Yagi director element position question

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kruser

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Hi,
I have a 12 element 850 MHz yagi made by Maxrad. I can no longer read the model number.
It is a typical yagi with a mechanical gamma match for tuning the driven element.
It has a single reflector that is the longest element and then a slightly shorter driven element and then a set of 10 director elements.
But... I was looking at this thing the other day while working on the rotor and noticed that the first two director elements are longer then the remaining directors. Directors 3 and 4 are both shorter then director 1 and 2 and 5 thru 8. I call the reflector element 12 and the other end of the boom furthest from the reflector and driven would be element 1.

I think I've seen a yagi design like this but I'm not really sure. I know in many yagi designs for UHF and higher that the directors are of equal length on some designs or they get shorter as they get further from the driven element (not as drastic as a log periodic though). What do you guys think, is maybe this antenna assembled wrong and I should swap the elements so the shortest is at the front of the boom? Or does this arrangement have a benefit that I'm unaware of like maybe narrowing the beam width or something?
It can be easily corrected other then the lowering of the antenna part as it is all bolted solid aluminum and stainless construction and all machined fasteners.
It just seems odd that the first two directors are longer then the next 8 behind them.

I purchased the antenna brand new from a local 2 way shop and it came unopened from Maxrad back in its day so you would think it is by design. I wish I could reach it from the roof but cannot. So swapping the elements just for tests is kind of hard and that is why I figured I'd ask here first. Once I get it down then the elements are easily swapped but I'm then too low for any type of accurate comparison testing. The antenna works but never did seem to have a tight beam width like I've seen with other similar designs. It seems to have many side lobes were I can still peak the signal so it is hard to use this one for nulling out a tower when monitoring a simulcast system as it is acting more like an omni antenna.

Opinions please! Should I swap the director elements and put them in order of shortest to longest back to the driven element or leave it as is?

The reflector (element 12) measures 7-1/4 long. The Driven element (11) is 6-1/2 inches long, Director #10 is 6 inches, Director #9 is 5-3/8 inches and directors #8 through #5 are all equal at 5 inches in length.
Director #4 is 4-15/16 inches, director #3 is 4-7/8 inches, director #2 is 5-1/4 inches and the first director (#1) is 5-5/16 inches.

Spacing between all elements is equal at 3-1/2 inches except between the reflector and driven which has a shorter 2-7/8 spacing.

I would think the shortest director should go in position 1 and then get longer as it progress's back to the driven element.
Thoughts please.

I'd take a picture but I don't think I could get at a decent angle were the longer elements would show up. It is hard to notice it by eye alone due to the height. Maybe if I can get a decent background behind it somehow they would show in a picture.
 

kruser

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What you describe is normal for a large Yagi and I would not touch the antenna unless you have computer modeling software and an antenna range to check the results.
prcguy
That is why I asked! Much easier then pulling it down to swap the elements and have to put it way back up again just to find it did nothing or made the antenna perform worse.
If I could reach it from a ladder then I may have tried since swapping the elements can be done in minutes with a 5/16 nut driver only.

Anyway, since I made the post, I pulled out some Antenex and Laird branded yagi's. They are both direct end feed models but like you said, they both have a longer director all the way out at the far end. I can't believe I've never noticed this before after all these years. Must need glasses :evil:
Of the two I just pulled out, the longer length is barely perceptible to the naked eye but a tape measure sure reveals it. So thanks for confirming my thoughts that it is correct and not an assembly error. Heck, I paid dearly for the 12 element years ago and surely Maxrad would not have assembled it incorrectly :)
 

k8tmk

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You could have built an outstanding 10-element yagi for pocket change per an article in May 2007 PopComm. Very easy to build, and works great!

Randy
 

k9rzz

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What you describe is normal for a large Yagi and I would not touch the antenna unless you have computer modeling software and an antenna range to check the results.
prcguy
+1

Some of the directors are longer because it was made that way!
 
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