[NLRS] Wait a sec - 1296 Rhombic

[Dr. Gerald N. Johnson, electrical engineer]

17dBi is nothing to sniff at for a small antenna. But an array of yagis
of equal length and breadth should have a clean pattern in the range of
24 to 27 dBi.

The noise temperature of a typical yagi is on the order of 25 to 40 K.
The noise temperature of the rhombic termination is 285K. Suppose half
its noise power radiates that still leaves a noise temperature from the
termination of 143K. Then there are ALL those lobes (something like 96 I
think I counted) with more power in them (while transmitting) than in
the main lobe, and so their noise contribution looking at the warm earth
is at least another 143K making the Rhombic antenna have a noise
temperature of about 285K, e.g. 3 db NF. At that point the .4 dB NF
receiver at the end of the .2 dB loss feed line is wasted by the noise
from the antenna.

Yes, wire and bamboo poles is cheap, just deafens the receiver with
noise. The 5.5 and 6.5 wavelength wires and their angles in the Laport
Rhombic are picky because he reduced the sidelobes (in E and H planes
only) by carefully aligning wire first sidelobes to cancel (thereby
quadrupling the total number of lobes and he didn't look at those lobes
up about 45 degrees and off the side about 45 degrees that are only 7 dB
down from the front lobe). That makes construction critical.

Lobes at 45 degrees up, down, right, left with 1 dB less total power
than straight ahead don't contribute to finding signals from odd
directions except the occasional satellite.

The rhombic can be effective as a transmitting antenna, but as a weak
signal receiving antenna its a waste of any good low noise radio. A
parabola, if fed for maximum gain, can be poor for G/T also because of
too much feed spillage seeing the ground.

Before liking the Rhombic, look at the entire 360 degree three
dimensional pattern. Looking only at E and H planes misses many of the
stronger oddly polarized side lobes that contribute all that receiver
noise and nothing to communications.

DL6WU yagi designs are fairly forgiving, not as picky as the classic
K2RIW yagi designs.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.