Thread: Tall Wifi Antenna

I know what you want to do. I get it. Anyways, do some creative engineering. Get your best wifi omni antenna. Get a cheapo CB antenna from radio shack, a nice big one, or whatever matches your other antenna. Build something that will bracket that antenna to the top of the other one, or run it right in back of it, truck side.

Problem is, you probably want to mount it low on the truck, and the truck will shield it from the other side. Just try to make the wifi antenna look like the cb antenna.

You will gain nothing by making a wifi antenna 6 ft long. You could also get a hollow fiberglass pole, to match the other side, run your coax up through it to a proper antenna at the end. Just get one of those small wifi antennas, and put it inside the tube, hot glue it in place or something.

Good luck!

WHOOOAAA Trigger...Lets to back to Antenna Basics 101.

*** First the math for the middle of WiFi band (2412-2462 MHz):
Wavelength is:
300,000,000 meters/sec. divided by 2,437,000,000 cycles/sec. = .123 meters
or simply 300/2400 MHz = .123 meters
For a quarter-wave radiator:
.123/4 = .0308 meters or 3.08 cm
Converting to inches:
3.08 cm = 1.20 or 1-3/16"

*** Second a quarter-wave dipole plane and ground plane antenna:
A dipole is to opposed radiators, like a "T". It has UNITY GAIN or zero gain.
A ground plane replaces one of the radiators where the ground plane is at least 2 wavelengths in diameter.
Therefore a 9" pie pan with a radiator poking up through it works fine as a UNITY GAIN antenna compared to a dipole.

*** Third - What is Unity gain?
Depends on the standard you are comparing to.
Commercial communication antenna manufactures have always used "gain over a dipole" comparison in the past.
WiFi antenna manufactures use "gain over an isotropic source" comparison.
I'll skip the explanation and just give figures for comparison where dBi and dBd stand for decibels of isotropic and decibels over dipole.
0 dBi = -2.15 dBd 2.15 dBi = 0 dBd (a dipole or ground plane)
3 dBi = .85 dBd 5.15 dBi = 3 dBd is 2x the gain over a ground plane
6 dBi = 3.85 dBd 8.15 dBi = 6 dBd is 4x
9 dBi = 6.85 dBd 11.15 dBi = 9 dBd is 8x
10 dBi = 6.85 dBd 14.15 dBi = 10 dBd is 10x
So if you want to compare real life antennas, build yourself a simple ground plane for your standard.

*** Fourth - What is a "long antenna"?
You can "stack" the radiating elements for more gain. Each radiating element must be spaced 1/4 wavelength apart.
If length is not a problem, you can use an out of phase element for the spacer; but it is a problem as in mobile antennas, then you replace the spaced with a physically short (but still an electrical quarter wavelength long) coil of wire.
Each additional radiating element ARRAY gives an additional 3 dB for gain (a doubling).
There is such a thing as a "long wire antenna". Its called a Zepp Antenna (Just Goggle for the damn explanation)

*** Fifth - What is an ARRAY?
Its the correct multiples of quarter wavelength radiators

Tilted on the side, a ground plane looks like this:
---|

A two element co-linear antenna (3 dB gain):
---***---|

A four element co-linear antenna (6 dB gain):
---***---***---***---|

An eight element co-linear antenna (9 dB gain):
---***---***---***---***---***---***---***---|

Now...on to my responses...............

EBFoxbat is looking for a cosmetically matching antenna mast.
"Can I get anotehr tall antenna tuned fo wifi that is similar in length? I'd like to have one on each side. thanks."

gizmomkr answered with:
"Even the ones listed on that site are 2 ft max. Most CB whip antenna I see are closer to 6."
*** CB (27 MHz 0r 11 meter antennas can be just about any length. From a 9' (108" or 96" with 6" spring) quarter-wave ground plane to an inductively loaded whip (usually wire wound around a fiberglass rod) usually 4' long.

longwoodtrail pondered:
"Is it not possible to create a 6ft antena for 2.4ghz range? What I am trying to ask is, is this outside the parameters for WiFi, or do manufacturers simply not make them?"
*** No. It would be 60 element co-linear antenna. The losses from all the joints would defeat an gain.

S80 calculated:
"So the optimal length for a 2.4GHz antenna should be 4.9 inches or 12.5cm."
*** If you are thinking of a simple quarter wave antenna, no. It would be 3.08 cm.

erazor offered:
but as far as i know you could also make tha antena x * 12,5cm so you can have 25, 50, 100 but an larger antenna means lower signal as said optimal is 12.5
*** I'm not clear what you're saying. See my response to longwoodtrail.

kiltjim theorized::
"In thoery, you could build an antenna that is 6 foot in length, but you would need an extremely low velocity factor, or an extremely large gain."
*** See my response to longwoodtrail. Velocity factor has NOTHING to do with it, you're thinking of the LOSS FACTOR. Velocity factor is used in another part of the calculations altogether.

and added:
"Also, the difference in appearance between the CB whip and a rigid WiFi antenna is going to be significant. You are not going to find a whip, it just does not exist."
*** Take the 6' CB antenna and drop a piece of 1/2" PVC over it. Stick a USB WiFi dongle in another 1/2" piece of PVC and mount it on a spring.

"I knew you could do it..." <-- Mister Rogers, circa 1980

nevtxjustin lectured:
lots of information

damn.. awsome post.. i learnt so much.. .. good work

Originally Posted by kbyrd

So, I've used the 48" 12dBi antenna on this page. It was mounted on top my house (I helped get this wifi ISP going). It worked great.

What I don't get is that based on what you wrote it shouldn't have really been a 12dBi gain? It was quite a bit of gain (even with 25 foot LMR-400 run between the omni and the radio), I could get a signal with my laptop down the road quite a ways.

I'd like to know because my plan was to install a shorter one inside the car.

Finally, a question about polarization. If I mount the omni in the car, I really need to keep it upright to match the polarization of my AP right? Does this matter?

yea it does matter which way you have the omni mounted. In the most simple of terms you can think of it as having a disc pattern around it that the radio waves come out of. Say you were able to change the polarization in your AP, your omni would still only be receiving in two directions laying ont its side (Because towards the ground and the sky really dont matter) so it would be useless unless it was pointed correctly, in which case you should be getting directional antennas for that use anyway.

I am a minor partner in a WISP http://www.wtechlink.com in Eastern Oregon. From our downtown base site, I can pick up our signals 5 miles away just using the antenna in my laptop. I just use a small 6" 5dbi antenna on my car and will pick up AP's quite well (and picks up our towers for miles)