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cactusjackslade

900MHz vs. 2.4GHz - Any conclusive comparison?

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I've been using a 1000mw 2.4GHz downlink with Diversity Rx for my video feed for a couple years now and it works pretty well, but I have been contemplating switching to the ORACAL so I can go to 900MHz hoping that I can get better obstacle penetration.

I also use my wireless for live brocast on a helmet cam for bicycle races and would like to improve the reception when obstacles interfer (trees, small hills buildings etc).

Does anyone have a good (real use) comparison between a 1000mw 2.4GHz and a 500mw 900 MHz system with similar receivers?

Thanks in advance, and thanks RC-CAM for a great forum :)

CJS

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oh that is much more complicated.

since you are using SOO much power and probably not extremly far,

your problem is multipath fading.

ok you say you use diversity, but what is the distance between the antennas ?

if you have an even number of wavelength the diversity is useless !

use ½ or 1½ or 2½ and so on distance !

also if your multipatch fading is comming in a phase in parallel with your two antennas, but 180 Degrees out of each other = a hole !

so to avoid this compleetly you need 3 antennas mounded in a triangle.

ok back to the 900 vs 2400:

lower frequency travel longer at same power.

and at lower frequency it is much cheaper and easyer to make a good receiver,

and at lower ferquency the antennas length is longer, will pick up more air space = better.

but again I am 100% sure your problem is multipatch fading, due to wrong use of your antennas, it is not that simple and easy, and not that well documented out there.

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Antenna distance (apart) vs. angle?

What about antenna angle, here is my set-up, I am trying to be compact but maybe I need to revise my patch antenna configuration? Suggestions?

Thanks,

CJS

post-8-1179168646_thumb.jpg

Edited by cactusjackslade

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what is your center ant to center ant distance ?

are both your antennas horizontal polarized ? and pointing both up/down,

that will also explain alot, specially if they are often placed horizontal,

with same distance to ground = useless diversity, same bad-phase will hit them both at the same time,

try to position them 45 degree, so they are NOT horizontal to the ground,

and make the distance from center to center 1½ wavelength or 2½.

right now I think you are close to 1 wavelangth making it bad.

also a patch antenna MUST be free from stuff arround its sides,

so it will help them both with this extra 3-4 inches distance you need,

but please measure and calculate and make it exact.

if your patches are not the special circular types, you MUST rotate one of them 90 degree from the other, your gain will be 26db with this feature, so be happy if they are the normal vertical/horizontal types.

for non diversity systems one circular polarized is a great advantage !

for diversity systems it is better to use two horizontal/vertical mounted.

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Wow, I'm very impress.

If i need to install a diversity receiver, I will think twice. I will certainly ask Thomas to give a look at my installation. It's much more complicated than I tough.

Cactus, if you was not knowing how to position your antennas, you are like 99% of us as I discovered.

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I found the spec's on what I have:

To me it looks like rotating these particular ones may not help... but maybe moving them your recommended distance apart and staggered 45 degrees?

Q: How would one figure the center to center distance apart - what is the formula for 2.4 Ghz?

Electrical Specifications

Frequency 2400-2500 MHz

Gain 14 dBi

Horizontal Beam Width 30 degrees

Vertical Beam Width 30 degrees

Impedance 50 Ohm

Max. Input Power 25 Watts

VSWR < 1.5:1 avg.

Lightning Protection DC Short

Mechanical Specifications

Output Connector "N" type Female

Mounting 1-1/4" (32 mm) to 2" (51 mm) dia. masts

Vertical Tilt 0-60°

Weight .95 lbs. (.43 Kg)

Dimensions 8.5 x 8.5 x 1 (inches)

216 x 216 x 26 (mm)

Radome Material UV-inhibited Polymer

Flame Rating UL 94HB

Polarization Horizontal or Vertical

Operating Temperature -40° C to to 85° C

(-40° F to 185° F)

RoHS Compliant Yes

Wind Survival >150 MPH (241 KPH)

RF Antenna Gain Patterns:

post-8-1179295148_thumb.jpg

Edited by cactusjackslade

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Just as an add on, if you mount the patches so they are angled apart as in the picture you will not get maximum diversity effect but you will make the capture area wider.

Thomas, I see your thinking about mounting the aerials at 1, 1/2 or 2,1/2 wavelegnths apart but it dose not matter to much. If the signal is comming at an angle to the aerials then this will throw the phasing out anyway. If you can be sure that you are always pointed directly at the aircraft then it may well help.

Also you say to mount the aerials at 45 deg to the ground but it dose make a difference if you are doing long range flying. At only 1/2 mile range and 1000ft the angle is 21deg and at 1 mile range and 500ft the angle is less than 6deg.

I would try the patches mounted as Thomas said rotated 90deg to each other, both pointing exactly at the same aera and at an angle to the ground depending on the range of the flight. 45deg for very close but maybe high flights and 10deg for maximum long range.

Hope this helps :)

Terry

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See here what I mean:

two-patches.GIF

both patches you are using are the horizontal or vertical types,

depending on how they are mounted,

just see you have the antenna wire out the same end so they was mounted the same way.

just change that like my picture.

it is a know fact that a horizontal to a vertical mounted antenna will add 26dB extra loss !

and since your plane can be in any angle depending on how you fly,

it will be really bad to have both your patches in the same polarization.

it is another fact: that a horizontal to a circular polarized antenna will add 3dB extra loss, but also 3dB loss if the horizontal is then tiltet up to vertical position,

that is why so many people see great benefit when using them circular in a non diversity system.

ok the distance between your antennas is really important.

use the wavelength calculator to find the distance in the system you use,

I am in the metric world and you might not be :-)

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YES !! super, please try to fly at about the same range and same height as you did the last few times,

and then please compare the picture and how many drop outs you get,

have you any videos to compare with ?

and can you record with your system ?

it is a bit subjective to jugde the results,

so try to make it all as fair as possible, and try for example to count the number of visible drops in 1 minute, and compare that value.

I am really eger to hear about your results,

if poossible make / release a few , before-after clips so we can DL them ?

and jugde for our self ?

GREAT to see my text did generate a vision in your mind exactly the same as the vision I have i my head,

The power of letters and their combination is strong in our family !

Edited by ThomasScherrer

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I have a specific "route" I fly often, as I have been taking construction progression photos. I should be able to easily tell if the reception has improved... also I do not think I have any past video to compare it with, so it will be very subjective.

post-8-1179507362_thumb.jpg

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WOW that picture is a video grap ?

or a digicam ?

tell us the truth and nothing but the truth :-)

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WOW that picture is a video grap ?

Do you actually believe what you're saying? :P

Digicam of course, not even HD would give that ;)

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WOW that picture is a video grab?

or a digicam ?

tell us the truth and nothing but the truth :-)

I wish! If I could pull that off I could lose 11 ounces of digital camera!

It's with a Canon S80... a bit heavy, but the best digital cam I've use so far.

Still a south wind today :( ... maybe I can test the nex antenna setup in the next couple days....

Edited by cactusjackslade

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well Kilran, I have seen some of your video graps pictures,

some are close in quality, if I recall right ?

ok maybe they was a bit smaller in size also.

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Nah the ones you saw are certrainly my digicam ones too, like this one:

http://i18.photobucket.com/albums/b105/Kil.../PICT2757_t.jpg

Here's an HDV grab: http://i18.photobucket.com/albums/b105/Kil...rah/cricri1.jpg

A SD camcorder one:

http://i18.photobucket.com/albums/b105/Kil...Clipboard03.jpg

And some good SD KX131 grabs:

http://i18.photobucket.com/albums/b105/Kil...ipboard02-1.jpg

http://i18.photobucket.com/albums/b105/Kil...Clipboard01.jpg

Reminds me I really MUST edit that great footage...

The biggest problem with video grabs is this %&ç/ interlacing. If you want a good grab you MUST take a frame that has the least possible movement. Very hard sometimes, even if you have 25 potential chances per second, it often happens you have no steady shot of the subject you want... :angry:

Edited by Kilrah

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please see those files from today:

added RSSI output into the audio line. (I dont need audio)

http://www.webx.dk/rc/video-wireless/rssi001.jpg

http://www.webx.dk/rc/video-wireless/rssi002.jpg

http://www.webx.dk/rc/video-wireless/rssi003.jpg

http://www.webx.dk/rc/video-wireless/rssi005.jpg

see the two receivers.

http://www.webx.dk/rc/video-wireless/rssi004.wmv

this clip show why antenna placement distance is so important to be right.

http://www.webx.dk/rc/video-wireless/rssi005.wmv

to be 100% sure to have good signal, when indoor :

you need actually 3 RX units, since multipath fadings can happen on all 6 walls

reflections.

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hey again cactus

any luck with test flights ?

got any new videos ?

or any info on how much all this helped or not in real life usage ?

teory is a nice thing , but I also like personal opinion and real results :-)

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now I have tried with 3 receivers

http://www.webx.dk/rc/video-wireless/rssi0...06-triangle.wmv

all 3 mounted in a triangle, 2½ wavelength each side,

the video is an indoor test with super wierdo reflections from all walls !

conclution: outdoor it is impossible to make a nulling on 3 antennas at the same time if mounted in the right distance.

Edited by ThomasScherrer

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Antenna distance (apart) vs. angle?

What about antenna angle, here is my set-up, I am trying to be compact but maybe I need to revise my patch antenna configuration? Suggestions?

Thanks,

CJS

Catus: It would appear that you have both 14 DBI patch antennas vert. polarized. Work ok for you. I have mounted horiz. I have one of these antennas on a standard RX (Not Diversity) I have been 2 miles away!! Still received. What a test.

On my future hobbies diversity RX I have two patch antennas. Mounted horiz. and 70 degrees up. About the same outward angle as your setup. 18 inches off ground and in front of pit area. That way when people walk by in front will not cause drop outs. Also low becasue for take offs and landings while flying FPV VR.

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hey again cactus

any luck with test flights ?

got any new videos ?

or any info on how much all this helped or not in real life usage ?

teory is a nice thing , but I also like personal opinion and real results :-)

I have now flown twice with the new Rx antenna config, but without side by side comparisons, or a strictly regimented range test I think it very hard to tell how much an improvement this has made, but I do think it is an improvement.

When I reviewed the video the low level at far distance reception seems to have improved... meaning maybe a 75-100 feet above tree line and about .4 miles away in a "RF crowded" area. I had zero drop outs and very little interference.

The worst it got was when banked over, turning back towards me when the Tx antenna on the plane is pointing away from me.

Was it worth the change, I think so and I will keep it in theis configuration.

Thanks Thomas and all for the suggestions.

I still am curious about a 900MHz system however!

CJS

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