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Parabolic Antenna Question


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Hi guys/girls

I was planning to buy an antenna like this one http://youtu.be/00CC-cTZbTA on episode.

Could anyone point me a quality one/s and places i can buy them?

If they have even more DBI signal better but not super mega expensive ones and nothing beyond 10 miles and 10 would be already a lot.

I was planning on getting a tripod to put the parabolic too.

Since i live outside USA wanna buy it where i can avoid those expensive custom taxes, maybe uk or germany but i would like to check usa ones too.

I have another antenna i bought (this one) http://dx.com/pt/p/2-4ghz-100w-18dbi-directional-high-gain-18-unit-antenna-for-wifi-wireless-network-sma-34759

Bought it bit before i see on hak5 the table yagi one on episodes since i had one equal and i couldn´t get much signal(even my alfa awsu036h had 4 times more) i decided to mail hak5 to see if they had any on sale in the hakshop, but no. Later i see on episodes the parabolic one and i said to me it was more one like these.

My doubt too is with witch type of antenna i have a stronger signal a parabolic or yagi one?

Thanks

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You have to be very careful when asking for a stronger signal with regards to antennas - they don't amplify, they focus - it's almost exactly the same physics as a flashlight with mirrors behind the bulb to focus the beam. (Admittedly, this can be confusing as both amplifiers and antennas use gain, but they represent different things)

Be careful with using dBi as well - that term means decibels over (an) isentropic radiator (antenna) - effectively a 'point source' which radiates a perfect sphere of signal - any deviation from that sphere incurs a change in the rated dBi of the antenna - you can get a 9dBi directional antenna - a cheap 2-3 element yagi, for instance, and you can get a 9dBi 'omni' - a folded dipole or a 1/4 wave whip antenna or similar - both have the same gain, but entirely different radiation footprints.

As it so happens, people pay my company lots of money to install these kind of systems - We used to use an antenna called a 26T-2400 by a company called Andrews - it had about 23dBi gain and wasn't mad expensive, but it's been discontinued. You want to look for a '2.4ghz parabolic grid antenna' on the internet - there's a few on ebay amongst other sources.

Looking at what you've got already - the yagi - I don't understand what you mean by it not having as much power as the alfa unit:

even my alfa awsu036h had 4 times more)

The antenna will focus and direct the signal with it's 18 dBi gain - not add output power.
The alfa has a maximum output power of 1W / 1000mW / 30dBm (dBm is decibels relative to 1mW) depending on the modulation scheme you're using. In the real world, 802.11a/g uses OFDM and as such the alfa 'throttles back' to 250mW. 802.11b, however, uses CCK - the alfa will tx at 1W in CCK mode. This comes with a warning, however - you have legal limits that vary from country to country about the maximum Effective Radiated Power (EIRP) on the unlicensed (ISM) radio bands, of which 2.4GHz is one. EIRP is a function of antenna gain * output power - by attaching an 18dBi antenna to a 250mW transmitter, it's likely you're breaking the law...

Something else worth considering:

"- Impedância de entrada: 75 Ohms" (from the yagi antenna webpage)

Could be causing you issues - as far as I'm aware, the alfa has an output impedance of 50 ohms - matching a 50 ohm transmitter to a 75 ohm antenna causes standing waves to form in the transmission line - in other words, you lose power and possibly damage the output of your transmitter (alfa).

Neither design of antenna will have a stronger signal, particularly. Parabolic Grid antennas are great as they handle inclement weather much better in permanant installations (wind passes right through them) but are expensive because of the fact that they're designed for this. Yagis are great as they're light and cheap and perform well, but not as well as the parabolic reflector that's intended to be permanant.

Have you ever considered making your own? For about 10 Euro, you could build an axial mode helical antenna out of plumbing pipe and wire - here's a design that'll get you about 18-19dBi depending on construction. You get that old-school "Did it myself" hacker feeling too when it all works. I designed an array of these (4 in parallel) that didn't cost a ton and I used for satellite tracking (stepper motors/etc) with roughly 24dBi gain (diminishing returns after 1, really)
Maybe you could try a cantenna - they're cheap and well documented!
Research, read, hack and ask dumb questions!

If all else fails, give me a shout on IRC. I can talk about this all day. The guys in #hak5 know it all too well.

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  • 4 weeks later...

Hibby:

Like your response! Good info.

How does one match an antenna (yagi, parabolic, panel, rubber) with a wifi usb adapter

such as the ALFA usb adapater AWUS036NHA on the Hak5 site.

My understanding is that one sums the db from the adapater and the antenna to

get the maximum db. Please correct me if I am incorrect. I'm also getting

the sense that one also has to match the ohm of the adapater with

that of the antenna?

Does anyone know what the maximum legal Tx power in Canada?

I've done some research on the Industry Canada website but I'm still struggling and clearly

there are people on the forum who know their stuff very well and I was hoping

that someone might be able to provide the info.

Comparing between a yagi and a flat panel antenna (Alfa 14db flat panel on Hak5 shop)

with both having the same dbi, would both have the same theoretical distance, excluding

the issue of degree of beam width?

If it does have the same theoretical distance, again assuming that all other external

variables are the same, (i.e. line of sight, no obstructions, etc, etc), would one

have a better advantage over the other?

I'm hoping that you can provide an thorough and well thought out explanation as you

have already done. I need all the help I can get.

I would like to get a yagi, but if I can get a flat panel, which is a little easier in terms

of placement, then I'll go that route.

Hope you can help.

Thank you.

Regards,

Lou

P.S. All yaggied on this on this one.

Other online references to this stuff for a beginner would also be appreciated.

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  • 2 weeks later...

Right, so - There's a few things here to look at. There's a couple of types of matching that you cover.

First off: Electrical.

The alfa should have an antenna out port. You can assume, because I tell you so, that it's output impedance is 50 ohms. The laws of electromagnetism state that conductance is most efficient when the load is matched with the source. In english, this means that your antenna has to be at 50ohms, or close to 50 - not doing so will cause you to lose transmit power and receive sensitivity.
The background on this is: The impedance value is frequency dependent - this is why we don't call it a resistance. Your antenna has a certain frequency at which it transmits most efficiently, and at this point the impedance happens to be 50 ohms. By ensuring your output, transmission line and antenna are all matched, you minimise your losses and maximise your potential output and efficiency - in real English, you lose less power to the magic power eating pixies. When the load, line and source aren't matched, you get waves reflected from the antenna back to the transceiver and standing waves in the transmission line, in this case the alfa card - this can possibly damage your card and further reduce overall transmit efficiency. (There's an excellent, visual experiment you can do to prove this using an oscilloscope and small power supply that I'll explain another day if someone desires). Electrical matching is important as you're minimising your losses. Remember, though, that we're working at really low power here - 1W. A 3dB loss really isn't that bad. I work with systems that are often hundreds of W and sometimes in the kW region - shitty standing waves and a 3dB loss there is really fucking bad news for the hardware and financial bottom line!

Secondly: Signal Matching.

You talk about summing dB - yes, it's very important, but be careful when you do it! Remember dB are a dimensionless number - they indicate a ratio, nothing more. Ensure you consider what 'type' of dB you're using. Antennas are often measured in dBi - decibels relative to an isotropic antenna (a theoretical 'point source' that radiates in a perfect sphere. impossible), but can be measured in dBd, decibels relative to a dipole (the simplest antenna) and less frequently dBq, decibels relative to a 1/4 wavelength 'whip' antenna. Watch out for dBm - this is a power level relative to 1mW and not to be used calculating proportional losses. The key things you're interested in are line loss - the loss, in dB per meter, of your coaxial cable / transmission line.

Multiply that by your output power:

Power out = Power in * 10^(loss/10)

and you get your power inserted into the antenna - we'll call this insertion power for the next section!

Thirdly: Antenna Gain!

Always, always, always remember this: Antenna gain is passive.
Repeat after me: Antenna gain is passive! Antenna gain is passive!
What does this mean? It means that by using a directional antenna, you are not gaining any power. You are focusing the beam, and the output of: (antenna gain * insertion power) is called the EIRP: "equivalent isotropically radiated power", but for short we can call it ERP or Effective Radiated Power, and is measured, like all power, in watts, W.

Note the word Effective. That means that the power 'is like' the output of the isotropic antenna at that insertion power. Remember the isotropic antenna? The impossible perfect sphere of radiation?

All you're doing with a directional antenna is focusing your beam to give the effect of a higher power transmitter and higher sensitivity receiver.

You calculate your ERP with:

Insertion Power * antenna gain = ERP.

And antenna gain == 10^(antenna_gain_dB/10) for all intents and purposes. Transform the gain in dB into a proportinal number by un-logarithming it.

It's worth noting that I get all concerned about these things and have to consider them because at the high powers I work with, and with the transmission line lengths I work with they really, really, really fucking matter. At your 1W output (maximum legal output for 2.4GHz ISM band, also known as "wifi"), the effect is drastically minimised, and it will only be equivalent to -1dB or -2 dB total loss, about 15-30%, which isn't that much. If I lose that much, it could be 300-400 watts, which is a lot of money to waste, and a lot of heat to generate!

If you're really, really interested, I'll go into more detail with actual proper maths on my blog. But I'd advise, if you're still here dear reader, to get an Amateur Radio license with your local Ham Radio club. You will learn how radio works, and be legally entitled to build high power transmitters, antennas and all sorts of groovy shit. It's not just about talking to lonely old men late at night - you can build every part of your data transmission system from modem to radio to antenna and communicate with people on the other side of the globe. How cool is that?
I learned everything I knew about RF there from some of the smartest, most creative people I've ever met at my local radio club, until I became a radio professional. Then I realised I knew more than most folks in the building anyway!

I got distracted. Did I even answer your question?

Edited by Hibby
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