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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 14th Nov 2018, 3:40 pm   #1
G6Tanuki
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Default Metal fence-post earth.

I recently had to replace some wooden fencing and while I was driving in one of the 20 or so galvanised metal supports (see attachment for photo - the wine bottle is included for size-reference) it occurred to me that the surface-area of a single support that was to be embedded in the ground was rather large - much greater than that of the traditional metal earth-rod used in conjunction with a typical MW/SW aerial.

The supports are now all coupled together using a length of 2.5mm single PVC-insulated wire, which is buried a couple of inches below ground to keep it out of reach of the strimmer. The house-end will connect to my Eddystone 840A 'bedside radio' in the hope of me getting some transatlantic MW DX later in the winter.

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Old 14th Nov 2018, 4:00 pm   #2
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Default Re: Metal fence-post earth.

Nice bit of lateral thinking there, with dual-purposing and connection diversity. I'd always thought that the traditional rod must offer marginal contact area with the soil. I trust that the wine in the bottle didn't last long post- (!) completion....
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Old 14th Nov 2018, 4:02 pm   #3
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Default Re: Metal fence-post earth.

Be aware of safety problems associated with introducing an alternative 'earth' into a domestic electrical system. The RSGB booklet on this basically advises "don't do it, but if you must, use a qualified electrician". It recommends using ungrounded antennas, such as dipoles. The details of the danger depend on how your house is wired.
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Old 14th Nov 2018, 4:03 pm   #4
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Default Re: Metal fence-post earth.

Nice to see galvanised ones - all the "Metpost" ones I've used have been painted.
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Old 14th Nov 2018, 4:03 pm   #5
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Default Re: Metal fence-post earth.

Yes, it seemed a shame to let such a good surface-area of buried metal just sit there supporting fenceposts when it could have a bit of RF flowing through it too.

[The remaining wine found its home in a chicken casserole]
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Old 14th Nov 2018, 4:07 pm   #6
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Default Re: Metal fence-post earth.

I'm assuming that the 840A is somewhat like the 670A here, with a mini patch-panel offering chassis and high voltage capacitor isolated signal earths.

None-the-less, a very worthwhile point to raise, and one deserving careful and thorough appreciation in any added "radio earth" situation.
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Old 14th Nov 2018, 4:10 pm   #7
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Default Re: Metal fence-post earth.

Quote:
Originally Posted by G8HQP Dave View Post
Be aware of safety problems associated with introducing an alternative 'earth' into a domestic electrical system. The RSGB booklet on this basically advises "don't do it, but if you must, use a qualified electrician". It recommends using ungrounded antennas, such as dipoles. The details of the danger depend on how your house is wired.
Yes I'm aware of the safety-implications of 'extraneous' earths: here it's "TT" and there are full-service RCDs on both consumer-units so things are a lot safer than the old PME-style setups where loss-of-Neutral could cause bad things to happen.
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Old 14th Nov 2018, 6:16 pm   #8
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Default Re: Metal fence-post earth.

Quote:
Originally Posted by Nickthedentist View Post
Nice to see galvanised ones - all the "Metpost" ones I've used have been painted.
Yes - I chose these (from B&Q) because they are galvanised rather than painted. Though the reddish-brownish paint on the Metpost types does seem quite good (I've got some of their bolt-down-to-concrete supports on my wood-store and they're showing no sign of rot after 10 years).

I think these galvanised ones (and the pressure-treated fenceposts which I've also drenched in creosote/old engine-oil) will still be OK when I'm consigned to the Bide-a-Wee Home for the Hopelessly Bewildered.

Where I've connected the earth lead tags to the posts I've used a stainless-steel coach bolt through the metal into the wood, and slathered everything with semi-fluid moly-grease before tightening up so they should stay corrosion-free for a while. No "rusty-bolt" QRM-causing diode effects here!
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Old 14th Nov 2018, 7:31 pm   #9
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Default Re: Metal fence-post earth.

Red Oxide - great paint!
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Old 14th Nov 2018, 10:16 pm   #10
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Default Re: Metal fence-post earth.

Isn't it possible to isolate RF earths with a capacitor when it enters the building (if you have one of those PME systems)?

As for old engine oil as a preservative the oil is often acidic, so may not be the best presservative.

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Old 14th Nov 2018, 10:21 pm   #11
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Default Re: Metal fence-post earth.

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Originally Posted by Tractionist View Post
Red Oxide - great paint!
But be sure to remember that the term "Red Oxide" now refers only to a shade of colour and has nothing to do with chemical composition (unless is it very old stock and still has lead in it).

The "Red Oxide Primer"sold at a very good price by Screwfix utilises zinc phosphate to inhibit corrosion. It has the "4CQJ" 5 Star seal of approval. It is actually made by the Johnstone's Company but packaged in small tins for Screwfix.

(no connection with either company)

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Old 15th Nov 2018, 3:37 am   #12
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Default Re: Metal fence-post earth.

The other thing to be aware of is that the galvanising on the post can/may react with acid in the soil because of the current (however small it may be) passing through it and cause all sorts of strange symptoms.

Probably not in the short term, but definitely in the long term (rust/corrosion makes a good parasitic semiconductor).

The copper wire joining the posts may also react depending on how you connected it to the posts.
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Old 15th Nov 2018, 5:52 am   #13
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Default Re: Metal fence-post earth.

Zinc (plating)
Mild steel
Stainless steel
Copper wire
A tiny bit of rust somewhere

Used engine oil (could be acid or base, and with numerous additives)
Creosote
Soil

“No "rusty-bolt" QRM-causing diode effects here!”

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