[TowerTalk] Folded dipole

K4SAV RadioIR at charter.net
Tue Aug 25 15:05:10 PDT 2009

Maximum gain at 90 degrees elevation for a 40 meter dipole is at 27 ft 
above average ground.  Anything between 23 and 33 ft will be within 0.1 
dB of that.  It doesn't vary a lot with ground quality, although EZNEC 
thinks that 15 ft is the max gain point when over salt water.  That may 
be a simulation error (I don't know).  Some people may think max NVIS 
gain is achieved at a lower height because of a NEC simulation using a 
Mininec ground.  With that ground, EZNEC will give you a gain of 19.8 
dBi for a 40 meter dipole 1 foot above ground (due to massive simulation 

I don't think there is any justification for intentionally putting a 40 
meter dipole lower than about 23 ft.  Originally there was a reason, or 
so the story goes.  It doesn't apply to amateur radio however.  As you 
put the antenna at a lower height, the low angle lobes get attenuated 
more than the high angle lobe gets attenuated.  So when you don't want 
to be heard by the enemy at great distances, you put your antenna very 
low, then crank up the power until you can just barely maintain 
communications with the close friendly troops.  Actually cranking down 
the power is almost as much help as lowering the antenna.  If you go 
from 25 ft to 3 ft, the gain at 10 degrees elevation decreases by 12.4 
dB while the gain at 90 degrees elevation decreases by 10.4 dB (over 
average ground).

Maybe if you don't want to be heard by those unfriendly DX stations, 
this would be a good idea.

Jerry, K4SAV

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