (and even run to run to some degree). I used RG-6 which is about 82%. That suggests 1/4 wavelength is about 21.42 ft. Cut it somewhat longer than that, then trim to get the notch where you want it. I ended up at about 21.6 ft long.

Here's two screenshots taken with SdrUno and an SdrPlay  RSP1A receiver, I used a homemade noise generator as the signal source. The top screenshot is without the notch filter, the second is with.  The notch is not going to just take out 9330, but also a good chunk of the 31 meter band as well, but the rest of HF is mostly unaffected (you will also get a notch at 3x the design frequency or about 28 MHz). It's fairly deep, 20 dB or so. 

Thanks to CommSigma for sending me in this direction.

(https://i.imgur.com/OMd2xGi.png)
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: R4002 on June 25, 2019, 1851 UTC
Thanks Chris and COMMSIGMA.  I use both your KiwiSDRs on a pretty regular basis, and the WBCQ signal on 9330 kHz is insanely powerful (especially on the Westminster, MD KiwiSDR). 

A notch 3x up in frequency?  9330 kHz x3 is 27990 kHz (27.990 MHz).  Hmmm....would this mean a similar sized notch effect just below the 28.000 MHz 11 meter freeband / 10 meter amateur band interface?  While there isn't a lot of activity in the Americas in the upper portion of 27 MHz (unless the band is really busy), the UK FM band, CADS and WPAS bands (Irish Church Radio, UK FM CB, etc.) are 27601.25 to 27991.25 and 27605 to 27995.  Would the UK FM Channel 19, 27.78125 MHz / 27781.25 kHz be notched out as well? 

While most 11m CB activity tops out in the 27.8 MHz to 27.9 MHz region (channel 40 on the high band is 27.855 MHz), the 27.8 MHz to 28.0 MHz region is heavily used by 27 MHz Church transmitters and other 11m DX targets coming out of Europe, Ireland and the UK. 
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: Σ on June 25, 2019, 2322 UTC
I haven't put a 9330 kHz filter on my SDR yet. I'm watching to see if the overloading is regularly occurring or not. I currently have one tuned to 7435 kHz to reduce the overloading affects of R. Marti that did occur almost nightly. Ideally, if I could rig up a coax switch on a timer that would only add them at night during the high signal strength time... hmmm. :)
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: Pigmeat on June 26, 2019, 0238 UTC
Buy a real radio with knobs. They come with notch and noise filters installed.
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: ChrisSmolinski on June 26, 2019, 1258 UTC
Buy a real radio with knobs. They come with notch and noise filters installed.

Looks like Fansome hacked Pigmeat's account.
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: i_hear_you on June 26, 2019, 1549 UTC
I was listening to 9330 yesterday afternoon.  The broadcast sounded "hot," heavy audio compression squeezing the voices of both weeneys as they spoke, room noise pumping during pauses.  I understand heavy processing for intelligibility while QRP (if I don't make use of serious compression and EQing with my 15W I'm usually not understood,) but pushing a half-million I hope they decide to ease it back some.  It was extremely fatiguing to my ears.
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: ThaDood on July 01, 2019, 1836 UTC
You could always build a DIY Notch Filter.    http://www.wa4dsy.net/cgi-bin/lc_filter4?FilterResponse=Bandstop&poles=6&CF=9.330&cfunits=MHZ&cutoff=.020&funits=MHZ&Z=50       I've used a site like this to calculate a 7 Element Chebychev filter when I've built the 13.560MHz mW TX. One trick to really bring this filter dead-nuts perfect? Use all variable CAP's. The inductors you can wind yourselves.   http://www.wa4dsy.net/filter/filterdesign.html        There are other filter design sites out there, but this one works pretty well.
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: R4002 on July 10, 2019, 1425 UTC
I was listening to 9330 yesterday afternoon.  The broadcast sounded "hot," heavy audio compression squeezing the voices of both weeneys as they spoke, room noise pumping during pauses.  I understand heavy processing for intelligibility while QRP (if I don't make use of serious compression and EQing with my 15W I'm usually not understood,) but pushing a half-million I hope they decide to ease it back some.  It was extremely fatiguing to my ears.

I concur.  It does seem like they're running some pretty heavy compression...but there's differences from program to program, so there's also the possibility that the audio is being provided to WBCQ already compressed.  Who needs dynamic range anyway? [kidding]   with 550,000 watts carrier power, you can afford to have some dynamic range and make the audio sound nicer, especially for music. 
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: i_hear_you on July 10, 2019, 2100 UTC
...but there's differences from program to program, so there's also the possibility that the audio is being provided to WBCQ already compressed.

I was listening to Bossman and his wife (Alan and Angela) who, I assume, should know better!

A question to the old-timers:  was this what it was like during the good 'ole days of shortwave broadcasting?  Were the Cuban, Russian and Chinese (and American, for that matter) propaganda stations shooting around the globe with 500k watts like the WBCQ super station?  I reckon I could RX WBCQ with a salty shoestring.

Back on topic, are any of you suffering from overloading?  I haven't noticed any on my end, but I'm still quite green and probably wouldn't know it if I saw it.  Are you talking about de-sensing on adjacent frequencies?  Images on other bands and so on?
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: Σ on July 10, 2019, 2139 UTC
Radio Moscow has some serious flame thrower transmitters. Sometimes the audio was very poor. I don't remember them splattering like China though. The old days of the late 70s and 80s were a lot of fun. You could hear tons of countries broadcasting and the content was often very good.
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: skeezix on July 11, 2019, 0248 UTC
When are they going to switch to C-QUAM?

Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: ChrisSmolinski on July 11, 2019, 1225 UTC
When are they going to switch to C-QUAM?

They could even switch to DRM and the audience would not get any smaller.
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: Ray Lalleu on July 11, 2019, 2052 UTC

A question to the old-timers:  was this what it was like during the good 'ole days of shortwave broadcasting?  Were the Cuban, Russian and Chinese (and American, for that matter) propaganda stations shooting around the globe with 500k watts like the WBCQ super station?  I reckon I could RX WBCQ with a salty shoestring.
There were lots of high power transmitting stations. Many have been dismantled. The highest power was in  Grigoriopol with 1000 kW transmitters in SW. Now set back to 300 kW modern TXes  under Babcock management. Also don't forget the high gain antennas, and anything else carefully planned to give the strongest signals in the target areas. Look for the Radio Liberty in Playa de Pals, now dismantled.
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: Josh on July 11, 2019, 2107 UTC
When are they going to switch to C-QUAM?

They could even switch to DRM and the audience would not get any smaller.

I second DRM use! Who wouldn't love hidef pics of Bro Stare?
Title: Re: Notch Filter for the 9330 kHz WBCQ SuperStation
Post by: redhat on July 12, 2019, 1337 UTC
When are they going to switch to C-QUAM?

They could even switch to DRM and the audience would not get any smaller.

At least C-QUAM is AM compatible...there would be something to hear during fades, unlike DRM.

+-RH