Ham radio RFI to modem

Tutor

Ham radio RFI to modem

Hi Everyone,

 

I'm looking for technical help on this!!

 

My uncle is have severe problems with his Ham radio knocking his ATT U-verse off the air. I'm trying to help him, since he feels local ATT techs in Maumee, Ohio are pretty much clueless about RFI. I do design work, so I have no problems with building something for him.

 

I've cleaned up my own ADSL system with home made low pass and band reject filters, and I can run over 6,000 watts or more ERP (1500 watts into an antenna over 6dBd gain) pointed into the modem without any RFI issues on any Ham band. Prior to building and installing my filters, just 100 watts ERP would unsync my 2Wire ADLS modem.  

 
I am in Georgia and he is in Ohio, so any help I can offer will be by building a custom filter, or pointing him to a competent ATT tech in the Maumeee, Ohio area.

 

My uncle is in a congested city neighborhood with short normal copper Telco wiring to the house and modem. Because of his city lot, he has no opportuningty to relocate his antenna (nor should it be his obligation to do so just to correct an ATT equipment design problem).

My specific questions are:

1.) if a filter that knocks the 80M DSL carriers down to useless levels is likely to help, by fooling his modem into ignoring his 80M SSB signal?

2.) if there is a software fix that he can get ATT to use that causes his modem to ignore 80M signals?

 

3.) Short of any of that, is there a way to get an ATT tech that understands RFI out to his house??

He enjoys his radios, and would like to continue to enjoy a hobby he been in for 60 years.  I'd really like to help him resolve the ingress problems with the ATT system he has.

 

Thanks,

Tom

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Message 1 of 16
Expert

Re: Ham radio RFI to modem

U-Verse uses more frequency spectrum than standard DSL. A U-Verse VDSL line will use frequency components from DC up to about 8 MHz, which overlaps the 40m, 80m, and 160m bands.

If you can build him a filter, it would have to shield against RFI over this entire frequency range, although frequencies above about 5-6 MHz are less important. Using twisted pair or shielded wiring will help also.
Message 2 of 16
Tutor

Re: Ham radio RFI to modem

Thanks for the reply.

 

The filter does not have to notch a wide range. It only has to notch the DSL carriers in the band he uses.

 

Filter construction not an issue. I need to know how the system will behave if we take the carriers down into the noise around where he operates, say dropping those carriers 35 dB or more below the other carrier levels.

 

If that won't cause the system to ignore his signal, then I'd like to find an ATT technician who has some knowlege of problems like this. I saw a claim that firmware could be changed to ignore particular frequency ranges.

 

 

Any help on these points would be appreciated!

 

 

 

Message 3 of 16
Expert

Re: Ham radio RFI to modem

OK, I think I see what you're saying.

VDSL (and ADSL/ADSL2+) use DMT/OFDM, so there are hundreds or thousands of individual carriers in the U-Verse signal. I think what you're wanting to do is intentionally create narrow-band noise on the DSL line to force the modem to not use DMT carriers in that narrow range. That way, the HAM carriers in that range will not interfere with the modem.

As far as I know, if your filter is fairly narrow-band around the HAM carrier frequency, producing a narrow slice of noise, then the modem will figure out during training not to use those carriers.

The only frequencies I know you need to leave alone are:

(If he's on ADSL) - 276 kHz (pilot tone)
(If he's on ADSL2+) - 452 - 475 kHz (pilot tones)
(If he's on VDSL) - 2.484 MHz (pilot tone)

As far as I know, there are no firmware changes that can be made to make the modem ignore certain frequency ranges. The only way to force it to not use certain frequencies is by the use of noise so that the training process believes that insufficient SNR exists in those frequencies.

If he has a VDSL modem model 3800HGV-B, 3801HGV, i3812V, or 3600HGV, you can use my tool UV Realtime ( http://www.uvrealtime.com ) which will provide you with a bitloading graph, showing the bit allocation on each frequency. This can assist you in verifying that your filter is successfully notching the used frequencies.
Message 4 of 16
Tutor

Re: Ham radio RFI to modem

Thanks for the reply!

 


VDSL (and ADSL/ADSL2+) use DMT/OFDM, so there are hundreds or thousands of individual carriers in the U-Verse signal. I think what you're wanting to do is intentionally create narrow-band noise on the DSL line to force the modem to not use DMT carriers in that narrow range. That way, the HAM carriers in that range will not interfere with the modem.

 

Close! What I actually want to do is remove the carriers inside the band he operates, so they fall into random noise. I do not want to inject noise, because anything strong being injected will likely leak out of the poorly shielded wiring system and cause interference to his amateur radio system.

 

The entire problem centers around use of low frequency copper pairs and poor system design to carry radio frequency signals, so the fact something can leak into the system also means stuff can leak back out. 

 


As far as I know, if your filter is fairly narrow-band around the HAM carrier frequency, producing a narrow slice of noise, then the modem will figure out during training not to use those carriers.

 

Technology and expense limits filter response to notching maybe 20% of the center frequency. If I center a band reject filter at 3.700 MHz, it will have 3dB attenuation points of  3.3 MHz and 4.07 MHz. The primary amateur band he enjoys is from 3.5 MHz to 4.0 MHz, but he mostly operates near 3.6-3.8 MHz.

 

I can get 40 dB attenuation over that range, which I hope would make the VDSL system ignore any signals that suddenly appear in that range.  

 


As far as I know, there are no firmware changes that can be made to make the modem ignore certain frequency ranges. The only way to force it to not use certain frequencies is by the use of noise so that the training process believes that insufficient SNR exists in those frequencies.

 

It is unfortunate ATT sells a system that has a very weak point without having easy-to-find resources to help people. After all, this is technically and legally 100% ATT's responsibility.  

 

If he has a VDSL modem model 3800HGV-B, 3801HGV, i3812V, or 3600HGV, you can use my tool UV Realtime ( http://www.uvrealtime.com ) which will provide you with a bitloading graph, showing the bit allocation on each frequency. This can assist you in verifying that your filter is successfully notching the used frequencies.

 

Thanks! I'll try that if I go up there soon, but in the meantime I will probably build a filter. I assume the line impedance is typically 100-150 ohms and the connectors are standard telco type connectors?

 

He can just plug a filter in and see if it works.

 

Otherwise, he might wind up having to change services.

 

Thanks for your help,

 

Tom

Message 5 of 16
Highlighted
Employee

Re: Ham radio RFI to modem

Has the tech actually been out to look?


Does transmitting affect any other devices in the house? (like in the analog TV days when 10 and 50 meter transmissions used to kill the TV image ...)

 

More than likely, the drop needs to be replaced with a shielded drop, and the grounding / bonding from the VRAD forward needs to be verified.

 

In nearly every case with RFI into the U-verse system I've worked, it's been an unshielded drop, and / or a busted shield on the F2 (the segment of the cabling system betwen the cross-connect (X-Box) and the "drop" (the segment from the "last pole" to the side of your house. Note that the "last pole" can be an actual pole, or it can be a pedestal ("can") feeding an underground drop.

 

I've also seen some NIDs with lots of "tails," long sections of untwisted and / or stripped pair (all act to accept RFI/EMI). Proper termination is critical.

 

Ham radio operators (I am an Extra Class, since 1990) need to check their stations too. Many don't run filters on the antenna line to suppress spurious radiation, many don't have proper single-point grounding of the cable shield and equipment, many have improperly self-soldered PL259 / SO239 connectors which screw the shielding capabilities of the cable. Because the RFI can soak the electrical system of the house, line filters in surge protectectors have to be of high quality and in good shape. The techs should have / usually have TrippLIte surge bar / filters on teh truck, or can get them.

 

Many fail to properly choke the power feeds, interconnects, speaker lines. There have been many cases where the HAM's .... ah...um..er..."frugality" and desire to get something done DIY and or cheap create an / some ingress / egress point(s) for EMI/RFI.  The ARRL has a number of documents up on their site, some deal with HAMs and U-verse and have some great suggestions.

 

It's true that many techs are not too savvy regarding RF radiation and propagation, but there are some documents available to them to assist their troubleshooting, and internal resources in Support to provide guidance, if necessary.

 

A notch filter doesn't work, because if you filter the offending 160 / 80 / 40 meter signals, you are also filtering the desired signal. In most cases an external ferrite / choke on the UTP also fails for the same reason (assuming you have the right type of ferrite for the freqs you are trying to suppress). Screened UTP (that's like "shielded" Unshielded Twisted Pair) or shielded cable on the U-verse stuff is usually more likely to scew something up as to fix it, because it must be properly grounded / bonded, and the equipment must be designed to accept it. i.e., UTP systems are 100 ohm +/- 10% characteristic impedance, Shielded Twisted Pair (STP, not sUTP) has a characteristic impedance of 150 Ohms +/- 10%... so integrating it into a 100 Ohm system would require a BALUN or the extra impedance lump would further reduce the desired signal and introduce an unbalance within the system that makes it more susceptable to interference ingress / egress.

Employee Contributor*
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.
Message 6 of 16
Contributor

Re: Ham radio RFI to modem

Hi Tom,

 

Judging from the date, I can only assume this problem has been resolved. I had a similar problem here in Athens Ga.. 

 

The solution was that I installed a balun from the 2 wire feed to coax.. The coax was grounded at the box. The coax then fed the modem indoors. (diplexor is installed indoors as well)

 

ATT had all the parts.

 

Hope this helps,

 

Jim

Message 7 of 16
Tutor

Re: Ham radio RFI to modem

Wow, I just spent 30 minutes entering a fresh Ham Radio related RFI/TVI issue in my home using the 3800HVG-B Gateway, then had to start again after resetting my password..........

Long story short, we've had U-Verse for 8 or 10 years (before that was Comcast and no issues).  I've had to refrain from "hamming" during popular TV viewing hours due to screen and audio blanking out on multiple TVs as I either speak (SSB) or use CW, 100Watts output, on all bands 40 through 10 Meters.  Let me say that my U-Verse Gateway resides in the same room as my ham station, and shares the same cable/wire locations through the wall and into the attic space.

I've attempted every diagnosis I can think of as regards the radio gear; everything from assuring good grounding, sound/clean coaxial connections, installed a low-pass filter at the antenna feed point; have reconfigured and relocated my ham antennas, etc......... As a last resort a few weeks ago I invested in a new technology transceiver capable of only 15 Watts output, feeling this should allow my continued hobby activity while not interfering with prime-time TV.  Last night I proved that theory is not working, as my wife observed that even at such low output power, here TV programming is not viewable.

 

It was today that I landed upon this Forum thread, and from what I glean here, it would appear ATT has an obligation to investigate and determine what they might do to resolve my problem. For example, I believe it was here where I read that teh twisted pair service from the junction box outside the house should be a shielded cable........mine is not.  Additionally, I wonder if there might be other findings relative to my ATT installation which could be cause for concern.  I recall several years ago that ATT trenched through the neighborhood and laid new copper to each home, but I thought it odd that no-one came to connect that new copper to my internal wiring.  Unless mistaken, that copper is simply tucked inside and not connected to anything inside the terminal box on the outside wall of the house.

 

Now, I must echo a question I saw voiced here previously, and that is to know how I communicate my issues to ATT Service in a way as to be understood and to assure me that a qualified technical person will be dispatched to investigate my concerns.  I recall on separate occasions when a service rep came for a set-top box issue, I was amazed at the total lack of electric/electronic knowledge evident in those persons.  What may I do now to attempt a resolution to my TV interference issues? 

 

Thanks,

Mike

Boca Raton, Fl

 

[Edited for privacy-please do not post personal or unique information such as but not limited to full names, employee ID numbers, email addresses, phone numbers, account numbers, etc.]

Message 8 of 16
ACE - Guru

Re: Ham radio RFI to modem

Mike, have you tried transmitting into a dummy load to see if the problem is along the feedline/antenna setup or elsewhere?

 

Is it possible to try the rig in a different room?

 

If you still have a problem running only 15W then there is some kind of direct path of interference that needs to be found. I suspect there could be a grounding issue.

*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.
Message 9 of 16
Tutor

Re: Ham radio RFI to modem

I ordered a dummy load which should arrive in a couple days. I do have the
capability to carry a rig to the back porch and operate from there, and that
would have been my next step....However, somehow I have chased the symptoms
away (at least temporarily), and I'm not even sure how. Sort of out of
frustration yesterday I pulled the Gateway partially off the shelf, actually
so I could read the model number off of it. While fiddling with it, I
jiggled all the cables and plugs on the back side of it, and did the same
with the set top box adjacent to it. I then positioned the TV in an
adjacent bedroom so that I could see it from my Radio operating position,
and Lo and behold I observed no interference on that TV while transmitting.
Thinking this might be a fluke (that perhaps that TV had not previously been
affected by the RFI/TVI, I went to another TV in the house, and since I
could not see it from my ham shack, I turned the volume all the way up,
keyed the ham rig, and did not notice any disturbance to the audio from that
TV... As a conclusive test, when my wife came home, I asked her to monitor
each TV in the house while I operated my ham rig, and she reported no
visible or audible evidence of the problems I had been enduring for a few
years.... I've operated the ham rig extensively today while the wife is
watching her TV programs, and she claims there is no problem what so
ever....Go figure. Maybe there was a cable on the back of the Gateway just
marginally making contact, and my subtle disturbances cleared up the issue.

I feel totally baffled that this has turned out this way. Thanks for your
kind response and suggestions. I will retain your contact information in
case I have a recurrence of my problems. I have a feeling however, that
before I cry Uncle, I will surely give all the plugs and cables a good once
over before I speak up.

Cheers and thanks for your suggestions.



Mike

Boca Raton, Fl

 

[Edited for privacy-please do not post personal or unique information such as but not limited to full names, employee ID numbers, email addresses, phone numbers, account numbers, etc.]

Message 10 of 16
ACE - Guru

Re: Ham radio RFI to modem

OK great Mike.

As I mentioned, getting interference at low power means an obvious problem.

You may have just stumbled upon the solution without knowing.

 

A dummy load is a very useful device and it will be useful down the line.

 

I have been licensed since the late 70's in 2 countries operating out of houses, apartments, army bases and even aircraft using FM, SSB and CW into home brew wire antennas as well as multi element yagis.  While these things can sometimes be a challenge, with a little perseverance they can usually be solved.

 

Good luck and 73.

 

ZS6xxx and KK6xxx

 

 

*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.
Message 11 of 16
Contributor

Re: Ham radio RFI to modem

Mike: Has your issue reoccured?

 

i too have RFI/TVI issues with my Uverse. it exists on 40 meters. in the past i've had service come out and check out the system. they've grounded, tested and i still have the problem. i have used clip on toroids on incoming and outgoing lines. i am awaiting delivery of donuts to  put on the incoming lines.

 

my 2 lines coming in to the box are 2 sets of twisted pair. they appear to be unshielded. they claim the cat5 nside the house are shielded. 

 

as someone asked previously, whom do you call and what do you say to them?

 

WB9GMO/Roswell, GA

Message 12 of 16
ACE - Guru

Re: Ham radio RFI to modem

Michael,

Have you done a dummy load test?

 

As mentioned above the results of this can be very useful.

What type of antenna are you using on 40m?

What is the SWR like?

At what power level does the problem occur?

 

 

*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.
Message 13 of 16
Tutor

Re: Ham radio RFI to modem

Well Tom, first let me confess that you are talking way above my head when
discussing your filter design knowledge. Let me cut through the chase with
this scenario of my success story.

I live in south Florida, but that seems irrelevant to the particulars. I am
a ham operator, going on 60 years. I never had a RFI/TVI problem until the
past few months. I subscribe to ATT U-verse for my Internet, TV, and
Digital phone service. For some reason, my little 100W HF ham rig started
interfering with the TVs (we have 5 of them in the house) about 6 months
ago, and then I noticed that even with my little 15W HF rig, four of the TVs
would blank out and frequently the set top box on each would be knocked off,
requiring a power-off reset. The 5th TV was never affected. This TV was
the only one connected via CAT5 to the ATT Gateway located on an adjacent
shelf in the room.

I pursued my issue with a RFI/TVI contact at ARRL who put me in touch with a
Senior Engineer at the old Bell Labs. This guy happened to be a ham, so we
were immediately on common ground in our conversations. What follows was my
path to closure on this issue:

Recognizing that the TV hardwired via CAT5 was not affected, the Engineer
suggested I purchase a long length of CAT5 cable to directly connect from
the gateway to each TV separately to determine if that bypass technique made
any difference. It was gratifying and certainly no surprise to witness the
positive results of this test. There was absolutely no TVI/RFI when a TV
was directly connected via CAT5 to the Gateway. By the way, at Amazon I
purchased the 100' CAT5 cable for $8 or so.

The next step was to place a service call with U-Verse, reporting the
problem in such a way as to imply that the installed first generation
Gateway and set top boxes were technically outdated and were not able to
deal with RFI/TVI interference.

The service rep was quickly on the scene, but he was just as quick to voice
his personal ignorance and misguided advice when the first words out of his
mouth were: "It's not our problem. You cannot operate ham radios in the
same house as the U-Verse service.".... My blood pressure immediately hit
the mad zone as I asked to speak to this person's supervisor.

It came as no surprise when the supervisor attempted to embellish this young
techie's explanation of who owned the problem. I then reminded him I came
from Comcast where there were no issues, and I can easily go back to
Comcast... It was then that the young service rep was instructed to replace
the Gateway with the latest (4th generation I think) model, three set top
boxes (old version connected via coaxial cable and splitters in the attic)
replaced by WIFI set top boxes, and the final failing TV had CAT5 cable run
from the service box immediately outside the bedroom wall. The techie then
ensured CAT5 was routed from the service box, replacing the old copper
twisted pair to the new Gateway box. Additionally, an independent WIFI
transmitter was installed adjacent to the Gateway box to provide the
wireless link to the WIFI set top boxes.

And the end result is that I do not have any indication of RFI/TVI on any of
the 5 TVs in the house, even when running the 100W rig.

I suppose one could conquer this issue by engineering an assortment of
filters. My personal opinion however would be to attempt the obvious
diagnoses to determine if updated ATT equipment would help remedy the
situation before next considering if CAT5 or WIFI is the ultimate
solution... I think at a minimum you should ask ATT to ensure CAT5 is used
from the service box on the exterior of the house to the Gateway box. I
would also investigate if the suspect equipment in the house is the older
generation, as in my case being the first generation, outdated stuff.

You must be careful about demanding the coaxial cabling to be replaced by
CAT5, as that is easily put back in your lap as a customer responsibility.
My request for service was incorrectly documented as a request to run CAT5,
and when the service rep arrived, he was trying to tell me it was going to
cost $200 or $300 to honor my request. When all was said and done, my total
problem was resolved without any associated charges. I suppose however, if
you find your problem to be similar to mine, and you cannot wrangle WIFI set
top boxes out of them, then perhaps the cost to run CAT5 is the cheapest way
out.

I hope this response can be of assistance. I'm willing to discuss further
if needed. Good Luck.

Mike




Message 14 of 16
Tutor

Re: Ham radio RFI to modem

Thanks Mike and Others,

 

Let me clear up a few things:

 

1.) The station with then problem is on a city lot. He has a dipole about 50 feet high. Sometimes he is barefoot, sometimes he runs an amplifier. Either way, his radio wipes out the Uverse when operating on 75 meters.

 

2.) He could ground until the cows come home and it would not change the RFI. The RFI is because the telco lines are in the nearfield of his antenna. He has no options to move the antenna, his lot is 60 feet by 120 feet, the telco lines are overhead and  run on one short side and one long side of his lot. The drop comes in overhead.

 

3.) It's not spurious, it is his fundamental signal. The problem is the modem uses carriers INSIDE  Ham bands. When he transmits the 80 meter frequency range carriers ATT uses are wiped out, and this causes the modem to unsync. The entire problem is rooted in the modem being a receiver that actually uses carriers in the Ham band he is on.

 

There is nothing that can be done at the transmitter to fix the modem, because the problem is ingress into the modem via the telco lines. The telco lines cannot be filtered, because they use the same frequency range he uses.

 

It appears to me the only solution is to have ATT not use carriers in the 80/75 meter amateur band. I thought perhaps someone at ATT would know a way to disable the 80/75M carriers, so the modem only used frequencies away from his operating frequency.

 

It's been years since his problems with Uverse started, and he still has them.

Message 15 of 16
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