Interesting thread.
I have a few thoughts on what I've read.
Those the tried shielding under the batteries may have actually seen an increase of noise as the foil was closer to the source of the noise which is likely the SD card or the signal lines feeding it.
The foil could very well act as an antenna instead of a shield which may explain why placing it under the batteries increased the noise instead of reducing it.
I hunk of metal placed near a noise source can work both ways, as an antenna which would increase the noise or as a shield which would reduce the noise which is the obvious goal here.
What I don't think has been mentioned is if anyone has tried grounding the shield tape to the radio's ground plane?
I'm not talking about running a single wire from the ground of the PCB but rather several ground points and try and build an actual metal shield or lid over the SD card area like you see with a true shield that has been soldered in place over RF sensitive areas to keep noise in or out.
A properly installed metal shield can be seen in N9JIG's pictures in this post:
http://forums.radioreference.com/un...436hp-discriminator-teardown.html#post2120656
Best seen in his last two pictures of that post.
The shield cover is properly soldered at several points along its edges.
I think that is what is needed to get the best shielding of the noise maker in the 436.
Of course there is not much room for doing this plus it would make removing the micro sd card rather hard!
The chance of shorting something out would also be very high so I don't actually recommend doing this.
The metal retainer latch over the SD card is shielded when it is closed but I can't tell how well by eye.
It does not cover the entire card also nor does it cover any of the PCB traces leading to/from the card socket that have a black colored coating applied over them. I think this black coating is just a coating to prevent shorts in that area of the board being as it is exposed when the batteries are out. I doubt that black coating is also an RFI/EMI shield coating. I think it was applied as a protection for the PCB more than anything. It would keep moisture from getting to the copper traces and help stop corrosion of the copper traces and through holes all around the SD card area.
Looking at N9JIG's pictures in the post I linked to above, the blackish colored conformal coating seems to have not been applied very evenly and some of the PCB areas show the coating to not exist at all. It looks like it has been washed away in his 2nd to last photo around IC104 or the area between the two IF filters marked 50E and 50G and the shielded metal cover just above those two ceramic filters. You can actually see all the copper traces feeding IC104. I did not read his thread but after seeing that faded away black coating, it almost looks like he may have sprayed the area with a contact cleaner product which could have dissolved the conformal coating that is now gone.
Perhaps he did this on purpose to get at the copper traces for soldering in wires for a discriminator tap which I think was the topic of that thread. It's odd if it came from the factory looking like that.
You can also see the exposed round test point all over the boards in his pictures. You don't see any exposed test points in the visible area around the SD card which is good so nobody shorts anything out while swapping the card. Even all the thru holes all around the SD card are loaded up with the black colored coating to prevent people from shorting something out in the card socket area. A tiny hunk of wire falling into the area could do damage if it shorts something out so the coating would reduce that risk. If you did drop a tiny hair sized hunk of something conductive into the card area, it would eventually work its way into the radio and probably still short something out with all the exposed test points and shiny soldered components that are not covered with a coating.
I do wonder if the steel card retainer cover itself is not the source of the noise. It very well could be acting like an antenna and radiating any noise coming from the card even though it appears to be grounded to the chassis when closed.
Sometimes an ungrounded shield cover may work better than one that is grounded. That may explain why bearcat's battery door shielding works while the attempt at shielding below the batteries did not. It could have been conducting to the cards metal retainer or acting as a passive antenna and re-radiating any noise coming from the card or around it being as the foil tape was much closer to the noise source..
Has anyone powered a 436 up via a USB cable without batteries installed and probed around the card area with a shielded probe lead such as would be used with something like a frequency counter or oscilloscope?
That may be a good way of finding the noise source as just the tiny probe tip would be your pickup antenna.
As strong as this noise source sounds, that should work for picking up the noise and narrowing down the source of the RF noise.
If there is no noise when powered from an external USB power source and no batteries installed, then I'd suspect something is leaking noise back to the batteries which could also explain why the foil tape inside the battery door works. Probably a voltage regulator in the power supply stage of the radio if this is the case. Or the battery voltage level monitoring circuit maybe. The batteries would be acting like an antenna.
The bottom battery cell is grounded to the radio chassis and the top battery is the B+ source of power. The middle battery floats between the other two. I don't think they use any type of individual cell monitoring in the 436 so there should be nothing hooked to the middle battery other than the lower and higher batteries. The middle battery does sit nearest to the majority of the SD card and its socket so that may be why the noise seems to be shielded the best when covering it with the foil tape if the noise source is the SD card or something radiating from one of the copper PCB traces going to the card socket. The upper battery could also be a noise radiator as its metal housing is also floating below B+ and the positive terminal of the center cell.
Anyway, if someone has a way of figuring out how the ground the battery door shield like many are installing, that may be worth testing to see if grounding the shield helps or makes thing even worse.
One could also take the radio apart and spray the area that the battery door touches when installed with an RFI/EMI shielding spray coating and also maybe spray the door instead of using foil tape, that would make an electrically conductive connection between the battery door and the radio chassis. You would need to apply the spray coating so it electrically connects to the radio chassis when the boards and case are screwed back together. Just need to be careful as that shielding spray coating is conductive. They recommend two coats with a dry time between the coats to make the most efficient shield.
I would try and test if actually grounding the battery door foil tape shield helps before applying any conductive spray coatings as the spray coating would be hard to remove once applied if it makes things worse.
For those using copper foil for the battery door, they could solder wires to the copper foil.
For those using an aluminum foil tape shield, you could solder a wire to the lowest batteries negative terminal in the radio where the batteries negative terminal is soldered to the PCB and make a coil of bared wire laying on top of the battery where the battery doors anti-rattle foam is located on the battery door. When you close the battery door, the pressure from the foam should cause your ground wire to connect to your foil tape when it smashes them together. Just don't make it super tight to the point where the pressure could cause the wire to cut through the batteries outer jacket and create a short.
I would only do this for creating a conductive ground between the foil tape and the radio chassis for a quick RF noise test and then remove it when done testing. If the wire cuts through the batteries skin, you would have a direct short to ground. Even though they are only AA size cells, a direct short would heat something up pretty fast as todays NiMH cells can produce some decent amperage and get hot fast.
Probably be a good idea to lay something thin and strong and non conductive between your temporary ground wire and the battery it sits on.
At least a couple layers of electrical tape if nothing else.
If the RF noise goes back to being bad when this test is done, I'd not spray the case with any type of conductive spray coatings made to reduce or eliminate RFI or EMI. Stick to just trying different foil tapes on the battery door alone.
Later today, I'm going to try and do some probe tests with and without batteries installed and see if I can find the actual source of the noise. If I can, I'll try some various shielding methods including the original design the bearcat used.
I also don't know what spray coating has been messing with but I've had good luck with a product from MG Chemicals called Super Shield. I've used it on several plastic cased devices I've owned over the years and it worked remarkably well in most instances. I did run into a few devices that did not like to have the coating be grounded to the devices chassis but just applying the coating and leaving it float without a connection the those devices chassis did work.
I learned to always check and see if an eclectically grounded shield helped or not before I ever applied any spray coatings.
Mouser and Digikey both sell MG's products.
I've used Nickel based, Silver based and copper over silver spray products.
The silver and copper over silver products did the best job but none of them are cheap.
There are others that also make similar aerosol based EMI/RFI protection products that may be better but I've only used those made by MG.
Be back later when I have some time to do some sniffing and testing unless someone beats me to it.
