5.8Ghz antenna trouble shooting.

[OrangutangSurfer]

I have purchased several different CP antennas and have used them on a couple different 200mw transmitters. I have successfully flown to about 1000ft with out to much video loss. I have heard rumors with a 200mw Tx and some good CP antennas 2km was possible, and I am way short of that.

So... I bought a diversity Rx a cheap helical and patch antenna to try my luck, I set them up at different times with my CP antenna. The field i'm in is approximately 1000ft wide and 2000ft deep and runs off over a lake, so there are homes on 3 sides and a wide open area past the park over the water. despite the clear area the closer i get to the lake 500-600m range the video starts to give out. and a hard bank can get pretty scary pretty quickly. the Helical and Patch antennas don't seem to give much improvement and when the diversity rx switches to them there is significant static in my picture vs the cp antenna.


Here's where it gets interesting... I now I have a CP antenna and a stock dipole (rubbery ducky) antenna on the diversity Rx. after flying recently my spotter observed that the Rx almost never switches to the CP antenna, my range didn't improve much but the quality of my signal improved greatly. he decided to give the setup a try and replaced his CP antenna with an extra stock 5.8 antenna I had and he experienced a much improved video quality and some range improvement while using a CP on his mini quad tx and the dipole on is Rx. I found this very strange and am considering getting a higher gain dipole antenna to try and improve my range. Where else can I trouble shoot to improve range? continuing to buy antennas that don't deliver is getting expensive.

is this good practice running a CP antenna on the plane/multirotor and a dipole on the Rx side? Is there something significantly wrong with my setup that would cause this configuration to be my better quality picture? I tried to google about mixing antennas and didn't find anything that recommended it to improve video range so I must have something wrong in my setup.

thanks in advance

-Jes

 

[Klinger RC]

I personaly never use dipoles. When you bank you plane and the dipoles are no longer parallel, your range is completely destroyed. I prefer to use a helical or patch on the Rx with a cp antenna on the tx. You have diversity so maybe use a high gain dipole AND a patch or helical. That way when you bank it switches to the helical and you retain good signal quality. Those cheap helical antennas are probably crap and never have, and never will, work. Go with some thing quality like on of these:

The five turn helical from Video Aerial Systems: http://www.liftoffrc.com/store/p41/5.8ghz_5-Turn_Helical.html
Or you could choose the ImmersionRC Patch: http://www.liftoffrc.com/store/p8/SpiroNET_5.8ghz_13dbi_Patch_Antenna.html

 

[Craftydan]

OS,

Are you sure you've paired the antenna polarization correctly?

Any CP paired with any Linear pol antenna will have a constant 3dB gain loss, regardless of orientation (assuming you're not sitting in a null - a blind spot). Half the signal gone, just for the mismatch, but only half. That equates to a roughly 40% reduction in range (the signal strength static you would see at 100m will now be at 60m).

A left hand CP and a right hand CP have a "Near Infinite" (meaning really big) dB gain loss from the mismatch. In my experience (listening into RHCP signals with a LHCP antenna on a strong RX and TX) it's painful to watch a L/R mismatch at range on the same freq, but not impossible. move off freq and the signal drops to nothing pretty quick . . .

. . . but this would explain why it "works" on a single RX, but your diversity RX prefers the linear pol antenna

 

[OrangutangSurfer]

Weird.... I have an email regarding a reply but it isn't yours dan, and it's not on this thread....


Anyway. the CP antennas I'm using I purchased as a set. they were listed as boscam cloud spirit but came in a MR RC or something or another box, I was a bit disappointed and likely I'm suffering the effects of a cheap knock off. as well as the same problem using the helical and also the patch. Like anyone else I don't want to drop $40 on a set of fat sharks to only go another 100ft.


the other thread mentioned not using dipoles because of the loss of signal in a bank. however using one of the dipoles on the Rx side has produced a clearer signal with a slight gain in range. I can only assume that I have a bad Tx or crappy antennas.



I suppose, since I already have a diversity Rx. I could try that and a pair of fat sharks from HK: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=49826

I would assume that these are "quality" antennas and If I was looking to hit that 2km mark these would provide it. Until I commit to buy I'll do some more testing with the stuff I have and see if I can better determine where my fault is. the Patch antenna I purcahsed from Amazon was also in a set with a CP for the tx side. I had been using the boscam knock off's on the Rx/Tx with the patch on the 2nd Rx antenna (diversity, but with poor results), perhaps I'll get better range using the the antenna packaged with the patch. and a dipole.
http://www.amazon.com/gp/product/B00LTOC9SM/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1


the area I'm flying is a long rectangle. if I fly to the side or behind me it is within a few hundred feet or less but out in front of me is about 2000ft before I hit the lake and i'm certain my radio range won't allow me much further than that. I only want to stay in the field area but I lose video signal as I approach the far side of the field. anyway. I'll switch some antennas around and give my other 200mw tx a try and report back.

- Jes

 

[joshuabardwell]

For the record, both CP and linear antennas will have a null at the top of their emission pattern. The problem that CP antennas get you away from is cross-polarization loss when the plane banks. The point being that the orientation of the CP antenna does matter, just for a different reason.

In the interest of science, consider switching the CP and the linear antenna with each other on the diversity RX, to rule out the (unlikely) possibility that one of the receivers is defective. Bear in mind that the loss between an LP and CP antenna is only 3 dB, so it's possible that the CP antenna you've got is poorly built and is losing 3 dB to bad tuning, in which case it would be on par with the LP antenna.

 

[Craftydan]

Weird.... I have an email regarding a reply but it isn't yours dan, and it's not on this thread....

OS,

The "missing post" was moderated because the eager young man who made it posted links to his own store . . . it's fine to share links to nice products you find or vendors to self identify, but suggesting wares to fix a problem without self-identifying that you are the seller is a bit sketchy. The story is a touch more complicated than that, but that's the gist.

Joshua,

I *did* mention the nulls, but considering the size of a typical null and how ANY point in space with a linear cross polarization(which only requires you tilt one antenna 90 degrees) is "near inifnite", I'd say liner pol is always sensitive to antenna orientation and CP is nearly insensitive to it.

 

[OrangutangSurfer]

Thanks for the info guys, I know if I fly above me I'm going to have some signal loss. I'm more concerned with my range, I'm not trying to go far I'm just trying to go "far enough"

I am using the immersion RC OSD with GPS. It reports a distance of about 1100 feet that's about 300 to 400 m or about 1/5 of a mile. I should be able to reach 1 mile I don't really need to go that far but clear signal to at least a half mile would be ideal. When I try to search about similar problems most people seem to report about the same range, 300 to 400 m tops 200mw transmitter and using CP antennas.

What I found interesting as I mentioned above is when I added the dipole to the receiver side I would get the diversity switching between the CP antenna and the dipole but it most often use the dipole my range didn't really increase but the quality of my picture throughout that range was much better. If I understand Dan correctly my range should have decreased 40%. Perhaps the CP on the rx side is garbage.

Looks like I migt just suck it up and buy a set of fatshark skew planar v2's as they seem to get good reviews. Thanks Dan for the heads up about the deleted post. I'll steer clear of that sneaky business.

- Jes

 

[Craftydan]

I'd go with Joshua's suggestion and swap antennas on the VRX before tossing any gear -- it's obvious it's not the VTX antenna, but it might be the VRX's port.

I'd also not count it as a sketchy business . . . just a new businessman not quite aware of ethics yet . . . not recommending, just not letting him do things I wouldn't let another vendor do.

 

[joshuabardwell]

I *did* mention the nulls, but considering the size of a typical null and how ANY point in space with a linear cross polarization(which only requires you tilt one antenna 90 degrees) is "near inifnite", I'd say liner pol is always sensitive to antenna orientation and CP is nearly insensitive to it.

Sorry, Dan--I didn't mean to suggest you'd missed anything. I just wanted to reiterate that cross-polarization was not the only factor in play. When I fly directly overhead, I notice static on my feed even though I may only be at a height of 50 feet. (EDIT: Honestly, this is certainly in part because of the attenuation of the copter's body.) Likewise, if I am flying directly towards myself with my copter and I go from hovering to fast forward flight. The extra 45 degrees of tilt makes a noticeable difference in some cases. I would say that the main difference is that CP antennas are only sensitive to the null at the top, whereas LP antennas will also be sensitive to cross-polarization nulls even when they are facing edge-on to each other.

 

[Craftydan]

I would say that the main difference is that CP antennas are only sensitive to the null at the top, whereas LP antennas will also be sensitive to cross-polarization nulls even when they are facing edge-on to each other.

yup. Null loss vs polarization loss is a difference between whether the antenna can see the signal (so it can get into the wire) -- relative position to each other in space -- or whether the EM wave being built in the wire cancels or builds on itself.

One extra tidbit to go with that . . . even with the CP/LP mismatch of 1/2 the signal power, you *do* gain the immunity of the CP antenna, since theoretically CP is a time-based twisting mix of both linear polarizations. you do not however pick up the immunity to reflected signals . . . nor can a diversity RX reject the RHCP/LHCP mix for two pilots on neighboring frequencies if it's got a linear pol antenna in the mix.