Yagi antenna boom thickness, element length, and element diameter - Page 1
Forum section: electronics / rf-microwave

author: tkamiya
author_rank: Super Contributor
timestamp: September 08, 2020, 05:43:14 am
content: I've been curious about this since I saw 2.4GHz Yagi antennas on Amazon like this one:https://www.amazon.com/Tupavco-TP512-Wi-Fi-Antenna-2-4GHz/dp/B008Z4DNFC/ref=sr_1_3?dchild=1&keywords=2.4ghz+yagi&qid=1599543123&sr=8-3There are many brands and models available, but common theme on all of these is that elements are quite stubby and considerable amount of element is INSIDE conductive boom. In some cases, QUARTER of element length is inside the boom. Typically, length of element is measured from tip-to-tip but when that much is inside a conductive boom, I don't know what would happen.My experience on Yagi is mostly for HF. I've built 21MHz 4 element, 50MHz 3 element, and various adaptations of similar configurations. But my understanding was, element diameters needs to be sufficiently small compared to length. Otherwise, Q will be too low making resonance shallow, diminished gain, dialectic factor (wrong term) will be low making ratio even worse, and effects of boom will not be negligible. If I were to make one, I'd use something like 2mm rod for elements and non conductive material for boom.On the one I gave a link above, spec says it has 30 degree horizontal and 25 degree vertical beam width. (I would guess 6db point?) That seems rather wide for 13 element yagi. On HF, 5 element would have something similar to it. I wonder if this is the effect of above sizing issue? Also, FB ratio is only 16dB.... 3 element yagi on HF would be better than that.Has anyone tried these antennas? I'd rather not buy one just to see if it works. I don't have an antenna range to properly test it. I'm just curious. I don't need one as I only have limited use for wifi even in my house.

author: Whales
author_rank: Super Contributor
timestamp: September 08, 2020, 08:12:54 am
content: I remember seeing some opinions that the commercial wifi-yagis are not any good or "can't" be any good. The size of the elements does look a bit comical. I'm also eager to see test results if anyone finds any.I did end up trying to make my own at one point, but I found helicals much easier/better. Lots of pics and info athttps://halestrom.net/darksleep/blog/032_helix_wifi/, make sure to check the comments too.Failed yagi (pictured in non-intended use location):Working helical:Yagis need balanced feed (unless I'm missing some clever tricks), so you need a balun or something similar to get them working with a wifi card's unbalanced coax output. I wonder what the item you link to does to fix this? It's clearly an unbal coax connector, but there is a box where it feeds the driven element, so maybe magic is hidden in there.EDIT: Since writing this article I'm come across more stuff about people using high gain antennas at too high power (in built up areas like cities too, making it worse). Obligatory: please don't use a directional antenna like anything linked to in this thread without first reading up on what problems it can cause neighbours.

author: tkamiya
author_rank: Super Contributor
timestamp: September 08, 2020, 08:53:07 am
content: You can feed the drive element of Yagi using Gamma-match. The radiation pattern will shift just a little to the direction of the matching stub. But it will take care of balanced/unbalanced issue.Balanced/unbalanced issue is actually not that big of a deal. I built plenty of dipole/yagi/quad without one. You can also use ferrite choke to force this, too.Please see voltage balun / current balun elsewhere on the topic. It's too involved for me to explain it.

author: TheMG
author_rank: Frequent Contributor
timestamp: September 08, 2020, 01:18:14 pm
content: That's quite common for such commercial antennas. Yes, some performance is sacrificed, but in this case the mechanical robustness is more important, since the antenna is intended to be outdoor mounted and you don't want ice, hail, birds, etc bending the elements or boom.For the ultimate performance at these frequencies, you would normally use a parabolic antenna. Way better gain and directionality than a yagi.

author: tkamiya
author_rank: Super Contributor
timestamp: September 08, 2020, 01:52:07 pm
content: Or a corner reflector....I was just thinking, for some bands, there are antennas naturally suited than others. For example, making a Helix for HF band will be crazy. I used to see huge Log Periodic for HF on embassy grounds in 80s. Wow, they were BIG.I can see ruggedness requirement for commercial implementations. I've seen 800MHz Yagi that used solid materials and they were welded/braised together. It's just that proportion of 2.4GHz version is such that I'd doubt they even work. I may just buy one and try it out.

author: tkamiya
author_rank: Super Contributor
timestamp: September 08, 2020, 01:57:43 pm
content: To Whales....I'm not sure if using high power output and beam antennas are mutually exclusive. I'd think in unpopulated area, that may be just the right thing to do. On flip side, using lots of power and using omni-directional antenna will splatter in all directions, too.Of course, using just the right amount of ERP (effective radiated power) and consider locations and circumstances are the key even if law allows it.By the way, I just love your design. Using common components and getting performance out of it. I remember my own old days. Bamboo sticks and wires were my choice of materials. (Yes, I grew up in Asian country!) Once, I made a 2 element ZL special for 21MHz like that. Boom was a piece of wooden stud. After 6 months, it rotted and split in half!