[Antennas] dbi for horz bi-square

Wed Oct 3 19:53:59 EDT 2007

At 03:51 PM 2007年10月03日, Sam Morgan KA5OAI wrote:
>I'm trying to setup PropNetPSK in preparations for this weekend and 
>one of the questions ask is the dbi of my antenna. It's a 190' 
>horizontal loop at a height of 20' It resonates @ about 5.310mhz so 
>If I'm running it on 30m it's a bi-square, I think? So what's it's dbi?

Sam,
Assuming it's fed at a corner and is over average ground, the maximum 
gain should be about 5 dBi gain at a takeoff angle of 51 degrees in 
both directions through the "side" corners (the corners 1/4 way 
around the loop from the feedpoint). Since this may not be the most 
useful takeoff angle for paths over 300 miles, it's worth looking at 
the pattern at say, a 20 degree elevation, where the peak gain is 
about 0 dBi, also bidirectional through the "side" corners. The gain 
at 20 degree elevation on a line through the feedpoint corner and the 
opposite corner is about -3 dBi. (At 10 degree elevation, those 
gains drop to about -6 dBi and -9 dBi.) Gain straight up is -1.7 dBi.
The two-wavelength resonance is about 10.65 MHz, where the impedance 
is near 90 ohms (assuming #14 bare copper wire).
BTW, a bi-square is normally oriented with the plane of the antenna 
vertical, but most importantly, the corner away from the feedpoint is 
an open circuit. The pattern is totally different than a closed loop 
and the feedpoint impedance is Hi-Z. If you opened the corner away 
from the feedpoint, the gain is mostly straight up, where it is about 
8.4 dBi. At 20 degree elevation, the gain is about -3 dBi in the 
direction opposite the feedpoint and -4.5 dBi toward the 
feedpoint. Off the sides the gain is about -10.5 dBi at 20 degree 
elevation. The feedpoint impedance is over 3000 ohms at 10.125 Mhz.
73, Terry N6RY