AlmostHandy
Member
Is there a limit to the number of director elements a Yagi antenna has?
A Yagi Question
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The limits are dictated by size, weight, mechanical stability, cost, performance requirements, etc. I've never read anything to indicate that there are electrical limits, but I'm sure that at some point, it's just not cost effective or practical to add another element.
The only limit is the length of the boom.... and the ability of the supporting structure to stay up in the air. Throw a couple hundred on.
I don't know the practical limit off the top of my head, but there is a point of diminishing return. When yagis are made, the people who build them usually take into account how much gain they need, how long of a boom they can reasonably use, the weight/wind load of the whole assembly, and of course the cost.
You may see yagis with a half dozen elements on a fairly short boom to around 24 elements on a long boom (for the diehards requiring high performance). But again, you have a diminishing return after a certain number - any after that usually add more cost, length to the boom, etc for less of a db gain and no improvement in directivity.
Mike
hoser147
Member
thinking about your next build?
Here is an example of a long Yagi with numerous elements. As you can see it requires a slightly different mounting design.
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AlmostHandy
Member
lol yup.
I wonder if I can make a yagi for Wifi. Could I use #12 Solid coper wire with an 8 foot long piece of copper pipe? The reflector is only 69mm long, so I have some ideas about suspending the whole thing inside of a piece of 6 inch drainage pipe. Would that work, or would PVC attenuate the signal.
The calc I'm using says that you can bond the elements directly to the boom, and refers to them as parasitic. I haven't been able to find any information about this type of element/boom connection on any of the homebrew pages. None of the amateurs even mention bonding the directors to the boom. Is there some reason that insulating is better? Better gain? Better aperture? Better SWR?
N
N_Jay
Guest
Why don't you go the other way?
What is it you want to do?
What gain is required?
THEN
Decide what antenna provides that gain best.
Yes, I know.
Doing things ion the logical sequence is out of character for this forum.
LOL. But you speak the truth!
"Is there a limit to the number of director elements a Yagi antenna has?"
In a word, no, the more the merrier. Well, at least until it falls on it's nose because you didn't provide proper support. (;->)
In a word, no, the more the merrier. Well, at least until it falls on it's nose because you didn't provide proper support. (;->)
AlmostHandy
Member
rofl! I know, I know. I should try to do it the right way. It's just really fun to build them. I think I almost enjoy making antennas as much as I like to use them.
Now I have to figure out who I want to listen to.
N
N_Jay
Guest
ERICMYERS
Member
You can use wire and water pipe, but it's going to be massively overbuilt boom and flimsy elements if you do.
Yes, you can bond reflectors and directors to the boom (or not). You insulate the driven element from the boom though if you use a metallic boom.
My most recent one was from 1/4 inch brass rod boom, 1/8 inch brass rod reflector/directors, copper wire driven element shielded from the boom with shrinktube. tuned to 774 mhz, with the elements actually cut carefully to as close to 1/64th inch as I can be with my measuring tools and dremel. Frankly, I think I wasted a lot of time trying to be that precise. It's just under 3 feet long, and it's heavy and floppy for an end mount. It's not going to be mounted outside, so I'm not worried about its physical ability to withstand the weather.
Next one will be PVC or aluminum boom. Cheaper, stiffer, and 1/4 inch brass rod is difficult to drill precisely (not impossible, just slow and aggravating and very little margin for error) even with a drill press.
It does look cool though.
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One of the best, actually probably THE best 2.4 Ghz antenna I ever made was a helical. You can find design plans by searching google. There are numerous ways to make one but I wanted something light weight and rugged. I used a clear plastic tube that goes over florescent light tubes to protect them from shattering on people. I think it was a $1 at home depot complete with nice end caps. I cut it down to around a foot and a half. I got a roll of that sticky copper tape used for stained glass projects and wound it on the tube in the proper spiral and spacing. A small metal disc was placed on the back with a BNC bulkhead. It was connected with a small flat piece of metal at the base for the matching tab and that was all there was to it. It worked incredibly well for all 2.4 Ghz signals. It was used to intercept microwave video and outperformed an Antennex 2' tall vertical. The best part is that it was circulary polarized so there wasn't as much loss in that regard. I still have it on a shelf at work but the back plane broke off. I will try to remember to take a pic of what's left to give you an idea in case you decide to mess around with that type of design someday.
AlmostHandy
Member
Good point. If the directors are going to be >70mm long, the boom only needs to be 6-10mm thick, max. I can easily keep the boom >1000mm long total, so it doesn't need to support a lot of weight.
What is the difference between isolating the directors from the boom, as opposed to connecting them directly to the boom? Does it matter?
nycrich
Member
In easy laymans term , the gain curve will level off to the point that more elements hardly makes a difference and the mechanical structure to support it well be impractical. Also the beamwidth will be very narrow and directional making it very difficult to pinpoint the source if you are not in a fixed position( as opposed to using a rotor). You will also start to pick up interference from close frequencies that are further away and on the same frequency in another area( Eg. another state) in that direction
prcguy
Member
A crude way to figure how the number of elements affects a Yag is is every time you double the number of elements you will increase the gain about 3dB. This depends on element spacing and other factors but should put things into perspective. If you have a 10 element Yagi, adding 2 more element is not worth the hassle. If you have 20 elements its going to take 40 to make it worth considering.
prcguy
prcguy
k8tmk
Member
I built an 800 MHz yagi from a construction article in Pop Comm about a year ago. I used welding rod and a wooden boom (my next one will use an metal boom). I believe the article provided plans for three different options concerning the number of directors. Element dimensions were provided for each option. I believe the article was in the June or July 2007 issue, but I'm not sure now.
Anyway, The darm thing was side mounted on a TV tower and it works great!
Randy, K8TMK
Anyway, The darm thing was side mounted on a TV tower and it works great!
Randy, K8TMK
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