How much horizontally polarised skywave is there and how well does it
propagate?
Would it be worth constructing an aerial that favoured skywave?
73
John Rabson G3PAI
----- Original Message -----
From: "Dick Rollema" <[email protected]>
To: <[email protected]>
Sent: Saturday, January 03, 2004 11:30 AM
Subject: RE: LF: Re: "T" versus "L"aerial
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: [email protected]
>[mailto:[email protected]] Namens Dick Rollema
>Verzonden: maandag 29 december 2003 16:37
>Aan: [email protected]
>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: <mailto:[email protected]>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
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