Greg,
A couple of thoughts about your question and some observations
1. My 20m and 15m 5L OWA's are on 36" diameter rings on 24" face Rohn
65. The SWR curves are perfect. Mounted between D1 and D2.
2. My 10m 5L OWA is also mounted the same way and shows higher swr than
planned and the minimum is up 500KHz. I think there are two possible
reasons - 1. interaction with the ring or tower and 2. interaction with
the guy grip used on the Phillystran boom truss. My bet is the latter
since these yagis were previously side mounted and the Philly was
terminated with compression sleeves. I found this is ok in a factory
note. So someday I will change to sleeves and have more data.
3. The Wilson per its dimensions is not a modern OWA design. Further,
I'll venture an opinion that the tower placed between DE and D1
maximizes whatever is the interaction. My advice would be to redesign
as an OWA and then place the tower between D1 and D2.
4. There are some rules of thumb to deal with grounded element mounting
plates in the books (Lawson & Cebik?). They might work for big swage
diameter changes. My modeling of a swaged 80m dipole came out ok in
NEC4 without fudging.
5. One rule of thumb I've read for modeling is openings less than 1/10
wavelength look like solid sheet conductor. I think that is in the
EZNEC manual re making wire grids. So I don't think you need a
complicated lattice model since your openings are 1/20 WL or less. I
did model the Rohn 65 as a triangular solid sheet and found no
interactions with my antennas placed as described.
Grant KZ1W
On 4/26/2019 10:57 AM, Jim Thomson wrote:
Date: Thu, 25 Apr 2019 00:37:24 -0500
From: Greg Best <gregbestconsultant@yahoo.com>
To: towertalk@contesting.com
Subject: Re: MODEL FOR TOWER
<Thanks to you all for your questions, thoughts, and input. My original question was
about interaction with the tower and really asking the question as to whether someone
besides me has addressed the issue before. (Better to take advantage of work done
<before rather than recreate the wheel.). For your further consideration, the tower
face at 60 ft is about 30 inches and it is 3 sided. The mount would likely be fixed on
EU or a small robust sidearm with ?rotator on a post? realizing that I would not be
able to turn it <fully around. All of the bracing is an X.
<braces using 1.5 inchs angle steel above and below the 60 ft level. The spacing
between the DE & D1 is 106 inches, so I do have some room. The balance point is abt
63 inches from the DE. The 20 m beam is a Wilson 204CD. I am not familiar with the NEC
command to ?<mirror ? the wires. Is it ?copy? or something else?
<Tks. Greg N9GB
## If you are using the OEM wilson 204CD ele dimensions, and ele
spacings, they are 99% likely to be incorrect. I owned several wilson
yagis back in the day..and none of them worked to an optimum level. And
everybody else that I knew who owned any wilson yagi product stated the same
thing.
You folks that model this stuff in software, how do you factor in the
myriad of swages on each ele ??? How do you model the actual swaged
portion, where it jumps down 2 or more sizes of tubing. I once asked K6STI
about this when using several versions of his infamous YO program, and never
got a straight answer. Do you take the average or what? Considering the
sheer total number of swages used on a typ 20m yagi, like the wilson or
Hy-gain 204BA, I can see it being very easy to screw everything up while trying
to model an optimum electrical design.
## and how do you propose to tweak the ele lengths, and or spacings, to
factor in.... aka compensate for the 30 inch face width of his tower..at the
60 ft level ? He may as well use an optimum design for 60 ft above real
ground...mount it, and be done with it. The vertical legs of his tower will
have minimal to no effect on his 20m els..since the legs are at right angles to
the els. His tower uses X bracing, and based on my own past experience
using a 33 inch wide tower with X bracing..and side mounting a 20m yagi, their
will be no interaction, due to the tower. The X bracing is all at an angle...
its not horizontal. The effort should be put into modeling the old wilson
204CD..and coming up with better
ele dimensions...and spacings.
Jim VE7RF
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