Message-ID: <676EC95B2D174FF79B59FC2718AE9C5F@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> <9E37EAD7DC7A43E08EFF3EAF82788F58@gnat> <957601B31AB3407BB82F82515E15A9F1@AGB>
Date: Sat, 7 Nov 2015 16:23:11 -0000
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Subject: Re: Re[2]: LF: Capacitive top hat question
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Status: O

I think you are waving your arms around G  :-)) There is no "transformation" 
the elevated coil merely reduces the voltage in the section below it. This 
reduces the current forced into lossy environment near the feed and verticle 
(as Mike found) The top section has a remote end with is the high voltage 
point, that point drives most displacement current through the "load 
capacitance" to ground. The capacitance is a physical thing.....two 
plates....it does not vary with frequency !! At the feed point the elevated 
coil cancels some of the capacitance so the capacitance must be measured 
well below the resonant frequuency, or the inductance allowed for. This can 
all be calculated easily. This is why I always recommend measring the 
parameters of an untuned aerial . You dont then get confused by the tuning 
elements, it is a simple capacitance in series with a resistance (which is 
predominantly loss)

If you have a copy of the old Peter Dodd LF experimenters Sourcebook there 
is an interesting reprint of an article from 1926 about the Nauen VLF aerial 
system.. Short of money they couldnt afford unlimited amounts of copper so 
they had to use what they could afford efficiently. The found with a top 
load the best point for the ground rods was under the remote end not under 
the feed point!!

Alan
G3NYK

----- Original Message ----- 
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
Sent: Saturday, November 07, 2015 3:53 PM
Subject: Re: Re[2]: LF: Capacitive top hat question


> Im thinking of  transformation, caused by  a top loading  coil . That 
> would appear to  offer a  higher  ground capacity  from the  top  wires , 
> after the  coil , lowering the  feed  z at one side and  higher at the 
> other ?
>
> G,
>
> --------------------------------------------------
> From: "Alan Melia" <alan.melia@btinternet.com>
> Sent: Saturday, November 07, 2015 3:24 PM
> To: <rsgb_lf_group@blacksheep.org>
> Subject: Re: Re[2]: LF: Capacitive top hat question
>
>> 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
>>>>>
>>>>>
>>>>
>>>>
>>>
>>
>>
>