Morrison,
Dean Straw is correct about the modeling of the Log-Yag for 20 meters:
about 6.7 dBi in free space. His recommendation that one choose a
"regular" Yagi as a more simply constructed and higher performing antenna
in this case is also correct.
To verify this, let me note a couple of sample Yagis for 20 from my files.
Both use 1" aluminum tubing and therefore would have to be adjusted for
stepped diameter construction, but they indicate what a 3-element monoband
Yagi might do on 20. All gain numbers are free space in dBi for direct
comparison with the Log-Yag.
The first design is for a Yagi having a direct 50-ohm feed with a low SWR
across the band:
Element Length Space from rear of antenna
Reflector 36' 0
Driven 33.4' 12.5'
Director 30.5' 22.54'
Performance
Frequency Gain dBi F-B dB Feed Z SWR (50-0hm)
14.0 7.0 21.3 49-j9 1.19
14.175 7.1 22.2 47-j0 1.07
14.35 7.3 20.8 43+j12 1.34
This 24'-boom antenna shows a smooth performance curve that varies only
insignificantly across the band. It surpasses the Log-Yag using only 3
elements and can accept a direct coax connection with no matching circuit.
However, its gain is far from the maximum for a 24-boom 3-element Yagi. On
the other hand, when scaled for 10 meters, this antenna permits direct coax
feed for nearly band-edge to band-edge performance. It is adapted from a
Bill Orr design.
The second design is for a Yagi more nearly (but not necessarily fully)
optimized for good gain and F-B:
Element Length Space from rear of antenna
Reflector 34.6' 0
Driven 33.0' 10.46'
Director 30.5' 22.54'
Performance
Frequency Gain dBi F-B dB Feed Z SWR (25-0hm)
14.0 7.9 20.8 27-j13 1.67
14.175 8.1 27.3 26-j1 1.05
14.35 8.4 17.7 23+j13 1.72
Note the steeper curve of gain and F-B, as well of the feed impedance of
this antenna. The SWR is cited for a 25-ohm reference, since, with a 2:1
match, this curve would likely be similar to the resultant 50-ohm curve.
Average gain is 1 dB better and average F-B is about the same (although F-B
varies more widely). A Beta, Gamma, or T match would all work, as would a
1/4 wl section of 35-ohm feedline (made from parallel 70-ohm cable).
For practical designs already set up for stepped diameter construction, see
K6STI's YA program, which I understand is still included with the 18th
edition of the Antenna Book.
In essence, the old Log-Yag design took the hard road to a good match when
all one required was some resizing and respacing of elements for a standard
Yagi to get a no-matching-circuit design.
If anyone with a modeling program wishes to see progress in Yagi design,
take some designs from older ARRL Handbooks and Antenna Books from the era
BL (before Lawson) and model their performance. Good home brew designs
were either luck or the product of infinite patience in moving things
around and retesting carefully. Nowadays, we just let the computer do the
work, while we warm up the hack saw to cut the tubing.
I hope the comparison is useful in getting a perspective on Yagi design
potentials.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://funnelweb.utcc.utk.edu/~cebik/radio.html
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