Dear John,
Computer simulation takes place over perfect ground and shows appreciable
horizontally polarised radiation under high elevation angles for the
inverted-L aerial . But even at say 20m height of the horizontal
part of the aerial this is only 0.0009 of a wavelength so the aerial is
almost on the ground. This means that over real earth such strong
currents will be induced in the earth under the aerial that almost all
power in the horizontally polarised field will be converted into heat in
its resistance.
Another point is that for DX the take-off angle should be as low as
possible and that is certainly not achieved this way.
As a ground wave (surface wave) a horizontally polarised cannot exist
over a perfect ground and over real earth is so weak that it is of no
practical use.
The question has come up before on the reflector and as far as I can
remember the conclusion has always been that this is not a viable
solution for producing a useful sky wave.
73, Dick, PA0SE
At 10:39 4-1-04, you wrote:
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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