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

["Dick Rollema" <d.w.rollema@freeler.nl>]

To All from PA0SE,

Mike,  PC4M, wrote: 

At 02:50 1-1-04, you wrote:
> Dear
> Dick / Bob and Lofers,
>  
> Does the computer calculate the earth losses in the return path from the
> aerial system to the transmitter? If it would then the earth losses in a
> T should have been significantly less then the L alternative. There are
> two separate return currents 
> (parallel resistance) and each with a smaler physical length (lower
> R-earth)  in a T system resulting in more ERP if compared to an L
> system.

In the computer simulation no resistances were included. That means that
the 1 kW fed to the aerial is completely radiated. Even an extremelly
short vertical with no top load would do so and produce the calculated
29.9mV/m at 10km

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.

The subject of losses in the earth  and surrounding objects has been
treated very well by Jim, M0BMU, in his e-mail.

73, Dick, PA0SE


> -----Oorspronkelijk
> bericht-----
> Van: owner-rsgb_lf_group@blacksheep.org
> [mailto:owner-rsgb_lf_group@blacksheep.org]
> Namens Dick Rollema
> Verzonden: maandag 29 december 2003 16:37
> Aan: rsgb_lf_group@blacksheep.org
> Onderwerp: Re: LF: Re: "T" versus
> "L"aerial
>  
> To All from PA0SE
>
> Bob, ZL2AC wrote:
>
>
>
> Dick PA0SE,
>  
> Fine on the test result.  As you
> stated, the tested T has twice the amount of top loading wire (2x 20
> metres) than the L (1x 20 metres).
>  
> It would be interesting to know if a T
> is better than an L for constant length top loading i.e. what the
> difference is if the upwire joins at the end or the middle of the
> horizontal top wire (theory suggests the T is better as there is minimal
> horizontally polarised component).
>  
> Bob,  I cannot answer your
> question by a practical experiment but used computer simulation instead
> by means of K6STI's program Antenna Optimizer.
>
> I modeled two antennas with a vertical element of 20m.  One an
> Inverted L-antenna with a horizontal top load wire of 40m. The other a
> T-antenna with a top load of 2 x 20m.
> Both antennas  without losses, over perfect ground and fed with
> 1kW.
>
> At a distance of 10km (so well outside the near field region) and over
> perfect ground both antennas produced a vertically polarised field of
> 29.9mV/m. The horizontally polarised field was zero; but this is to be
> expected because over a perfect conducting ground a horizontal field
> component cannot exist.
>
> 73, Dick, PA0SE 
>
> Original message:
>
>
> To: LF-Group
> Sent: Sunday, December 28,
> 2003 3:09 AM
> Subject: LF: "T" versus "L"aerial
> To All from PA0SE
> Further to my e-mail of 26 December I measured the field strength as
> radiated by the aerial in 
> Inverted L-configuration. From this I found EMRP = 57
> milliwatt.
> This confirms the
> benificial effect of top loading. The T-aerial radiated
> 140 milliwatt. 
> So going from a single
> 20m top load wire for the "L" to 2 x 20m for the "T"
> resulted in an improvement by a factor 2.46 (3.9dB) in radiated power.
> The vertical part of the "T" consisted of an open wire feedline
> of 11m with the two wires connected in parallel in the attic shack. For
> the "L" one of the feedline wires was removed. I assume this
> did not appreciably affect the EMRP.
> 73, Dick, PA0SE