[TowerTalk] 15 meter yagi height for 6000 miles

[Steve Hunt]

Frank,

You may not have the resources to raise or lower the antennas, but tools 
like HFTA are very useful for assessing trade-offs and making 
cost/benefit choices.

Take a look at the chart in Section 2 here - it formed the basis of a 
QST article a while ago:
http://www.karinya.net/g3txq/hexbeam/height_2/

That chart allowed me to quantify the performance against height for UK 
Short, medium, and Long-Haul paths on 20m and 10m. It allowed me to make 
the following sort of observations:

  * There is no single "best height" for the antenna;
  * On the long-haul UK/Oceania path there is no substitute for height.
    As we lift the antenna from 20ft to 100ft we get a 10dB improvement,
    both on 10m and 20m; and the shape of the curve suggests that more
    height would bring further dividends. But note that half of that
    10dB improvement has been achieved in lifting the antenna from 20ft
    to 40ft;
  * On the intermediate UK/USA path, 50ft-60ft looks to be about
    optimum. Anything higher produces a degradation in the 10m HFOM, and
    the 20m HFOM is within 1dB of its peak;
  * On the short-haul UK/Europe path, the optimum height is between 30ft
    and 40ft. If we go much higher the 10m performance drops
    significantly. But notice, if we could get the antenna up to 100ft
    the 10m performance would recover again because the elevation of its
    second vertical lobe would begin to match the high arrival-angles on
    this path.

That chart assumes flat ground around the antenna, but you can do 
exactly the same thing using HFTA for your particular site characteristics.

Of course, it's long-term statistical data - but isn't that what you 
need when making choices about a long-term investment like a tower?

Having said that, it's also very important to look at the detailed 
"mapping" between the antenna elevation response and the 
Angle-of-Arrival statistics - the sort of charts shown in Section 1 on 
that page; sometimes it might be more important to avoid a null falling 
at an elevation where there is a significant chance of signals arriving, 
than to squeeze the last fraction of a dB out of the long-term Figure of 
Merit!

73,
Steve G3TXQ



On 02/03/2012 23:50, Frank wrote:
> I guess I did not fully understand what HFTA was all about.
>
> Now that I do, I am not convinced it is entirely useful, at least not to
> me since I don't have the resources to raise or lower my antennas at
> will.  I just raise them up to 1/4 wavelength at the lowest  frequency
> of interest and hope for the best.
>
> So far that has worked for me.
>