[TowerTalk] Tribander losses

[Pete Smith]

For whatever it may be worth, following data were developed using NEC-2 and
carefully-converged models.  Interpretation is up to you guys:

1.  Following gain figures are for the Force 12 C-3.  Considering the very
small currents flowing in any elements but the ones specifically designed
for the band in use, I think it's fair to describe the C-3 as a 2-element
yagi on each of the 3 bands 20-10, with effective boomlengths of 11 ft on
20, 8 ft on 15, and ~6 feet on 10.  Gain in free space was:

20  6.84 dBi
15  6.46
10  6.22

Gain at 97 feet above normal ground was:

20 12.34 dBi
15 12.21 dBi
10 12.00 dBi  

Compare those numbers with the following, which reflect an idealized
TH-7DXX model for each band.  The effective boom length is 24 feet, and the
antenna has 4 elements on 20 and 15, and 5 elements on 10.  In each case,
elements 2 and 3 (from the back) are driven, with about 2 feet between
them.  In each case, no traps are included -- in effect, each antenna
represents the parts of the antenna that are "supposed" to be in use on a
given band, with perfectly cylindrical elements.  In my view, this probably
represents the best performance achievable from the TH-7, given perfect traps.

Free space:

20 7.58 dBi
15 8.06
10 8.36

97 feet up:

20 13.11 dBi
15 13.74
10 14.14

Note that the absolute differences between the two antennas remain very
similar from the free space case to the above-ground one - about .75 dB on
20, 1.6 dB on 15, and 2.1 on 10.  My horseback guess is that these results
show that the TH-7 is compromised toward 15 meter performance (a reasonable
thing to do), and that, in general, the results are not as good as one
would expect from a 24-foot boom.

Can anyone advise what gain should be expected from 3 monoband elements on
a 24-foot boom on 20 and 15, or 4 elements on 10 (assuming reasonable SWR
bandwidth and F/B performance across the band)?  

73, Pete Smith N4ZR

The World Contest Station Database 
is back up and running at
http://www.qsl.net/n4zr 



--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com