4x7jxx6 array on air since the end of October 2003
Elevation plot courtesy of K0GU
Array composed by 4 x 7JXX6 antennas
Extimated gain on free space 16.6 dbd (18.70
Extimated gain with ground gain 21.7 dbd
Center of the double "H" placed at 17 meter above ground
Horizontal spacing 7,50 meter
Vertical spacing 6,50 meter
· First lobe on H-plane at about 4,6°, second lobe at about 15° with
4,5 db of
· Secondary lobe on E-Plane at -18 db
· Average front to back ratio 22 db
The new array, completed at the end of October 2003, is strictly optimazed for EME traffic: the gain increased up to 16,6 dbd, the lower secondary lobes and the better front to back ratio should clearly improve the capability of the system for this peculiar kind of traffic (notice the polar plot above and the clean pattern of the array).
"double H" maintains the previous features with
just a little increase of the spacing. Infact I prefere to reduce the secondary
lobes to obtain a cleaner pattern even if this way cause a little degradation of
gain (less than 0,5 db): using a horizontal spacing of 8,5 meter and vertical
spacing of 7,20 meter I could achieve 17 dbd of gain, but the secondary lobes
would be too high and the internal noise of the array would not be optimal for EME traffic.
Moreover, limitating the spacing I can use the double H of the previous array
wich is quite light and robust. Due to the increased weight of the structure,
now weighting almost 200 Kg, I replaced the old
with a new stronger and compact
system powered by 380 volts current.
The elevetor piston is described here.
Here you can see a picture of a single antenna 7JXX6:
The prototype antenna is 9,40 meter long boom yagi, capable of 11,35 dbd of gain with 23 db average front to back ratio. It has excellent electrical performance and a return loss of 40 db at 50.130 MHz.
The following screenshot shows the strongest echo I ever get off the Moon with the new array: -7.9db when the Moon was only +0.7° above horizon and -10 db of degradation!.