A friend replaced her Comcast TV box with an over-the-air antenna but continues to use Comcast Internet service. This leaves her Comcast-supplied splitter with an unterminated socket - does it matter? Should I bother to terminate it?
(A barrel connector isn't a good replacement for the splitter as the splitter provides a grounding point.)
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It is a good idea to do so.
Cable-TV systems often use frequencies on the cable that are assigned to non-television services for over-the-air use (e.g. aircraft, ham band, etc.). If you have loose or unterminated cable connections, some of the cable-TV signal will "leak" out and can be radiated. This can cause interference in those other services.
The cable companies usually have "signal leak" detection teams driving around occasionally, looking for emission from their cable transmissions which ought not to be present. I've heard of people who've had their cable company employees arrive at their door, and insist that their in-home wiring either be fixed or disconnected.
Also, if you don't terminate a splitter port, you'll create an impedance mismatch at that point, and this can cause a signal "reflection" within the cable. Depending on the quality of the splitter (its port-to-port isolation) this reflection can cause "ghosts" in an analog TV transmission, multipath distortion in analog FM, and inter-symbol interference (leading to a higher bit-error rate and possible dropouts) in digital TV.
75-ohm F-connector terminators are cheap and widely available. It's good practice to use them on all otherwise-unused splitter ports.
If the FCC catches them (usually because of a complaint from the user of those frequencies) they can be heavily fined, so usually they are agressive at tracking down "leakage".
Geoff.
-- Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379
tinues to use Comcast Internet service. This leaves her Comcast-supplied sp litter with an unterminated socket - does it matter? Should I bother to ter minate it? (A barrel connector isn't a good replacement for the splitter as the splitter provides a grounding point.) --- This email is free from viru ses and malware because avast! Antivirus protection is active. http://www.a vast.com
COMCAST might even furnish your friend with a splitter.
Oops, I meant furnish your friend a terminator
Yes, it matters.
Yes, you should terminate it.
Sorta. Apparently she has a 2 way splitter installed with one port left unterminated. A barrel connector is a very good replacement. However, you're correct that you'll need to do something about the grounding point. If this is indoors, just remove the grounding wire, wrap it around the barrel connector, and hold it in place with a small hose clamp. For outdoors, use a proper grounding clamp: For outdoors, also don't forget to recreate the coax drip loop so that water doesn't run into the connector block, or into the house.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Careful. Comcast has an early termination fee.
-- Jeff Liebermann snipped-for-privacy@cruzio.com
150 Felker St #D
Thanks for all your replies. It's an indoor splitter. Since I had a 68 ohm resistor and appropriate connector (but no hose clamp), I just built a terminator. Factory-builts are cheap - if you want 10, 25 or 100 but I only needed one.
Not good enough. If you just crammed a 68 ohm resistor into the F-connector jack, you probably have a fairly good approximation of an antenna. It's going to radiate junk, and pickup more junk (ingres). If you insist on doing it like that, wrap the resistor with some insulating tape, and then cover it with aluminum foil.
Yeah, you're right that nobody sells them in small quantities. The best I can do on eBay is 10 pcs for $3.85 including shipping.
Looks like you're semi-local. Send me a mailing address and I'll stuff a few into an envelope for you. You could also buy one of the outdoor grounding blocks, and use it indoors. Lots of options that doesn't involve a leaky do-it-thyself terminator.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
One time I was looking into switches that couldn't be accidentally toggled, and I thought of type F connectors.
I'm sure I took a bit of wire for the center conductor, then a bit of thin copper sheet. PUt the wire into the centre of the copper sheet, soldered it, then soldered the copper sheet to the outer part of the connector. So switching was by screwing in the connector or unscrewing it.
I'm blank about soldering the copper sheet to the outer part of the conenctor though, but I can picture the finished connector.
That's why I tend to keep junk. Terminators may seem like the most mundane thing found in a pile of junk on the sidewalk, but if you need one, you actually save money and effort, while the 60gig iPod that I found somewhere else needs a battery in order to be useful.
Michael
No need to build your own. Most switch manufacturers make "locking lever" type of switches. I've used them in various devices where I didn't want any accidentally toggling. They're also common in aerospace and military equipment:
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Do you have any idea what the wavelength is at 600MHz? It's half a meter (Meters times megaHertz equals 300). That makes the length of a quarter wave is 1/8 meter or about 5". A tiny resistor is not going to radiate efficiently enough. If the "splitter" is a hybrid the unterminated port will cause higher attenuation through the splitter. Maybe a single dB. Not important when receiving.
You'd be better off getting a DC block. The splitter loses at least 3 dB.
A DC Block is nothing more than a capacitor between two connectors. Some use more than one capacitor to extend the usable frequencies. I've used up to five to have a flat response from 1400 MHz at 50 Ohms for a production test line. The insertion loss was about .1 dB at each end of the usable range.
-- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge.
Whether it radiates efficiently or inefficiently, it still radiates. Assuming a 1/2 watt resistor, with one lead wrapped around the grounding screw, and the other shoved into the F-connector center wire, there will be about 1/4" of exposed wire between the center pin and the 60 ohm resistor. That's not much, can probably be tolerated, and will be sufficient to slightly increase base line noise level in the area. I would hate to think what might happen if EVERYONE decided to use leaky terminators.[1]
I like to play ham radio (VHF/UHF) in my vehicle when driving. I constantly hear the digital hash as I drive repeatedly by specific locations. At first, I assumed that these were leaks from the overhead cable plant. Nope. When I started banging on doors, I found some really creative wiring and connector crimping. Most were caused by distribution amps with unterminated cables, where the coax cable becomes somewhat of an antenna. I also found either a defective drop or tap on the pole (Comcast didn't tell me what they found, but they did fix it).
Obviously, a leaky cable is a much longer length of wire than 1/4" of exposed center conductor. Yet, after the gross leaks were fixed, I still could still hear the test tones on the borrowed Trilithic something leakage detector. I had to install terminators on literally everything to reduce it to zero. Admittedly, I was working well below the FCC Part 76.605 leakage limits: but I needed to get the radiated junk down to a level where I could use my radios in the neighborhood. Overkill yes, but necessary.
It's also a problem in the other direction, which is called ingress. Nearby transmitters, such as mine, can easily get into the cable system, and interfere with reception. Once it gets in, there's no easy way to get rid of the signal. The best approach is proper shielding. It isn't just my transmitters, but broadcast stations, intermod with cordless phones, RF remotes, wireless security cameras, and other RF devices. Mostly, they cause problems with cable modems, which use the higher RF frequencies, but in extreme cases, can affect TV reception. It's also difficult to see with a spectrum analyzer: Can you see it? Spoiler:
More: "Characterizing Signal Leakage from an All-Digital Cable Network"
Without a termination, the splitter begins to resonate with the coax cable which can be made to radiate. I'm not sure of the unterminated losses, but I suspect it's more than 1dB. I'll try it on the bench when I have time (mostly to clean off the accumulated junk from the bench).
A DC block won't do anything useful except block DC. I think you might have meant a straight through barrel connector.
[1] Incidentally, you might be amused to look at my study of monopole antenna gains versus length. The title includes the wavelength. For example, 0_750 is 3/4 wavelength long and 5_250 is 5.25 wavelengths long. If you look at the antenna gains, you'll notice that the gain doesn't really drop very much for shortened antennas. From the antenna patterns: Wavelength Gain dBi 0.250 5.19 0.125 4.85 0.050 4.75 That's a 0.45dB gain loss in an antenna that's 1/5th the length of a 1/4 wave radiator. That might explain why I'm concerned about even small amounts of exposed center conductor.-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Heh. I remember a discussion hereabouts, some years ago... it might even have been you posting, Jeff?... in which the poster reported having detected cable-leakage problems in his town, reported them to the cable company, and was being ignored. He found that even the cables system's central facility (where the downlinks and primary head-end amplifiers were) was leaking like a sieve.
He visited the office, asked to speak to the lead engineer, and explained what he'd found. The engineer poo-pooh'ed and said "No, our system is tight, we're very careful about that." The OP took out his
2-meter radio, keyed down... and the signal monitors in the next room went crazy, as the 2-meter signal leaked into the system and saturated the first-stage amplifiers. Picture quality went to hell all over town and the phones started ringing...
Not me. I've reported specific leaks, with photos of my test equipment and attached reports to Comcast. However, that's when I had a friend employed by Comcast to expedite the process. Even so, it would usually take a month for any kind of response. I no longer have an inside track into Comcast, so I don't know what will happen if I tried that today.
That definitely wasn't me. I would never do that. The most sensitive point of the cable system is at the head end. That's where all the gain is buried, where all the numerous RF sources come together, and where there's a high probability of nearby RF sources creating problems. I also doubt the story. I'm quite familiar with the Comcast head-end in Scotts Valley. Nobody gets anywhere near the equipment unless they have a very good reason for being there. A leakage complaint doesn't qualify. Also, there's no "lead engineer". All the Comcast engineers are either in Denver, or outside contractors.
If you sit in the parking lot at the Scotts Valley office, you'll hear some RFI coming from the building on VHF and UHF. I've walked around
2 sides of the building with a portable spectrum analyzer and didn't see anything really disgusting. However, I will admit that everything above 700 MHz was problematic due to the proximity of a nearby cell site. That was also many years ago, so things may have changed (or deteriorated).So far, what I've seen of the local Comcast plant, it's quite well done. There's some junk cable Comcast inherited years ago north of Boulder Creek that should have been ripped out and replaced, but otherwise, it's quite well done and shielded. Unfortunately, Comcast is using more and more outside contractors, with minimal oversight, resulting in some rather lousy installations. I have more photos that are worse, but I think you can see the problem.
Also, there are problems with the roadside amplifier and pull boxes. These are targets for vandals, copper thieves, and sloppy installers. I've found them broken open and reported them to Comcast. I usually get an instant reaction, not because of the potential loss of customer service, but because there's high voltage on the big cable (to run the amplifiers) which could easily kill some idiot trying to steal the copper.
Incidentally, if you run into the test frequency that Comcast is using for leakage testing, I'm interested. It changed a few months ago when they went all digital and I haven't been able to pry it out of anyone.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Here's one kind of splitter:
And another:
Only the hybrid type leaks a bunch. The port to port isolation of the second type keeps unterminated ports from radiating.
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