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please help me understand antennas

mrQQ

Occasional Visitor
hello,

i want to ask a theoretical question which has been bugging me.

Say, we take a Netgear R7000 AC1900 router. There are tons of ads on internet for "high gain antennas".

What would happen if we were to replace antennas on the R7000 with some +9dbi antennas?

Are the antennas on R7000 somehow special with their length etc being precise for something?
Are they already "gained", eg amplified to the maximum usable power before noise floor becomes an issue?
Would it actually work and give some benefit?

I don't find much info about the subject. I found company called Amped who claim they do amplified routers, but cannot find reviews of them, except one user claiming it did nothing.
 
Higher gain antennae than the stock (usually 2dBi to 5dBi) antennae will narrow the beam pattern and give a longer range on the horizontal plane but decreased coverage in the vertical plane.

Think of the stock antennae as close(r) to a sphere of radiation / coverage. The high gain antennae would be more like a flattened donut type of coverage. Benefits and draw backs to both.

Usually drawbacks for most home users though.
 
Vendors try to provide the best coverage in a general use case - they're not trying to reduce range to sell higher gain antennas..

Since each location may be different, they work towards the best case - high gain antennas can work for specific locations, but the higher the gain, the flatter the pattern, and this can actually hurt coverage if client devices are not in the peak of the Radiation Pattern.

I'm attaching a pic I cribbed off antenna-theory.com that shows this very well - again, the higher gain will squish it flatter..

pattern.png
 
Higher gain antennae than the stock (usually 2dBi to 5dBi) antennae will narrow the beam pattern and give a longer range on the horizontal plane but decreased coverage in the vertical plane.

Think of the stock antennae as close(r) to a sphere of radiation / coverage. The high gain antennae would be more like a flattened donut type of coverage. Benefits and draw backs to both.

Usually drawbacks for most home users though.

Reme
Well.. the typical after-market "rod" type of "gain" antenna claims are 5-9 dBi vs. about 2 for factory antennas. The doughnut pattern at 5-9dBi is not very flat, whereas at 9-12dBi it is.
Numbers: envision 7 degrees on the vertical plane (180 degrees is horizon to horizon in a flat place). Stretch your arm out, then move it up 7 degrees. That's a 12dBi omni antenna. Go down to 5dBi (as are more common), and the vertical beamwidth is much more spherical, say, 20 degrees... less flattened doughnut.

BUT the big issue here is: often people have a shortfall of 20dB or more in signal strength. And you need 10dB excess to allow for intermittent blocks such as human-body blockage (attenuation in the path). SO, going from 2dBi stock to 6dBi "high gain", is just a 4 dB improvement - not much if your shortfall is 20+dB in certain areas.

the high gain antennas are over-marketed to the lay person and they sell at huge multiples of what plastic and wire cost if you buy from storefronts and the like, vs. other sources.
 
Well.. the typical after-market "rod" type of "gain" antenna claims are 5-9 dBi vs. about 2 for factory antennas. The doughnut pattern at 5-9dBi is not very flat, whereas at 9-12dBi it is.

Now that we're into dual band devices - a lot of these aftermarket antenna's have bad matches for 5Ghz...

Vendor provided add-ons, might help, but I would stay away from generic antenna's on the eBay or the List from Craig... RF design with antennas is a science, and in some ways, an art-form...

I would stick with the OEM provided antenna's myself and just move the AP to improve coverage to where the clients generally converge at...
 
I would stick with the OEM provided antenna's myself and just move the AP to improve coverage to where the clients generally converge at...
Agree.
A 4dB better antenna is not an alternative to a coverage-fill-in Access Point - which is a 30-50dB improvement.
 
Agree.
A 4dB better antenna is not an alternative to a coverage-fill-in Access Point - which is a 30-50dB improvement.

What most folks fail to realize, moving the AP a few inches to the left or right can make a huge difference in 2.4GHz - much more than a big black rubber dick, erm, rubber duck...

math follows...

c/f = (3e8 m/s)/(2.4e9 1/s) = 0.125 meters = 4.92 inches​

So move the AP to the left or to the right, might be surprised, lol...
 
What most folks fail to realize, moving the AP a few inches to the left or right can make a huge difference in 2.4GHz - much more than a big black rubber dick, erm, rubber duck...

math follows...

c/f = (3e8 m/s)/(2.4e9 1/s) = 0.125 meters = 4.92 inches​

So move the AP to the left or to the right, might be surprised, lol...

Same applies to router location too.
 
But so too, moving the client a fraction of a wavelength.

"Favorable and unfavorable multi-path". The joy and sorry both, of MIMO.
 
Moving a few inches makes a huge difference...is this the basis for rotating the antennas of a router to find the best direction/position?

2 follow-up questions:-
a. how 'huge' can this gain be? signal reception going from 'barely receiving' to 'significantly strong'?
b. does anyone here actually have this experience in a real life situation? would love to hear a story on this.
 
Well, it probably won't be huge, but it will be significant, and it's due to some physics related to RF propagation and fading - specifically Rician fading

That, and multi-path as well, where two reflected paths can cancel themselves out sometimes, and sometimes they constructively assist - generally, without getting too deep, is I suggest moving either the AP or the client about a foot, and sometimes that can make all the difference - depends on the environment.

That's not going to solve for poor coverage - remember, RF loss over distance is not linear, it's a 10LogN function (that's why we measure RSSI in decibels, BTW)

Rician Fading - You may have seen this first hand on your mobile phone - where it can go from 4 bars to 2 bars in a couple of feet..
 
Moving a few inches makes a huge difference...is this the basis for rotating the antennas of a router to find the best direction/position?

2 follow-up questions:-
a. how 'huge' can this gain be? signal reception going from 'barely receiving' to 'significantly strong'?
b. does anyone here actually have this experience in a real life situation? would love to hear a story on this.

Moving the router or client will make a small difference. But tomorrow, things will differ. That's why you need AP placement, etc., to have lots of excess signal strength to accommodate changing attenuation and multipath conditions. But don't go euphoric if it works better at this instant.
 
Moving the router or client will make a small difference. But tomorrow, things will differ. That's why you need AP placement, etc., to have lots of excess signal strength to accommodate changing attenuation and multipath conditions. But don't go euphoric if it works better at this instant.

True - and the best thing is to put the AP close to where people actually are...

Too many times, we see folks throwing money at bigger/faster AP's from the far side of the house/apt when spending a bit of time/effort to move things around...
 

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