[UK-CONTEST] Fw: Vertical radials

[Clive GM3POI]

----- Original Message ----- 
From: "Clive GM3POI" <gm3poi2 at btinternet.com>
To: "David, G3YYD" <g3yyd at btinternet.com>
Sent: Tuesday, October 13, 2009 12:02 PM
Subject: Re: [UK-CONTEST] Vertical radials


> Another important point is that using shortened 
> verticals whatever the type increases the need for 
> longer radials. As the efficiency of a vertical is 
> dependant on the losses and they increase with 
> lower heights. Typically if you want your short 
> vertical lets say 0.1 wave to be like a full 
> quarter wave vertical you will need radials 0.4 
> wave in length. Hence my 160m radials are typically 
> 70m long. 73 Clive GM3POI
> ----- Original Message ----- 
> From: "David, G3YYD" <g3yyd at btinternet.com>
> To: <uk-contest at contesting.com>
> Sent: Tuesday, October 13, 2009 11:24 AM
> Subject: Re: [UK-CONTEST] Vertical radials
>
>
>> The best book to read on radials is Low Band DXing 
>> by ON4UN and read the
>> section on verticals.
>>
>> The key thing to remember about radials is they 
>> act to screen the
>> underlying lossy soil from the RF current (note 
>> current). A radial
>> system of any sort shape or whatever should be 
>> thought of as a ground
>> screen. Therefore the spacing between the radials 
>> ends is dependent on
>> wavelength of the RF current. The smaller the 
>> wavelength the closer the
>> ends of the radial need to be. But the shorter the 
>> wavelength the
>> shorter the radials need to be for the same 
>> screening effect.
>>
>> The other key item is the distribution of the RF 
>> current. Assuming the
>> radiator is a quarter wave long or less then the 
>> current maximum is at
>> the start of the base of the radiator and then 
>> declines as we move out
>> from the base of the radiator. With loss 
>> proportional to current squared
>> this means that the most gain in terms of reducing 
>> loss with the ground
>> screen is near the base of the radiator. As the 
>> ground screen extends
>> further out from the base the reduction in loss 
>> per unit length drops
>> very quickly (remember the square law). In other 
>> words relatively short
>> radials will get you rapidly to the point where 
>> loss reduction is not
>> worth the extra effort and cost. The distance 
>> depends on just how much
>> the extra dB is valued and the loss of the 
>> underlying soil, but
>> generally an eight of a wavelength length is 
>> sufficient and a quarter
>> wave giving another 1dB (poor soil conductivity 
>> giving more loss
>> improvement with the extra length).
>>
>> The spacing of radial ends to achieve maximum 
>> screening is 2.5m at 160m.
>> Doubling this to 5m spacing will increase loss by 
>> about 0.5dB (some soil
>> and length dependency) but half the effort and 
>> half the wire. For higher
>> bands reducing the radial end spacing pro-rata, 
>> i.e 1.3m on 80m
>> (1.8/3.5*2.5).
>>
>> As for material, avoid using buried galvanised 
>> wire. I tested it at my
>> location, London clay,  and it dissolves away 
>> within about 2 years due
>> to either soil chemicals reacting with the zinc or 
>> through the battery
>> effect with the other metals it is in contact 
>> with. I use 1mm
>> multi-strand copper wire plastic coated. Give 
>> Westlake a call and see
>> what he has got in stock - it will not be on his 
>> web site. For RF
>> performance the wire diameter can be very small as 
>> so many are being
>> used in parallel. It is the physical strength 
>> required to avoid breakage
>> that determines wire diameter.
>>
>> I also suggest the coax feeder is run under the 
>> ground screen to
>> minimise coupling between the radiator and the 
>> coax outer. This needs a
>> bit of pre-planning i.e. lay the coax before the 
>> radials!
>>
>> Do not use radiators over a quarter wave long as 
>> the peak RF current
>> moves out from the base and that means the radial 
>> lengths need to
>> increase to meet this situation to keep ground 
>> losses down (more cost
>> and effect for no gain). If you want more signal 
>> strength then use more
>> radiators to form a vertical beam.
>>
>> David G3YYD
>>
>>
>>
>>
>>
>>
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>