On Sat, 2 Oct 1999 wa4dou@juno.com wrote:
>compromise of characteristics that i'm looking for in a yagi. But the
>Tennadyne T-6 has a lot going for it too! I do remain skeptical about the
>gain claim of 5.1 dbd. But tonight i am reminded that ,since no one has
>"defined" dbd as it relates to the Tennadyne T-6(and Tennadyne products
>in general), i can't be sure that the reference standard isn't a dipole
>in free space as opposed to a dipole at a given height above ground in
>the "real world." My own thinking on that subject leads me to believe
>that its gain probably is more like 3-3.5 dbd of gain, with the "real
>world" dipole as the standard.
I can report what K9LA reported in the modeling graphs he presented in his
article in the latest (V. 6) of the Antenna Compendium series. On 20, the
gain looks to be 4-4.5 dBi free space. On 18, a bit over 5 dBi. 15
meters looks to be in the high 5's (5.5-5.75 dBi). Likewise for 12
meters. On 10, the gain goes rapidly from just below 6 dBi to just above
it, but there is an SWR cut-off slightly below 29 MHz. This is subject to
the limitation of squinting at a very small 1-column-wide graph covering
14-30 MHz.
I cannot vouch for the accuracy of the model relative to the original,
since I do not have detailed mechanical and electrical specifications on
the T-6. As well, I have a couple of questions about the model with
respect to NEC practices--for example, whether the TL transmission lines
between elements are well-centered, since the model specifies 10 segments
per element, whereas centering normally requires an odd number of segments
per wire in NEC.
Nevertheless, the numbers are not dissimilar to some I have found in my
own models of some LPDAs that show up in handbooks that have either a
similar number of elements or a similar boomlength.
I have seen some good models of LPDAs for 20 through 10 meters using about
a dozen elements and a 30' boom with gain about 7 to 7.1 dBi free space on
all but 10 meters, where it varies from 6.5 to 6.7 dBi. For free space
gain, subtract 2.15 dB to get dBd. The resulting number should hold up
for dipoles vs. other antenna types over ground at the same height and
elevation angle within 10%.
For reference, a good 2-element driver-reflector Yagi can be designed for
monoband use with a gain of about 6.1-6.3 dBi free space with about 1/8 wl
spacing. 3-element monoband Yagis with short booms can achieve 7 dBi free
space gain, while long boom versions can average a tad over 8 dBi free
space gain. Short means about 8' on 10 meters and long means about 12' on
10 meters--and you can scale from there to other bands. Both have 20 dBi
F-B as the design factor that limits gain. Let the F-B go to pot, and you
can get more gain from a 3-element Yagi. Once more, subtract about
2.15 from the dBi to get free space dBd. Thought these comparators might
be useful.
These notes are based on free space models and have nothing to do with any
maker's claims of performance, or the methods by which they might justify
those claims.
Hope this is useful.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / Tel: (423) 938-6335
1434 High Mesa Drive / \/ \/\ ----/\---
Knoxville, Tennessee /\ \ \ \ / / || / http://www.cebik.com
37938-4443 USA / \ \ \ \ || e-mail: cebik@utk.edu
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