One way to understand the gain, bandwidth, F/B, and SWR trade offs is to
play with a design with AutoEZ by AC6LA using the optimizer. Then set
the weighting of the criteria at different frequencies of interest.
Then have it optimize. Repeat with different weights and frequencies.
I've done this with a number of my designs using NEC4.2/EznecPro and
setting many variables to optimize. Many useful designs can be
generated, what I "best" is a matter of what you want. I have learned
there is no free lunch.
One good place to start is with published designs and see how they can
be made "ideal" for your specific needs. There are a number of optimized
designs on various web sites by home brew designers/builders.
How the "old-timers" managed to optimize must have had a pretty
painstaking learning curve. Now the PC "super computer" on your desktop
can do several hundreds of 10 variable trade off calculations in a few
minutes. AutoEZ uses a sophisticated algorithm to find a global
optimum, but it still possible to optimize to a local one, as with any
optimizer.
I have great respect for those that have created successful multi-band
interlaced designs. Those makers who share their design and NEC4
results are the ones I would chose when buying an antenna. As with any
commercial product there were trade offs to have the design appeal to
the largest audience, however marketing discerns "appeal".
Grant KZ1W
On 5/23/2021 13:31, Mike Zak wrote:
Is there a paper, article, YouTube video, Powerpoint deck, chapter in a book,
website, message-in-a-bottle, or anything else, that clearly and succinctly
describes the trade-offs and interactions between yagi gain, pattern, and match?
A 15 minute session with EZNEC, and a candidate yagi design, is plenty adequate for showing that
there’s some serious systems engineering involved in coming up with the right set of
trade-offs, starting with “which problem are we trying to solve?” But few of us are
ever going to capture the ideal design and then fabricate it. So maybe if there was some insight
around these tradeoffs we could all do a better job with our selections of commercially available
products.
Did W4RNL ever do anything along these lines? Or W2PV? How about the Telrex
archives? Anyone have any suggestions for something that is readable by mere
mortals?
Mike, W1MU
From: TowerTalk <towertalk-bounces@contesting.com> on behalf of Jim Brown
<jim@audiosystemsgroup.com>
Reply-To: "jim@audiosystemsgroup.com" <jim@audiosystemsgroup.com>
Date: Sunday, May 23, 2021 at 3:11 PM
To: "towertalk@contesting.com" <towertalk@contesting.com>
Subject: Re: [TowerTalk] OptiBeam OB2-40M
YES! SWR is NOT an indicator of antenna performance.
73, Jim K9YC
On 5/23/2021 9:40 AM, Richard (Rick) Karlquist wrote:
On 5/23/2021 7:29 AM, VE6WZ_Steve wrote:
I have received a few direct email questions about this thread…..
Just to be clear, the reason for inductor switching at the Yagi
elements is NOT only to get a “good, low SWR" match at the driver.
The “SWR” and “matching” at the feedpoint is really NOT the issue.
The real issue is tuning the REFLECTOR across the band. That is what
determines the gain and F/B performance.
The PARASITIC element needs to be incremented across the band to
maintain maximum gain and F/B performance.
Yagi SWR has very little to do with performance.
Steve, ve6wz
Another great posting from Steve. It seems like this point
has to be constantly repeated on TT to counteract marketing
hype and wishful thinking. When I first got my MonstIR,
I did some experiments where I set the tuning for 40 CW
and then QSY'ed to 40 phone, without retuning the MonstIR.
Of course the SWR went way up. I then used the custom
tuning feature to tweak just the driven element for SWR.
This did improve the VSWR somewhat, but the gain and F/B
were still lousy. SWR bandwidth doesn't equal pattern
bandwidth. And the MonstIR is a full size 3 element
design (yes, 70 foot elements), so it will tend to do
better than a shortened 2 element.
Rick N6RK
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