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Old 24th Nov 2020, 10:54 am   #8
Ian - G4JQT
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Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,320
Default Re: How effective is an earth for reducing noise on AM?

Adding an earth may help, but most of the noise is probably being picked up on the wire antenna. A screened lead may also help, but unless you try some sort of matching(*) where the wire connects to the centre of the coax you are in effect just shunting much of the signal to earth via the capacitance between the coax centre and the outer braid. (This doesn't happen with matched systems like TV antennas and active antennas like the Wellbrook loop.)

There are a few active antenna designs out there much cheaper than the Welbrook loop to build yourself. Wasn't there an article in the Bulletin a year or two ago?

The Mini Whip by pa0rdt has a good reputation. Only about £40 (£14 if you supply your own waterproof housing) although you can build one yourself. It might be successful if you can locate it in the least-worst area for noise. It's not directional like a loop or active dipole so you can't null out the worst interference.

Unfortunately active antennas can be very expensive because it's essential that they are well designed so they don't overload and cause more problems than they solve.

(*)On the other hand there are baluns which attempt to match the long wire to the coax. Again you can buy or make.

https://m0ukd.com/homebrew/baluns-an...re-balun-unun/

https://www.iw5edi.com/ham-radio/317...ongwire-baluns

https://www.google.co.uk/search?sxsr...dZCOZbizRhEEVM

etc.

All of these antennas use coax which is designed for a 50 ohm receiver input. Our vintage sets usually just have an antenna wire input, the impedance of which will vary greatly across the LW/MW/SW, but in practice won't matter too much.

Last edited by Ian - G4JQT; 24th Nov 2020 at 11:12 am.
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