FPV Video Antenna Testing

From RCwiki
Jump to: navigation, search

How to test

Ideally, you need a spectrum analyzer. If you want to go cheap, you can get a TV card with the RTL2838 chipset. Order appropriate connectors to be able to connect your antennas on Ebay.
Then download RTLSDR scanner, connect the card and run it.

If you have made a set of antennas, one way to determine which channel to use is this:

Which channel are the antennas tuned at?

Steps:

  • install the antenna on the Tx. Install the Rx's antenna on your spectrum analyzer
  • set the Tx a few meters away from the analyzer, in a wide open space without metal objects
  • turn everything on
  • for every frequency, get a few measurements and record the max or min or average or mean AND at which frequency it occured. Make sure you don't stand between Tx + analyzer, and try to keep everything still and same between measurements
  • when you wrap around the frequencies, you can test again one or two channels that you already did, just to make sure you're getting consistent results
  • at this point you could switch to stock antennas, and repeat all or some of the above steps to have a baseline to compare your antennas

Which channel interferes less with other equipment?

So, you may be using LRS (UHF) control link, or a GPS that is sensitive to interference. Theoretically you should have used filters and enough spacing, but equipment may still interfere with each other. So you should test the effect of the various components on each channel of the FPV link.

Example: Using 433Mhz OpenLRS may cause some noise if 1300Mhz video link is used (433 x 3 = 1299). Either a different video channel can be used, or the LRS's frequency can be changed to e.g. 443. Then, 443 x 3 = 1329 which should be enough spacing. This is still applicable if filtering is used, if the RC Tx is close to the vRx. If separation is enough, it shouldn't be an issue

Other considerations

Environment can affect the choice. Maybe something else is operating at one of the frequencies, which would "raise the noise floor". It may be good practice to scan the spectrum with Tx OFF, to determine if some of the frequencies should be avoided

Final Test

As a final test, one could check all channels while looking at the RSSI, measured by the vRx to be used. This will probably give the closest to real life results, and it SHOULD agree with the results of the above tests.

1.2/1.3GHz test results

Here are some results of my tests with a 400mw 1.2/1.3 Ghz video transmitter and a few antennas. All values are the best of 3 scans. Measurements should be considered with +0 to +3 dB deviation. Remember, all of the results should only be compared with ones done at the same day, time, setup etc. They should be used for relative comparison. If someone is very careful and controls many variables, we could establish a baseline (e.g. "you should be getting -10dB at 10 meters distance with stock antennas") but it will not be very accurate.

Channel comparison

Using one set of Cloverleaf and Skew Planar Wheel antennas, all the frequencies where tried. Results:

Channel Frequency Max dB
4 1080.28 -29.2
5 1120.39 -38.69
6 1160.25 -22.01
7 1200.25 -19.29
8 1240.24 -20.70
9 1280.34 -18.87
H 1320.23 -26.42
C 1347.57 -25.57

Measurement with Stock antennas both on Tx and Rx, on channel H: -30.77dB @ 1320.25Mhz
Conclusion: For my set of antennas, channels 7 and 9 are the best choice. Also, making new antennas was worth it! 12 dB increase and no multipathing!

Also, look at the frequency of channel C. It should be close to 1360, but it's 1347. This shows my Tx has some kind of problem... All the other frequencies are spot-on so it's not a problem of my spectrum analyzer.

Various antennas on Tx

With the stock antenna on the TV card, the following measurements where made (channel H, 1320MHz):
Stock antenna on Tx: -20dB
Skew Planar Wheel on Tx: -27dB
Cloverleaf on Tx: -18dB
Result: Cloverleaf (even homemade) is the best on the Tx

Various antennas on Rx

Test 1

With the stock antenna on the Tx, we simulate what the Rx would be receiving (channel H, 1320MHz):
Stock antenna on TV card: -13dB
Skew Planar Wheel on TV card: -10dB
Cloverleaf on TV card: -18dB
Result: Skew Planar Wheel (even homemade) is the best on the Rx

Test 2

At 30m away, 400mw BevRC vtx with home made cloverleaf, channel 9 - 1280 MHz, readings with the following antennas on the spectrum analyzer:

  • Stock antenna: -17.00 dB
  • DIY Vee: -13.34 dB
  • DIY SPW: -14.84 dB
  • DIY Crosshair: -1.19 dB !

Crosshair is definitely good. Maybe the SPW needs to be rebuilt!

BevRC 1400Mhz filter

Two tests with this filter show that it does attenuate signal a bit. Setup: Using my Antennas (CL & SPW), channel C (1350Mhz):
With filter on Tx: -14.5 dB
Without filter on Tx: -13 dB
Things should be better at lower frequencies.

5.8GHz test results

Various antennas on Tx

At 3m away, 200mw Boscam video transmitter at a channel that transmits around 50mw. Measured by the ImmersionRC RF power meter, equipped with a rubber ducky antenna for all tests.
Antennas on Rx and range of dB values read (lower is better):

  • DIY SPW (thin wire): -32.5 to -36.5 dB
  • DIY CL (thick wire): -33.5 to -36.5 dB
  • SZ AOMWAY SPW: -28 to -30 dB
  • SZ white cased CL: -41.5 to -43.5 dB
  • Rubber ducky: -30.5 to -32.5 dB

A lot of deviation was observed, although the environment was stationary. Outdoors testing should be done.

Conclusion: The AOMWAY SPW is a descent receiver antenna.

Various antennas on Rx

At 3m away, 200mw Boscam video transmitter at a channel that transmits around 50mw, equipped with a rubber ducky antenna for all tests. Measured by the ImmersionRC RF power meter.
Antennas on power meter and range of dB values read (lower is better):

  • DIY SPW (thin wire): -25.7 to -26.1 dB
  • DIY CL (thick wire): -21.4 to -21.8 dB
  • SZ AOMWAY SPW: -27.2 to -27.5 dB
  • SZ white cased CL: -27.9 to -28.1 dB
  • Rubber ducky: -25.6 to -25.8 dB

Conclusion: It's worth making 5G8 antennas, even without the proper tools to measure them. Just don't use thin wire.