DIY high gain omni antenna

Hi all,

After I have successfully made some wifi antenna's I now want to make a high gain omni for a hotspot. What is the highest gain I could get with what design? There are some commercial types sold as 12 dB gain??? but what I found on the internet to make yourself is

7 dB tops.

Thanks

I found these sites but what design are those?

Yes I thought of that too after I posted this message :-)

That is kind of interesting, what about mounting such an antenna upside down?

What can be done to improve or create some downtilt.

Maybe I will start with making a simple one like this:

it help to extent such an antenna to lets say 16 sectors? Also they mention some brass tube design but I can't find info about that, if it can improve the sensitivity I like find some details about it.

Thanks for the quick reply

Edmund hath wroth:

End fed vertical collinear. Sorry, no plans for anything better than

8dBi. Please don't build the one consisting of alternating pieces of coax cable. The antenna is twice as long as it needs to be for a given gain. The antenna also has the same loss as the coax cable it's made from, which costs a few dB gain. If you're thinking of building your own 15dBi omni, you'll find that the bandwidth is sufficiently narrow that you'll need some test equipment to get it right. The reason can, patch, and biquad antennas are fairly easy to build is that they're broadband devices and therefore can be built rather sloppily and still work. Even badly built, they're still better than the stock antennas that come with most cards and wireless routers. However, that changes when the gain increases. Cut tolerances become critical and details that are often ignored are no longer insignificant. I just tried some assorted antennas on my Wiltron 610d sweep generator and VSWR bridge. The one's I built are fairly awful as compared to the commercial antennas. Photos later...

That sorta works and I've done exactly that. I've also tilted antennas in desperation. The usual problem is that the antennas are not quite sealed and need a place for water to go. That's usually a small hole in the base. Mount the antenna upside down and the pipe slowly fills with water (mostly from condensation).

Not much with an end fed collinear antenna. Lots with a dipole array using phasing lines or tilting the individual dipole elements. I have lots of books on the topic, but am too busy right now to find references on the web.

Ugh. That's exactly the type of antenna methinks is a problem. Good luck cutting the sections to the exact correct length.

The brass tube version is more accurate. URL returns nobody home. Here's some copies I found |

| the balun on the base.

OK what designs are there that gives a shorter length? ( with a vertical polarization ) Like this maybe

Right I will start with a lower gain antenna then.

In the mean time I've seen designs with tilted dipoles.

You are right, it won't be easy, or at least it will be time consuming to cut these sections.

| Note the balun on the base.

This will be a lot easier to make, still quite some work to do and I have to find cable and tubing from the right size. I think it is a lot easier to make the other model and add some sections to see how that works out in vertical gain loss. However I don't understand the math they are doing :-) In

first section is 1/4 they say but it is 2/4 L. And why the last part is shorter I really don't know. And what if I want some additional sections, is that simple

3/4 for each section except the last one? How about the direction of the "loops" here are all winded in the same direction (right handed direction) and an 3/4 wavelength. While here use 1/2 a wavelength and changed winding direction, although that makes some sense to me, why isn't the distance between the loops 1/2 wavelength or 1? Not sure about the diameter of the loops either, is the length of the wire from the loop 1/4? It seems to be :-)

Edmund hath wroth:

I found the "brass tube" collinear article: |

This is more like what's inside commercial antennas: |

Yep, that's much better. Basically, it's half wave radiators, with half wave "delay lines" in between to keep the radiators in phase. The coils can be almost any shape as long as they're a half wave long (and they don't overlap). However, the antenna is missing a balun at the base, and could probably benefit from some ground radials. It will also have considerable uptilt, but at low gains, will not be much of a problem. The cut lengths shown are really rough and will probably result in an out of band tuned antenna. (I did the calcs for the required precision in a previous rant). In other words, it's a tolerable design for a small and simple antenna, but not so great for a high gain omni.

Can I suggest an alternative? Try a Franklin sector antenna: |

| | | is 13dBi gain, 120 degrees horizontal beamwidth, 5 degree vertical beamwidth, and quite easy to build and customize. The required balun is a challenge, but not impossible if you use 0.085in or 0.141in semi-rigid coax. The antenna is symmetrical and therefore has no uptilt problems. If necessary, it can be mounted at a small angle to optimize coverage area. Three of these 120 degree antennas, with a power splitter, can simulate an omni with adjustable downtilt. Also, note the NEC2 plots and test results which means that someone actually went through the trouble of calculating and measuring the antenna.

Actually, what I think you need to do is figure out some way of testing the results. What I've been doing (due to lack of suitable test equipment) is setup an antenna range that is not affected by nearby reflections or Fresnel Zone edge effects. There's an access point on a mountain top about 5 miles away that I can see on my spectrum analyzer and with Kismet. I've taken the time to make signal strength measurements with some commercial antennas of known gains. These are my reference antennas. When I check the gain, I first compare the signal strength with my reference (which happens to be

8.0dBi). If Kismet shows my antenna under test to be 2dB better than my reference, then it probably has a gain of 10dBi.

I recently added a Wiltron 610D 10Mhz to 4GHz sweeper and VSWR bridge to my collection. About $500-$800 on eBay. Now, I can test for VSWR and tune the antenna onto frequency.

Yep. That's one way. Different length phasing lines to each dipole is another. Some antennas are made from brass or copper tubing, which can be internally tilted.

What math? The numbers are rounded off to 1 significant figure.

Good observation. The design is wrong. The length of each section should be 1/2 wavelength long except the very top which is 1/4 wavelength.

Well, you got me there. The design looks correct, but it's upside down. The connector should be at the other end with the 1/4 wave section at the top. It will work with the 1/4 wave section at the antenna connector, but then it needs a ground plane, which is absent.

It's very much like a wire version of the alternating coax cable vertical collinear, except that every other half wave section is a replaced by a half wave coil. If you look at the plans for the coax version, you'll see that the coax connector end is 1/2 wave and the top is 1/4 wave. Weird. These are both easy enough to model. I'll do it if I have time.

That's a totally different idea. Those are horizontally polarized loop antennas with 1/2 wave phasing sections in between. The loops radiate, while the vertical sections do not radiate (much). The vertical sections are usually coax cables, but can be done otherwise. In the various end fed collinear designs, it's the vertical sections that radiate, while the coils do not.

The length of the loop is 1/2 wavelength. You can build it like a hairpin if you want. You can also overlap the turns, but then the mutual capacitance of the turns tends to shorten the loops. If you build it out of coils, it's very important to keep the coil stiff and immoveable as small changes in position have a big effect on tuning. Best to use an oversized single turn or hairpin style.

's OK, I wouldn't mind to make this but I like to know what on what calculations all this is based. I seems magic to me. In the feedline they use a 1/2 lambda = 61.5 mm and in the elements they use 1/2 lambda = 56 mm???????? Beats me. The we have the length of the coils with the straight ends which are

1/4 lambda to the middle of the coil? ( hope you understand what I mean ) Where is the diameter of these coils comming from (6mm) ? Then to top it off, the total length of the element part 1/2 lambda =56mm + the 1/4 lambda of the coil part + the remaining part of the coil seems to be 98 mm. Not to mention all the different diameters of all parts. I don't ask you to explain all this :-) I only say I don't understand all this.

So does that mean the radiators are the straight parts, they are not

1/2 a wave here. And the delay line is that the coil? or the other way around? The coil is not 1/2 wave either, it is 10 mm diameter which comes close to 1/4 wave.

Not overlap? like in a spring or the above model you mean?

However, the antenna is missing a balun at the

I will do that some time later, for now I prefer a really omni.

I agree but that is not so easy here, there are a lot of buildings here and no free sight in any direction :-(.

Now you really got me confused, the design is correct? Why? They use 3/4 wavelength for the straight part and a diameter of 15 mm or less for the coils. That gives a straight length of 47 mm for the loop/coil.

Let me know what direction the coil/loop has to be then, I wonder what comes out of this simulation.

Forget I asked about that before I give it all up :-)

OK here you answered my previous question except for the fact that a diameter of 10 mm or 15mm isn't even close to 1/2 a wavelength.