Message-ID: <9E37EAD7DC7A43E08EFF3EAF82788F58@gnat>
From: "Alan Melia" <alan.melia@btinternet.com>
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> <AC8A5CA3A2DB4EDBA6E3FE8CB9B23A7B@AGB>
Date: Sat, 7 Nov 2015 15:24:25 -0000
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Subject: Re: Re[2]: LF: Capacitive top hat question
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Status: O

Measured ....the only type that matters :-))
Alan
----- Original Message ----- 
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
Sent: Saturday, November 07, 2015 2:31 PM
Subject: Re: Re[2]: LF: Capacitive top hat question


> 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
>>>
>>>
>>
>>
>