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2
's 20M5-125: The inside story
(Customizing for maximum advantage)
The design concept behind the 20M5 and the 20M5125 is to allow you to CUSTOMIZE the antenna
to your needs and location. The latest computer techniques have been employed to produce the
attached set of patterns and associated element spacing and lengths. The program used allows
optimization of the Yagi not only for specific bandwidths, gain and F/B, but also for any height
above ground and stacked pairs.
Look over the optimization choices carefully and make a note of the one that best suits your needs.
The assembly manual that follows has element tip and spacing settings for your choice. We are
sure that you will end up with an exceptional antenna and you will also learn more about Yagis in
the process.
The computer program used to generate the plots and optimize the Yagi is YO (Yagi Optimizer)
created by Brian Beezley, K6STI. The figures generated by the program have been checked
against NEC and generally are within a few hundredths of a dB.
You may be accustomed to seeing inflated numbers on spec sheets and in advertising. Don't
worry, the inflated numbers may be referenced to Isotropic (dBi), which is 2.14 dB higher than a
dipole reference (dBd), or including as much as 6 dB of 'GROUND GAIN'. Other possibilities
include errors or ''marketing" guys that simply decided that a higher number would sell more
antennas.
Actual realized gain, in the case of plots over perfect ground, may be somewhat less than the
figures indicates. Most of us don't have perfect ground. Not only does the quality of the ground
itself affect the actual number but, in most cases, the things above the ground. Houses, buildings
and heavy trees all eat up the part of your pattern needed to generate ground gain.
To put this in perspective take the case of a station using a 5 dBd tribander up 70 feet located in
open farmland of normal ground. The net gain at 13 degrees radiation angle is probably 10 dB.
Compare that station to the 8.6 dBd monobander at 70 feet in the middle of a housing tract with
perhaps only 2 dB of additional ground gain for a net of 10.6 dB. In a pile-up it would be a flip of a
coin who would get through first. However, reverse the situation and the monobander would be
ahead by miles.
Understanding this fact of life may not ease your frustration, but at least you can understand why
some small stations do quite well and why some big stations are almost unbeatable. The flexibility
of the 20M5 design at least allows you the get the most you can out of your QTH.
STACKING ANTENNAS for more gain can be most beneficial, however, some designs don't stack
very well. Pattern and gain don't come up to expectations. The YO program allows us to optimize
the gain and pattern for a stacked pair of 20M5125s. With the availability of rotating rings and
rotating tower sections, more and more Amateurs will be taking advantage of the benefits of
stacking. Think about it. The increased vertical capture area reduces fading and the compressed H
plane pattern increases the efficiency of the signal as it reflects off the ground and ionosphere be-
cause the energy is reflecting off these mediums at a more grazing angle. While many computer
programs don’t indicate a reduced angle of radiation for stacked Yagis, our results over the years
indicate a greatly reduced radiation angle for stacked arrays.
20M5-125 ASSEMBLY MANUAL
