[TowerTalk] tuning the beam

[T A RUSSELL]

Yep,  build and install works pretty well for most commercial antennas.

For those who like to tinker,  hanging the beam with the boom
pointing straight up  is a good technique.  There was a piece in
QST and possibly a handbook which related distance between
the ground and the reflector to prevent ground interaction.  

Unfortunately, I do not recall where I saw it.  Basically, the better
the antenna F/B ratio, the less the spacing required to prevent
detuning by the ground when hanging vertically.  It is fairly easy
to rig this by tying a rope on a tower, up 50 ft or so, pulling it out
from the tower and attaching the director end of the beam to the
rope.  Adjust as necessary to get the desired elevation.

My vague recollection is that with good F/B (>20 db) only a few 
feet of elevation of the reflector was necessary.  With moderate 
F/B (>10 db) the reflector needed to be 10 ft off the ground.

These recollections go way back so it would be good to verify
the numbers for yourself for serious antenna tuning.

SWR measurements with the antenna only a few feet above ground
and horizontally polarized are generally meaningless.

de  Tom  N4KG
............................


On Sat, 22 Aug 1998 21:07:44 -0400 "Joe Natale" <k1jn@99main.com> writes:
>
>I recently posted a question regarding tuning a triband beam prior to
>mounting it on the tower. I built it on a mast about four feet above 
>the
>ground. I wanted to know if I should try tuning it at four feet before
>mounting it. The antenna is a Cushcraft A4S with the 40m add on kit. 
>Thanks
>to all that replied. Answers fell into three categories: 1-tune it on 
>the
>mast at four feet where I could easily reach it, 2- turn it facing 
>straight
>up with director just off the ground and tune it, and 3- just build it 
>to
>Cushcraft specs and mount it on the tower without tuning. I tried 
>number 1,
>and got OK readings on 10 and 15; 20 and 40 were way low (out of 
>band).  I
>tried adjusting the antenna without much change. I didn't try number 
>two, as
>I couldn't turn it up alone. I reset it to the specs in the manual and 
>today
>we put it on the tower at 75'.
>To my pleasant amazement, the 2:1 SWR points were 10m 28.0 to 29.4, 
>15m 21.0
>to 21.450, 20m 14.0 to 14.350 and 40m 7.080 to 7.220. Guess the number 
>three guys were right!
>

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