SOLVED: Running an R7 in the BSM Beast - Mazda CX-5


Don't hate me just because I'm beautiful 🤣
Intermediate User
Oct 25, 2014
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Vancouver, BC
Running an Uniden R7 in a Mazda CX-5 with it's BSM's is a test of one's sanity, and marriage, if you're in an area where K band is used for radar. The issue appears to be the rear BSM system that continuously spews 24.123/4 GHz, and in my case with firmware 1.35 turning K down to 30%, Rear to 30%, MRCD, K Block, K1/2, TSF all turned on, it was still unusable in the CX-5.

I was inspired by @SixPackABS radar absorbing material that he applied to his R7 case to address the problem:

and @DC Fluid solution of blocking the rear horn with metal based tape to stop any signal coming into the rear horn:

I was seeking the benefits of both systems where I could still have functioning arrows when possible, and took a very crude waveguide filter approach. The intent was to reduce the strength of the 24.123/4 GHz BSM entering the rear horn, without affecting most other signals. I mocked it up for testing which can be viewed in the same thread that I've listed for @DC Fluid. Wanting to make a more discrete system that I could share with others seeking a similar solution, I continued experimenting with a waveguide filter, but mounted inside the R7 case.

Here's the items I used (No beer can, and no machinist, so I had to go with Diet Coke :beer:)


Using my calipers I scribed a circle into the light gauge aluminum from the coke can. Ignore the measurement (radius of the circle) on the display. Despite it being theoretically correct, it did not provide enough attenuation when mounted inside the R7, yet it worked well in the exterior mock-up. The final version has a 7.0 mm (~9/32") diameter hole, and I'll detail why I think that is later in the article.

. PXL_20210406_152526400.jpg

I did the final cutout with an exacto knife following the track created by calipers.


I then removed the four screws in the bottom of the R7 case, and removed the base, along with the front face plate, which comes off freely. Then I removed the lens covering the rear horn by removing the two brass coloured screws. The base of the lens clips into place, so have a look at that before forcing anything as it comes off/goes on with little effort. I then trimmed the coke can piece with the hole I created, to cover the face of the rear horn, and a tab to secure it into place with the lens and its two screws. This is now a crude waveguide filter.

PXL_20210406_153328922.jpg PXL_20210406_153558430.jpg PXL_20210406_155634737.jpg

Holding the aluminum piece in place, I slipped the lens back into position, and secured it and the aluminum piece into place with the two screws. I created the two holes in the aluminum by just pressing an awl through it. Finally, I replaced the R7's face plate and bottom plate. Again, there is nothing to force into place.

PXL_20210406_161330795.jpg PXL_20210406_161649978.jpg

In testing however, the waveguide filter provided little attenuation to the 24.123/4 GHz rear BSMs without having to reduce the K and Rear drastically. I'll leave out the details, but eventually a 7mm diameter hole, with the hole centered over the left side (viewed upside down; opposite side as the mute and mark buttons) of the rear horn, and the remainder of the rear horn covered completely.

I think the radically difference in the mock-up hole diameter and the interior version has to do with the lens, which focuses the radio signal into the horn increasing its sensitivity. Hence, my filter was facing a stronger signal that required more attenuation than when mounted on the outside of the R7 case. The reason I centered the hole over the left side of the horn is it appears smaller than the right side of the horn, indicating it handles the higher frequencies. I assume this to be the Ka band, but it could very easily be both K and Ka, with the larger right side being X band.

I could very easily be wrong on that point, but I installed the waveguide filter without trying it in the center, as I had for every version previous, and it worked. Good enough for me as I had put too much time into it already - The dog liked all the car rides for testing mind you. :laugh:

Now I can run K at 90%, Rear @ 90%, MRCD, K Block, K 1/2 on (TSR off) and I get the odd FRONT 24.123/4 GHz if there is vehicle close behind me to reflect the signal forward. Previously I never noticed a front alert, presumably because the rear anttena had the stronger signal. However, dropping K down to 80% pretty much solves it entirely.

Running pass two speed signs transmitting at 24.151 & 24.153 GHz respectively (they drift significantly through the day, and we much lower the day before), anecdotally I seem to have more range (the heartbeat was beating more quickly and regularly with this modification) from each speed sign. The front arrows of course operated normally, and would switch at or slightly past the sign, with the rear arrows being delayed, but still flipped to the rear.

What do this mean to the final operation of the R7? As the frequency gets lower in the K band, the rear antenna will be less sensitive, and a dead block to X band which isn't used here. As the frequency increases (as in Ka), the rear antenna will receive those frequencies as if the aluminum waveguide filter wasn't there. The front antenna will operate just as it always had, and the CPU won't be dragged down looking at blocking false alarms which should help it sniff out weaker signals.

Big thanks to @SixPackABS, @DC Fluid (who doesn't even own a CX-5, and offered up his International RD for testing!) and @Mithheru, for all of their testing, advise/input, and refining of what settings to use to prevent/reduce CX-5 BSM falsing that everyone gets to deal with when they're passed (not a typo :rofl:) by a CX-5.
Last edited:


PSL +5
Intermediate User
Aug 8, 2018
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Just a comment that the way the K blocks work on 137 there would be no reduction in performance compared to without K block. This is because the K blocks are implemented differently in 137, and written directly in machine code, rather than relying upon separately compiled code, so actually the 137 firmware handles K blocks more efficiently than US 135 without them!

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