Message-ID: <2B340C0B9A8644159051241F5E7CE88F@AGB>
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <fc7eccec0902260816u68aa7394lc2adc64e2440b62e@mail.gmail.com> <88184BDCCE244700B62ACEBF39F34582@JimPC>
In-Reply-To: <88184BDCCE244700B62ACEBF39F34582@JimPC>
Date: Fri, 27 Feb 2009 00:19:07 -0000
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Subject: Re: LF: Re: LF Antennas - back to basics
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Jim,

Yes I can confirm this :-

>>>>
increasing the height of the top loading, even if only in the middle, does
lead to substantial improvements. One benefit of doing this is that the
effective height and radiation resistance is increased by increasing the
average height of the antenna wires
>>>>

I made a substantial improvement to the  original inverted L system,  35 
feet x 40 foot  top , by adding a loading  coil to the  top of the  vertical 
section (actually  the 40 feet  of top wire wound round a coke bottle) and 
providing 2 x 40 ft capacity wires 6 feet apart back to the  house.

Earth is everything that conducts in the garden bonded along with a couple 
of 12 ft alloy scaffold poles sunk in,  to one point   along with a couple 
of 50 foot  ground lying wires as well, feed is via a auto transformer and a 
variometer in series , I did use a parallel tuner but this flashes over 
above 50 watts ,

A bonus , is the  system now works well on 1.8 and  3.5  Mhz as well . but 
with a very high Q , 20 Khz  qsy on 3.8 is enough for the pa to trip on vswr

Notably, Gary, has modelled  his array , similar 35 ft vertical section, 
using mmana  and he favours the  loading coil to be placed mid section, the 
offset capacity section of my array dose produce a slight slew of the 
pattern, but  nothing too radical

tnx- G ..

--------------------------------------------------
From: "James Moritz" <james.moritz@btopenworld.com>
Sent: Thursday, February 26, 2009 11:43 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: LF: Re: LF Antennas - back to basics

>
> Dear Andy, Dave, LF Group,
>
> My experience is that improvements to the ground system soon reach a point
> of diminishing returns where the ground connection losses are small 
> compared
> to other losses - I would expect you have reached that point already. In 
> my
> case, literally filling the garden with wire made at best about 0.5dB
> improvement by reducing loss resistance, compared to having about 5 ground
> rods spaced a few metres around the downlead and loading coil. However
> increasing the height of the top loading, even if only in the middle, does
> lead to substantial improvements. One benefit of doing this is that the
> effective height and radiation resistance is increased by increasing the
> average height of the antenna wires. If you are in a fairly built-up
> environment, the improvement is probably more than you would expect from
> calculating the effective height from the dimensions of the antenna 
> itself,
> because increasing the physical height of the antenna puts it further 
> above,
> or nearer the top of, screening buildings and trees etc., that are
> surrounding it. Also, moving the antenna further above loss-causing 
> objects
> leads to a modest reduction in the loss resistance. In my case, with an
> inverted L about 10m high at the ends and sagging to 9m in the middle,
> propping up the middle of the span with a fibreglass pole to about 14m
> increases ERP by 3 - 4dB.
>
> As to possible advantages and disadvantages of a loop compared to a
> vertical, in general the loop ought to benefit from lower dielectric 
> losses
> due to the generally lower voltages. The directional pattern is often
> helpful on receive. But the figure-of-eight directional pattern could also
> be a drawback for a transmit antenna where it isn't practical to rotate 
> the
> antenna to avoid having nulls in awkward directions. Also, one has to 
> think
> about the scale. AA1A's loop is quite big in overall dimensions compared 
> to
> G4JNT's vertical. The radiation resistance of a loop is proportional to 
> the
> square of the area, which is proportional to the square of the linear
> dimensions of the loop conductor - so when scaling down a loop, one would
> expect Rrad to reduce much faster than Rloss due to reduction in 
> perimeter,
> and so efficiency of relatively small loops to be poor.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
> ----- Original Message ----- 
> From: "Andy Talbot" <andy.g4jnt@googlemail.com>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Thursday, February 26, 2009 4:16 PM
> Subject: LF: LF Antennas - back to basics
>
>
>> Firstly, bearing in mind there is no scope whatsoever to raise one end
>> of the capacity hat which is defined by the apex of the house roof,
>> and the other end would also need some major sugery to raise its heigh
>> substantially, that only leaves the middle, ie the height of the
>> actual radiator.    It wouldn't be too difficult to raise this to 10m
>> or even more with a fibreglass pole, but will I be throwing away all
>> the advantage by having the top-hat drooping down to 7m?
>>
>> Secondly, pictures of other peoples antennas aften show a substantial
>> grounding plate immediately under and around the antenna base.  Just
>> how far out is is worth going with a really substantial base.   I
>> could cut more conductors into the ground; while the ground is still
>> is still soggy in March may be a good time to do this.
>>
>> Any ideas please ...?
>>
>>
>> Andy  G4JNT
>> www.g4jnt.com
>>
>
>
>



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