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Re: [TowerTalk] Measuring resonance of a yagi element

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Measuring resonance of a yagi element
From: jimlux <jimlux@earthlink.net>
Date: Wed, 22 Apr 2020 08:34:23 -0700
List-post: <mailto:towertalk@contesting.com>
On 4/22/20 8:00 AM, Grant Saviers wrote:
Good point.  Modeling Yagis and optimizing with Auto-EZ is instructive in how much one change changes a lot else.  Watching the automatic optimizer try to find the several dimensions of target values shows this.  Having designed a two band interlaced 30/17m I gained enormous respect for those designers with proven performance multi-band designs. A lot of hours at the PC.  Plus I wonder about how designers got it right in the "old days".  My solution to the mount modeling problem is all insulated elements.


In the "old days" people would make all sorts of claims, regardless of whether the antenna worked well or not. If the sunspot number is >200, given 24 hours you can probably "work the world", and things like crummy F/B or gain that's 4 dB less than you think don't make a lot a difference.

That's why ARRL started requiring either range measurements or a model in ads that claim gain.

Beyond that, lots, and lots, of cut and try. If it's your *job*, e.g. at NBS, in pre-computer days, you've got some theory to work with, so you've got analytical equations for mutual impedance as a starting point (and tables for the Si() and Co() integrals you need).

Then, you make up a test matrix and systematically vary lengths and spacing and make lots of measurements. That's what a "good" antenna range has over a "not so good" - the ability to run the AUT up and down quickly. If you can run an azimuth pattern in, say, 10-15 minutes, and then change lengths in 10 minutes with another 5 minutes to run up, and 5 minutes to bring down, you can get a dozen tests done in a day.

A couple months later, you'll have the data you need to produce an "optimized" design.

You'll have previous knowledge of the range, so you know how high to put the AUT so that the "ground reflection" effects are known. At JPL, the two longest ranges (1200 and 3000 ft) that are regularly used are over a canyon between the transmitter and receiver and both have adjustments of the height of both ends. But still, there are anomalies when someone piles a bunch of steel light poles in a flat spot half way between the end points.
https://mesa.jpl.nasa.gov/1200_Foot_Range/index.html

We haven't done many HF antenna measurements on the JPL range, though. UHF (400 MHz) and up, for the most part.


One can see why modeling is popular.




Grant KZ1W

On 4/22/2020 06:38, Chuck Dietz wrote:
Hi Tom,
I don't think you are taking the right approach. I don't think you can just
"tune" one element of a Yagi antenna. You need to measure all the element
sections and placements including the element mounts and run them through a
computer design program. Changing any dimension will affect the other
dimensions to some degree. If it is a commercially sold Yagi, the file will
already be available and can be used to optimize the antenna.

Chuck W5PR

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