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From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <1843908200.20151104162925@chriswilson.tv>, <CAA8k23QB1RxnKaYXrYq0YVscvzEsNstciTwTkW=D6Dvt2fLUYQ@mail.gmail.com>, <8E31BC8EB75546E59F99E35AFAAA0DEE@AGB> <563DD93A.22662.37C4EA@mike.dennison.ntlworld.com> <514333482.20151107111905@chriswilson.tv> <79AAC450664E4A62AE4F7F72331E43BC@gnat>
In-Reply-To: <79AAC450664E4A62AE4F7F72331E43BC@gnat>
Date: Sat, 7 Nov 2015 14:31:16 -0000
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Subject: Re: Re[2]: LF: Capacitive top hat question
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Status: O

doubling the capacity ofan aerial halves the ground loss

Actual or  effective  capacity  ?

G,

--------------------------------------------------
From: "Alan Melia" <alan.melia@btinternet.com>
Sent: Saturday, November 07, 2015 12:32 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: Re[2]: LF: Capacitive top hat question

> I think that maybe too much emphasis is placed on specific structures. At 
> these frequencies any structure of conductors can be resolved into an 
> equivalent vertical and horizontal arrangement......even a continuously 
> sloping wire!. As Mike says the horizonal portion does not radiate 
> appreciably because of the reflection in the close-by ground.
>
> Predominantly horizontal conductors will inrease the capacitance of the 
> aerial to ground and an extra run of wire will have most effect if 
> separated by around a metre to reduce interaction between to two. 
> Ball-park figure an extra 6pF per metre. The actual configuration of 
> connection is unimportant for the horizonals form a "skeleton" plate. Note 
> whereas Rugby LF station had originally caged wires between the 850 foot 
> masts to increase the capacity, after the rebuild the internal area with 
> the masts was "laced" with straight single wires. This produced more 
> capacity and was easier to maintain the the high windage cages.
>
> Then the more horizontal wire the higher the aerial capacity, so the 
> smaller the inductance needed to resonate it ......and lower coil loss. 
> However another effect not well modelledin aerial synthesis programs 
> isthat doubling the capacity ofan aerial halves the ground loss. There are 
> mesurements on my web site confirming this, under spiral aerials. Halving 
> ground loss is very difficult to achieve with extra groundrods or 
> "radials" at LF/MF unless it is poor to start with. The only casewhere 
> this is not useful is over very good ground, a high water-table or 
> possibly sea water.
>
> As to feeding Chris's loop as a loop..... the size is much less than a 
> wavelength and is too low compared with the wavelenth to radiate 
> efficiently. Any uncancelled radiation will probably be vertically 
> upwards, much of which will escape the ionosphere never to return.
> Its performance at HF where distance above ground is of  the same order as 
> a wavelength will be totally different.  Phase difference round the loop 
> will lead to a totally different pattern of radiation.
>
> Alan
> G3NYK
>
>
> ----- Original Message ----- 
> From: "Chris Wilson" <chris@chriswilson.tv>
> To: "Mike Dennison" <rsgb_lf_group@blacksheep.org>
> Sent: Saturday, November 07, 2015 11:19 AM
> Subject: Re[2]: LF: Capacitive top hat question
>
>
>> Hello Mike,
>>
>> Saturday, November 7, 2015
>>
>>
>> Thanks for the info Mike, as always! Is there any real benefit in
>> having the vertical section centralised within the top hat capacitive
>> array, be it a horizontal loop, random horizontal wires, or a plain
>> single wire?
>>
>> And is there much point in struggling to get one corner or side of a
>> horizontal top hat loop higher than the rest, or the same for a single
>> wire? I have some tall trees, but unfortunately not two tall trees
>> opposite one another across free space, so whatever capacitive hat I
>> fix up is likely to be significantly higher at one end or one corner.
>> There's no point in struggling and risking life and limb to get one
>> end as high as possible if it ends up only as efficient as the lowest
>> end or point.
>>
>>
>>> To all intents and purposes a short (in wavelengths) vertical
>>> attached to an arrangement of horizontal wires is a simple
>>> (capacitively) loaded vertical.
>>
>>> The horizontal part will radiate, especially if it has a vertical
>>> component rising higher than the feedpoint, but most of that
>>> radiation will be cancelled by its reflection in the ground.
>>
>>> Several unbelievers have tried using purely horizontal transmitting
>>> antennas and have had poor results. The beauty of amateur radio is
>>> that you can prove something to yourself, but it won't change the
>>> laws of physics.
>>
>>> Of course, every electrically small Marconi that is not in free space
>>> performs in a complex way, but the result of that complexity is small
>>> compared to the predominent omnidirectional radiation from the
>>> vertical section. There is very little difference between various
>>> arrangements of capacity hat, so long as you follow the rule to put
>>> up as much wire as possible, as high as possible and covering as much
>>> ground as possible.
>>
>>> Mike, G3XDV
>>> ==========
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> -- 
>> Best regards,
>> Chris                            mailto:chris@chriswilson.tv
>>
>>
>
>