[TowerTalk] STACKED 40 METER BEAMS MATCH

[Michael Tope]

On 11/25/2010 2:30 AM, Jim Thomson wrote:
>
> ## The ARRL  HFTA program is flawed somewhere.   Apparently it uses  simple
> ray analysis in the far field only.   In the old w2pv literature /notes,  w2pv  modeled
> a  20m yagi at 150'.... and  varied the  height of the lower yagi  from 65/75/85'.  The BIP
> gain peaks when using the lower yagi at 75'..and drops  off at 65/85.     HFTA on the
> other hand shows the gain increasing, as the lower yagi  keeps  getting higher and higher.
> Try it, it's  bizarre.   Gain  is lowest, when lower yagi is  say 65'...then  just keeps  on increasing, as
> you raise it!
> When the bottom yagi is a 130', it's  still  increasing.   [top one stays put at 150'].  Since the HFTA
> program does not factor in near field interaction/mutual coupling, etc,  that might explain the dismal
> results.  [ it was w2pv who 1st suggested the BOP  mode].   Who knows,  maybe at the top of the
> next cycle, [ when angles are usually a little higher], BOP may actually work...more often.
>

I believe you are correct about HFTA, Jim. From the HFTA manual:

    "The internal Yagi model in HFTA is a simple mathematical model.
    It does not compute interactions between individual Yagis in a
    stack---HFTA
    assumes that each antenna is a "point source." For antennas stacked
    more than about a half wavelength apart this is not a problem. For
    example,
    you should be cautious specifying spacings less than about 20 feet
    on 20
    meters (and proportionately scaled on other bands) because of
    mutual-coupling effects between real antennas.


An interesting experiment would be to compare the output of EZNEC with 
HFTA (terrain set to flat ground) using the 20 meter stacked Yagi 
parameters you give in your example. To get the best comparison it would 
be helpful if you had the physical characteristics for the Yagi's that 
Dean uses as the basis for his antenna selections in HFTA. I would 
expect to see good agreement between the two programs at wider Yagi 
spacings with the results diverging significantly as the spacing was 
reduced.

73, Mike W4EF...................