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Reflector Element Length
- Thread starter spongbob
- Start date
The instruction I have show each side of the quad type reflector is to be 9'6" exactly, and it should be held within 1/16" inch.
SB, I built a homemade beam using a 190" long Moonraker boom, and it was the best performing beam I ever owned. It also used a yagi dipole element for the reflector...and it had great rejection.
Is your boom 190" long or a bit longer? I ask, because I ended up using the element spacing that this MR'r boom provided me, with its 67.5" - 55" - 67.5" dimensions...on a tapered boom.
I first tried an old Wilson Shooting Star no taper boom that was 192" inches long, but I never could get the match or performance adjusted like I wanted even using Maco's dimensions of the time. The old Wilson boom was straight and a bit shorter...from end to end with an internal support piece in the middle. This provided for some new ideas of the time for optimization.
So, I was using some hot shot optimized settings out from some magazines back then, but I could not get the beam to work right...like I wanted.
When I got the MR'r boom, I found it limited to where I could use the Wilson hardware, just like the MR'r hubs did...that is where only the ends of the MR'r boom could accommodate the hubs. I decided to try it anyway even though I would not be able to try and optimized it later.
I found this beam, with its more traditional spacing setup, worked much better for tune and performance...even though the elements sat on top of the boom instead of in the center like the Moonraker boom does.
If you are interested I will make an Eznec model of your exact dimensions, and maybe the results I will get you closer to how it might work in the real world.
And, if your idea does not fit with the idea I noted above I will understand, if you wish to use your original idea as your setup. I will build it your way. Maybe the model will help give you some clues.
Good luck...whatever you do.
The basis of a beam is the driven element, typically a 1/2 wave long and center fed, or a 1/2 wave dipole. If you add an element, a director or reflector, then the input impedance of that dipole/driven element is lowered. The more elements added causes that input impedance to be lowered even more. Additional elements may only cause a slight lowering, but the impedance still gets lower.
That means that with a multielement beam/yagi an impedance matching device of some kind is used to increase the input impedance to something closer to that desired 50 ohms impedance. A gamma match is probably the most common method of doing that.
- 'Doc
That means that with a multielement beam/yagi an impedance matching device of some kind is used to increase the input impedance to something closer to that desired 50 ohms impedance. A gamma match is probably the most common method of doing that.
- 'Doc
SB, I was not suggesting that you make changes to your antenna.
Try 'Doc's 5% longer recommendation for your reflector first. That is the type of information you asked for in your original post here.
In trying to setup a beam from scratch...I suggest you make only one change at a time, and for sure once you get close with the tune.
If you feel your beam is not where you want it to be. At the stage you're in now, with a decent match that is a little off the center from what you want, you will get off track if you start making multiple changes to your beam. You won't likely be able to tell what change did what.
Just a note:
Here shows the difference in the gamma I used on my beam and a Maco 5kw I have. There are notable differences.
I'm not suggesting the 5Kw won't work on your beam, but I found it would not get nearly as close as the stock gamma that came on the old Wilson in the picture.
I tried the 5Kw on my beam and it tuned OK, but for some reason I could not get it as close. Back when I did this however, there were no portable antenna analyzers available...so i don't know how close I could have gotten.
All I can think of is to try changing the length of the driven element just very slightly, shouldn't take more than '+/-' and inch or so. That would be a sure way of chaning SWR. You might also take a look at what may be close to that antenna (what it's close to) that may be affecting things and not a lot of trouble to change.
Sorry not much help...
- 'Doc
Sorry not much help...
- 'Doc
Just try lengthening the driven element a little and see what happens. Changing the spacing and or element lengths with change the match as well as f to b , gain and resonance,as said earlier, change one thing at a time.+ 5% on the reflector will get you into the ball park but will not be optimal, particularly the Front to back ratio.
Just did a quick sweep of your measurements with yagicad ( using 3/4 inch diameter elements and 3 inch boom, just guessing) it works and is resonate . It does show the driven element is 1/2 short but that is beyond the accuracy of the program. If you make the reflector 18' 8" it shows a better Front to back.
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