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Author Topic: Noise, Noise, Noise!  (Read 2945 times)
N8ESP
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« on: September 25, 2012, 09:33:48 PM »

I have an OCF Dipole -- one of the 80-6m variety.  When I first got it, I didn't have anyplace to really put it, so I ran the long leg along the top of my fence and tossed the short leg up in a convenient tree.  I expected nothing from it, but ended up surprised at how well it actually worked.  I made a decent list of contacts, including a fair amount of DX -- all on 100 watts or less.

After several weeks, I finally got a vertical mast (aluminum army surplus poles) installed and put the centerpoint of the Windom up there.   It is about 20-25 feet high.  The long leg follows the same fence as before but is up in a tree, and the short leg is tossed across my roof (shingles).  The antenna works, but I have terrible noise (S7 and above) on every band except 15 meters.  I have to turn off my preamps and turn ON attenuation to reduce the noise to a level where I can stand it enough to hear stations.  The antenna works (still making DX contacts, etc), but I'm looking to solve the noise problem.

It is going to be raining the next few days, so I thought I'd ask for opinions on a couple things I thought of trying.

1) I was thinking about pulling the short leg off the house and toss it into a tree. However, if I do that it will be at only a 45-60 degree angle relative to the main leg.  I've read a couple of posts on other forums that indicated that contact or proximity to shingles can cause issues. This is the easiest option and takes almost no effort.  However, I have no idea what the effectiveness related to the massive noise levels would be, nor am I entirely sure what folding back the short leg would do to the radiation pattern.

2) I have also read that issues with the shield connector on the feed line can cause noise problems, so I could cut down the feed line and re-do the solder connections just to make sure.


I've already tried:

- Ugly Balun (loop o' coax) to try to remove common mode currents.  This did absolutely nothing.
- Turned off everything in the house and ran only the radio.  No change.
- Tried a different receiver -- same result...  Copious amounts of noise.


I'm getting really frustrated with this noise.   I have some friends who post on "the book of faces" and they'll be talking about 20m "booming" or 10m being "wide open" -- and all I hear is noise and the strongest of the strong stations (who I can usually work, by the way).

Thoughts?
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WX7G
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« Reply #1 on: September 25, 2012, 09:50:48 PM »

I suspect the noise problem is not the antenna, it is your local RF environment. As you raised the antenna you improved its performance and consequently its ability to receive desired signals as well as noise signals.

If you want less received noise you will have to hunt down the source(s) of noise and eliminate them. The first thing is to see if the RF noise source is in your house. To do this you can power the radio from a 12 volt battery and shut OFF the main AC breaker to the house. If the noise drops the RF noise source is in your house.
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W8JI
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« Reply #2 on: September 26, 2012, 05:06:51 AM »

You could have external noise propagating in, but OCF antennas have significant common mode current on the feeder. This makes the feeder and everything connected to it an "antenna", and it can feed local noise into the receiver.

I'm not saying that IS your issue, just that it could be.  Smiley

An ugly balun would do next to nothing to suppress common mode. It can take thousands of ohms to suppress commonn mode, depending on the system. It might require a ground at the house entrance if you have a shielded cable, and a string of beads on the antenna side of that ground.

Also, the type of balun can make a huge difference, as well as what antenna wire goes to what side of the balun.

This might not be the issue, but it commonly is a contributor with OCF antennas.
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G8JNJ
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« Reply #3 on: September 26, 2012, 07:10:13 AM »

Hi,

I'd agree with Tom. OCFD's are very difficult to tame in terms of common mode current on the vertical feeder. This makes them very susceptible to noise pickup on RX in an urban environment.

Try using a properly balanced antenna.

I have a 100ft doublet with a good 1:1 Ferrite cored balun specially optimised for the LF bands (I've built it and measured it, so I know it works properly) at the bottom of the vertical feeder section. I can also add an extra 33ft section to one leg with a wire jumper to quickly convert it into an OCFD.

The noise floor on 80m is about 10dB higher when configured as an OCFD. So for the majority of the time I leave it configured as a doublet. The extra 1 dB or so I should gain on the LF bands, when configured as an OCFD just isn't worth it.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
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KG6YV
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« Reply #4 on: September 26, 2012, 10:42:58 AM »

I have used a 133 ft. Carolina Windom for 15 years (similar to a plain OCF dipole).

1.  You should not have a problem if the short leg is at a slight angle.  An included angle between the long and short wire of 120 degrees works.  Mine has the longer wire at about 150 degrees angle to the short wire. 

2.  Your noise may be high relatively however consider that with any of the modern hi-tech transceivers you should use no preamps "at all" on 160M, 80 or 40M....  I run my FT-2000D in IPO mode (no preamp) and most evenings run an additional
6db of attenuation.  I still hear everything fine and my noise floor is about S-2.  I even work DX in the window on 35M with that configuration.  Preamps are only needed on 20M and above.

Greg
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AC5UP
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« Reply #5 on: September 26, 2012, 11:12:27 AM »

I had good luck with a no-frills 136' OCF / Windom for several years and zero problems with excessive noise or feedline radiation. The balun at the feedpoint was a 4:1 dual-core Guanella and note that I said "no-frills". Not a Carolina Windom or anything tricked out beyond the original design. It would not load on 15 or 30 Meters but worked well on the bands it should have with a no-tune match. I took my time in pruning it and recorded the SWR on all band segments of interest before deciding to cut or not cut another inch. Found the sweet spot within an hour and it was plug & play from that moment on.

I'm a proponent of the K.I.S.S. principle and was never disappointed with the OCF despite the ends being slightly 'drooped' to fit the available space.

I'd also like to agree with KG6YV regarding the use of a preamp on the lower bands. Like many of you my noise floor isn't particularly high but well above the threshold of the radio. A preamp at the lower frequencies will add noise and will degrade the signal to noise ratio even if it is state of the art quiet. The " Keep It Simple, Stupid " principle says that if you don't need it don't add it to the RX chain because there really is such a thing as too much.

Experiment with the preamp and attenuator settings on your rig some night and you'll find that S-5 can sound far more comfortable against a slight noise floor than 10 over 9 with the noise at S-5. You paid for 80+ dB of dynamic range, don't be afraid to use it.
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N8ESP
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« Reply #6 on: September 26, 2012, 04:32:00 PM »

Well, one thing I cannot do is buy anything.  I just got laid off last week, so I'm on a personal "spending freeze."  I'll have to make do with the radio/antenna combo that I've got until the job situation gets rectified.

Unfortunately, tossing the short leg into the tree would reduce the long-to-short angle down under 90 degrees -- probably close to 60 -- so I suspect that there would be some undesirable cancellation going on as a result. 

I'm not entirely sold on the "shingles add noise" theory that I read about, simply because there wasn't any science backing it up.  I don't know if there could be some weird triboelectric effect at work that is the genesis of the theory.    Pulling the line off the roof isn't an irreversible decision -- I'm just not sure there is any value in doing so, especially given the acute angle it would form.
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WD5GWY
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« Reply #7 on: September 26, 2012, 05:22:48 PM »

If push comes to shove, put the antenna back where you had it and it worked.
That is, if you cannot find the source of the noise or due to where it is coming from, and you can do nothing about it. At least that way, you will be able to use the antenna and not be out any money. (well, other than the cost of the poles)
 I'm betting that it's something in or near the house. Just as a test, move the leg that is on the roof, off, and away from the roof and see if the noise goes away.
If it does, put it back and do the A/C mains off test. Then, turn the mains back on, then turn off one circuit at a time until the noise goes away.
Good luck, sometimes interference can be a bear to track down. And other times, very easy to find and fix. Hope this one is easy for you.
james
WD5GWY

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N4JTE
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« Reply #8 on: September 26, 2012, 05:56:34 PM »

The OCF is a tricky antenna under the best of conditions, what you have described is the worst of conditions. Get the thing off your roof and find a way to get both legs in the air.
Bob
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K4SAV
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« Reply #9 on: September 27, 2012, 06:40:28 AM »

Judging from your description, you could have multiple sources of noise.  You may have to find all of them before the noise gets to a tolerable level.

1.  W8JI's post describes a common problem with this antenna.  This antenna requires a very good choke (maybe more than one) and an ugly balun won't do.  You didn't say which OCF dipole you have so there is no way of knowing if it has a choke or not.  Bending it around into strange shapes only makes the problem worse.  Even a center fed half wave dipole can have common mode problems if it is folded into weird shapes because it will no longer be balanced.  Turning off everything in the house might not make any difference if this is the problem.  Noise on the house ground can be the source and turning things off doesn't change that.

2.  Shingles don't usually cause noise (a moving wire in contact with a shingle might) but shingles are usually on top of a house and the things in the house cause noise.  You may have to move the antenna away from the house. 

3.  The noise could be radiated from a source a significant distance from your house.  In this case you will have to find it and get it fixed. 

4. The noise could be from a bad connection somewhere in the antenna system.  That one can be a very large noise source.

Any or all of these sources are possible.  When you test any of these possibilities and don't see a change, that doesn't mean it's not a problem.  It means it's not the largest source.  When you find the largest source and fix it, there should be a change, although it might be only a small change.  Then you have to find the smaller sources.

Jerry, K4SAV
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KC9TNH
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« Reply #10 on: September 27, 2012, 09:02:33 AM »

Also, the type of balun can make a huge difference, as well as what antenna wire goes to what side of the balun.

This might not be the issue, but it commonly is a contributor with OCF antennas.
Dovetails with what I was advised by the maker of the 4:1 current balun that's on my OCFD, who took pains to advise & email, as well as label same - short & long leg connections. Runs long ways on the lot, short leg nearest the street and some neighborhood o/h power lines (not dangerously so, just closest). That said, unless the last of the antique on-my-hit-list CRT TV's is on in the house, my OCFD is as quiet as an end-fed 20m contraption all the way at the back of the lot. After trouble-shooting a few household anklebiters (wall-warts, a flourescent that was going bad, etc.) the OCFD is a pretty versatile antenna for me. A suburban S2 noise floor is pretty good for me (75m). I suspect quality coax and a quality balun meant to be on an OCFD might be contributing to that.
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
K4SAV
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« Reply #11 on: September 27, 2012, 09:25:13 AM »

That said, unless the last of the antique on-my-hit-list CRT TV's is on in the house, my OCFD is as quiet as an end-fed 20m contraption all the way at the back of the lot.


Since an end fed antenna is the absolute worse choice for common mode current problems, that isn't much of a recommendation for the low noise characteristics of an OCF.
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G8JNJ
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« Reply #12 on: September 27, 2012, 09:30:49 AM »

That said, unless the last of the antique on-my-hit-list CRT TV's is on in the house, my OCFD is as quiet as an end-fed 20m contraption all the way at the back of the lot.

As end fed antennas in an urban environment tend to be the best noise magnets ever - I'd fully agree with your observation  Smiley

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
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KC9TNH
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« Reply #13 on: September 27, 2012, 09:46:54 AM »

Since an end fed antenna is the absolute worse choice for common mode current problems, that isn't much of a recommendation for the low noise characteristics of an OCF.
Didn't say an OCFD was inherently quiet, just that I'm happy with mine.  As to the end-fed, well, it's fooled me then, or I'm lucky, or  something. It is what it is. It hears really well, goes where I need, and critical sig/audio reports are glowing. [/hijack]
 Smiley
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
W5WSS
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« Reply #14 on: September 27, 2012, 10:35:07 AM »

The antenna system could be susceptible to common mode and the rig could be at a higher voltage potentialthan the earth around the rig in either case some techniques can help: One can experiment with a single wire radial connected to the transceiver earthing lug routed away and just for over cautious safety I always use insulated wire and a wire nut screwed on the end.

When one routes the radial keep the wire isolated and insulated from ground. and have no other ground connection connected at the radio lug when listening

Try different lengths and connect and disconnect from the rig safety ground lug as you listen on all your bands

Also if you are already using a safety ground rod and wander lead try double checking to see if noise is changed by disconnecting and re connecting while listening.

Any changes even if it quiets down is pointing to common mode displacement issues as the culprit.
,
One can use a wander lead to connect to a safety ground rod when away rather than  permanently if it helps quieting.
But warning...Only choose that remedy if you are committed to safety and disciplined in reconnecting during stormy potential. and your attempts at the antenna do not help.
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