WiFi antennas - seeking info.

[ElectricMirk]

Hey there EEVblog forum members .

So I've suddenly gotten into another tech project..  making WiFi antennas!. So far I've made ..

1x Cantenna (needs to be re-done)
1x Bi-Quad feed for sattelite dish, following this - http://martybugs.net/wireless/biquad/
2x Yagi antennas, 15 and 20 element following this - https://www.ab9il.net/wlan-projects/wifi6.html

And all work pretty damn good, epcept the cantenna  my record is the yagi picking up a WiFi signal 2.6km away (not useable signal tho.).

Now I am in the process of making a 3rd yagi, using this calculator - http://www.vk5dj.com/yagi.html

What I seek..

Many of the sites I found are partially working, maybe some are from 1998!, many of the calculators and programs too. Some designs I find are too simple and seem lacking, other sources seem only understandable to the one who wrote it.
So anyone got suggestions for sites and calculators/designers?.

Also, are there any affordable tools/meters for measuring and testing?. I've seen some for 10.000$ and one for 100$ so far(just tonight and yesterday), but totally lost about what options there are, usually I find what I seek quite fast.

A more basic question - maybe.
In some articles/totorials, etc. I've seen that sometimes the coax cable is essentially short-circuited and other times not.. I've heard one guy on youtube say it's very bad, but then also design himself many antennas where it is short-circuited. And I don't get it..

example.
On my Yagi antennas it is essentially short-circuited on the folded di-pole.. but in a Cantenna design it is seperated, also on the bi-quad feed I made for sattelite dish.
The director? elements on my yagi can all be connected together on metal rod, or they can be isolated..  what's the difference?.

[ElectricMirk]

Pictures needed ofcourse, makes things more interesting

[hagster]

Hi ElerctricMirk

Those antennas look pretty good. The online calculators for yagi antennas are generally ok and have a strong basis in empirical data. Also calculators for patch and helical antennas are fairly good.

I really recomend the free 4nec2 for simulating these designs before you build them. This will quickly tell you the kind of gain you can acheive. It also allows you to see the currents and predict SWR etc. You will learn more from playing with this than anything else.

Really good cheap measurement tools dont really exist. Things like the miniVNA or the N1201SA or 2101SA seem like the best of the cheap tools, but they are far from perfect. A spectrum analyser with tracking generator is also a good tool if you can afford it especially if you get an external directional coupler too. Its also worth getting a cheap SDR radio(sadly the very very cheap RTL dongles dont cover wifi bands).

I think I know the youtuber you are refering too. I would be very wary of the things he says. He sounds highly confident in his skills and knowledge, but lots of what he says is very wrong.

For really good info (without the RF superstitions) go to http://www.antenna-theory.com/m/index.php

As you seem to understand there is no problem with having a DC short on an antenna. Its the RF impedence that matters. A short looks like an open circuit a quarter wavelength away.

For a yagi the e-field should be in line with the elements. This creates a voltage difference between the two arms and hence current flows back and forth. There is no e-field(at least we dont want there to be) along the beam and hence no voltage difference exists and no current flows.

The classic antenna with a DC short is the PIFA antenna. With this you can tune the impedence by moving the feed towards of away from the short. By doing this we change the ratio of current to voltage seen by the feed (voltage would be zero at the short). As Z = V / I ,we can use this to tune the impedence(Z).

[ElectricMirk]

Hi ElerctricMirk

Those antennas look pretty good. The online calculators for yagi antennas are generally ok and have a strong basis in empirical data. Also calculators for patch and helical antennas are fairly good.

I really recomend the free 4nec2 for simulating these designs before you build them. This will quickly tell you the kind of gain you can acheive. It also allows you to see the currents and predict SWR etc. You will learn more from playing with this than anything else.

Really good cheap measurement tools dont really exist. Things like the miniVNA or the N1201SA or 2101SA seem like the best of the cheap tools, but they are far from perfect. A spectrum analyser with tracking generator is also a good tool if you can afford it especially if you get an external directional coupler too. Its also worth getting a cheap SDR radio(sadly the very very cheap RTL dongles dont cover wifi bands).

I think I know the youtuber you are refering too. I would be very wary of the things he says. He sounds highly confident in his skills and knowledge, but lots of what he says is very wrong.

For really good info (without the RF superstitions) go to http://www.antenna-theory.com/m/index.php

As you seem to understand there is no problem with having a DC short on an antenna. Its the RF impedence that matters. A short looks like an open circuit a quarter wavelength away.

For a yagi the e-field should be in line with the elements. This creates a voltage difference between the two arms and hence current flows back and forth. There is no e-field(at least we dont want there to be) along the beam and hence no voltage difference exists and no current flows.

The classic antenna with a DC short is the PIFA antenna. With this you can tune the impedence by moving the feed towards of away from the short. By doing this we change the ratio of current to voltage seen by the feed (voltage would be zero at the short). As Z = V / I ,we can use this to tune the impedence(Z).

Thanks .

I downloaded the program, perhaps it is a bit beyond me at the moment , but it looks really comprehensive and it works.

Right about the tools, it seems it's fairly easy and cheap to get meters for the lower frequencies, but WiFi is a different story.

[Lord of nothing]

Why do you build your own antenna when an chinesse is available that cheap?

[hagster]

Because a lot of the chineese antennas don't work(very well).

The antennas that do work often look identical to those that dont. Often with the exact same plastic mouldings.

This goes for the big yagis as well as the cheap monopoles and clover leafs.

The problem doesnt appear to be limited to china either.

[ElectricMirk]

As Hagster said, then I also heard that many of the cheap aren't that great . Also because it's fun making, the Yagi's I made also cost me less than the cheapest I think.

But yes, could totally buy myself out of this, and also will I think if I need it for something very important.

[ikrase]

You might want to explore what to use for first person view. They have a lot of circular polarized antennas to reduce multipathing.

[Howardlong]

Regarding the Cantenna, there were a lot of designs doing the rounds some years abo based on the Pringles can which turns out to be nonsense as the can’s diameter is too small.

I did a lot of work on 2.4GHz antennas about 2000/2001, before WiFi really took off, this was predominently for narrow band satellite ground segments.

Making some antennas demands accurate dimensions, such as Yagis, including element lengths and element diameters. As a result they tend to be relatively narrowband in that respect. Additionally, particularly with yagis, it’s a classic engineering squidgy balloon problem, you fix the impedance but then the gain or front to back goes off, and vice versa. Or the bandwidth decreases.

To make a serious go of it you really need a VNA to get the matching right.

Possibly the easiest most reproducible way to get gain is with a dish. That way you only need to worry about a relatively simple feed compared to the complexities of a yagi. There’s also little detuning from the dish itself on the feed, somyou get your feed to work then put it at the dish’s focal point. At 2.4GHz this only works reasonably as a solution for dishes of about 60cm and larger, so about 20dB of gain or more.

When making the feed for the dish, you want to be careful not to over or under illuminate the dish, so choose a feed with the right beamwidth. A patch is not a bad solution, assuming the dish f/D makes this reasonable, but fabricating one accurately takes effort. Just be careful about the feed’s bandwidth. Alternatively a simple dipole with plane reflector would be a very simple solution.

Comparing antennas is difficult in practical terms, but you don’t need an anechoic chamber. You can do your own outside range, but you’ll need a signal source, horn antenna and RF power meter. This setup should take into account ground reflection when measuring.

Trying to compare antennas to within an order of magnitude without a range is frankly a waste of time.

[metrologist]

I built the same bi-quad using a screen mesh with lips for the backplane, and mounted it to the feedhorn mounts on a C-band dish. Worked great! Actually, the bi-quad itself worked so well, I decided to make a double bi-quad for the 2M band.

[David Hess]

In some articles/totorials, etc. I've seen that sometimes the coax cable is essentially short-circuited and other times not.. I've heard one guy on youtube say it's very bad, but then also design himself many antennas where it is short-circuited. And I don't get it..

example.
On my Yagi antennas it is essentially short-circuited on the folded di-pole.. but in a Cantenna design it is seperated, also on the bi-quad feed I made for sattelite dish.
The director? elements on my yagi can all be connected together on metal rod, or they can be isolated..  what's the difference?.

A coaxial short may be open at high frequencies as with a shorted 1/4 wave stub.  An open 1/4 wave stub looks like a short at RF and is open at DC.

The advantage of an antenna which includes a DC short is that it prevents a build up of static charge which can damage the receiver and also makes it more immune to EMP.  So for instance an antenna like a J-pole which has a continuous DC short even to its mounting bracket can be earth grounded for lightning protection; that might not save your radio but it can prevent a fire.

[hagster]

The potential gain of a cantenna is limited by the apperture of the open end. Increasing the length beyond a certain point will no help.

A dish is a great way to increase the apperture. While a good acurate dish is hard to build, you can repurpose a sat tv dish for the task as they are frequency independent.

I did some NEC models of the biquad antenna and built a couple. It is a very good antenna.

[Lord of nothing]

  reduce the Output Power help to say inside the legal range. 

[usagi]

the Chinese n1201sa is legit, it holds up well against my expensive Rigexpert AA-600 and my Agilent n9342cn.

[gregariz]

To make a serious go of it you really need a VNA to get the matching right.

When I do this kind of stuff at work I model the hell out of it with very expensive solvers, then I prototype it and finally test it. There is a couple hundred grand in tools being used. Microwave antenna design is really not for the hobbyist - doesn't mean you can't have a go and have fun but as Howard suggests you won't really be able to fully have faith in it without the tools to measure it. Just because and antenna matches doesn't mean much as efficiency or pattern may be screwed up and at 2.45GHz we are talking about fractions of a mm making a difference.

Some advice regarding making a dish is not a bad one. Aperture antennas have their gain's set by the size of the aperture, then you are left with what may be a simple feed. My suggestion would be, as a fairly safe way to generate an antenna you can be proud of would be to build a large horn (if you are seeking several km). Its larger than a dish but the feed mechanism is simpler, just being a monopole. That Cantenna should have been the easiest to make work - you need to cut a slot down the can moving the probe backward and forward for maximum field strength, you then need to adjust the length of the monopole for maximum field strength - once you are happy you've peaked it, solder the connector there - thats about as good as you can do without the test gear. Any efficiency losses will be purely down to this process. Like a horn if you follow the calculations correctly, you need not worry about the horn gain, and so you are left with the relatively simple task of matching a monopole into a waveguide transition - a bit simpler than a dish feed where you still need to worry about feed pattern.

[hagster]

As mentioned, I modelled the biquad antenna in 4nec2 and it really does perform pretty well indeed.

To back this up, the modelled antenna measures roughly 10cm x 10cm and so has an aperture of about 100sq.cm. Using the formula[gain_ratio = 4PI / (lamda^2 * area) ] This equates to a gain of 9.23dBi. My simulation shows 9.17dBi gain. That is an aperture efficiency of 98%.

In short you will really struggle to find an antenna of this size with more gain.

note - You need to rename the BIQDFEED.TXT to a .NEC file to run with 4nec2

[metrologist]

I built my bi-quad for 13cm ham band and made a crude measurement with two dipoles and spec an. to verify about a 9dBd gain - of course not very scientific, so it merely qualifies the result.