The AM Forum

The disney station in town, listed at 50KW on 1580kc, Is tearing up my little Soft66LC2. Can someone recomend a low cost solution for a recieve only filter?  I dont need an expensive Transmit capable filter.  This is a $100 radio... 

Most I have round only offer 1 to 8 DB at 1580 and I doubt thats going to help me.

C

Clifton Labs, Jack Smith.
I know someone who has one. Maybe he will sell just the board and parts to save money. He just sold me 4 interface board kits for his e probe antenna.
Saved me a lot of money.

Hi

Maybe this will help,a wavetraps circuit.

C resonant is around 300 ,400 pf resonated with a coil at interfering Frequency.
(parallel resonance)
Maybe using two or three wavetraps in serie ?

Gito.N

The previous post shows a parallel tuned cuit tuned (I assume) to the frequency you wish to recieve. I prefer a series resonant circuit tuned to the offending station.  This is basically a notch. This works quite well in a high Z antenna system, placed ahead of the receiver input terminal.

I second Jeff's suggestion.  I built two of these notch circuits to use with a pair of Hallicrafters SX-101 receivers to remove a 1650 Khz. station out of Iowa that would make it into the Halli first IF when operating on 160. 

Any details? Does this apear in the handbook?  I saw one that uses three caps and two 3.3 chokes that might work.   I cant see spending $200 on a production filter for a $100 SDR.

C

On the cheeep.

If you can find an old radio to  'gut'....  rip out the loopestick ant and tuning cap ... and there's your filter.


klc

Thats a great idea..  Hmmm.  Now I need a POS old transistor radio to gut.

C

Expensive but a very good one, if interested; schematic and explanation are below. This filter was designed by Doug DeMaw.

hi

Jeff,You are right using a series resonant circuit.if the circuit is connected between the Areal input(antenna input) and ground.

But if you look at the attachment picture .it used a parallel resonant circuit  between the Antenna and the the areal input ,and made a high resistive  impedance at resonance (high Resistance at the tuned frequency)but not for another frequency.

Vincent ,if the Radio  Reciever  is design with a known input impedance like 50 Ohm,than actually we can make a high pass filter ,passing only the hF frequency,there are many high pass  filter design in the ARRL handbook

Gito.N

I plan on reading through the handbook tonight. I am using the little SDR reciever now and it really needs  a BC band filter in front of it. Its fantastic for $100 but lacks those filters. 

I cheated by setting the bandpass filter to 5.5mhz to 10mhz.  This stopped the BC stations cold, however, I lost about 20DB of sensitivity on the 75 meter band.  Even so. I get images across the spectrum.

C

Hi Gito,
of course you are right. I do not know this receiver so I thought it had a 50 ohm Z antenna input, like so many other receivers. I was completely "out of tune"!

Here is what I am dealing with.  1580KC Disney radio blasting in over the 75 meter ham band. 

C

Hi

Rereading the Article,it says A 50KW broadcast station in town? usually broadcast station is out off town ,to avoid RF interference /minimize interference to electric apparatus in town (audio amplifier ,telephone ....).

A high power BC station must have harmonics frequency ,even it's very low to the fundamental frequency ,it's high enough  to made it into You  radio reciever.
 
If the cause is the harmonic frequency /or the product of it that comes in Your Radio reciever   and in the  75 meter band ,than nothing You an do,since filtering it,is also filtering the 75 meter band.

Gito.N

That picture is of an image.  The BC station stays centered when I change Frequency.  The BC station is mixing with the oscilator? 

The filter will work.  I changed the default setting from 1.2 to 5.5 mhz to 5.5 to 10mhz and the BC station disapeared on the 75 meter band. HOWEVER, 75 meter band performance really suffers!

Today I wound chokes for Audio line, and USB cable.  No change.  Only way to remove the image is to unhook antenna.
C

possible some suitable parts for the filter may come from an -old- inductively tuned AM car radio. They had 3 slugs IIRC.

HI

When You Changed the bandpass filter to 5,5 mHz to 1o Mhz of course the BC station disapeared , because "it's frequency"/the product frequency" You recieved  is around the 75 meter band ,or around 4 mhz,

and of course  75 meter (3.8 Mhz?) band suffers , because the frequency below 5,5 Mhz is block  by the band pass filter.

Gito.N

Yes. You state the obvious.  However, this proves that the BC band filter will work. Maybe tomorrow I will experiment.  I woke up this morning with a damn chest cold.  Canceled my trip to family and am stuck home alone sick.  Ba humbug.



C

Hi

Yes the BC band pass filter works It eliminates the fundamental frequency and it's harmonic below the pass band.
That also mean,that the distrubing frequency is pick up through the antenna

A the transmitter produce  harmonic frequency that is filter out by it's output network,but  not 100% eliminate it. and if some of this harmonic gets into Your Radio ?
 It's a 50 Kw transmitter in town(it means "close" to Your house),if the the harmonics frequency  that falls in the 75 meter band has only 2 watt power,it still can distrub the radio. (50000:2 = 25000 =...DB)

It's like this someone is transmitting in the 75 meter band,and We want to block him by a band pass filter.Yes We can but of course we also blocks all the other transmitter which works in the 75 meter band.

So if it's a harmonic Frequency ,that falls in the 75 meter band ,than we can do nothing about it.

Maybe using  a high Q notch filter?

Of course I can be wrong.


Gito.N

I think the problem is the BC station mixing with the SDR radio.  The entire BC band and the 75 meter band is destroyed.  However, If i move the filter up or use a higher band, Then I dont have any interferance.

I understand you are suggesting this can be the second harmonic of the BC station which the BC band filter will not help at all.   


C

Clark,
is this a complete self contained SDR. RF 'front end' / I.F. ?? then the usual computer sound card hook up?
Some SDR's are digitizing the RF and maybe skipping the sound card and going USB to the 'puter.
I'm a little out of touch with any advancements.
Is this an Elektor SDR??

http://www.elektor.com/products/kits-modules/modules-%28-9x%29/software-defined-radio-%28070039-91%29.91475.lynkx?gclid=CK7Gm5Wyna0CFcNo4AoddQEDmA

And Gito, it is very possible that a 50KW AM station would be very close to people living close by. A developer near Washington DC built an apartment building right next to a directional array, 50KW, 1500 AM station. Used to be WTOP, NEWS/TALK radio.

There's a little TV shop near by making lots of money getting rid of RFI to consumer electronics and telephones. There is nothing on the WEB about this. I talked to the chief engineer, when they were having terrible coverage problems with their AM signal. They re-built the ground screen and it helped a little. Their weird directional pattern made it almost impossible to listen locally.
The engineer said booo-hooo on them. The radio station was there first. Sorry 'bout that!!!

ITs a self contained Receiver Fred.  It can hook to the IF but I do not use it that way.  Its called a Soft66LC2.  I own a bunch of recievers and almost all of them get some kind of interferance from the Station. My R390 and R390A are the worst.  Sp600 a bit better but I can here it. The Icom 756Pro does not get bothered to much. The FT101 does.  It really depends on where you are on the bands and what radio you are on.

The SDR being the worst of the bunch as it does not have any Hardware filters on the front end. 

The station is right around 8 to 10 miles from me.  The pattern plots show its beaming right at me though.  There are hams that life closer and never hear the station. But being in front of that directional array has caused me trouble for years.
C

OK Clark
Being that you mention several receivers getting overloaded from this station, it MIGHT be better to spend the $$$ on a real BC blocker and then your entire station plus the little guy will be happier without front-end overload.

My loop antenna has an optional BC blocker. Apparently part of their loop antenna options.

http://www.pixelsatradio.com/uploads/RF-PRO-1B-Specifications.pdf

A $100 dollar add-on preamp with BC High pass filter.

I own an I.C.E. model 123B Bandpass Pre-amp.......$48......

http://www.iceradioproducts.com/reconly.html

Might also knock out the BC band for $48.00

On 'The CHEEP" causes frustrations and cursing. I'm tired of both!!

Something to consider
Fred

Thanks for the links Fred!

C

FYI, Fred, The Ice page you sent seems to be products they no longer make. They only make the products on the main page now:

http://www.iceradioproducts.com/

ICE is now owned by Array Solutions. Mike Koss, owner of ICE, passed away in March 2011.

I kinda guessed that they where purchased.  They had alot of neat products listed.

C

I was wondering why my I.C.E. front-end protector was always taking months to come back from repair.
Looks like Array solutions is offering something better.

http://arraysolutions.com/Products/bcb_filters_top1.htm

That filter has only 10 DB or so at 1580. I doubt it would work.

C

A few years ago, 2002 to be accurate, I endeavored on a filter design that yields a minimum of 40 dB attenuation to all signals below 1702 kHz ... so it passes 1.8 MHz and above through 30 MHz with low insertion loss and better than 20 dB Return Loss:

http://www.scribd.com/doc/34925080/AM-Broadcast-Band-BCB-Brickwall-Filter-160M-10M-pass-Design-and-Performance

If it is constructed with the larger toroids and 500 V Silvered-Mica caps it will take 100 Watts transmit power down to 1.8 MHz too.


A more generic description that demonstrates the attenuation but is light on technical or construction details can be found here:

http://datos.s3.amazonaws.com/usblsb/descargas/AM%20Brick-Wall%20160%20Meter%20Filter.pdf

de Jim WB5WPA  Merry Christmas!

PS. Four or five protos were built, and they worked better than expected! W3DQ near Wash DC and WTOP (at the time) tested a unit as he is a big 160 M op.

Great info. And thanks for the connection to WTOP.

Hi


The way we think ,a strong MW transmitter (50 Kw) transmitter is transmitting near us,it goes into our Radio reciever ,and this signal is the cause of the all  distrubing frequencies in the band in our radio.Meaning that the cause of all that is this MW transmitter.

And Clark can get rid or at least weaken the transmitter coming in the radio by a band pass filter,and proof with a 5,5 to 10 Mhz  band pass filter,the band below 5.5 mhz is "clean".

Logically thinking when we get rid of this fundamental frequency ,than the radio must be clean since the interference is produce in the Radio is caused by the fundamental frequency.that gets in the radio.

 The point is if we can ellimate the cause,then the trouble is gone.

Yes a band pass filter or BC filter can get rid for all the BC station  that get into the radio .The purpose is blocking the bc station signal gets into Our reciever.(fundamental frequency)

But what about the harmonic frequency,or frequency by rectification..Remember is a power full transmitter and close. to Your House

Harmonic by rectification (from  ARRL}

Even though the transmitter is completely free from harmonic energy,it is still possible for interference to occur because of harmonics outside the transmitter.These result from rectification of fundamentall-frequency current induced in conductors in the vicinity of the transmitting antenna........
.
And harmonic means a second harmonic ,third harmonics,and all the product all  harmonic  frequency mixing  together. The harmonic frequency can very small power compared the fundamental frequency ,but enough to interfere.

This frequency is higher than the bandpass filter,so it gets into the Radio.

If this harmonics falls in the 75 meter band? it is not filtered out out by the pass band.



Gito.N

My experience in/around Dallas (with several 50 kW stations in the area and host of others at various other power levels and distances including a 1700 kHz night-time transmit location just a few miles away) is that most of them have little in the way of 2nd (or 3rd) harmonics of note; I have, however, encountered in-band 'trash' (in the AM band) by malfunctioning transmitters, about 4 years ago I reported one (with detailed info as to direction, where the 'trash' was observable with a spec an and what station the trash was 'coincident' with) to the FCC and the next week the station was busy remedying the problem (they were off the air for a few days).

There is trash presently, in the area, just below 1800 kHz that I have not attributed to any particular 'source' yet.

Of course, if I was setting 5 miles away in the 'boresight' of a DA 50 kW it might be a different story! As it is, 770 (5 miles to the east) and 1080 (50 kW at 10 miles) and the nighttime 1700 (2 miles) are the ones that hold any potential for inflicting interference my way.


Jim de WB5WPA

I am far from an expert, but I have an opinion anyhow...

This link's filter: http://www.scribd.com/doc/34925080/AM-Broadcast-Band-BCB-Brickwall-Filter-160M-10M-pass-Design-and-Performance

is essentially a Cauer or elliptical filter. If you take out the cap on the leg to ground you have a ordinary 2nd order HP filter per section, each section adding so two sections, 4th, then 6th, etc. The problem with the usual filter is that the stop band is not down that far AND the slope of the stop band is less than one would want.

The solution is that one makes one of the elements in the filter (series C, L to ground) into a notch filter. In this configuration that would be usually the one to ground. This makes the slope of the transition from stop to passband rather much sharper.

One can simulate these filters rather nicely in one of the free SPICE programs, like LTSpice. You could take an existing filter example and just dork the values and add the extra C to make it into that notch. For things like this, LTSpice is great - of course you need it set up with the operation codes to show the results properly (that's a bit obscure) but as I said you can use a generic example that produces a freq response curve for ANYTHING, and change the parts to be your filter, cut and paste!   ;D

In filters used for "important" applications they care about things like ripple, the depth of the first drop, etc... maybe even phase. For the application of notching out a strong local AM broadcast station this all is probably totally unimportant.

So, what I'd suggest is putting together a two section Cauer or eliptical high pass filter with the first drop set to bottom out just about dead center on the offending broadcast frequency. I'd bet you can get 40-60db of attenuation without getting very precise. Also it will be pretty insensitive to input and output impedance if ur a little clever in the input and output loading... you can always add another section. Even with at two section filter one can spread the center freqs plus and minus the target frequency, this ought cause the filter to still provide good attenuation wrt frequency in the event it sees somewhat significant changes in input Z or output Z.

These filters can be built empirically - what happens as you vary the L and C values around a center frequency is that the shape of the filter changes a bit, and the depth of the stop varies from maximum to about 20dB less, but since max is around 60dB down, that's not too shabby, especially since the attenuation goes up with the number of sections. So, get in the ballpark and dork the C until you get it on frequency, sweep it (if at all possible) and get the best you can, which is probably more than good enough!

The article was for a broadband broadcast band filter, you probably only need one that aims like an arrow for the offending BC frequency. Much easier.

My 2 cents...

                  _-_-bear

"is essentially a Cauer or elliptical filter."

Yes, stated so on pg 4. Thanks.

I also admit to using EESOF ESYN (as stated) to initially synthesize the components values, but, it takes 'skill' in picking certain key indices to hit the design points like IL in the passband, nail the 40 dB goal *at* 1.7 MHz and keep the ultimate attenuation better than 40 dB in the stopband; for some of those 'goals' modeling (I used Touchstone) with multiple iterations (many runs) tweaking and changing seeking all the while to meet goals and, indeed, they were met.

So, you're 'talking' to the designer, here. Any questions I've got answers (although it's been a few years since the design, test and field work was done) ...

de Jim WB5WPA

PS. As to 'SPICE', I would guess by now you've made it to the page where the SPICE simulation results for component currents at critical points were listed ... linear simulators like RFSIM99 are nice for quick analysis of filters, but, when it comes time to evaluate the power handling capability one turns to SPICE simulation runs ...

Jim

Very good info Jim,
How can you help Clark out?? Gotta filter to sell?? Not many people have junque boxes with the components needed to build such a nice filter.

He may not need the ultimate 40dB supression.

And Clark would you be willing to try the Array BP pre-amp on the condition that if it doesn't work for you you get a refund??

I'll do this............EDIT::::
I tested the model 123B and there was very little difference atten. BC band freqs. And the unit is only for 160M. It was essentially a pre-amp for a K9AY loop antenna.
Back to the drawing boards.

Fred, if one can find or wind the four inductors the rest of the components are just a bunch of caps! And a combination of values is acceptable, like I had to do to get close to some particular values.

And for receive only, the requirements for High Q components goes down as one can usually tolerate a little more loss on receive on these low frequencies, and the toroids can be the smaller ones and use more like 20 or 24 ga. wire to wind ...

Just come up with the correct uH values and she should fly ... not as optimum as if using the specified components, but again receive isn't as critical ...

Adjustment should be made to 'hit' the important 'notch' freqs shown on the schematic by adding or removing turns on the toroids to get as close as possible, within a turn, so come up with the values of the SM caps and then tune the toroids on the shunt arms to 'hit' those four notch freqs ... and the filter should fall right into place from there (she should meet RL value specs in the passband beginning at 1800 kHz on up).

I added a compression trimmer for the arm that tunes close to the 1700 kHz notch freq, but this is because we in this area needed to kill the 1700 kHz station and also to meet the 40 dB spec *at* 1702 kHz without having to touch that toroid for tuning, just a quick adjust of the trimmer and bingo!

de Jim WB5WPA

PS Should probably come up with a small PC board using small toroids for a receive-only model ... I'll look into that ...

Jim,

I wasn't trying to be critical of your filter or article.
What I did was to jump right to the schematic and response curve to see what it was. Then I posted. The original post stated that there was one particular frequency BC station in close proximity. So, he doesn't appear to need a broad, AM BC spectrum, filter at all. And he only needs it on receive.

So, his constraints are greatly reduced.

He can get away with maybe two sections of filter centered around the BC stations frequency - in many ways a much simpler filter to design and build than yours. Different problem, different design goals.

Hope this makes sense?

                    _-_-bear

Clark, do you have an attenuator? I wonder how many dB you need for the spurious signals to disappear. This will tell you what sort of filter you'll need.

Hi C:
I heard that some guys use the link antenna coupler to help with broad cast interference.  The link coupler is band pass design.
Chuck

Hi

If the transmitter is clean.free from harmonics,than a filter that can make the fundamental frequency low enough not to overload the front end of the Radio ,makes the Radio "clean".

Cause this interference is caused by overloading the input front end of the radio and

The interference is build in/produce in the Radio itself cause the overloading.

So if that's the cause, a simple notch filter ,than can weaken the input signal,can solved the problem

But if it's an harmonic frequency that causes the problem ,that's another story

Gito.N

That slipped by my grey matter !!!!!!!! YES an antenna tuner will be kinda like a filter.

But Gito brought up a thought about 2nd harmonics.......Any way to listen for the second harmonic on any of these radios?? 2X the broadcast freq.


It's taking this problem apart and do some troubleshooting. To see if it is the station's fault or just fundamental overload to your station.......
Crappy ground conditions/diode action can make life miserable too, Clark.
Do you own a portable radio, Sangean type of receiver, to listen for the same problem? If the portable radio is not affected, then your station needs a looksee.

Fred

Hi guys.  I have been very very sick.  I currently have no voice still and lot really feel bad.  This is twice this year I have been on the verge of the hospital. I have had no energy to do anything with radio. 

Gito:
I think the main problem is the primary frequency.  I can listen to the station the second harmonic, however, Its not blasting in or causing me trouble.

Steve, My brother has a few rack mount attenuators here.  I dont know how to use them and have not really looked at them close up. However, one is push button and one has many rotary dials across the front. I will see if I have the hook up cables here to experiment with them. 

Fred. I would be willing to try anything here.  I have a design that a local ham friend sent me that looks very simple to make.  Its a notch at 1580. I will ask if I can post his schematic. 

C

Good luck with the nasty BC band interference.

Hope you feel better.

OK Clark
Not goodness to get the same bug twice geeesh.
Get well first.

Fred

Thanks Fred.  I Just cant kick this thing. 

Any thoughts on this unit?  I understand its popular with DXers because they can run two bands and radios will not interfere with each other.  I like the fact that it will switch auto or manual, Has an SWR meter which is nice and has these filters for both in and out. I would place this behind the ICOM PRO/SDR radio switch so when transmitting, the icom would go through the bandpass and when recieving it would go through the bandpass.

I wish I had hard data on DB and design.  I can only go on the fact that I am told by a DX/contester that he can run two radios on two bands at once with out any interference.

http://www.rmitaly.us/index.php?main_page=product_info&products_id=18

That box is a lowpass filter. That will not remove your 1580 BC signal. You need a highpass or a notch.

I asked that question and I am told it is a band pass.  So each knob is a low and high at those band edges which would imply that it would filter everything below the indicated frequency 2-4-7,5-15-22-30-60 MHz

Let me shoot off an email to the company

C

If so, you are set.

Clark you might need an antibiotic regimen to kick it out. Probably a shot and then about 14 days worth of pills.
A teacher at school who handles one of my special needs kids has been dealing with something in her lungs the last two weeks. And another neighbor nearby has a similar ailment, something going on in her lungs. Pneumonia in one lung.

Fred

Excellent info on various BC traps, high pass, and notch filtering

http://www.dxing.com/tnotes/tnote06.pdf

Thanks fred.  I am going to just go to the doctor/clinic tomorrow.  Someone mentioned something called a Z pack that the doctor gave them and in days they where normal. 

C

Carr's "Figure 3" (pg 6) High-Pass filter looks pretty good.

He says it has a cut-off frequency between 1700 and 3000 KHz and blocks some some AM BCB signals while passing all the shortwave frequencies.

" Figure 3 shows a design used for many decades. It is easily built because the capacitor values are 0.001 μF and 0.002 μF (which some people make byparallelling two 0.001 μF capacitors). "

Figure 3 below:

(http://i39.tinypic.com/fxb9s1.png)

http://i39.tinypic.com/fxb9s1.png

W7TFO knocked on the Door today and handed me a JW miller 812-bc1 wave trap.  This looks like an IF can.  It has an adjustment to fine tune the trap.  I clip leaded the filter inline to the antenna ahead of the SDR and BINGO!  Problem solved. I can no longer here the Disney station anywhere on on the spectrum. I have attached Screen shots of the Primary signal with and without the Wave trap installed inline and of 75 meters with and without the filter.  As you can see 75 meters was not usable without the wave trap. 

BIG thanks to Dennis for this little Gem.  I need to mount it to a box with connectors now!

C

These shots show 75 meters with and without the wave trap.

W7TFO wins the "Victory Over Disney Award"

Clark, Now the fun begins.

I bet you're close to heaven now.

Fred

I sure am. The SDR works great now Fred.  The ECSS, is really cool.  I can use the AM mode, Choose the Sideband I want and the software tracks the carrier back and forth auto!

C

I put a Galvanic isolator on the sound card input.  This is a simple 1 to 1 600 ohm stereo transformer.  This took the peak out seen in the plots.  The LO of the SDR shows up there without the isolator.  This Dropped the noise level of the radio down 2 full SU.  Its now very nice to use on SSB and AM.  Not bad for a $100 radio. 

C

Friggin' old tech, saves bacon every day! :D

73DG

Sure does.  It really worked perfectly Dennis. 

The SDR is alot of fun now with out that LO signal in your way and with the BC station out of its ears.  One I learned how to adjust the AGC threashold, The noise disapeared and the stations came up.

Now I need a TR switch with 100DB pad so i can have this thing inline all the time.

C