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Re: LF: Re: "T" versus "L"aerial

from [Dick Rollema] [Permanent Link][Original]
To: [email protected]
Subject: Re: LF: Re: "T" versus "L"aerial
From: "Dick Rollema" <[email protected]>
Date: Sun, 04 Jan 2004 15:29:15 +0100
In-reply-to: <006401c3d21e$16191420$03c828c3@captbrian>
References: <[email protected]> <000001c3d009$acf7a620$c7e47f50@Smisan> <[email protected]> <006401c3d21e$16191420$03c828c3@captbrian>
Reply-to: [email protected]
Sender: <[email protected]>
To All from PA0SE

Bryan, G3GVB wrote:


Then the simulation must surely be wrong.!
 I spent a long time reading a  160/80 m antenna book by ON4 ??  before
realising that the performance of various antenna configurations confidently
reported as  if they actually existed, were in fact only the result of
computer simulations.

The simulation is certainly right under the conditions used as input.  One of the conditions I used was that no resistance existed in the aerial system. This is certainly not realistic; but the effect of resistance is that the actual power radiated is less than  the simulation indicated because power is lost in the resistance of the wire and in the earth that forms part of the return path for the aerial current. But this does not affect the shape of the radiation pattern. 


That you can get vertically radiated horizontally polarised radiation for
_nothing_ using the simulation program has to tell you something about its
limitations .

I had a look at the radiation pattern of the horizontally polarised field. In the case of the "L" there is maximum under 90 degrees elevation: straight up.
For the "T" there is a minimum under 90 degrees. But the pattern is split into two lobes with maxima under 45 and 135 degrees, so there is no complete cancellation.

The radiation pattern for the vertically polarised field over both perfect and real ground is identical for "L" and "T".  When you consider the top load wires as capacitors to earth it is reasonable that the total capacitance is the same for a single wire of 40 m or two wires of 20 m in opposite directions.  So the "extra" current in the vertical leg due to the top loading "capacitors" is also the same. And because the current in the vertical part of the aerial generates the vertically polarised field it is logical that the radiation patterns of "L" and "T" are identical.


Both the book and the programs are very  good indicators but not _absolute.
truths_

Very true.  But both are still very useful as long as the limitations inherent in the modeling of an aerial are kept in mind and the results make sense.

73, Dick, PA0SE



----- Original Message -----
From: "Dick Rollema" <[email protected]>
To: <[email protected]>
Sent: 03 January 2004 11:30
Subject: RE: LF: Re: "T" versus "L"aerial
>

>
> Bu the point raised by Bob, ZL2CA, was that the current in the single wire
> topload  of the "L" would generate a horizontally polarised field. In the
> "T" the currents  in the two topload wires flow in opposite directions so
> the horizontally polarised fields caused by these currents would at least
> partially cancel each other.


> The horizontally polarised field is radiated as a sky wave and the power
in
> it detracts from that in the vertically polarised field of the ground
wave.
> If the above reasoning were correct it could be expected that the "T"
would
> produce a stronger ground wave than the "L" because less power disappears
> in the horizontally polarised sky wave.


> ****The simulation has shown that this is not the case.****
>
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