[TowerTalk] Tribander losses + 3L Yagi design
n4kg@juno.com
n4kg@juno.com
Tue, 18 Jul 2000 04:37:19 -0600
Tribander Performance vs. 3L Yagi - de Tom N4KG
An excellent characterization of the 3 element Yagi can be found
in figure 2.9 of YAGI ANTENNA DESIGN by W2PV.
Jim characterized 3, 4, 5, 6, 7 element monobanders with boom
lengths from 0.1 WL to 0.7 WL for 3L and out to 1.5 WL for 4
through 7 elements. Equal spacing was assumed for all elements
and tuning curves were plotted for reflector / director tuning
of +/- 2, 3, 4, 5, 6, 7 %. Plots of the 3, 4, and 6 element data
were included in his book.
Table 2.4 lists band center gain and F/B vs. boom length in
small increments, optimized for a 4 % bandwidth. Figure 2.12
shows gain vs. boom length for 3, 4, 5, 6, 7 elements.
"Optimum" performance (gain) for a 3L monobander occurs with
a boom length of .35 to .50 WL. Optimum F/B occurs with a
boom length of 0.25 to 0.30 WL.
For the 3L Yagi, boom lengths of .35 to .40 WL provide excellent
gain (8.0 to 8.5 dBi with a slight upward slope vs. frequency) and
moderate F/B. Peak gain can reach 9.5 dBi with +/-2 % parasitic
element tuning but the bandwidth is extremely narrow and there
is little tolerance for modeling error. With parasitic tuning of
+/- 4 or 5 %, 8.0 to 8.5 dBi can be achieved over a 4% bandwidth
which is plenty for 20 and 15 Meters and the DX portion of 10M.
My Telrex 3L20 exhibits 8.1 dBi at 14.0 and 8.5 dBi at 14.3 in
free space, almost identical to the 204BA gain curve. W2PV's
data is for equal spacing of the elements. As the DE is moved
towards the director, gain decreases but feed impedance increases.
As the DE is moved towards the reflector, gain increases but feed
impedance decreases.
Popular tribander boom lengths are 14 ft (A3, TA33, TH3), 18 ft
(A4, CL33, TA34, TH5), and 24 ft (PRO57, TA36, TH6, TH7).
BOOM 20M 15M 10M Peak 3L Gain (20, 15, 10M)
14 ft .2WL .3WL .4WL 7.0 7.5 8.5 dBi
18 ft .25WL .4WL .5WL 7.5 8.5 8.5 dBi
24 ft .35WL .5WL .7WL 8.5 9.0 9.5 (4L) dBi
Note that most 18 and 24 ft tribanders use a separate
reflector on 10M which reduces the effective boomlength
on that band.
It is probably safe to conclude that tribanders fall somewhat
short of the maximum achievable gains for 3L monobanders.
Gain-bandwidth seems to be compromised in short boom (14 ft)
trapped tribanders, especially old designs.
Note that the achievable gain for 2L Yagi's is 6.5 to 7.0 dBi,
which is quite competitive compared with short boom 3L
designs (.2 to .25 WL).
The TH6 and TH7 use a separate 15M director which also
reduces the 15M boomlenth to approximately 20 ft (.43WL)
which is still widespaced for 3L on 15M.
Another effect of the 15M director is to reduce gain on
10M by approximately 1 dB, thus reducing the maximum
achievable gain on 10M to 8.5 dBi. I verified this by modeling
a 4L 10 using the TH6 element spacing and then inserting
a 15M director at the appropriate position.
Comparison tests between my TH6 at 80 ft and Telrex 3L20
at 75 ft are virtually indistinguishable on 20M. The TH6 also
"plays well" on 15M when wave angles are low (band openings).
On 10M, my 5L10 Monobanders at 35 and 62 ft are noticably
better than my TH6 at 80 ft.
de Tom N4KG
# # # # # # # # #
On Tue, 18 Jul 2000 01:07:19 +0000 Pete Smith <n4zr@contesting.com>
writes:
>
> 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
>
>
________________________________________________________________
YOU'RE PAYING TOO MUCH FOR THE INTERNET!
Juno now offers FREE Internet Access!
Try it today - there's no risk! For your FREE software, visit:
http://dl.www.juno.com/get/tagj.
--
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com