Antenna Basics

Very informative & necessary info

Much appreciated. Please maintain the flow.

On a lighter note, two antennas met on a roof, fell in love and got married.
The ceremony wasn't much, but the reception was excellent.
 
So anyways.. Im not exactly jumping at the opportunity (like many others i think) to go and purchase a piece of fancy wire for R550 so does anyone know if its possible to construct your own antenna for the iburst desktop modem?

As far as i can see the only thing that looks like a problem is that little connection point to the modem. And then all you need is a piece of 50ohm (or 75? or wot?) antenna cable no? :D
 
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Omni's

Long time coming, but as promised….the omni antenna.

An omni looks like a vertical broomstick handle, and does exactly what its name says…..radiates in an omni-directional pattern….360 degrees……all directions.

There are a few mis-conceptions about omni’s, so let me describe the pattern as follows…..
Take a short pencil (that’s your antenna), stand it vertically on the table, and slide a doughnut over it (a doughnut with a small hole), so that the doughnut lies flat on the table, with the pencil through the hole, held upright on the table.

The doughnut, around the pencil (the antenna) is the radiation pattern….about 2-3dBi.
You’ll notice…if you were to cross-section the doughnut, it would form two circles, each side of the pencil.

Now flatten the doughnut somewhat……the radiation pattern gets flatter (all around the antenna), but extends further (ie makes a bigger circle)……that’s about an 6-7dBi.

Now really flatten the doughnut….making a huge circle, so the signal travels very far….still omni-directionally around the pencil……that’s about a 12-13dBi.

What do we notice?……the 2dBi omni radiates all around 360 deg, but has significant radiation almost vertically aswell.
This means…..if you had a wireless router in your study downstairs….with a 2dbi omni standing vertically……chances are, you would receive that signal upstairs in the bedroom aswell.

With the flatter pattern 6-7dBi, the signal is still omni-directional, and travels a lot further horizontally, but a lot (most) of the near vertical radiation is gone….so this antenna on your wireless router, would probably reach your neighbor……but you wouldn’t get a signal upstairs…….get the idea?

The 12-13dBi, would give you great distance, omni-directionally….which is great, but the signal pattern would be very flat…like a frisbee…….so if your mast isn’t all that sturdy, and the wind blows the antenna over slightly, you signal would overshoot, or hit the ground way before your intended target.

Your typical omni is a low gain antenna…..meaning….for a given size of antenna (say 600mm long) the yagi antenna will exhibit much higher gain than an omni…but not omni-directionally. (nothing for nothing with antennas)

Omni’s are great antennas…especially for iburst, because you can “see” many towers, so if one should drop out, you can still connect with other towers in the vicinity.

The disadvantage is that you also receive the unwanted noise or interference from all directions which could affect your connection speeds, and of course, if you use a high gain omni….the installation must be rock sturdy.

If you only had one tower in your area……the omni would not be the best choice.
 
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PDonut said:
So anyways.. Im not exactly jumping at the opportunity (like many others i think) to go and purchase a piece of fancy wire for R550 so does anyone know if its possible to construct your own antenna for the iburst desktop modem?

As far as i can see the only thing that looks like a problem is that little connection point to the modem. And then all you need is a piece of 50ohm (or 75? or wot?) antenna cable no? :D
Click to expand...

The radio amateur community correctly thought as you do, and they, through years of tinkering with various antenna designs, are responsible for much of what we know about antennas today, and many, many antenna designs were born in some enthusiastic radio ham’s backyard.
To their advantage though…much of their designs were for use at way lower frequencies than those of WLAN and iBurst, where the margin for error is much wider.
Let me explain…..i-Burst works on the 1780-1800Mhz frequency band. The wavelength of that frequency is about 160mm. Most antennas are based on the half wavelength which would be about 80mm.
This means that if you were to build a very simple, almost no gain dipole antenna for i-burst, it would be 80mm long.
But if you cut the piece of aluminium skew and ended up with a piece 79.5mm long, your antenna would work wonderfully well on 1820MHz (eg only), and miss the whole iBurst band. And this becomes even more critical as the desired gain increases.
Thus the difficulty in building “home brew” antennas.

For a manufacturer…..this is no problem. If required, we have some fancy techniques to shift the operating frequency of an antenna either way, if we miss cut by a fraction of a millimeter, and we use some very expensive test and measuring equipment to make sure our antennas operate where, and how they should.
This is of course outside the scope and capability of your average “home brew” antenna builder, so for all practical purpose, building a WLAN or iBurst antenna, and making sure it works properly, is near impossible.

I know the R550 you mention sounds like a lot to pay for a piece of aluminium on a pole, but the price of an antenna is much more than the sum of its parts.

I hope this was helpful…..albeit not good news.
 
PM me if you wish to build your own corner reflector...

PDonut said:
So anyways.. Im not exactly jumping at the opportunity (like many others i think) to go and purchase a piece of fancy wire for R550 so does anyone know if its possible to construct your own antenna for the iburst desktop modem?

As far as i can see the only thing that looks like a problem is that little connection point to the modem. And then all you need is a piece of 50ohm (or 75? or wot?) antenna cable no? :D
Click to expand...

It is simple, but it would costs you about R 350 in materials and you would have to spend some time in constructing it.

EDIT: Don't PM me, here it is...

Regards

Michael
 
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@Radiant Antennas
Is there an aerial shop near Cape Town? I'm looking for one for the VHF band.
 
Using MIMO Wireless Tech...

Hi RA / bdt

I need some help with my reasoning….

I refer to a conversation we had some time ago, which might relate to this, re-read the entire thread if you up for it, or the post below.

who.is.michael said:
Good to see it helped...

If you have spare cash and want to further improve your signal either purchase another Yagi of the same type from the same supplier and mount them side by side about 16 cm apart (boom-to-boom) and combine them to the main feedline using a “combiner” module, this will narrow the horizontal beamwidth and increase the gain by approximately another 3dB or purchase two higher gain Yagi’s .

However the best cellular antennas can be found in Ireland, reasonably priced (60 EUR) however the shipping cost (125 EUR) is rather high. The shipping cost would be the same for ten antennae, so if you can convince a few iBurst users to purchase as well and split the shipping costs it can be well worth your while.

Insulated Yagi’s

Parabolic antennae

Hope this helps.

Regards

Michael

EDIT: To reply to your post below; as it is not completely accurate. Narrowing the beamwidth would allow for less “noise” from unwanted towers and other sources, therefore an improved signal (> -3dB) allowing for a higher throughput.

Would appreciate your feedback once you have tested it – it is especially relevant where there are several base stations with a difference in signal of around 10dB.
Click to expand...

OK, so have look at http://www.cocacola.za.net/antennae.htm the document regarding MIMO, page 3 & 4 mainly, if you brave the complete document.

After reading this, it seems LOS is not such a good thing when it comes to iBurst? Maybe if you have a poor signal at a range of > 2Km, then yes?

Since the base station is making use of a multiple antenna configuration , surely it will be an advantage to have the CPE also making use of multiple antennae - to truly benefit from the MIMO/MAS technology, however should one use the double Yagi configuration, as mentioned above would it assist with MIMO/MAS?

And would this be the best way of going about - making full use of the MIMO/MAS technology of a base station, with a fixed land-based antennea configuration? Or would a parabolic dish not be the most suitable antenna for use with iBurst, considering MAS?

How's the CRA coming on Graeme and maybe you would like to comment on what Poynting has done with the Omni on page 6 of the document Data Sheet Poynting?

Would welcome any suggested reading.

Thanks

Michael
 
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