Antenna Basics

R

Radiant Antennas

Member
Joined
Dec 1, 2006
Messages
26
Reaction score
0
As promised, the first of a few basic antenna tutorials aimed at providing at least some understanding of what an antenna is, the various antenna types, and why they may, or may not do what you thought they would do for you.

There are a few basic parameters to consider when choosing or comparing antennas. One of them, and possibly the most widely mis-understood is VSWR.

VSWR (Voltage Standing Wave Ratio) is quoted as a ratio to 1…ie 2:1 or 3.2 :1.

Without going into too much of the technical details, it tells us how well the antenna will “match” to your transmitter, modem or radio etc.
1.1 :1 is a perfect match and rarely achieved.
1.5 :1 is an excellent match. 2 :1 is a very good match. 2.5 :1 is still a good match and 3 :1 is still an acceptable match. Anything higher than this usually indicates a fault and should be avoided.

If an antenna is well matched, all the power from the transmitter will arrive at the antenna (and be radiated). If the match is not particularly good, some of that power it will be “reflected” back down the cable to the transmitter. The higher the VSWR, the higher the reflected power. Excessive reflected power can damage your equipment (although modern electronics provides protection against this condition) and of course…..reflected power is not radiated.

VSWR is often thought of as extremely important, and an absolute indication of antenna performance (the lower the figure, the better)….but the reality is, if it is less than about 2.5:1…that’s just fine…even 3:1 is still acceptable. At Iburst frequencies, typical values are around 1.5 – 2 :1.
Note: An antenna with a VSWR of 1.5:1 (quite low)…may not perform better than an antenna with a VSWR of 3:1 (quite high).
Loosely speaking......gain and VSWR are not related.

You have heard the term 50 Ohm....transmitters are designed to have an output impedance of 50 Ohms.
To be a good match, antennas are designed to have an input impedance of 50 Ohm.
To join the antenna and the transmitter together.....we use 50 Ohm cable...thus ensuring a good match.


Coming soon…..Antenna Gain.
 
What a self-appointed idiot?

If you are going to bore us and continue treating us all as idiots, at the bare minimum please ensure 100 % accuracy of all your facts.

On the bottom right of your post there is an option to edit your post, before you ask again.

PS. Everyone on the forum has internet access and we can use Google.
 
You have a valid point.....but you wont believe the number of phone calls and e-mails I get every day, enquiring about these very things.
I am new to i-burst and this forum....and I see very little else but complaints about the i-burst service....very few people are constructive and offer assistance.
My effort is merely to provide a little understanding, and while it may not help a worthy expert such as yourself...maybe just one person gets some benefit.

If you think it is not valid...I will drop it....no problem.
 
Radiant, just ignore the idiotbuster, post what you want as some people here might gain some useful info from your posts.

After all it is but one thread that those not interested can skip without making negative comments.
 
Knowledge is Power! Although I'm not sure what I've just red.
 
maybe just one person gets some benefit – You?

Forgive me MOD’s

You won’t believe it, but I’m not interested to know what happens at your job. If you are only seeing the uttering’s of frustrated users it might be because of iBurst experiencing issues lately - try using the search option and have a good look at what is available on the forum.

“very few people are constructive and offer assistance.” – Totally untrue, browse the forum. I can assure you if you decide to visit a bit more often you will realise that most forumites are extremely knowledgeable and supportive.

Your effort to contribute is appreciated, just up the level a few notches and skip the boring bits – give us your practical knowledge on what works and when.

The users that require more will surely reply to your posts if it is of value
 
MrH said:
Radiant, just ignore the idiotbuster, post what you want as some people here might gain some useful info from your posts.

After all it is but one thread that those not interested can skip without making negative comments.
Click to expand...

Agreed. Please keep going Radiant Antennas.
 
Nice thread! Hoping to see some pictures too.

Mods, please delete IdiotBuster's threads.
 
Antenna gain

I’ll admit VSWR was a boring and mostly irrelevant place to start, but it had to be done.

Gain, on the other hand is more relevant to the subject at hand.
You’ll remember your teacher once said in a physics class…”energy can neither be created nor destroyed”. If this is true, how does an antenna exhibit gain? It cannot create signal.

An easy analogy would be the torch….a maglite, the type that can be rotated in front to sharpen the beam.
Taking the front off, leaving the bulb open an exposed, light radiates equally in all directions…upwards and sidewards, and if the torch body wasn’t in the way…backwards.

This is the isotropic radiator…the ‘i’ in dBi, to which all antennas with gain are compared.

If you put the front back on the torch and screw it all the way down, the beam is large and quite feint, and shining only forwards.
The torch reflector has ‘taken’ the light from the direction it isn’t required, ie the back and sides, and added it to the light in the direction it is required…ie out forwards, which made the light somewhat brighter than the open globe, and able to travel further…..ie gain.
Lets say this represents 5dbi.
Now turn the front so that the beam is slightly sharper…ie narrower and brighter…..this would represent maybe 10dBi. Now turn it so the beam is as narrow as it can be and as bright as possible…this could be 20dBi.

What have you noticed? In order for the beam to be brighter, and travel further, the beam has become narrower……this is gain.
The amount of gain is stated in dBi, because the light intensity radiated is compared to the light intensity radiated by the open bulb….ie a 10dbi torch radiates light in a certain direction which is 10dB’s stronger than the light radiated all over, by the open bulb.

With an antenna, this focusing effect means that the signal travels further, but in a much narrower beam.

This has its disadvantages too. If, for the purposes of diversity, you wanted to be able to receive a signal from two towers, and those towers are situated at 12 o’clock and 3 o’clock from your location…….the beamwidth of a high gain antenna of 17dBi would be too narrow to receive from both towers, even if pointing in between the two.
So, a lower gain directional antenna of say 6-8 dBi, would probably be more suitable.

Lower gain antennas are designed as such on purpose, for the above reason.
A person that designs a high gain antenna of 20dBi is not necessarily a better antenna designer than one who designed a 15dBi……Its what they set out to achieve, given the constraint of size (remember, normally, the bigger the antenna, the better), and cost….versus performance.

What about omni-directional antennas….how do they have gain?
Soon….
 
Radiant Antennas please keep it up. I would love a consolidated antenna reference and I am sure many others do as well.

Ignore IdiotBuster he is the first morpheme of his name.
 
The Panel Antenna

Before we get ahead of ourselves with the omni- might be better to start with an antenna that you are probably all familiar with.
The Panel antenna is a wonderful antenna….its fairly small, exhibits good gain…the one you know is an 11 or 12dBi, (and a good one), and is not too expensive and easy to install.

A panel antenna, usually consists of a patch antenna in front of a metal backplane (just a simple flat metal plate), which focus’s the radiated signal forwards, and provides gain. (Like the reflector of our torch)
Because the backplane is only a flat plane, some signal is radiated off the sides of the antenna, which is why a patch is usually not a very high gain antenna, and exhibits quite a wide beamwidth.

Typically…..a patch antenna exhibits about 8dBi of gain. The i-burst panel we are familiar with has two such patches, combined to make one antenna.
Doubling up on the antenna like this, increases the gain by 3dB…it also obviously doubles the size of the antenna, and the backplane also has to be doubled up in size too.
Doubling the size of an antenna takes our 8dBi to 11dBi.

If the panel antenna was doubled again (ie 4 little patch antennas inside…you would get another 3dB’s more in gain, taking our panel from an 11dBi to a 14dBi.
If the panel was doubled again…another 3dBi…our panel is now a 17dBi.

Those figures sound familiar?...…that’s why most panel antennas you see advertised are either an 8, 11, 14 or 17dBi.

To my knowledge…no-one has been brave enough to try a 20….I think the physical size would be prohibitive…..not to mention, as the size increase, the losses start to increase too, making the exercise futile.

The patch antenna is an ideal candidate to explain ‘Front to Back ratio”, which is the third important aspect of directional antennas………up next.
 
Front to back ratio

The electromagnetic world around us is full of signals, all around us, everywhere you go, some very powerful, some very weak….from Europe, the USA, from all over the world. From your neighbors wireless intercom, to your own kitchen when the wife turns on the micro-wave oven….Your son welding in the garage, and the poorly suppressed cars driving past your house….all these sources of interference conspire to make your modems job difficult.

Some of you have taken the trouble to measure the signal strength received by your modem, and figures in the order of -70 to -80dBm appear to be quite acceptable…..even good….but how much of that measured signal is actually wanted signal, and how much of it is noise…interference. It’s difficult to tell, and you would need some pretty sophisticated measuring equipment to find out, but the point that I’m try to make is that the signal received by our antenna, isn’t always the signal you want.

Modem’s, radio’s…just about any wireless device that receives a RF (radio frequency) signal and extracts data or information from It, needs the ‘signal to noise’ ratio of that signal to be quite good in order to extract the data efficiently.

Lets say (and I really don’t know so I am using demonstrative figures) your i-burst modem needs a signal to noise ratio of 10dB to extract data at 500k.
If the modem received a signal from the antenna of -70dbm (very good you would think), but the received noise level was -76dbm, then the signal to noise ratio would be 6db, and the modem may not cope with 500k, but have to slow down to 300k…(figures are only as example)….thus irrespective of what level of signal you measure on your modem, the signal to noise ratio is what determines the quality and speed of your connection.
Most areas I am sure are pretty noise free…but if you are receiving good signal level, and the tower you are connecting to is not overloaded, yet you are still suffering poor connections and drop outs…maybe the source of your troubles is interference.

What has this got to do with antennas……well if you are using an omni-directional antenna, which is it definitely advantageous for the purposes of connecting to multiple towers, this omni antenna will not only receive the wanted signal from all directions, it will also allow noise in from all directions.
You may be receiving a fairly good signal from one direction…but you maybe receiving noise from all directions….get the point?

This will (no…could) decrease your signal to noise, and therefore decrease your modems performance.
Note…if you are in a good signal area where the strength of the signal far supercedes the level of any received noise…then there are advantages to using an omni…if you don’t have this luxury, then a directional antenna with good “front to Back” performance is required.

Front to Back Ratio…only a directional antenna can have what is known as a ‘front to back ratio’. In the case of our panel antenna, this is a measure of the signal leaving from (or arriving at) the front of the antenna, versus the signal leaving from (or arriving at) the back of the antenna.

‘Front to back’ is applicable for both the signal you transmit and the signal you receive.

Undesirable interference causing signals may be kept out of your system by using an antenna with a good front to back ratio.
Providing that interference arrives at the back (or sides) of the antenna, it will be mostly attenuated (suppressed).

If you were to point your panel antenna at a tower, and measure the received signal…then turn the antenna around so that the back of the antenna is facing the tower, and measure the signal level again, you would notice about a 20dB drop in signal level…..that’s the front to back ratio of the antenna. (of course…you have to be sure there is no tower behind you when you turn the antenna around, otherwise your measurements will be skewed)
Because a panel antenna has a nice large backplane, the actual active element is shielded from the rear entering signal. Some signal does creep around the backplate, but generally, if the signal in or out of the back of the antenna is lower by 15-20dB’s, that’s fine.

Thus…an antenna with good gain characteristics will ensure a high level of the wanted signal from the desired direction, and the antennas high front to back ratio will ensure minimal reception of the interference.
 
Last edited:
Great job Radiant. You really have a great way of explaining and putting your message across.
 
IdiotBuster said:
Forgive me MOD’s

You won’t believe it, but I’m not interested to know what happens at your job. If you are only seeing the uttering’s of frustrated users it might be because of iBurst experiencing issues lately - try using the search option and have a good look at what is available on the forum.

“very few people are constructive and offer assistance.” – Totally untrue, browse the forum. I can assure you if you decide to visit a bit more often you will realise that most forumites are extremely knowledgeable and supportive.

Your effort to contribute is appreciated, just up the level a few notches and skip the boring bits – give us your practical knowledge on what works and when.

The users that require more will surely reply to your posts if it is of value
Click to expand...

Well.. just to note, you seem to have only made two very negative posts. So I'm not sure why you are defending the forum in terms of being positive or constructive.

I agree some practical knowledge would be useful though. Most of us cant really use what is in this post. It would mainly be useful for understanding antennae.
However antenna gain etc will be interesting to read, so go on :)
 
EHV, for those interrested it's far more important to know how and why certain things work, so the stuff some might find boring (or maybe can't grasp) is as important as the final outcome of what the antenna is supposed to achive at the end of the day.:p
 
Empowerment

The motor vehicle industry made a very wise move some years ago...not any individual manufacturer, but the industry as a whole, set out to educate the car buying public.
Technical terms such as ABS, RPM, torque, horsepower, kilowatt, fuel injection, normally aspirated, 4WD, TDI, OHC, A/C and all the others, are now understood by every man and woman in the street.
Even my wife can now read through a few car brochures and make up her own mind on what car would be right for her, and both the industry and the consumer has benefitted because of it.......thats what this is about.
 
Last edited:
Yagi's

This antenna type is very well known to all of us….our TV ‘aerial’ (antenna is the correct term) is a Yagi (or, as correctly pointed out, Yagi-Uda), named after two of the early antenna pioneers..…Mr Yagi and Mr Uda of Japan, who were the clever guys that designed this antenna almost 90 years ago…..and you thought antenna technology was cutting edge?
Many…if not most of the antennas we are so familiar with today, were designed more than 50 years ago.
(Have to pad this with irrelevant information, as there’s not a lot to say about a yagi).

To me….and I know some may differ, the performance of a yagi antenna and the performance of a panel antenna, is near exactly the same.
I am not talking about company xxx’s yagi compared with company yyy’s panel, I am talking about the antenna designs per se.
(I will leave it up to the experts and installers, who have way more experience with the various antennas made locally, to tell you what, in their opinion, is the better choice).

A Yagi, consists of a driven element…a dipole (that’s the bit the cable is connected to), a reflector- the long element behind the dipole, and positioned in front of the dipole are the shorter directors.
Depending on the desired gain of the Yagi, it could have more than one reflector, but common practice is one……and many directors…..the more directors, the more the gain, the longer the antenna (again, the higher the gain, the bigger the antenna, or in this case, longer the antenna).
All of these elements (reflector, dipole and directors) are mounted on a single boom.
The boom has no electrical effect on the antenna, and is not used to connect all the parts together…it merely is a way of getting them all mounted in line at the right spacing…if they could be made to hang in air…all in a row…that would work too.

If we compare the performance of a known 11dBi panel antenna with a known 11dbi yagi antenna, they will measure up near exactly the same.

The decision one makes when choosing between either of these antenna types is for the most part a personal one…..I think a panel mounted on the wall is less obtrusive than a Yagi…..but then….how many walls just happen to be facing the tower.

In size, the typical 11dB panel (for the i-burst frequency band) would be about 100x200mm in area and about 40mm thick, while an 11dB yagi will have a boom length of about 500-600mm, and be about 80mm high.
Any claim of extra-ordinary higher gain, from an antenna of similar size…is probably not truthful……..Miniaturization has enhanced many technologies……antenna design is not one of them.
 
You must log in or register to reply here.
Back
Top
Sign up to the MyBroadband newsletter
X