W4EF wrote:
>Thanks for clairfying that, Rick. I also noticed that when modeling the
>inverted-L compared with a symmetrically top loaded vertical with the
>same vertical height. This inconsistency between the radiation
>resistance of the inverted-L compared to the pure vertical really bugged
>me, but I didn't know how to reconcile it (it seemed like you were
>getting something for nothing by using the inverted-L instead of the
>symmetrically top-loaded vertical). Your explanation makes a lot of sense.
>
>
>
>
No you don't get something for nothing. If you look at the radiated
patterns you can see that. As far as the ground loss difference between
an L and a T, the answer is unclear as to which has the most ground loss
(even though NEC says the T has the most ground loss). Even though the
radiation resistance of the L is greater than the T (as calculated in
free space), and the current going into the ground at the base of the L
is less than for the T, it is not obvious that translates to greater
efficiency when placed over real ground. The current density in the
ground for the L is spread over a wider area than the T. Does that mean
an equivalent ground loss resistor placed at the base of the antenna to
simulate ground loss, should be a higher value than for an antenna such
as a vertical which has currents confined to a smaller area and
therefore higher ground current density at the base of the antenna? I
think it does, but I have no method of determining what that number
should be. I do have a guess that the difference will be very small in
this case, but maybe not so small for a very tall vertical or long L.
Unfortunately you cannot rely entirely on NEC to calculate accurate gain
numbers for verticals, Ls, Ts, or any antenna that uses the ground for
return currents for one half of the antenna. Most people know that NEC2
and NEC4 both underestimate near field ground loss, sometimes by a large
amount, so for a more accurate answer, you can simulate an almost
perfect radial system and manually enter a ground attenuation resistor
based on measured data which simulates the loss in the actual radial
system used. The measured data that is available was generated for
verticals, not Ls, or Ts, or tall verticals, so the accuracy may be
questionable when used for these antennas.
The bottom line is that for vertical antennas NEC has significant error
sources, but it is pretty good for evaluating trends, and good for
comparing antennas that are close in their physical configurations, and
it is also very good for evaluating antennas where the ground loss is low.
As for the differences between an L and a T, a few conclusions can be drawn.
1. A T antenna produces greater ground wave signal strength. You will
never find a broadcast station using an L.
2. A T antenna has a lower radiation resistance than an L.
3. An L antenna has a front-to-back ratio. NEC says that an L in its
best azimuth direction produces a greater far field signal strength at
all elevation angles than does a T. In the opposite direction, the T
produces a greater signal strength at low elevation angles than does the
L. That is NEC's conclusion. You can argue with that if you like, but
to prove something different you either need measured data, which will
be very difficult because the difference will only be 1 to 2 dB, or
maybe even less, and over a skywave path at low angles, or you need to
explain why in this case there is an increase in ground loss even though
the radiation resistance also increases, and therefore NEC is in error.
I will be all ears on that last one because I think there may be some
validity to that statement, but I don't know how to evaluate the
magnitude of the error and whether or not it is significant.
Jerry, K4SAV
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|