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Radio & Equipment Questions

This section includes questions on cabling, antenna, pigtails and radiation / power requirements. If your question is not here, try looking through all of the questions which are listed on TheEvolvingFAQ.


What equipment do I need?

A wireless card or an access point, some low loss cabling, and an antenna.

For a two person link, you would need



So... how much does it cost?

From $200 if you are resourceful.

Very good question. Retail prices for 802.11b devices are averaged to around $100 for the client, $30 for the pigtail, $250 for an antenna and the necessary low-loss cabling required (or you can buy a range-extending product that looks like an antenna, but isn't called one for marketing reasons), and $90-$1500 for an AP.

On The Incredible Bulk, the prices on offer are much better. PCMCIA client cards are $55 to $135, PCMCIA cradles for desktop systems are $85, Pigtail and cable connectors for $40, low loss cabling LMR-400 for $5/meter, Antenna are sold at prices above $100, or you can make a Cantenna, a Waveguide Antenna from a tin can or a Pringles container for the cost of a few parts and an NConnector.

If you are lucky, and have some patience, you can create a small ad-hoc network between two people, with a modified GalaxyAntenna above your roofs, 10m of LMR-400 low-loss cable, and two 802.11b network cards for $250 each.

It costs most people a lot more, not a lot less than $300. Not a lot of the equipment and parts can be found in the bargain basements or spare-parts bins of your local radio parts or computer shop. They are on the shelf, sold at retail prices. Don't walk into a shop and ask for an EnterasysCards for $50 - they won't have one, and will laugh at you, while they charge you the retail price which is still well above $100. The only reason we managed to get some at that price was because a company bought oodles of them and then went broke.

There's no shame in paying the full retail price for your parts, for which you can expect a warranty and some level of customer support. However we have the opportunity to make bulk purchases as a group, which makes hardware a lot cheaper, and a lot more compatible for everyone when the group is using the same hardware. Everyone wins.

Retail costs for 802.11a devices are far less inviting. From $600 for a client device, to $2000 for an AP. Belkin make a 802.11a/b router that can bridge the two distinctly different networks together, and it also supports the turbo modes of the only chipset currently available for 802.11a devices.


Which cards have external antenna jacks?

Quite a few.

See the Hardware Database (Don't forget to add any hardware you have that isn't listed!)


What is low loss cabling?

It has less resistance to high-frequency signals.

LMR-400 is the standard we recommend. If you use just any old coax cable, you'll lose too much signal before it even gets to your antenna. Use the calculator linked in the in the CoaxialCable wiki page to guage the amount of signal loss over the distance you require.

A 3db loss is significant - being a logarithmic rather than linear scale, each 3db loss represents half your power. More signal loss over longer cable runs requires even better cable.

CNT-400 coax is generally cheaper than LMR-400, but has virtually identical physical and electrical specs.

If you HAVE to use another brand of cable, or cannot get hold of LMR-400, we suggest something better or equal to RG-8, which can be found in Dick Smith and Tandy branches. However we do not recommend it, because you lose a lot of signal, and as you lose that power, the cable will radiate it out as a poor antenna, and this will have an effect on the 802.11b device, possibly damaging it. You have been warned.

RG-213 can be found at Dick Smith branches and NConnectors are also available. The core of RG-213 is a smaller diameter than LMR-400 so the connectors are not suitable for fitting to LMR-400 cable, although it can be done if 0.1 increment drill bits are available. The RG-213 page has further information about the cables suitability.


What is a pigtail?

It connects your card to the cable.

Its an adapter to convert from the minature connector on your wireless card to an industry standard connector such as a NConnector. This allows any antenna to work with any card as long as you have the correct pigtail.

Pigtail vendors: Add your links here!


What is an antenna?

Oh you're just getting silly now.


What kind of antenna are there?

Omni-directional or various kinds of directional.

Omnidirectional,
Parabolic grid,
Helical,
Yagi,
Patch,
Panel,
WaveGuide (possibly the easiest of all antenna design, the basis for the Pringles antenna and Cantennas)...
Slotted Waveguide,
External linkothers


OK, I've now realised I don't understand what dB is...

A measurement of difference in power.

It happens. dB is the abbreviation for the decibels measurement unit. It is a measure of "loudness" or wave amplitude. In audible sound waves, this translates to "volume"; in radio signals it translates to "signal strength".

Visit External link"Using and understanding Decibels" for a quick guide.


What is this dBi Gain thing I keep seeing on antenna specs?

How much an antenna amplifies or concentrates the signal.

Basically it's how much more "powerful" the antenna is over an imaginary dipole antenna. I say imaginary because nobody makes one, but it would be an antenna that spreads the signal in every possible direction, in the shape of a sphere. Such an antenna would have zero Gain. So for example, a 24dBi gain parabolic grid is a very focused antenna, it narrows the signal into a fine beam of coverage in one direction. It doesn't use or provide any more power than a dipole, it just focuses all that signal into one spot, so it goes farther.

A good analogy would be that of the light bulb in a torch. Unscrew the cap and lens so you just have a bare bulb, and your immediate area will be quite bright, but it doesn't go very far. This would be akin to a 1 or 2dBi antenna. Now put the cap back on and the parabolic dish will focus the light into a tiny little spot, and shine it down a hallway. Your immediate area isn't very bright, except for that one bright spot down the hall. This would be akin to a 24dBi parabolic dish.

You can also use a water hose analogy (but then it gets even more phallic, if that's even possible).

Further information along with some calculation examples is also available.


Is an omni directional antenna pattern really omni directional?

Yes, in a horizontal plane.

It's more like a pancake. A low gain omni would look like a small fat pancake (a pikelet), while a high gain would look like that same pancake flattened out into a thin and long pancake (a crepe). Mmm... pancakes.

There will always be a small blind spot (usually where the cable comes in), but if that's orientated towards the ground then there shouldn't be any problems.


Is there a limit to the height my antenna mast can be?

Your local council and state planning bodies set this.

External linkThe Building Regulations 2006 states:
431 - Masts, poles etc
The report and consent of the relevant council must be obtained to an application for a building permit to construct a mast, pole, aerial, antenna,
chimney, flue, pipe or other service pipe which
�
(a) when attached to a building, exceeds a height of 3m above the highest point of the roof of the building; or
(b) when not attached to a building, exceeds 8m above ground level.


How far will my signal reach? Which antenna will see my mate x km away?

Maybe.

There are lots of factors at play here, but as a basic guide: Omnis and Cantennas (6 - 10dBi) will get you up to 2km, smaller directionals (12 - 19dBi) will get you around 5 kilometres, and larger directionals (20dBi+) will get you 10km or more.

This is of course affected by the antenna at both ends. The best case scenario is a well-aimed, correctly polarised high gain directional at each end, and the worst is where both ends are using only the built-in antenna in the device (typical range 50 - 100 feet).

A good antenna at one end will increase the range at which you can connect to it, whatever antenna you are using at the other end. For example, in a typical application like university campus, a powerful omni or sector antenna will allow students and staff to connect from all over the campus with only their client cards.

In the fairly common situation where one end has an omni and the other a directional, the directional antenna will improve the signal strength in both directions, compared to an omni at both ends. It will pick up more signal from the omni, and transmit a better signal back to it.

Of course, on a really long link, the curvature of the Earth becomes a factor. External linkSee O'Reilly


What is the range of an Access Point where the signal needs to go through walls?

Help! I'm answering this question with wild guesswork. Anyone with some real-world figures should fix this up!
Typically about 30m.

30m is a wild guess. If you are trying to send your signal through the walls of a tin shed, then chances are it won't penetrate at all. If you live in a Japanese house with paper walls, you probably won't notice the effect on range at all. For internal plaster walls held up by a wooden frame with no insulation, expect the signal to attenuate (weaken) by perhaps 3 decibels per wall - meaning it will be 1/2 strength after passing through 1 wall, 1/4 strength after 2 walls, 1/8 strength after 3 walls, etc. This attenutation (weakening) is in addition to the attentuation suffered when the signal travels through the air. Brick walls with insulation might attenuate your signal by as much as 6db.

The only way to get a reliable answer is to test your equipment to get real results.


What is the best type of antenna to link between X and Y?

That's like asking which is the best vehicle to get from A to B in!

If you want to get from one side of a large metropolitan city to the other, you might consider using many types of transport from your own feet, through public transport, to flying a helicopter at great expense. Similarly, there are many options available when it comes to selecting an antenna to get connected.

If you've bought an AccessPoint or a client adapter already, chances are a small omni-directional antenna has been included, allowing you to make connections from about 50m away in every direction given good conditions.

The cheapest long range antennas are the do-it-yourself style (homebrew). You have the freedom to build whatever you want, providing you have the expertise or can find someone to give you a hand. A Cantenna can manage a directional link up to about 5km in the right conditions and costs about $5. Do some in-depth technical reading, spend a fortune and years of your life and you could build an antenna array capable of sending signals to the moon - though it probably won't do much good at sending signal from one end of a house to the other. ;)

In the retail world you typically get what you pay for (and yes there are some bargains to be had). Spending around $150 should get you a 24dBi parabolic dish antenna capable of 30km links to a similar antenna (this implies purchasing 2 of them), but is probably a good investment if you are serious.


What sort of cable should be used?

Low loss coaxial cables should be used to minimise the amount of signal lost before reaching the antenna.


Can I get a link if there is no line-of-sight?

Sometimes, yes.

The only way you can determine what will work for your location is to borrow some equipment and do some tests with it. There are various grades of link - if you use the cheapest and lowest gain equipment your link may be low speed and/or marginal. If you want the fasted speed possible you'll want to get the send and receive the strongest signals possible. The problem with going through residential housing is there are people and cars and moving about in your link path which may make your link unstable. And if your link path goes through more than a small amount brick, concrete or hill, your're going to have trouble getting any link at all. You'll have to decide for yourselves as to how tall is too tall for a mast. Then go get a mast which is as high as you, your family and your neigbors will tolerate. Maybe a 1 metre mast extension on your TV aerial mast will be sufficient.

Every link is different, and the only true way to determine what equipment you'll need is to test with different gear. You might get away with a cantenna, or maybe you'll need a 25dB parabolic antenna. The only way to know for sure is to test the gear out. If you don't want to spend big bucks on gear see if you can get a hold of old Galaxy or Austar Pay TV 2.4Ghz dishes, which can be made to work with 802.11b/g with small modifications.


Do you have any tips when doorknocking to get someone's old galaxy antenna?

Use your manners and some common sense.

Be polite, courtious and respectful, when asking, and be sure that it's really a galaxy and not a strange UHF antenna.

---

Can't find the answers to your questions?
Try looking through all of the questions listed on TheEvolvingFAQ. If you still can't find answers, ask your questions here.


Version 15 (current) modified Fri, 02 Jan 2015 09:49:01 +1100 by CRC
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