Botmite Construction Details & photos:

Hacks & Mods

Antenna : 900 MHz, 1.2 GHz, 2.4 GHz; Patch, Yagi, Dish


Gain, db and power

   At essence, an antenna boosts the signal of the transmitter outbound or focuses the transmitter’s signal inbound to the receiver while excluding other noises.  The measurement of antenna gain is in db or decibels.   One can convert from db to power and this is useful.
   Gain is not some magical quality of an antenna where physics is violated. When rf power is radiated strongly in a particular direction or vector, it is at the expense of power output is lost in other directions.
   RF power radiation charts (aka: Smith charts) provide graphical measurement of the intensity of energy in both horizontal and vertical dimensions completely surrounding the antenna. 
   Perhaps you may find it interesting as to why an antenna enhances or detracts from signal power.  A telescope is a receiver of light waves ( electromagnetic waves at a higher frequency )-- the light is either reflected or passes through a lens. Here, the larger the aperture, the larger the gain. 
   In light signal analysis, there is no question as to the significance of the antenna or telescope, so too with a radio or transmitter. Where real-time radio control is concerned, an antenna’s significance is even more important. However, a return-to-home autopilot can mitigate the importance of radio control.

Antenna Frequency Coverage

   Whatever type of antenna you use, the frequency range of the antenna is typically within a band, such as, between 890 MHz and 910 MHz, and its harmonics to a lesser extent.  One can use an antenna, such as, a dish over a wide frequency range provided the focal point of that antenna is tuned and optimized to its used frequency.
   Note: antenna performance targets maximizing performance. So, while a 900 MHz antenna may have better performance over a 1.2 GHz band it takes much less antenna to make up the difference.

Diy Direct Dish conversion

Rule of Thumb

   Approximately every 6 db of gain in an antenna should double the range or a 400 percent increase of power transmitted.
   A dipole antenna signal strength diminishes with distance as does a light bulb, that is, by the inverse of the square root.
   Since aircraft carries the batteries for power onboard, antenna is especially important to lower power weight requirements.
   While 900 MHz offers the longest range per power, a small antenna improvement can readily offset the 900 MHz advantage in the 1.2 GHz and 2.4 GHz Range where water in trees and earth’s atmosphere especially absorb signal power.
   A directional antenna improves the signal to noise ratio by excluding all other sources of interference but requires alignment or tracking of antenna.

Reference Sites

  Antenna Calcs Link 1
Antenna Design Link 2


     Software tools focus on two things, primarily, one: radiation pattern and two: gain.

AirView9-EXT 900MHz low cost Spectrum Analyzer
  low cost Oscilloscope and spectrum analyzer:

Junk Antenna

     One should not take  advertised antenna or hardware gain specifications literally.  Here again one should test these antenna and not be disappointed by the deficient result.  That is, many antenna do not fit the published specs. Be familiar with how to calculate gain and what critical dimensions an antenna actually uses.
       Actively test at distance, with interference of all sorts to produce reliable and  effective results.


   Left ( Direct TV dish ): after plastic holder of dual antenna connector removed from antenna.  Many of these dishes can be picked up for low cost and converted to 1.2 ghz or 2.4 ghz - even picking up 802.11 wireless network connections. Note: low cost used and widely available. 
     Design: Typically a dish is designed to provide narrrow sharp continuous point to point communication.  Beam divergence may be between 1.5 degrees to 4 degrees.  Gain may be between 28 dBi and 36 dBi.  Side lobes and gain losses form is dish is not aligned within a closely with the copper wire at the focal point of the reflecting dish.
     To remove the signal feed, unscrew the plastic connector to the metal arm from the base of the dish. Now disconnect the cable to the round signal detector(s). Adding a new 2.4 GHz feed
ual -- or single -- focal point detector of the dish.  Next, you can build the extension to focus 1.2 ghz or 2.4 ghz signals either using a cantenna or a double quad copper wire shaped (62 mm per side) copper wire 15 mm above ground plane -- for lowest SWR -- and (110 to 124 mm square) copper circuit board, or brass, or non-soldered aluminum back-plate.
   Vertical or horizontal polarization can be maintained in a dish antenna by orientation of the double quad copper wire alignment.  Best performance for rc aircraft is with vertical alignment -- a 3 dBi penalty at the dish occurs with horizontal alignment, even higher dBi penalty occurs at the remote aircraft.

Yagi 1.2 GHz & 2.4 GHz Antenna

   Left 1.2 GHz and 2.4 GHz Yagi Antenna examples.
   Yagi antenna typically have a broader range of long range use for fpv camera receiver use.

 This RF signal is typically polarized horizontal or vertical - the receiver antenna’s polarization should match the  transmitter.   
   Polarization refers to which way the antenna signal is orientated.  Vertical polarization means the waves are moving up and down. Horizontal polarization means the electromagnetic radio waves move laterally across the horizon.   Antenna gain is measured in db or decibels - a log unit of measurement.
   Picking up the Antenna Handbook by the ARRL (American Radio Relay League) would be helpful but not essential for those seeking to understand more.  Dipole, yagi, dish and panel antenna are basic types of antenna used with aircraft. Be certain not to interfere with commercial aviation; it is care well spent. Internet sources may prove useful as well.

Patch Antenna

   A patch antenna resembles a clothing patch, typically a square or rectangle shape but from 110 cm to 2 feet square in size.  While typically not having the gain of either yagi or dish antenna, a patch antenna has a wider useful beam divergence.  So, a patch can cover more horizon for plane flying fpv but at lower power output or reception.
   Patch antenna are not used typically on remote aircraft as the directed point is typically fairly narrow.  The higher the db may mean better performance. In some cases, Patch antenna are motorized to track the flying model aircraft.

Pre Amplifier

   A pre-amp may be useful along with selecting a suitable cable as the losses may make the video output useless.  Note: a pre-amp may operate one way or bi-directional, one should investigate this carefully.

RF TV modulator

   A RF TV modulator may be used with pre-digital Analog TV’s. The output from the Analog TV receiver connects to the input side of the rf modulator. The output from the modulator then connects to the cable input of the TV. You may have to scan the TV channels prior to use if the analog TV is newer model. Newer LCD monitors: omit modulator.

spectral analyzer

While not as prevalent as an DMM (digital multimeter) or oscilloscope, a spectral analyzer measures and graphically displays the frequency range and characteristics of rf output.  Stand alone, USB and box to oscilloscope or computer via USB interfaces are typical options.

swr meter

   Standing Wave Ratio meter is a tool to measure the efficiency of the rf signal put into the antenna versus the reflectance of that signal back into the amplifier. Typically, one can measure power, swr, and modulation.

Diversity system

   To assure video signal quality, a redundant receiver may couple to a diversity receiver which picks up the best video signal and displays that signal automatically and is seamless. Two separate antenna couple to two separate video receivers and converge at the diversity mixer.  Result best displayed or recorded video.

   Connecting to your rc flying drone is a combination of components but does not need to be complex.  Use care, planning, and spotters to make this experience a success. Get whatever permissions in advance of your flight.  You may need to appraise the airport as well.  Your fpv flight should be a happy experience.

Base Station Display & Recorder

  Base Station - Starting with HiDefinition TV which has all standard video inputs. Left side display consists of standard NTSC TV with modulator connected above TV.
   Not shown: laptop computer and interfacing cables from 900 MHz telemetry transceiver and the analog TV receiver.  Available LCD monitors typically do not have built-in ports to encompass a broad range of inputs from various sources at various speeds. Also, the HiDef TV monitor has port management built-in to the menu options. Caviat: be sure to scan the channels prior to use and set signal source via menu as this display needs to be told what to choose from prior to use.

DISCLAIMER: The information presented here is without warranty and no guarantee that it will work for you. No liability whatsoever is accepted by using any of this information on any legal theory or system of law.  Your construction is your responsibility as are the design and engineering.  All technical information presented here is for information only. Your use of this information is solely at your own risk. Should you use any of this, you agree to hold Rentnet, Inc. and all parties contributing herein harmless from any injury which may result or risk of injury or legal theory without fault or responsibility.

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