Hello and thanks for reading about my strange problem.

I am currently working at a house in which I can only get about four feet of range on an MRF-250/300 (both act the same), regardless of placement. After the folks at URC couldn't help (usually they solve my problems immediatley), I turned to the RC forums. I read several posts about what could be causing my problems. The house is full of lutron dimmers and is very near two radio transmission towers, both of which I have read will cause massive RF interference. Here is the weird bit; There is no RF interference!!! On two different MRF-250's and one MRF-300 all set to ID 0 DID NOT BLINK STATUS AT ALL!!! I sat and watched the things for five minutes while on the phone with URC and not one little blink, yet no matter where I place the remote I can only get about four feet of reliable range. I have tried three different MX-850 hand units to no avail. I have installed/programmed hundreds of URC remote controls and never seen anything like this before! Any help would be greatly appreciated.

My purely unscientific and unsubstantiated theory is that it's still an interference problem even though the light is staying off.

Assuming that you've tried different remotes, different base stations and the combination that doesn't work at the house DOES work on your test bench.

Could it be that the radio towers are on completely different frequencies, but their output is so strong that it's completely jamming all other nearby RF?

There's a tower in town here that broadcasts AM at 960Khz, but even if I'm tuned to say....1000 mhz, I can still clearly hear its signal as I drive past it.

Is it possible to throw in the towel and just use a hardwired IR repeater?

Incidentally, I'd be interested to hear if say an RTI or other competitor worked or had the same problems.

Crawl Monkey,

This sounds like a classic case of receiver desense. The receiver, MRF250/300, contains an Automatic Gain Control (AGC) circuit. AGC works by amplifying the level of the incoming signal, until a good signal is received. The MRF250/300 units are not very heavily shielded, and my guess is that you have so much stray RF flying around that it is bypassing the receiver stages, and getting into the AGC detection circuitry. This then causes the AGC to reduce gain, because it sees too much signal, causing you not to receive the signal you want.

The only way to combat this, if it is the problem, is to shield the AGC circuit. Because you will have difficulty shielding the MRF250, while not killing it's ability to receive signal, I suggest using the MRF300. Take the MRF300 and shield it. For starters, try good old aluminum foil. Wrap it up nicely. Fold the seams neatly over several times to achieve a good seal. You can poke small holes for the LED, or other things, just keep them small. Use the remote and the MRF300 and see if the LED comes on each time you push a button.

Good Luck!!! Hope this helps, I've got my fingers crossed for you!

hmmm, What exactly do you think your going to shield with aluminum foil? Aluminum foil has no ferrite in it, it doesn't shield very well. Aluminum IS a good conductor of electricity and transfers heat well - but shielding from RF, EMI, etc... not very well.
First, he would need to use something like Mu-Metal foil or Mylar foil - and you would have to ground it somewhere. Second, I doubt this will do any good - you still have an antenna that's necessary to pick-up the remote signal, that's also going to pick-up the interference (if your lucky you might get a few more feet due to out-of-band rejection of the antenna). If this is even the problem.

You didn't say what kind of radio stations they were. Are they AM, or FM, or a mixture of both? Depending on their distance & power, you just maybe screwed (can't defeat physics). I would think the Lutron would be missing a command every now & then if this was the case though. Does the Lutron work 100% every time? Also have you unplugged the Lutron's repeater and see if that changes anything? - they operate in the same frequency and although the Lutron only transmits breifly during a command, the Lutron repeater can pick-up noise and repeat as well.

Lastly, what components are you trying to control? HK's, Tivo's, etc... are known to give problems to Universal's base stations. Makes it seem like your not getting good RF when it's the strength of the IR. Did you try adjusting the IR output when using the MRF-300?

Brad, What I think I'm shielding with the aluminum foil is an electrical field. If I were trying to shield a magnetic field I would need ferrite, but I'm just interested in shunting the electrical field. It's not a perfect material, but it is cheap, readily available, and if this shows improvement, we could think about the best material to use if the aluminum turns out to be inadequate!


The Mu-metal would be the necessary item to shunt a magnetic field, but I don't think the issue is a magnetic field! For an electric field, all that is necessary is to shunt the electric field... It's called a Faraday cage. The Faraday cage does not need to be grounded to be effective. After 3 years of college level Electromagnetics classes, I do know what I'm doing in this area!


I'm focusing on receiver desense. If it is not receiver desense, this will likely not fix the issue. But if the interference is in band, and just as strong as what you're attempting to pick up, theres nothing I could do to help you. The only fix in this case is to supress the RF interference at the source. Another reason to focus on a cheap easy to try solution, If it helps great! If not, he's lost 10 minutes and $.50 in foil, BFD!


I would ignore IR for now, and look to get good repeatable results that when you push the button, the light on the front of the MRF-300 shows reception. Once this is working reliably, then worry about the IR, placing the emitter's etc. You want to reduce the number of variables you work with at a time.

Crack open your books again - Radio Waves Are Electric & Magnetic Fields Oscillating 90 Degrees to Each Other.

Saying "Ignore the IR" could be a big mistake. Anyone who's had to deal with equipment that had a very sensitive IR eye can tell you, the symptoms are simliar so you would think it's an RF problem. Moving around the room, the equipment would work intermittently. Seems like an RF problem, but it was the IR output from the base station varying with the RF input it was receiving. Move the IR emitters away from the eye a little (or pad them down) and it would work flawlessly.
Depending on the equipment effected, it could be an IR problem.

Just did some research: Aluminum is a little bit better at shielding than I thought. Also, store bought Aluminum Foil usually contains 1-2% Iron. Even so, I have never seen store bought alumnium foil used effectively for shielding.

This message was edited by Brad Humphrey on 08/15/05 01:02 ET.

I'm not an EMC engineer, but I did sleep at a Holiday Inn ... once.

Any good conductor can be used for RFI shielding--ala Faraday cage(any opening decrease effectiveness). It must be connected to the circuit ground. How it is connected to the ground is very important for it to be effective.

In any case, with the nice long emitter cables, and power supply cable potentially acting as antennas any shield is effectively nullified without addressing these unintentional antennas. These would need to be filtered with ferrites.

MX3000 - 899.00
MRF300 - 199.00
Labor to install and program - 500.00
Several days of unbillable labor due to RF problems - 800.00

The feeling from using an RTI remote and having it work 1st time....Pricless

URC is not a sku with us anymore as an RF based remote, had only a few actually work....not 100% but work

Every job with URC and RF has resulted in lost labor in troubleshooting
as an IR remote its awsome and no problems add the rf and its like flushing money down the toilet

Looks like user errror

I don't need to crack open the books to know that. But the circuit will primarily pick up the field due to voltage, the electric fields.


IR could be a problem. But the MRF250 and MRF300 both include an LED to tell you when the RF link is good. Until each time you push a button, this LED lights consistently, and doesn't light when no button is pushed, focusing any attention on the IR is a waste of time. Mixing troubleshooting of RF and IR together will only cause a longer troubleshooting process.


I have seen aluminum foil used on many occasions. I've worked at open field test sites for Zenith Data Systems, and Compaq. Copper Mesh has little or no iron content either, but it is the material of choice for a faraday cage!

I am an EMC engineer, and try not to stay at Holiday Inn!


Lou, connecting to ground is helpful, but not absolutely necessary. The Faraday cage shunts the field around the device inside the cage, even with no ground connection.


It can help with rejection of leads into and out of the cage to ground the circuit, to the correct ground. But this involves knowing which ground is correct ;-)

I didn't really want to get into a discussion here and try and determine if the noise is common mode, or differential, or some combination, so I've tried to keep things simple to start out.

I didn't think he likely had ferrites in his tool box, but if he has them and knows how to use them, I doubt he'd be here asking questions! I would use them myself, but I was trying to take an approach of using the easy things to try first.

Being an EE myself, I still can't come up with better advice than 'bcf1963'. My only thought would be check the emitters. If one is shorted it would draw most if not all the current leaving the remaining good ones with little or no juice. Check them all with a meter. Just my $0.02..........

Chad brings up a good point, loss of power or a bad power connection can look like very strange bahavior. You said you get the same behavior with both the MRF250 and 300, but make sure you used different power supplies with each. If you had a bad supply, and just plugged both units into it, this could make both units look bad.

Chad mentioned a shorted emitter, an even easier way to check this... just disconnect the emitters entirely! At this point you're just troubleshooting RF. If the LED doesn't light when you send a command, you've got an RF link problem.

The boys over at Belden are going to be really disappointed when they read this and realize that none of the cables made using copper or aluminum for shielding have done any good. Since the dawn of time, electronically speaking. Faraday will also be unhappy because his cages are universally made of copper mesh.

Aluminum is crummy at shielding or stopping a constant magnetic field, for instance from a magnet. Once the magnetic field is varying or flopping back and forth as does the field associated with any sine wave or other signal going plus and minus at some rate, for instance 60 Hz or a radio carrier, aluminum works well because the varying field generates a voltage in the aluminum, which is then shorted in the aluminum, effectively stopping the field.

Think about it: how many models of chassis, from, say, LMB and Bud, are NOT steel? MOST of them, by far. And they all shield. Faraday cages are typically made of copper mesh because the resistance of the copper is lower than that of steel, so it will shield better than steel. But only from waves, not from constant magnetic fields.


Mu metal is used to lessen constant magnetic fields because the standard shielding process -- a piece of metal looking like a "shorted turn" * -- will not attenuate constant fields. Mu metal does NOT have to be grounded unless it is being used to drain off electrostatic fields. RF fields will be stopped by a shield, but grounding is only necessary to drain electrostatic fields. There is almost always an electrostatic field on a shield, however, because there is always a difference in distance from the source from one end of the shield to the other, allowing such a field to arise...as the RF is shorted out.


Otherwise...
I have never thought of receiver desense as described above, and it is a really helpful concept.

Another thing, though: digitally modulated signals could be present at the receiver, not show up on the LED, but still muck things up. An analog TV will not show anything at all on UHF channels, but if you add an analog signal on one of those channels, say by modulating a camera signal, the analog signal will look like hell. The receiver shows nothing without an analog signal but will not let the analog signal come through properly.

* a "shorted turn" is an analogy; if a turn of a transformer is shorted, the energy that should go to the secondary will be shorted through this winding, and the transformer will be much less effective, i.e. the shorted turn will take a device that depends on no shielding of its AC signal and turn it into a pretty good shield between the primary and secondary. You can also see this effect if you short one secondary winding of a transformer; the others, although not connected to that shorted secondary, will be tremendously reduced in output. It is best to learn about this with line level audio, or with RF transformers, because they will tolerate shorting of turns due to the power level being low. High level audio or power circuits generally are destroyed when turns are shorted, so you can experiment with shorting turns with these, but be ready for destruction and/or fire.

This message was edited by Ernie Bornn-Gilman on 08/16/05 11:30 ET.

Since URC came up with the MRF-300 I never had a problem until today. I was doing and install today in the tallest building in the area(44 Stories) an as soon as I hooked the antenna to the unit, the status light in the base unit turned on steady as well as the RF in the antenna At the beginning I though it was some interference due to the fact that I had the antenna in a different room conected through CAT-5e to the base so I decided to connect the antenna in the same room with the base but the steady status light was still there. I move around the whole apartment with the base unit and the antenna and I wasn't able to find a single spot where the interference either dissapeared or got attenuated. Went to the truck and brought a new MRF-300 and the problem was still there. I assumed that since that building is the tallest one in that area of Miami Beach there is a big chance that there is a transmiter in the building. Looking from the apartment's balcony I didn't notice any antennas in any other building around this one. URC uses 418 MHZ. I had never use the RTI remotes but looking into their specs I noticed that they use 433MHz and also that the RM-433 which is the receiving part has a filter to reject whatever data is not generated by and RTI remote, but first I'll try to find out with the people at the building if there is any kind of transmiter located there. In this particular job IR repeaters are not and option and I also have another two prospect customers in the same building.