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Old Aug 01, 2010, 02:33 PM
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Old Blighty
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Yagi vs Panel antenna (why choose Yagi?)

When you consider how much more practical a panel (patch) antenna is due to its small size compared to a Yagi, what advantage does a Yagi have over a panel antenna when both offer identical gain?
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Old Aug 01, 2010, 03:40 PM
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Well, generally it'd be pretty pointless to choose a Yagi with identical gain
to a patch. The usual benefit of a Yagi is that it provides much higher gain
than a patch is capable of when you get up over about 9dB. A high gain patch
usually ends up being quite large and heavy. A very high gain Yagi can be quite
long, but otherwise is pretty light, and of course very easy to aim exactly where
you want.

But assuming they did have equal gain, one benefit to a Yagi is that,
at least in the lower frequency bands (like 900Mhz and 1.3Ghz) they don't need
an external ground plane to be maximally efficient, while often the patch
antenna works best quite close to the physical ground. This means you can
mount a Yagi on a high tripod to get over ground clutter (grass/bushes etc)
and know you're getting the full gain. With patches it's often a balance between
having it high enough to get clear LoS, but low enough to get highest gain.
That's why I use diversity with one on a tripod and the other on the ground.

The most practical difference though is that Yagis just almost always have
a much narrower beam width (thus their higher gain) and it's very one sided in the
forward direction. You generally can't fly very far to the side or behind a Yagi,
while a patch sitting on the ground can almost act as an omni.

These are all generalizations though. I'm sure someone will jump in and
tell you that they can fly a mile behind their Yagi and they mount
their 900Mhz patch 7 feet up on a pole..

ian
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Old Aug 01, 2010, 04:04 PM
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Quote:
Originally Posted by Daemon View Post
Well, generally it'd be pretty pointless to choose a Yagi with identical gain to a patch.
With your introductory sentence, you pretty much summed up my suspicions.

When using an antenna tracker a panel antenna much surely have the edge here also, as it's so much easier to manoeuvre.

It's just I use an 18dB 2.4GHz panel antenna and when dealing with gains of this level most people seem to be using Yagis. I could never really understand why, that's all.
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Old Aug 01, 2010, 05:02 PM
BEOWULF
North vancouver, B.C. Canada
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lets say you have a yagi and a patch same dbi

It would be interesting to see how much further the patch can see behind it?

If the lobe is larger with the patch, behind the dirrected target, the plane, multipath would be more likely, than a yagi.

the advantages to yagi's is to help with the elimination of multipath

for video and rc control
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Old Aug 02, 2010, 03:06 AM
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Here is a link to my 2.4GHz 18dB panel antenna with polar traces…

http://www.solwise.co.uk/wireless-ou...panel-18pn.htm

And here is one to a 2.4GHz 18dB Yagi antenna with polar traces…

http://www.allendale-stores.co.uk/wi.../18db_yagi.pdf

With respect to multipath – is this a genuine issue which FPV flyers encounter? I’m only asking because multipath issues were something I believed only concerned reception in heavily built-up areas, like trying to get a GPS signal in a city? Most people fly from large open areas where there is little for the signal to bounce off.

Also, I’ve often read in these forums that a panel antenna needs to be close to the ground to work best. Again, is this really the case? I have no scientific evidence to say otherwise, but it’s something I’ve only ever seen stated in this forums and FPV use aside, panel antennas are widely deployed and they’re normally mounted up a pole just like every other antenna.
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Last edited by Ian Davidson; Aug 07, 2010 at 12:34 PM.
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Old Aug 02, 2010, 03:34 AM
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Quote:
Originally Posted by iandavidson99 View Post
With respect to multipath Ė is this a genuine issue which FPV flyers encounter? Iím only asking because multipath issues were something I believed only concerned reception in heavily built-up areas, like trying to get a GPS signal in a city? Most people fly from large open areas where there is little for the signal to bounce off.
Agreed, most folks also fly close in where it becomes more of an issue, esp. in an urban environment.

Quote:
Originally Posted by iandavidson99 View Post
Also, Iíve often read in these forums that a panel antenna needs to be close to the ground to work best. Again, is this really the case? I have no scientific evidence to say otherwise, but itís something Iíve only ever seen stated in this forums and FPV use aside, panel antennas are widely deployed and theyíre normally mounted up a pole just like every other antenna.
I've never seen the effect described. I always mount my panel/patch antennas high up so I don't have to worry about disturbing the Fresnel zone and hence can fly further out without having to worry about the antenna being blocked. Having the antenna on the ground means flying further out becomes impossible as the ground itself blocks the LOS required.

http://en.wikipedia.org/wiki/Fresnel_zone

Cheers,

Sander.
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Old Aug 02, 2010, 03:42 AM
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Thanks for the feedback Sander. I agree entirely with your statements.
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Old Aug 02, 2010, 07:24 AM
BEOWULF
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Quote:
Originally Posted by iandavidson99 View Post
Here is a link to my 2.4GHz 18dB panel antenna with polar tracesÖ

http://www.solwise.co.uk/wireless-ou...panel-18pn.htm

And here is one to a 2.4GHz 18dB Yagi antenna with polar tracesÖ

http://www.allendale-stores.co.uk/wi.../18db_yagi.pdf

With respect to multipath Ė is this a genuine issue which FPV flyers encounter? Iím only asking because multipath issues were something I believed only concerned reception in heavily built-up areas, like trying to get a GPS signal in a city? Most people fly from large open areas where there is little for the signal to bounce off.

Also, Iíve often read in these forums that a panel antenna needs to be close to the ground to work best. Again, is this really the case? I have no scientific evidence to say otherwise, but itís something Iíve only ever seen stated in this forums and FPV use aside, panel antennas are widely deployed and theyíre normally mounted up a pole just like every other antenna.
I wish I had a wide open area to fly as you mentioned. All there are are mountains and trees around here.

Look into "front to back ratios" and see if yagi and patch are the same?
You should be able to fly further behind a patch, than a yagi, thus create more multipath.

For closer flying patches work well, but for long distance yagis I have found work much better.
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Old Aug 02, 2010, 07:29 AM
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Personally I prefer a grid antenna.
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Old Aug 02, 2010, 08:20 AM
BEOWULF
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Here are someone elses drawings to help understand the benifits of a more dirrectional antenna, with less ratio going out the back.
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Old Aug 02, 2010, 09:21 AM
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Originally Posted by bracky72 View Post
Personally I prefer a grid antenna.
I wonder how a grid antenna compares in the performance vs practicality ratio?

From what I've seen, grid antennas offer mega gain, but an equally mega size. Not very antenna tracker friendly perhaps
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Old Aug 02, 2010, 09:23 AM
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I have had a lot of experiments with patch antennas and I can say that the performance is far superior when it it close to the ground. Ideally, you want the bottomw of your patch antenna 1/2 wavelength above the ground. For 900 MHz this is 6".

Just an idea of how much it matters:

A friend of mine (Shpoa) copied my exact ground station except he used the Oracle for a diversity controller (I used my homebrew diversity unit) and my antennas were 6.5" off the ground and his were 8.5". So everything else was the same, antennas, angles of the antennas, and even the same RX units. We both watched his plane and the best he could do was 1/2 mile before the signal went seriously weak, while I still had perfect picture. He then lowered his antennas to 6.5" (1/2 wavelength) and had great picture. He was able to keep track of my plane for over a mile where before he'd lose me at 2500 feet.

-Alex
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Old Aug 02, 2010, 09:27 AM
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Quote:
Originally Posted by iandavidson99 View Post
I wonder how a grid antenna compares in the performance vs practicality ratio?

From what I've seen, grid antennas offer mega gain, but an equally mega size. Not very antenna tracker friendly perhaps
Every antenna has it's disadvantages:

Patch: Needs a good ground plane, thus must be mounted low and can disturb the Fresnel zone when flying low.

Yagi: Most likely to receive multi path interference. Poor window to the sides.

Grid: Large and easily damaged. Has deep nulls.

Moxon Rectangle: Weak signal to the rear of the antenna (high F/B ratio)

Dipole: Shortest range

-Alex
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Old Aug 02, 2010, 09:38 AM
BEOWULF
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I have been useing a 1.2 yagi or a patch up on a pole, tilted 6 degrees, and have great success.


IBCrazy
can you further explain why a yagi with a narrower window to the sides and smaller back ratio would be more prone to multipath?

here is an interesting link
http://www.qsl.net/aa3rl/ant2.html
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Old Aug 02, 2010, 11:48 AM
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I believe the multipath issue is due to the fact that the Yagi thrives on reflections. The patch antenna is a different animal. The front panel is designed and sized for a specific impedance. Also the impedance of the face plate is different at different points on the element. Beyond those factors, I have no clue.
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