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If I had:

a dish that claims 24db of gain

a yagi that claims 24db of gain

a patch that claims 24db of gain

and they are all tuned to the same frequency, would you expect equal range out of each? Is one type of antenna any better than the other for any reason?

Thank you,

-dave

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If I had:

a dish of 24db gain - it'd be pretty tight in beamwidth

a Yagi of 24db gain - it'd be extremely long and unweildly

a patch of 24db gain - I'd be amazed!

All things being equal, any antenna with 24db gain would have pretty much the same 'reach'

Nigel.

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The dish will tend to have the cleanest beam pattern so would go top of my list.

Terry

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Thank you very much for your insight. Now on to my next question. Assuming I go with a dish antenna, I would like to calculate the expected gain for a dish of d diameter. From Wikipedia (http://en.wikipedia.org/wiki/Parabolic_antenna) I learned that the Gain G of a dish can be expressed as

G = ((4 π A) / λ2) eA

or

G = ((π2d2) / λ2) eA

Where

G = Gain

A = Area

λ = wavelength

d = diameter

eA = is a "dimensionless parameter called the aperture efficiency. The aperture efficiency of typical parabolic antennas is 0.55 to 0.60."

So for an example, if I look at a dish from Hyperlink (http://www.l-com.com/item.aspx?id=21730) that claims a gain of 30dbi, I get confused. Perhaps you can point out the error in my very limited math skills.

G = 30dbi

d = 150cm

λ = 12.2449cm

30 = ((π2*1502)/12.2449 2)eA

30 = 1481 * eA

eA = 0.02 very far from the .55 to .60 I expected.

So where did I go wrong? I would like a 30dbi gain dish but I am unwilling to spend $429. I might be able to make one using another large dish as a mold. Does anyone know the gain of a DirecTV dish?

Thank you for the continued enlightenment and education.

-dave

PS Happy New Year!

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Most of the claimed gain figures are pure BS and marketing fantasy.

Have a look at this for some ideas

http://www.wlan.org.uk/antenna-page.html

I don't think you are going to achieve 30db very easily.

Why do you *need* such high gain?

Wouldn't 15 - 20db suffice? as antennas in this range are much easier to find/construct with minimal equipment requirements.

Nigel.

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If you are simply interested in the overall efficiency of the L-Com dish design then try this calculator: http://www.csgnetwor...liceffcalc.html / When I plug in your numbers I get 70%.

I agree with Nigel -- the gain claims from many antenna suppliers need to be taken with a grain of salt. The antenna industry does not have a common method for measuring gain, few suppliers factor losses from the integrated cables and connectors, and so on. For FPV, antenna gains above about 18dBi are probably not practical due to the few honest opportunities to find them and the hardships in precisely aiming the antenna at the moving model.

.

Edited by Mr.RC-Cam
Fixed Link.

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Nigel,

That link is awesome! Thanks!

Thomas,

Your link is not working. :( I tried searching the site but did not come up with anything.

Perhaps I am over thinking this. I want a wide margin of error (+6db) for an upcoming mission. I need to fly several kilometers over ocean and can not have a video failure.

Given:

Rx sensitivity -80db for an acceptable picture

Rx antenna +10dbi

Tx +27 dbm

Tx antenna 0db gain

Cable loss 3db (1.5 on each side)

2510Mhz

10km

Then I'll have 120db of path loss and an RSSI of -86.4dbm.

http://www.distributed-wireless.com/calculators/pathloss_RSSI.html

So, theoretically, I need 6.4db more just to break even and 12.4db to be within my margin of error.

And if for some reason I need to go 20km? Pathloss goes to 126.4db and RSSI will be -92.4dbm, so add another 6db to the 12.4db wanted above = 18.4db. And this needs to be added to the 10db antenna I currenly have and that equals a 28.4db Rx antenna. Right? Maybe I am way off, please share your experiences, they are appreciated.

-dave

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Have a look over on RCG at scrtsqrl's similar mission requirements

http://www.rcgroups.com/forums/showthread.php?t=1324112

A thought occured to me - could you use a small 3 ele Yagi on the VTx?

If you have it mounted on a 180 degree servo and have it 'pointing' at you for the outbound flight, as you turn for home, rotate the servo through 180 and have it 'pointing' at you on the inbound flight?

3 ele Yagi has around 6dbd gain.

I use gain in dbd opposed to dbi because dbd can be measured and dbi is more or less theoretical.

Nigel.

Edited by Devonian

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Thomas, Your link is not working. :( I tried searching the site but did not come up with anything.

I modified the link. If it still does not work then cut/paste it into your browser.

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I agree with Nigel that once you are going over 10k or so there is a real benefit in pointing the TX beam. Even a very non critical 6db is a fantasic help and could be adjusted manually from your controller or by gps in reverse to the ground based systems that seem popular now.

Terry

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Your link budget seems to be a bit pessimistic, but that is perhaps the safest way to approach your project. BTW, -3dB coax cable loss is unusual (and easily avoided).

Here's a couple of tricks that may help you determine your range limits with low risk to your model.

1. Install a -30dB RF attenuator (e.g., Mini-Circuits VAT-30+) on the video Rx's antenna. This will allow you to test fly at close range. If this achieves > 2km range and the RSSI voltage is not at its limit, then you should be able to achieve >10km without the attenuator.

2. Configure a temporary test setup with two Rx's. One Rx will have an antenna that is at least -6dB lower performance than the other. Or, use the same antenna on both, but install an RF attenuator on one of them to range cripple it. Connect both to your video display using a A-B A/V switch box (if you have an Oracle Diversity Controller you can use its manual switching feature for this). Start your flight with the crippled antenna. Fly in a safe direction until you see its Rx's RSSI voltage hit the limit, or constant snow appear in the screen. Then switch to the good antenna and end the test; you have indirectly found the range limit distance. This information will tell you the distance your full range antenna will provide. Basically, crippled distance + better antenna's gain difference = max possible distance.

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I think it is a little better to use the 30dB attenuator at the transmitter. When you use the attenuator at the receiver you could be masking local interference during the test. Best to do it with the receiver operating at the normal sensitivity/NF.

However, you could also argue that using the 30dB attenuator at the receiver would be safer if you do it with a switch to quickly bypass the attenuator. This would allow you to test at the limit knowing if it got too bad to fly, you could switch back to a full strength signal.

OMM

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Looks like we all agree on that then ;)

Good point about using the attenuator on the TX though, I have reported in the past that I made the signal better by using an attenuator on the RX. I wont go into the details of how and why just now though.

Terry

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Thank you very much for the idea of using an attenuator. I just won a bid on a really expensive Pasternack stepped attenuator on eBay (won for only $60!) but I don't think it is rated up to 2.5Ghz. The fixed attenuators are not that expensive so I will look at those. I know Meca and MiniCircuits mentioned make descent ones. Are there any other recommended brands or brands to stay away from?

Thanks,

-dave

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