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Element spacing
- Thread starter spongbob
- Start date
Why did you lengthen the boom to 40 feet? What did you base that length on and what were you hoping to gain from it? You don't pick a boom length and decide later how to space the elements. You first DESIGN an antenna with proper element spacing and THEN add the spacings to find out how long your boom needs to be. You kind of put the cart before the horse.
More information is required!
What are you trying to achieve, more gain? more bandwidth? better front to back? Are you wanting to make a m108 out of a m107?
That small increase in length would make 3/5th of bugger all difference in gain. Rule of thumb is double the boom length for 3db.You could improve F to B with some re spacing though..
What are you trying to achieve, more gain? more bandwidth? better front to back? Are you wanting to make a m108 out of a m107?
That small increase in length would make 3/5th of bugger all difference in gain. Rule of thumb is double the boom length for 3db.You could improve F to B with some re spacing though..
OK I dont think you understand how complex an issue this is . Get it right and you might get 1/2 db improvement. but get it wrong and you could see 3db decrease or more.
You just cant add length some place and expect it to work Element length and spacing's are all interrelated and critical to the performance of the antenna . You can optimize for Highest gain , front to back , band width.or feedpoint impedance but not all(all are affected but never occur at the same settings )You can get hundreds of different designs depending on what you want to achieve .
I suggest you download this program http://www.yagicad.com/yagicad/YagiCAD.htm
it's free
Put in your current dimensions and try changing things one length at a time and watch what happens .Try moving the last director out first and change it's length.
Sory I dont have time to run this myself and give you a more conclusive answer.
You just cant add length some place and expect it to work Element length and spacing's are all interrelated and critical to the performance of the antenna . You can optimize for Highest gain , front to back , band width.or feedpoint impedance but not all(all are affected but never occur at the same settings )You can get hundreds of different designs depending on what you want to achieve .
I suggest you download this program http://www.yagicad.com/yagicad/YagiCAD.htm
it's free
Put in your current dimensions and try changing things one length at a time and watch what happens .Try moving the last director out first and change it's length.
Sory I dont have time to run this myself and give you a more conclusive answer.
More wind drag is about the only difference you will see with an additional 3 feet 5 inches of length.
Did a quick run up of the figures. if you add 10% to the existing spacings you will get an increase in signal of 0.2 of a db and an improvement of 5db in the front to back.with little change to the match .A bit of optimizing you might get a little more. Like I said in the first post 3/5th of bugger all. off course 5db improvement in the f to b can be useful to some.
If you push out just the last director to 40ft You gain only 0.07 of a DB and get 1.1 db improvment on the F to B. But the match goes out the window.(z drops about 10) and you will need to re tune.
If you push out just the last director to 40ft You gain only 0.07 of a DB and get 1.1 db improvment on the F to B. But the match goes out the window.(z drops about 10) and you will need to re tune.
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Exactly. I previously had a four element 11M quad on a 16 ft. boom, when I recently put it back up I went with a 20 ft. boom. After all the re-tuning was said and done, I now have the same antenna on a 20 ft. boom, no noticeable difference performance-wise.
Most people fail to realize that small changes to an antenna result in small differences. If it was that easy to add 4 feet and make a noticeable difference the manufacturer would have done it already.
even increasing(or deceasing) element diameter has a huge effect on gain/f-b ratio/lobe(huge= a couple db in some types of antennas ..in my case a owa beam) ...
only need to do that in say the program which vkrules (and myself) uses to see that (without changing other parameters )
the guy with the 16ft quad increased to 20ft ..it would have you just cant see it yet
only need to do that in say the program which vkrules (and myself) uses to see that (without changing other parameters )
the guy with the 16ft quad increased to 20ft ..it would have you just cant see it yet
Funny you say that about the OWA antennas. G0KSC says on his site www.g0ksc.co.uk that small changes to the antenn design result in small differences in performance thus making the antennas non-critical for demensions. I use a six element OWA on 6m and with the close spacing of the reflector driven and first director element smallish changes do show results but not huge differences however changing the fourth director on the end has very little effect on gain and no noticeable effect on the impedance.
Without a doubt something has changed, common sense dictates that changing the antenna's dimensions has to have some effect on performance. Theory would suggest an increase in gain due to the longer boom length. It's not that the antenna was a lump of coal before, and now it's a diamond due to a 4 ft. increase in boom length. It was a solid performer at 16 ft., as it is now at it's increased length. All I'm saying is that I'm unable to detect any appreciable difference in performance between the two boom lengths. Also there's the time span of a few years between the two configurations, so the exact the performance characteristics of the shorter version could be somewhat hazy.
Well size does matter. 16ft to 20ft is a bigger increase than 36 to 40. But like you said you wont notice it without direct comparison. Owa designs are more forgiving ,as are designs that use wide regular spacing.
Cb beams tend to be notoriously short for the no of elements. Even the 40ft Maco 8el is a "short" yagi
Seriously download any of the yagi programs it's an eye opener
Cb beams tend to be notoriously short for the no of elements. Even the 40ft Maco 8el is a "short" yagi
Seriously download any of the yagi programs it's an eye opener
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Spongbob if you have already lengthened the boom. my suggestion is to just increase existing spacings
These work.
ref 0
drv 91 inch
dir1 136.5
dir2 184
dir3 273
dir4 365
dir5 480
I would also shorten the reflector about 2 inches. cleans up the rear side lobes a bit and gives you a bit more gain ,but it does cost you some of the Front to back we just gained.
If you haven't don't bother. With these changers we are only up 0.4db
These work.
ref 0
drv 91 inch
dir1 136.5
dir2 184
dir3 273
dir4 365
dir5 480
I would also shorten the reflector about 2 inches. cleans up the rear side lobes a bit and gives you a bit more gain ,but it does cost you some of the Front to back we just gained.
If you haven't don't bother. With these changers we are only up 0.4db
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And that right there is the crux of the issue. Lots of effort for extremely little in return. If it made a few dB difference then that is another matter. Keep in mind also that you can optimize an antenna for maximum SWR bandwidth, forward gain, or front/back ratio but only one of them at a time. Any gain in one of those comes at the expense of the other two.
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