[TowerTalk] MODEL FOR TOWER

[Grant Saviers]

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 at yahoo.com>
> To: towertalk at 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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