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HF QRM: Comcast?

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kd7rto

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During 6 years of apartment living, I experienced such high noise levels that HF and VHF-Lo monitoring were effectively impossible.

Now that I'm back to owning a house, I thought that I could finally get back to listening < 50 MHz. Wrong!

I find that I am still experiencing S9+ of noise on my AR5000, which tapers off above 60 MHz and is gone by 88 MHz. The antenna I am using is a longwire, no balun, fed by RG8X, with a ground at the point where the coax exits the house. I have, of course, tried turning off all the computers in the house. No improvment.

Relocating the receiver and feedline to the detached garage helps, but I still see a lot more noise than I think I should.

I have been a Comcast subscriber this whole time, and I now suspect that the interference is most likely comming in through the cable.

Can anyone confirm that Comcast causes problems like this, or offer a better explanation?
 

n3ncn

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Not too familiar with it but I do know you need a good ground for the antenna. Can even be as much as hooking the ground portion to the little screw on a wall plate. Floresent lights are a bad source of noise for HF.
 

crazyboy

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Why don't you throw the main breaker to your house and see if you still get interference. If not turn the main back on and begin turning off each of the breakers until the interference disappears.
 

kb2vxa

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Hi ORZ and all,

"...fed by RG8X, with a ground at the point where the coax exits the house."

I suppose you're using a grounding stake with the shield connected by a very short piece of medium gague wire and the antenna is roughly an inverted L configuration, anything else and you have no ground at all. Depending on it's location relative to the noise source(s) and what the source(s) are fixing the problem may be anything from simple to impossible. Detective work is in order.

That can be a long, drawn out process with lots of bends in the road, impossible to give complete proceedure in this itty bitty space. You may start by walking around with an HF portable in AM or SSB mode for a start checking out the usual suspects including the house wiring that may be conducting and radiating the noise.

Cable TV is the least likely suspect, in fact I have never heard of it causing interference on HF. It can and does leak but the crap is on the TV channels that fall within the VHF and UHF bands, I have never seen a perfectly RF tight cable system. The hottest targets are within your own home followed by the neighbors and the tail end is the power distribution in the neighborhood. When playing detective always remember Sherlock Holms who gave the very best of advice. "When the impossible has been eliminated what remains however improbable is the solution." and "The truth often hides in plain sight." are words to live by.

I hate to bring up the worst case scenario but you just might be the victim of BPL. That's another can of worms you most likely know about unless you've been on Mars the last several years but it's something to consider the possibility. I don't know how to find out if there is a system operating in your area but I suppose the ARRL can help you find out.

As for me, I'm the victim of apartment house living and high voltage feeders so near the shoreline, salt spray and HV just don't mix. I sympathize with you, a couple of years ago we had a blackout and my station on battery power was so quiet you could hear a pin drop in Afghanistan.
 
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studgeman

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I will agree with all the other posters, do your detective work. I wouldnt eliminate cable TV however, as the cable companies now use HF spectrum on their lines for data services. Communications for on-demand, cable modem upstream etc. Though based on your description of frequencies and the roll-off of the nose, BPL also sounds like a prime candidate. Battery chargers, including the ones the cable companies use on the poles (gray metal cases) are also likely suspects. More than likely you will find a combination of things which contribute to your noise floor. Good luck
 
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As an experiment you might try disconnecting your CATV drop. At the house of course. I'm not telling you to climb the pole and do it there. Not that anyone really notices if you actually do such a thing. Especially if you bring your own hardhat, tool belt, orange cones, climbing belt etc....
 

kb2vxa

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Hi again,

"I wouldnt eliminate cable TV however, as the cable companies now use HF spectrum on their lines for data services. Communications for on-demand, cable modem upstream etc."

Sub Band has been in use for a long time but all things considered it's not likely to be the problem. I'm not denying the possibility, just that it's near the bottom of the list of suspects.

Something I forgot to mention is that a better way to pinpoint the source(s) once they have been traced to the general area is to use a VHF receiver in AM mode such as aero band with the squelch off. Signal levels are much lower and more localized and a directional antenna such as a small Yagi can pin it right to the spot at close range.

The ARRL is a wealth of information on the subject, hams have what amounts to centuries of collective experience amassed and at your disposal.

Oh Hostile, I got a chuckle out of that one. My pirate days are over but not forgotten and upon reflection I was lucky not to have fallen, brushed some high voltage or whatever. A word to the wise, gentlemen?
 
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kd7rto

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Thanks for all the responses. I think you guys are right, that I've got a grounding problem, rather than interference comming over the cable. My ground is currently attached to a water pipe, with no difference in noise floor when I disconnect it from the polyphasor. Next step will be to install a ground rod.

Questions: is a grounded polyphasor in the middle of the feedline sufficient, or do the antenna and/or receiver need to be grounded as well? If yes, do I need to run these grounds to the same rod to avoid ground loops?

A convenient service outage helped rule out Comcast. I also checked for leakage. At 109.275, and found no test signal. 145.25 is infamous for cable interfering with 2-meter, so I used it. I could detect a signal with an HT near the TV, but nothing on the AR5000 with either a telescopic antenna in the radio room, or with my new Hustler 5-BTV HF vertical hooked up.
 

kb2vxa

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Hi again,

Assuming the "Polyphasor" is one of thier lightning protection devices the best place for it is at the antenna feedpoint which should be at ground level (that inverted L I was talking about) keeping the connection lead short and direct. If you're going to divert current to ground there's no point diverting it through the coax.

Grounding the receiver then becomes a questionable practice especially since an RF ground is nearly impossible to achieve at lengths greater than about 10 feet from earth. Consider the RF "return current" flows in the shield, not some ground lead from the receiver so that is likely to be superflouous.

Considering you're a ham I suspect you're transmitting with that antenna so stating the obvious the ATU goes at the antenna feedpoint, not in the shack. A weatherproof "autotuner" is ideal but then there is the "dog house" approach using a remote controlled ATU or the tried and true go out in the rain with an antenna analyzer method. If nothing else you'll see what Rover has to put up with, lucky guy, you have indoor plumbing. (;->)

Last but not least, consider yourself lucky you detected no appreciable cable leakage! Once upon a time I got free cable just by switching the TV to "cable" and pointing the antenna in the right direction.
 

kd7rto

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Hi Warren,

A tranceiver will not be in the budget for at least a month. Getting a working antenna system is first priority. I haven't done anything with HF since around 1980, so I want to do some listening first and learn my way around the bands.

I would think that the Huster 5-BTV (vertical w/ traps) would not need a tuner, at least for 10-15-20-40 (on 80, it's supposed to have a very narrow bandpass).

Gary
 
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