DJI O4 & Avatar HD: Why Digital FPV VTXs Have Two Antennas (Is Double Better?)

Digital FPV enthusiasts frequently ask: why do some VTX units sport two antennas? This question matters because antenna design affects signal quality, range, and system reliability—key factors when you’re flying high and fast. Understanding dual-antenna setups clarifies what’s actually going on in the RF trenches.

TL;DR

Dual-antenna VTXs use either spatial diversity (sending the same data twice) or spatial multiplexing (sending different data streams simultaneously) to improve signal reliability or throughput. Single-antenna VTXs juggle transmit and receive on one port. Goggles typically have multiple antennas for reception but usually transmit on one or two. Despite the hype, dual antennas don’t always guarantee better performance—real-world differences are often negligible.

Single Antenna VTXs: The Basics

Single-antenna VTXs, like many Avatar HD and DJI units, handle both transmission and reception through one antenna port. They transmit video signals down to the goggles, then pause to listen for telemetry data coming back. This back-and-forth is a tightly timed conversation—think of two people taking turns to avoid talking over each other.

The single antenna serves a dual purpose: sending high-bandwidth video and receiving telemetry data. This simplicity reduces hardware complexity but limits how much data can be handled simultaneously.

Dual Antenna VTXs: Two Options for Two Antennas

Dual-antenna VTXs open up two main strategies for managing transmissions: spatial diversity and spatial multiplexing. Manufacturers like DJI and Avatar may use either or both, depending on the situation.

Spatial Diversity

Spatial diversity sends the same packet of data over both antennas at roughly the same time. Minor timing offsets let the receiver distinguish between packets from each antenna. This redundancy means if one packet arrives corrupted, the receiver can use the other, improving signal reliability.

This method doubles the chances of successful packet reception, which is crucial when flying in challenging RF environments or at longer ranges. The downside is that you’re effectively transmitting the same data twice, limiting maximum throughput.

Spatial Multiplexing

Spatial multiplexing sends different packets simultaneously across each antenna. Packet one goes through antenna A, packet two through antenna B, and so forth. This doubles the data throughput potential—if your single antenna link maxes at 10 Mbps, spatial multiplexing could push it to 20 Mbps.

The trade-off: no redundancy. If a packet gets lost on one antenna, it’s gone. This method is ideal when you want maximum bandwidth and can tolerate occasional packet loss.

Which Method Do Manufacturers Use?

The truth is, nobody outside the manufacturers knows the exact details. DJI likely switches between spatial diversity and multiplexing dynamically, depending on signal quality and distance. Avatar’s approach seems more basic, probably sticking to one method at a time.

This explains DJI’s reputation for robust, adaptive signal performance—its VTXs might switch modes mid-flight to maximize link quality.

Why You Must Use Both Antennas

If your VTX has two antenna ports, both must be connected. Running a single antenna risks damaging the internal power amplifier and degrades performance. Dual antennas provide multiple signal paths, which is vital for both diversity and multiplexing to work properly.

As for antenna selection, single antennas on each port usually outperform combined dual-antenna units. The classic "rabbit ears" setup—antennas angled apart—works well for both spatial diversity and multiplexing. This arrangement helps avoid signal nulls caused by aircraft orientation.

Technical Terms: Single In Single Out vs Multi In Multi Out

Single antenna systems are "Single In Single Out" (SISO): one antenna transmits and receives. Dual antenna VTXs and goggles operate as "Multi In Multi Out" (MIMO) or "Multi In Single Out" (MISO) depending on the hardware.

What About the Goggles?

Goggles have multiple antennas but handle transmission differently. Avatar goggles typically have four antennas for reception but only one for transmission. DJI goggles, like the V2 and Goggles 3, have four to six antennas, with two capable of transmitting.

Multiple receiving antennas improve signal reception by combining signals and compensating for polarization and orientation changes. On transmission, DJI’s goggles might also use spatial diversity or multiplexing, but details remain proprietary.

Polarization and Antenna Orientation: How Much Does It Matter?

Polarization mismatches—like left-hand circular on VTX vs linear on goggles—cause less signal loss in digital MIMO systems than in old analog SISO setups. The multiple antennas and advanced signal processing in digital FPV systems absorb and correct many polarization and orientation issues.

So, don’t lose sleep over left vs right-hand circular polarization or whether your goggles use linear antennas. Just match your antenna type reasonably, and you’ll be fine.

Is a Dual Antenna VTX Always Better?

Field tests over the past five years show the difference between single and dual antenna VTXs is often negligible. Dual antenna systems can offer benefits like redundancy and higher throughput, but they also generate more heat and noise due to extra power amplifiers.

If you’re flying in a dynamic environment with lots of movement and RF challenges, dual antennas with diversity can help avoid signal dropouts by mitigating nulls. For long-range fixed-wing setups where antennas stay steady, the difference may be zero.

In general, if budget and setup allow, dual antenna VTXs are a safe bet. But don’t fret if a single antenna unit fits your needs better—you won’t sacrifice much.

Final Thoughts

Dual antenna digital FPV VTXs employ clever RF tricks—spatial diversity and multiplexing—to boost signal reliability and bandwidth. While DJI likely juggles both methods dynamically, Avatar’s approach is simpler. Goggles add complexity with multiple antennas mainly on reception.

Always use both antenna ports on dual antenna VTXs. Choose antenna setups that maximize coverage and minimize signal nulls, like the rabbit ears configuration. And don’t obsess over polarization mismatches; modern digital systems handle these gracefully.

Ultimately, the best way to improve your link is to use quality antennas and keep your system well balanced. Dual antennas help, but they’re not a silver bullet.

Takeaway Box

• Dual antenna VTXs use spatial diversity (redundant packets) or spatial multiplexing (parallel packets) to improve links.
• Single antenna VTXs juggle transmit and receive on one port, limiting throughput and redundancy.
• Use both antenna ports on dual antenna VTXs to avoid damage and maximize performance.
• Multiple antennas on goggles improve reception and compensate for orientation and polarization.
• Real-world performance difference between single and dual antenna VTXs is often negligible.

This article was based from the video DJI O4 & Avatar HD Why Two Antennas ? - Is 2 Really Better Than 1!