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Author Topic: Noise on 80  (Read 3539 times)
AB8BC
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« on: February 20, 2011, 01:55:57 PM »

I'm using an Isotron 80 for 80 meter in my attic.  Not the best situation, but it's all I have for now.  I'd like to try using the MFJ-1026 noise cancelling/nulling device.  I have a bit of man-made noise coming from all over the neighborhood.  I am in a covenant restricted area.  I have read some reviews and they look decent enough to tempt me to try it.  Anyone used one with an external rx antenna within 20 feet of the transmit antenna?  That would be what I am limited to.  I have a very tight space for both of these antennas to be in.  Thanks for any advice.--  73  Kirk  AB8BC
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K2ZS
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« Reply #1 on: February 21, 2011, 06:20:08 AM »

You may want to isolate the source of your 80m noise as much as possible. I am limited to indoor antennas and found that CFL lights are a huge source of man made noise especially on 80m. I can eliminate 50% of my noise by eliminating them in my apartment but the complex uses them for lighting in common areas. The lights are on timers and I can see my noise floor jump whenever they come on.
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Antenna restricted?
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N3LCW
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« Reply #2 on: February 21, 2011, 06:30:22 AM »

Yes the MFJ-1026 or Timewave ANC-4 will eliminate the noise providing the proper sense antenna is used at the right location.  I used my ANC-4 with my attic antenna to eliminate household noise.  Adjusting the 1026 or ANC-4 will be touchy but they do work.  I found that you will need at least 20ft of noise sense antenna to work. 

You will find it will be necessary to readjust the noise filter as you move across the band most of the time, especially for different noise sources.

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WX7G
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« Reply #3 on: February 21, 2011, 08:42:28 AM »

I use an MFJ-1025 on 80 meters. The noise antenna is a Hustler 80 meter mobile antenna in the yard with only a ground stake for a ground. The measured efficiency is 1%. Placed 60' from the TX antenna it works well.

Now, the Isotron is actually not an antenna, it's a common-mode current excitation device. It is not your 80 meter antenna. The Isotron excites the coaxial cable and "ground" wire and they are the real antenna. Think of them as the antenna and now how far away can you place the MFJ-1025 sense antenna?
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2E0OZI
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« Reply #4 on: February 21, 2011, 09:23:20 AM »

I know many people consider the coax to be the radiating element in an Isotron, but how does that work if the Isotron is sitting on the same desk as the FT817 and the owner has DXCC?Huh?
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Men can only be happy when they do not assume that the object of life is happiness.
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WX7G
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« Reply #5 on: February 21, 2011, 12:17:22 PM »

In that case any wire between the radio and ground is part of the antenna. This includes the AC wiring in the house.

The Isotron and associated wires are simple to model with NEC. For an analysis and test of the Isotron see my two part article in AntenneX magazine. The testing includes field strength measurements of an Isotron all by itself - no feedline.
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N3OX
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« Reply #6 on: February 21, 2011, 12:31:45 PM »

how does that work if the Isotron is sitting on the same desk as the FT817 and the owner has DXCC?Huh?

Here's someone who's worked 142 DXCC with half a watt to a dipole:

http://jq2uoz.blogspot.com/2010/01/dxcc-qrpp-500mwdipole.html

You really don't need much radiated power to work DXCC, so it's not much good as a measuring stick to how good an antenna is.

How much the Isotron itself radiates is a matter of the band, but WX7G's assessment is a good one especially on 80m.
It is not "absolute fact" to say that it's an antenna current excitation device, but it is a good description of what the antenna USUALLY does.  The lower you go in frequency, the more important the common mode current on the feedline is going to be.  Tiny antennas get worse at a rapidly increasing rate as the size in wavelengths is reduced.  Here's an example of that for a particular short loaded antenna:

http://n3ox.net/files/eff_Q.jpg

The exact dB numbers depend on the antenna, but the general trend will be identical for tiny coil-and-cap-hat antennas.   The decrease in performance gets faster and faster and faster as you stay at one physical size and go to a lower frequency, making the antenna smaller in terms of a wavelength.

The fact of the matter is that you have to heavily shield things to keep them from being heard around the world in good conditions.  And as far as I know all the Isotrons are the same physical size.  A 20m Isotron with no feedline to speak of is going to radiate a lot more signal than an 80m version. 

The other thing I wonder about your anecdote, did this person with Isotron-and-QRP DXCC have a ground wire of any length?  It is possible to sit at the top of a vertical antenna. Grin  High Q resonant circuits can develop huge voltages, and this allows them to drive currents into almost any load.  They can compensate for large reactances of either sign by shifting slightly to either side of resonance.  So they're very good at driving currents on stray stuff around the shack.

Anyway,  you're running an 80m Isotron or any other antenna that electrically small indoors like Kirk is, it's important to know that feedline current is doing the lion's share of pickup and radiating.  You're basically threading a top loaded vertical through your house to the shack and then maybe to ground... or like WX7G says, the house wiring.



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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W8IXY
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« Reply #7 on: February 21, 2011, 01:40:58 PM »

Several days ago, here in Northeast Ohio, we had a thaw....several days of well above normal temperatures.  As soon as the thaw occurred, I began experiencing a pegged S meter with power line arcing noise, worst I have ever had.  It took out every band from AM BC through 6 meters!  Last evening, during a rain/ice/snow event, several 3 phase feeder line wood poles along a main thoroughfare caught fire.  Probably because of a winter's build up of road salt that had accumulated for months.  That's what I suspect has been happening to cause the noise at my QTH. 

I had pulled the main breaker at the house, and the noise did not change.  Last evening, the neighborhood experienced a power outage for several hours, and the power line arcing was, as you'd expect, gone.  I called the power company and scheduled a visit from one of their trucks.

You may have a similar situation....build up of conductive gunk across one of more insulators on nearby power lines.  There may be more than one source of that noise.   If you can, drive around the area near your QTH and see if you can find some poles or power lines where the noise appears loudest.  Start with an AM broadcast radio, then go higher and higher in frequency, if you have an HT that will receive AM on the 144 or 220 bands, zero in with that.  With a little luck you will find the noisiest pole and then be able to call the power company to report some line arcing.   Good luck.

73
Ted  W8IXY
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2E0OZI
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« Reply #8 on: February 21, 2011, 02:33:48 PM »

Thanks for the thoughts on the Isotrons - yes IIRC that fella was CW all the way, and rather experienced.

I'm using a W3EDP at the moment with 10w, from 80 to 15m. I'm a believer myself in "lots of wire as high as possible" but a mans gotta know his limitations.  Grin
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Men can only be happy when they do not assume that the object of life is happiness.
George Orwell
KB4MB
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« Reply #9 on: March 02, 2011, 10:22:15 AM »

Quote
I'm using an Isotron 80 for 80 meter in my attic.  Not the best situation, but it's all I have for now.  I'd like to try using the MFJ-1026 noise cancelling/nulling device.  I have a bit of man-made noise coming from all over the neighborhood.  I am in a covenant restricted area.  I have read some reviews and they look decent enough to tempt me to try it.  Anyone used one with an external rx antenna within 20 feet of the transmit antenna?  That would be what I am limited to.  I have a very tight space for both of these antennas to be in.  Thanks for any advice.--  73  Kirk  AB8BC

I didn't see this posted yet, but I will throw in the normal advice on the noise cancellers - I have one, and they are good at removing ONE noise source, not multiples.  I can hear the tv flyback transformer when I am on the vertical, so I can remove that with the canceller.  But if there was another neighbor with a dimmer making noise, I would have to choose one over the other.  I have a pole that makes noise now and again at s1 on the upper bands, and I can reduce that to s .5 (if there was such a thing)... better but not perfect (when I had a closer pole that was making s7 I could get it to s2, again better but not perfect).

Also, you can remove the signal you are trying to receive, so you have to have a compromised "sense" antenna so you get more bad stuff than good on it, and then you phase out the bad.

So, in short, good at removing local (your house noise) then just gunk in the neighborhood.
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AB8BC
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« Reply #10 on: March 02, 2011, 03:28:27 PM »

I ended up rigging (what I intended to be) a "noise" rx antenna out of an old dipole. It's not pretty by any means.  It's strung under the deck, back and forth in particular design.  Maybe 5 feet off the ground.

So I get in the house and it's tuned at 4.200.0 MHz!!  I go out, add a few more feet and whamo!  It's right where I want to be at 3.800.0 MHz with 100 kHz bandwidth under 2.0 swr.

So I decided to do a side by side comparision.  Low and behold, it is mote quiet and transmits the signal better than the Isotron!!  So since I have a ton of noise on the Isotron, it will now become the "noise" antenna for now.

73 and thanks for all the input.   Kirk.  AB8BC

Thanks for all the
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KB4MB
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« Reply #11 on: March 02, 2011, 03:45:28 PM »

Quote
So I decided to do a side by side comparision.  Low and behold, it is mote quiet and transmits the signal better than the Isotron!!

How do you figure it transmits the signal better?  Because of the swr?  Dummy Loads have fantastic SWR's...

As for noise, sure it is quieter - its almost on the ground!  Signal is shooting straight up and down (as is the receive).

Many think just because the antenna hears more noise it isn't as good of an antenna - but sometimes you hear more noise because you hear better period.  Think of it like a 20 yr old and an 80 yr old - the 20 yr old hears more of everything, including very far away while the 80 yr old can only hear the really loud or really close.
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AB8BC
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« Reply #12 on: March 02, 2011, 04:41:14 PM »

My deduction of it transmitting better is a real test on air.  I had both antennas ready on switch, and had a conversation with a couple different stations, switching between the 2 for about 5 minutes each qso.  Each time I went to the dipole under the deck, the signal got 2 and sometimes 4 more s units on the other side than the attic antenna.  The attic has more noise induced by far.  So for now, I'll experiment between the 2 until spring thaw lets me work other arrangements.  I was impressed with the dipole as its literally thrown togetheir.  What a kick!! 
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