Antennas: power loss on polarization mismatch

Most people in RC hobby knows that receiver and transmitter antennas should match polarization. But what would happen if there was a mismatch? For example when transmitter uses linear vertical and receiver circular left hand? There would be a loss of power, attenuation. How much? Look at table below. Remember: -3dBi means half of power is lost during polarization change and you need +6dBi power (4 times more) to increase range by factor of two.

 

Vertical Horizontal LHCP RHCP
Vertcal 0dBi -20dBi -3dBi -3dBi
Horizontal -20dBi 0dBi -3dBi -3dBi
LHCP -3dBi -3dBi 0dBi -20dBi
LHCP -3dBi -3dBi -20dBi 0dBi
  • -20dBi is practical value. In theory, all power should be lost during this conversion. But we are living in not perfect world and always some power will go though
  • Vertical – Linear Vertical Polarization
  • Horizontal – Linear Horizontal Polarization
  • LHCP – Left Hand Circular Polarized
  • RHCP – Right Hand Circular Polarized

Improve Boscam BOS 600 cooling with thermopad

Boscam BOS 600 is a 5.8GHz 600mW 32 channel video transmitter well suited for long range (> 1km) FPV flights. Its design makes the whole casing a radiator. This is good, since 600mW of RF means a lot of heat. Unfortunately, Boscams BOS 600 design has a serious bug there: so what if whole casing can act as a radiator, since connection between RF module and casing is poor? Just look at picture below.

Boscam BOS 600 poor cooling

RF chip is not even touching casing, gap is filled with a blob of thermal compound. Add far from perfect quality control and you will have overheating transmitter that will loose output power or even burn. But it can be fixed with a small piece of 2mm thick thermopad.

  1. Open casing by removing 4 screws on the sides of transmitter
  2. Clean RF chip and casing from existing thermal compound
  3. Cut a pice of thermopad similar in size to metal block screwed to bottom part of casingThermopad cut
  4. Remove protective film from thermopad and put it in metal blockThermopad BOS 600
  5. Assemble whole unit and screw both parts together. Thermopad will tightly fit the gap between RF chip and casing, making heat transfer easier
  6. As a final touch I have added an additional aluminium radiator that I’ve bought for my Rasperry Pis. But this is not needed, and can be skippedBOS600 radiator

Final note of thermopads. They come in different thicknesses. 1mm and 2mm should be fine for DIY projects, and can be bought cheap from eBay or any other internet store. Strip big enough to improve dozen of BOS 600 transmitters should cost not more than few dollars or euros.

DIY FPV LC power noise filter

Both electrical motor and ESC are source of electrical noise that influences all devices connected to the same battery. This is why, very often, on airplanes or big multirotors FPV circuit is powered from separate battery. On small or medium drones this can be hard to archive: additional weight will influence both flight performance and flight time. So, when your setup is suffering from a power noise manifesting itself as vertical bars or other image distortions on goggles/monitor, you can do 3 things:

  • use separate battery to power camera and video TX,
  • cut the noise using LC low-pass power filter.

I would choose low-pass filter. Cheaper and lighter. You can buy one for a few dollars/euros or make 10 by yourself for the same price.

LC power noise filter for FPV

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RedCruiser – DIY Depron airplane for FPV

Few months ago I’ve wrote about my first handmade Depron airplane. It was cool design. Super simple, with KFm-2 airfoil, durable and easy to fly. But it had few serious flaws:

  • motor mounted in front of fuselage is great for flight characteristics, but propellers breaks all the time. Even with prop savers,
  • big fuselage is not the same as fuselage with a lot of space inside.

Depron KFm-2 airplane with pusher propeller

That’s why, I have build a second Depron airplane. This time with pusher propeller mounted on a tower behind wing. Wing is almost the same design as in my first airplane. It is still an KFm-2 (Kline-Folgeman, modified) airfoil, 150mm chord, 12mm thick. But is a little shorter: 1200mm instead of 1250mm. The main difference is inside wing. Instead of carbon fiber spars I’ve used two carbon fiber 6mm pipes. That gives plenty of stiffness. Wing almost does not bend in flight, even during hard maneuvers. And generates more than enough lift to give nice, slow flying experience without danger of heavy stall. You really have to want to stall it. And event when it stalls, it recovers nicely: drops a nose, dives for a second or two, and you have lift again. No need to work with rudder, just let it dive and gain speed and then level.  Read More

Quick Review: FPV 1/4 CMOS HD Color Camera Module 600TVL 120 Degree Wide Angle

Usually I have absolutely nothing against buying cheap stuff from China. OK, maybe quality will not always be as expected, but usually it’s not worth paying extra 100% in price for 10% of extra quality. Usually, with some exceptions. Exception number one are mechanical parts like motors. Exception number two are FPV cameras. And the second one is new one. I’ve put it in place after using FPV 1/4 CMOS HD Color Camera Module 600TVL 120 Degree Wide Angle from Banggood for one month.

FPV 1/4 CMOS HD Color Camera Module 600TVL 120 Degree Wide Angle from Banggood Read More

5.8GHz FPV channel frequency table

For 5.8GHz FPV systems, there are 4 main bands divided into 8 channels each. That gives 32 channels free to use. So, in theory, 32 pilots should be able to fly FPV at the same time. Unfortunately, this is not true.

First of all, there are transmitters, and there are transmitters. Better, properly tuned, will broadcast in narrow frequency range. Cheaper, or just not properly tuned, will broadcast on much wider frequency range. The same goes for receivers. They can listen “only” to their frequency, but also to others. The result is, that a transmitter can pollute other channels. This is why, general rule is then 2 pilots should not fly on adjacent channels. If pilot A is using CH1, pilot B should use CH3 and so on.

Channel frequency for band (MHz)
Channel A (FR1) B (FR2) E (FR3) F (FR4)
1 5865 5733 5705 5740
2 5845 5752 5685 5760
3 5825 5771 5665 5780
4 5805 5790 5645 5800
5 5785 5809 5885 5820
6 5765 5828 5905 5840
7 5745 5847 5925 5860
8 5725 5866 5945 5880

Second of all, frequencies assigned to channels can overlap with channels in different bands. Just look at the chart below.

5.GHz FPV channel frequencies

Not only frequency separation between bands B and F is smaller than expected, there might me overlaps, channel 4 is crowded and band E jumps high and low. Nightmare that concludes: you should not fly CH4 on bands A and F. Also better not to use CH4 in band B. Just to be on safe side. On the other hand, band E (FR3) is not overlapping with other bands. Also band A is quite safe as long as you are not using CH4.