Topband: [Topband] Inverted "L" vs." T"

[K4SAV]

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