Broadcast band interference

[ka1tdq]

I just got an SDRplay receiver, and it really does a good job except for broadcast band interference on 75 meters.  I've got one station planted on 3870 (a popular AM frequency here in the west).  One good feature is that the software can cut out my power line hash that I get on 75 meters.  I'm using HDSDR and clicking one button clears everything up almost 100%. 

Does anybody have a good schematic for a well performing high pass filter?  I don't care about 160 meters, so it could possibly cut that too.  I just need 75 meters and up.

Jon

[w4bfs]

hi Jon .... one of the best off-the-shelf ham hpf is a EFJ kilowatt matchbox, especially for 75 mtr bcb overload...give it a try

[Steve - K4HX]

Any ARRL Handbook in the last 30 years has this info.

[ka1tdq]

I could go with the matchbox, but the radio itself is the size of a cigarette carton. I'm trying to go sleek and have everything hidden in the shack.

The ARRL handbook is a good reference. It covers this topic as well as basic electronic theory, construction practices, troubleshooting techniques and examples of solid state and vacuum tube design.

My friend from VK land emailed me a program to calculate a 7 pole filter with a cutoff of 2 MHz. That should work pretty slick.

Jon

[w4bfs]

all good info ... low power but keep the Q's high for best rejection

[KD6VXI]

Short leads,  feed through caps,  and put sections in their own boxes to maximize shielding,

--Shane
KD6VXI

[W3NE]

Your experience with the SDRPlay in a high noise environment is interesting because I have a tremendous noise level here -- S9+10dB on 75M.  Rice box DSP noise reduction helps but still falls short of what is needed and I'm always looking for something better (short of moving to a mountain top). Can you give a rough "S-meter comparison" of your experience with the SDRPlay vs. a conventional receiver?
TNX, Bob - NE

[ka1tdq]

I don't have a rice box receiver anymore to give you an S-meter comparison. I only have the SDRplay now for my receiver. You can YouTube some videos that do give you a side-by-side comparison, and the difference between the two is about unnoticeable.

I'm using HDSDR as the software for my SDRplay, and clicking the RF NB button seems to wipe out all the hash. It seems to be well written software. The RF NB level is also adjustable according to the level of hash at your location.

I've attached a photo of the filter I'm going to build per the software.

Jon

[KQ6F]

If I were you I'd set the corner frequency higher, say 3.0 or even 3.5 mHz.  You can do this since you don't care about 160M.  With your present design you'll only be getting about 20dB attenuation from the high end of the BC band. 

Rod

[MikeKE0ZUinkcmo]

I have two bcb stations nearby and I built this filter to combat the problem.   The assembled filter closely follows the graphic.   As you can see the upper portion of the BCB is only down about 30db, but this allows me to use 160 with some success as the big offenders are fortunately below 1 MC.

[ka1tdq]

Mine isn't as neat as yours, but it made all the difference in the world.  I modeled this for a 3 MHz cutoff frequency.  With it, I copied the AMI net with no problems.  Without it, the spectrum was filled with broadcast. 

Jon

[flintstone mop]

I have two bcb stations nearby and I built this filter to combat the problem.   The assembled filter closely follows the graphic.   As you can see the upper portion of the BCB is only down about 30db, but this allows me to use 160 with some success as the big offenders are fortunately below 1 MC.

This looks like a very easy solution to your problem. It will also allow you to enjoy 160M. Winter is around the corner and lower noise from mother nature. Shorter day light hours and QSO's will be returning around 8PM local time. 75M seems like a real challenge to enjoy, unless your favorite mode is CW.

[WB6NVH]

If it's just one AM station, how about a wavetrap?  A bit simpler to implement and ready made ones by Meissner or JW Miller show up on eBay once in awhile and would be really simple to implement.

[pa0ast]

You can even buy them f.i.    http://www.allenavionics.com/LC_F/Prec_Fil.htm
Those small LC filters  are very common in video equipment. Watch the in and output impedance ( 75 Ohm ), ad some resistors 27 Ohm to load them properly.
You have some attenuation in that case , but the correct curve.

[MikeKE0ZUinkcmo]

The allenavionics is A possible solution, but I suspect the cost is pretty steep, on the order of 30 bucks or better.   If a simple reject/notch filter does the job, a one off, in a bent up bud box like Jon's pic, might cost 5 bucks to make.

The filter I show cost about ten to twelve Dollars as I remember but the most expensive component by far was the cast Zinc box.  

Mike