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Sep 16, 2020, 12:44 PM
Austrian FPV Freak
schim's Avatar
Thread OP
Discussion

DJI FPV Goggles - which antennas


Here's my take on practical antenna-testing for DJI FPV goggles.
A lot of DJI users ask me, "which antenna should I use for decent range?".
DJI FPV Antenna Test improved! (GPS + SRT = good comparison of Lollipop vs Patches) (13 min 17 sec)


Stock DJI Antennas (in the air and on your goggles) are ok, but with the cheapest of options you can gain way better results. I swap the air units antennas for RHCP so I can use all my normal 5.8 antennas that I collected over the years. (DJI antennas are not really different from what we used in the analog world).

On the AirUnit you need: mmcx RHCP antennas (if RHCP is what you use on goggles).
I use either Speedybee or Foxeer Lollipop mmcx antennas.
Caddx Vista: attention - here you need "UFL" connector instead of mmcx!

Foxeer Lollipop stubbies would be very convenient, but my previous measurements indicate that they are especially weak "overhead" (in the antenna null).

Normal Lollipop (the ones with coax cable) are relativly small and have good performance.
2 of them cost around 20$ currently on banggood - so for 40$ you can have the goggles fitted with new antennas, if you dont already have these from analog times...
Watch out - you also need RP SMA to SMA adaptors!

Longrange combo:
2 omnis (the said Lollipop) on top (yes, on top - they look like horns there as well)
2 patches on the bottom ports (if possible use an 45 adaptor to point them straight or even up a bit!)

In this video you see how much of a difference you can expect between already tuned Lollis and the Patch combo. Since I now have GPS RTH safety, I can almost go to the edge of my reception.
With the SG325 iNav copter here I have the luxury of an SD card for blackbox logging - and therefore I get good GPS data (check Pawels tutorial on how to convert blackbox logs to GPX files for google earth:
How to show Blackbox logs on a map (7 min 10 sec)
).
And with my little script (
Antennatests with DJI SRT to Excel (12 min 11 sec)
) I can convert the subtitles (SRT) from the goggles to Excel files and make nice charts which show you the bitrate over time rather then just the number overlay. That makes it much easier to judge the performance over the whole flight.
SRT to CSV converter script: https://bit.ly/2Zr9Gs6

Test setup:
25mw, current firmware (as of 16.9.2020)
50mbit mode, high quality
SpeedyBee Antenna on Caddx Vista
Crossfire for control link
Standing next to a road free LOS to the fiel, nothing blocks my view to the copter.
"fly as long as you feel comfortable" - I returned when bitrate dropped below 5mbit too often.
Summer, low humidity, 28 all flights around 5min from each other.
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Sep 16, 2020, 10:03 PM
fly by night
BCSaltchucker's Avatar
I like using the TrueRC Singularities on my goggles. wee stubby antennas with extra gain and no adaptors needed.

but mostly because it allows the goggles to fit in my case without removing the antennas
Last edited by BCSaltchucker; Sep 16, 2020 at 10:10 PM.
Sep 16, 2020, 11:01 PM
Fly FPV, sleep; repeat
twinturbostang's Avatar
Schim: Do you still think there's a difference between the V2 and V3 lollipop antennas (with coax)?
Sep 17, 2020, 01:26 PM
Registered User
kuiperJ's Avatar
Foxeer Lollipop as I know it is an analog antenna.

Are 5.8ghz video antennas analog/digital specific or can we just use our old analog stuff?
Sep 17, 2020, 02:44 PM
fly by night
BCSaltchucker's Avatar
they are not specific to analog vs digital.

like the TrueRC Singularities - they have identical specs to the non-DJI analog version. Only difference is RPSMA vs SMA

https://www.truerc.ca/shop/5-8ghz-2/...rp-sma-for-dji
Last edited by BCSaltchucker; Sep 17, 2020 at 02:59 PM.
Sep 17, 2020, 03:05 PM
Oldie, not Moldie
Ssayer's Avatar
@BCSaltchucker: It says that they give better range than the stock antennas. Do you concur or are they about the same but with a smaller footprint?
Sep 17, 2020, 03:06 PM
fly by night
BCSaltchucker's Avatar
Quote:
Originally Posted by Ssayer
@BCSaltchucker: It says that they give better range than the stock antennas. Do you concur or are they about the same but with a smaller footprint?
they claim 50% more range . I never flew out to 13km to test this out yet, lol I have never had a lost signal with DJI FPV yet, in 5 months of using it, out to 4km.

I owuld probably be happy with the oem antennas if they fit in my case without having to remove the infernal things every time
Sep 17, 2020, 03:18 PM
Oldie, not Moldie
Ssayer's Avatar
Thanks for that. I'll have mine back early next week. I like tidy, so like you, fitting it in a case without disassembling is a major plus.
Sep 17, 2020, 03:30 PM
Registered User
Daemon's Avatar
FWIW, not everyone agrees that the best antennas for analog are also the best for digital
and this is why.
In the analog world, first order reflections cause destructive interference. We
generally refer to this as multipath interference. We use CP antennas because
every time the signal is reflected off a flat surface it reverses the polarity so LHP becomes
RHP and vise versa. An RHP antenna will heavily attenuate an LHP signal which
greatly reduces the effects of multipathing.

But some digital systems like home Wifi, 4G LTE, and DJI HD FPV use
MIMO which relies on multiple transmitting and receiving antennas, and inherent
in its operation is the idea that data can be moved through multipathing (reflections).
But if you're using CP antennas with a good axial ratio, then you're purposely blocking
as much of the first order reflections as possible which will inhibit MIMO to some
degree.

Those who've measured the performance of the stock DJI LP antennas found that they
have a fairly poor axial ratio, yet they still perform quite well in most NLOS environments.
We don't know if they're just not very good CP antennas because they're cheap
or they were purposely made to be not perfectly CP, so as to enhance MIMO performance.
Whatever the reason, they perform ok, while most modern analog FPV antennas will
have the very best axial ratio they can achieve, and that may not be best for DJI HD FPV.

FWIW, I run stock DJI antennas on the AU and a variety of different LHP omni's and
high gain antennas on the goggles, depending on the specific type of flying I do
(which almost never includes flying high in the sky with clear LoS).
I con't care for any of the stubby omnis because they're so very easily
blocked by my own head.
Sep 19, 2020, 05:16 AM
Austrian FPV Freak
schim's Avatar
Thread OP
Very good points (as expected from you) Daemon!

It was weird for me (in tests with rc car) that even wrong polarity doesnt matter too much in terms of bitrate performance.
Another guy now sent me antennas with 2 (quite DJI stock like) antennas for the airunit - but one being LHCP and the other RHCP.
To fit to this, his set of 4 patches also contains 2 L and 2 R polarized.
He got to 13.3km (which seems to be the next hard limit).
The other fun antenna-set I got is: 4 helicals in a 3d printed mount. Specially curious about the everyday performance of the lower turn (3) version. The 7turn is quite longrangy...
Sep 19, 2020, 05:17 AM
Austrian FPV Freak
schim's Avatar
Thread OP
Tommy asked me to help him analyze SRT files and ultimately invited me into one of his videos- how cool is that. Very nice guy, doesnt even bother to show a flawed test (which then again shows you how hard it is to test properly!).
The WRONG way to test antennas | AXII HD (14 min 6 sec)
Sep 19, 2020, 05:20 AM
Austrian FPV Freak
schim's Avatar
Thread OP
Quote:
Originally Posted by BCSaltchucker
I like using the TrueRC Singularities on my goggles. wee stubby antennas with extra gain and no adaptors needed.

but mostly because it allows the goggles to fit in my case without removing the antennas
I like how similar your DJI bag looks to mine. Now I'm thinking about permanently having bigger antennas on - which makes me search for another bag again. Did I mention that I'm a bag-nerd?
Sep 21, 2020, 11:49 AM
Registered User
kuiperJ's Avatar
I'm confused about the ports.

Is it diversity (quadversity?) or do some ports also transmit?

-Why have two directional antenna? It seems you would only need or want one correct?
-It seems that the "long range" DJI users are doing biquads on either the top/bottom left or top/bottom right but then I see axiis mounted on the two top ports and RCschim doing two bottom.

Is DJI especially sensitive to the need for diversity (quadversity)? Do the ports have different functions?


Coming from an analog perspective I ultimately removed antenna #2 for long range. I found that one directional was better than two directional. Is DJI different?
Sep 21, 2020, 12:55 PM
Registered User
Daemon's Avatar
It's complicated, and much of what you know from analog world doesn't apply.
The top two ports both receive and transmit. FCC docs suggest that one of
the two bottom ports can transmit as well, but it's never been measured doing so.
DJI HD FPV uses CDD (Cyclic Delay Diversity) plus MIMO (Multiple Input Multiple Output)
and implements an active retry mechanism so it's likely that all the ports are active in some
way at any given time.
CDD means that the same packets are transmitted from the VTX with a very short
delay between them, and the receiver uses whichever signal is received the cleanest.
MIMO means that it can transmit the same signal from both VTX antennas at the
same time with different chip coding, or with a small phase delay, and data can be
moved through reflections (same reflections that normally cause multipath
interference for analog).
The goggles transmit primarily to handle the retry mechanism. It is unknown if
the goggles request retries for lost packets, or if they just always transmit an ACK for
every packet, and the VTX retransmits any packet for which it didn't receive an ACK.
The retry mechanism is what makes the latency value increase.

Assuming all 4 ports are capable of receiving the signal equally well (we don't know that
for sure), in theory you only would need to use the upper two ports, but my subjective
impression is that the received signal strength indicator on the OSD, is tied to the bottom
ports, but I think it's mostly a placebo gauge. I've often had the received signal strength showing
zero bars, while I'm still running at 50Mbit/s. Basically if you want that indicator to show
greater strength then put your high gains on the bottom. If you want true max range (such that
the retry mechanism works at full range) then at least one of the upper transmitting
ports must have a high gain.

The folks who put high gain on one side (both upper and lower) and omnis on the other
side, are usually doing so for one of two reasons.
1. so they can fly both long range, and around themselves with roughly equal balance of
signal strength from one transmitting and one non-transmitting port.
2. So they only have to remove antennas from one side of the goggles to stuff them
into the bag.

If I need to go longest range with penetration (knowing I may be flying behind
trees a couple miles out) then I put high gain antennas on all ports.
Right now I'm using the AXII HD patches and I've got em on the upper ports,
with long right angle AXII omnis reaching from the lower ports up over my head.
Those who put the HD patches on the lower ports, are usually doing so, so their
shorter stubby omnis can be on the upper ports with a better view over their own head.

There's not been proven to be one best configuration for all use cases (like all things FPV).
It just depends on your needs and priorities.
Sep 21, 2020, 09:42 PM
Fly FPV, sleep; repeat
twinturbostang's Avatar
Daemon:
As many times as I've seen you explain this, you must have it cued up on your computer and ready for a copy/paste.

BTW, you mention running a right angle adapter on your antennas. Have a look at this: https://youtu.be/-uw5HNfkFVI?t=61
Before I saw that, I had no idea that a 90 degree angle connector could result in significant signal loss. Who knows the exact effect in everyone's setups. But it was enough to make me second guess putting in 90 degree adapters. I almost bought some to pull my Biquads closer in to the goggles (easier storage) along with a new 3D printed bracket. But decided instead to use some 45's and compromised a little in the size.

Quote:
Originally Posted by Daemon
It's complicated, and much of what you know from analog world doesn't apply.
The top two ports both receive and transmit. FCC docs suggest that one of
the two bottom ports can transmit as well, but it's never been measured doing so.
DJI HD FPV uses CDD (Cyclic Delay Diversity) plus MIMO (Multiple Input Multiple Output)
and implements an active retry mechanism so it's likely that all the ports are active in some
way at any given time.
CDD means that the same packets are transmitted from the VTX with a very short
delay between them, and the receiver uses whichever signal is received the cleanest.
MIMO means that it can transmit the same signal from both VTX antennas at the
same time with different chip coding, or with a small phase delay, and data can be
moved through reflections (same reflections that normally cause multipath
interference for analog).
The goggles transmit primarily to handle the retry mechanism. It is unknown if
the goggles request retries for lost packets, or if they just always transmit an ACK for
every packet, and the VTX retransmits any packet for which it didn't receive an ACK.
The retry mechanism is what makes the latency value increase.

Assuming all 4 ports are capable of receiving the signal equally well (we don't know that
for sure), in theory you only would need to use the upper two ports, but my subjective
impression is that the received signal strength indicator on the OSD, is tied to the bottom
ports, but I think it's mostly a placebo gauge. I've often had the received signal strength showing
zero bars, while I'm still running at 50Mbit/s. Basically if you want that indicator to show
greater strength then put your high gains on the bottom. If you want true max range (such that
the retry mechanism works at full range) then at least one of the upper transmitting
ports must have a high gain.

The folks who put high gain on one side (both upper and lower) and omnis on the other
side, are usually doing so for one of two reasons.
1. so they can fly both long range, and around themselves with roughly equal balance of
signal strength from one transmitting and one non-transmitting port.
2. So they only have to remove antennas from one side of the goggles to stuff them
into the bag.

If I need to go longest range with penetration (knowing I may be flying behind
trees a couple miles out) then I put high gain antennas on all ports.
Right now I'm using the AXII HD patches and I've got em on the upper ports,
with long right angle AXII omnis reaching from the lower ports up over my head.
Those who put the HD patches on the lower ports, are usually doing so, so their
shorter stubby omnis can be on the upper ports with a better view over their own head.

There's not been proven to be one best configuration for all use cases (like all things FPV).
It just depends on your needs and priorities.


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