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Old 15th Jul 2010, 9:20 pm   #21
Don Collie jnr
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Default Re: Short wave interference and noise.

Gidday Jim,

I've been having thoughts about a noise insensitive antenna that may be of interest to you. It relies on the fact that all man-made noise [QRM] originates from below the horizontal [horison], when your antenna is above all surrounding QRM sources.

Therefore if you make an antenna that has no V plane lobe at the horizontal, but just a single lobe at say 20 degrees above horizontal, and a solid ground plane [eg: a disc of aluminium extending radially below the antenna to prevent radiation from below reaching it], theoretically, I reckon, this antenna would pick up no QRM.

I understand that an electrically short [1/10 wave, say] vertical satisfies these criteria, and a circular 1/4 wave [or more] ground plane disc of aluminium would do for the ground plane. The above would not really be suitable for transmit, and would be a receive-only antenna, but QRM is a receive-only problem, as it is only the local QRM that is of similar power to the [weak] received signal - the transmit antenna could be placed below it.

If you connected the bottom of the vertical to a low noise preamp, and used good screened cable to a well screened receiver, QRM should be reduced IMO.

What do you think? I haven't built it yet, due to relative poverty, and other reasons, but do you think it would work? BTW, most lower frequency HF signals arrive at about 20 degrees above horizontal, most of the time.

Cheers,
Don ZL4GX

Last edited by Darren-UK; 16th Jul 2010 at 11:12 am. Reason: SFK.
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Old 17th Jul 2010, 3:57 am   #22
G4XWDJim
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Default Re: Short wave interference and noise.

Quote:
Originally Posted by Don Collie jnr View Post
Gidday Jim,

a solid ground plane [eg: a disc of aluminium extending radially below the antenna to prevent radiation from below reaching it], theoretically, I reckon, this antenna would pick up no QRM.
Thanks for the thought Don but this disc to be connected by something typically like a quarter wave length of wire at 80 metres say, to get it above the house will be way above ground potential and contribute all sorts of unwanted signals on its own.

Jim
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Old 17th Jul 2010, 8:53 am   #23
M0AFJ, Tim
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Default Re: Short wave interference and noise.

Just remember with the onward march of PLA the interference may be being radiated by every bit of mains wiring in your house. If your neighbours are using Power Line Adaptors the racket will be induced into your wiring, there is no suppression at the meter.
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Old 17th Jul 2010, 11:16 am   #24
Don Collie jnr
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Default Re: Short wave interference and noise.

Hi Jim. My last post apparently got lost somewhere. Regarding man made interference [QRM] : This is a local problem, ie it happens when a weak signal,
arriving from afar, is added to a local interference signal. This means that it`s only the local noise field that is a problem, not any noise local to the transmitter far away. If it were possible to screen out the local noise field, then the weak signal from far away would be recieved weak but clean.
Looking from the top of a pole [this antenna would be above surrounding objects] above the surrounding noise sources, you will realise that all QRM origonates from *below* the horison, so if it were possible to place an antenna at this height that did not receive signals from the horisontal, or below, no QRM would be received. None. A 1/10 wave vertical
antenna, over an aluminium disc extending 1/4 wave out from the center satisfies this criterion, as it has a null at 0 degrees, and a single lobe in the V-plane at about 20 degrees above the horisontal - about the angle at which lower MF band signals arrive most of the time. This antenna is insensitive to horisontally polarised signals, and refraction at the edge of the disc of vertically polarised signals causes a vertically polarised wave to impinge on the antenna as if it attived at 0 degrees - where the antenna has a null. What do you think, Jim? I haven`t built this antenna yet, but think it might just work. Regards,.......................................... ..................Don.
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Old 17th Jul 2010, 11:26 am   #25
Don Collie jnr
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Default Re: Short wave interference and noise.

Hello Jim, No the thing won`t generate interference of it`s own, as it has no power going out of it. A little unconventional for you Jim, perhaps? I agree that for 80M, a 20 m disc would be enormous, but the concept I think is a good one. There are other antennas about that are smaller, and satisfy the criteria needed............................................ .................................................. .......Don
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Old 28th Jul 2010, 1:53 pm   #26
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Default Re: Short wave interference and noise.

I was an avid DX-er back in the 90-ies. I used a great active vertical whip out in the garden and could easily monitor far away stations in Asia and North- & South America. Back then, RFI and noise wasn't a problem. There were very few modern gadgets and most of the gadgets at the time followed the RFI rules. In the beginning of 2000 things had changed dramatically. Allmost every electronic devise was transmitting enormeous amount of RFI. I quitted my DX-ing and turned to my wife, that I met at the time. After a few years I thought that I might re-enter the DX-ing hobbie again. Things had not changed. Still a hell hole of RFI. So I started looking for a solution.

I looked into noise cancelling by phasing. I found a SEM QRM Noise Eliminator, that I recently manage to blow both JFets. But before that it worked perfectly. It was nulling out a severe +20 dB RFI I have from a local network. I'm currently seeking replacement JFets, so I can have up and running again, and maybe build a second one. These SEM's are no longer in production and therefore hard to find. I suspect that MFJ 1025/1026 are equally great in phasing out local RFI.

If you have a SEM, beware of static from the antenna, especially if you have a longwire.

I have also seen a few homebrewed phasing units on the web:

http://www.kongsfjord.no/dl/dl.htm
http://users.netonecom.net/~swordman...ise_cancel.htm

To successfully phasing out RFI it depends greatly on the AUX antenna, the short antenna that only picks up the local noise. It is recommended to experiment to find the exact configuration and even to have several noise antennas that can be used depending on the source and frequency band.

Regards

Lasse
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Old 7th Jan 2011, 1:57 am   #27
marc_uk
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Default Re: Short wave interference and noise.

sorry i'm a bit late to join this discussion .. the box works well, yes, cheap constriction, (i live in a flat) and my 10m vertical on the chimney was great ..and our nightly 10m QSO net was unaffected by the awful streetlamp QRN outside my QTH. i couldn't kill the streetlamps' s9+ noise out on 6m, but 10m down was fine. i found i did a lot of tweaking, but i didn't realise there were multiple sources of interference back then, and never always got the 'null' of course.now my 101zd is less affected by the noise than the TS870, FT 920, FT 890, FT 817, FT 690. now a move of QTH looms and i feel i'll be buying another ..
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Old 7th Jan 2011, 3:30 pm   #28
Roger-G3YTN
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Default Re: Short wave interference and noise.

The current issue of SPRAT - the GQRP magazine - has a circuit for a 'plasma nuller', by GW0VMR, which he used to cancel noise from a plasma TV, using a second noise pickup aerial.
Claimed frequency range is 3 - 11MHz.

Might be of interest.

Roger
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