Question of the Moment
Our "Question of the Moment" is sort of like a "Question of the Day", but without the added pressure of any particular deadline.

Question

What is the best position for my transmitter and receiver antenna?
Answer
The short answer is "where ever you get the best reception", but that wouldn't make a very interesting Q&A  ;-)

Antenna position is probably the most fickle part of a video system installation, it can make or break a successful setup.  As a general rule, 'whip' antennas should be operated perpendicular to the ground (mounted vertically) for best results.  Getting the receiver antenna up off the ground can help improve results also, but I'm going to suggest a radical departure from this rule that might help you achieve better results with your 2.4ghz video system.

First let's consider the fact that video systems are getting smaller and smaller and small airplanes are becoming more popular.  As a result, people are operating the systems in local parks and schoolyards where they open themselves up to many possible sources of urban RF 'noise'... microwave ovens, wireless networks, and even the new breed of 2.4ghz cordless phones.  Ironically, in an urban setting, the typical 'best' receiver antenna installation can actually invite unwanted interference.  We need a way to filter out these unwanted sources of RF while still getting a good signal from our aircraft.  One way I've done this is by taking my 8dbi circular polarized patch antenna and laying it face up on the ground.  Under normal circumstances this would be considered a bad antenna installation, but in dense urban environments the antenna on the ground actually serves to narrow the antennas coverage pattern to an area overhead, effectively screening out RF interference coming from ground based sources.   The antenna sensitivity is very likely compromised by this placement on the ground, but we make up for this by using a moderately high gain antenna.  The result is excellent noise rejection and video reception when flying in a challenging urban environment.

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Question

'm confused about the issue of power.  How much power do I need to be successful?  Why does a Black Widow A/V 200mw transmitter seem to give better range than an 800mw transmitter sold on ebay?
Answer
"Power" is perhaps the single most confusing aspect of wireless video, and this is due in large part to how different vendors quote power output.  The best indicator of range of any wireless system is how much energy is actually radiated into the air and sent to the receiver.  We call this "Radio Frequency" power, or RF power for short.  The trouble is that not all vendors talk about RF power.  Instead, they talk about the power drawn from the battery which is not at all the same thing.  Stating the power drawn from the battery gives a much more impressive sounding number than stating the RF power.  If Black Widow A/V were to state the power drawn from the battery instead of the actual RF power, we would be calling our 200mw transmitter a 1200mw transmitter! 

 


Question
Why is the 'polarization' of an antenna important in our application?
Answer
Without going into the physics behind polarization, suffice it to say that when selecting antennas for our wireless transmitters and receivers, we will get the best reception when the polarization of the two antennas is the same.  So, a vertically polarized transmitter antenna will best be received by a vertically polarized receiver antenna.  The problem is that our vertically polarized transmitter antenna can easily be made horizontally polarized simply by rotating it 90 degrees.  In our mobile applications this happens all the time, and can manifest itself as snow in our video, or a complete loss of video.  The least practical way to solve this problem is to constantly rotate the receiver antenna to match the polarization of the transmitter antenna, but you can imagine that this is not very practical.  A better solution is to use a receiver antenna that is 'circular polarized'.  A circular polarized antenna will receive a horizontally polarized signal equally as well as a vertically polarized antenna.  This nifty trick does not come for free however.  Matching a circular polarized antenna on the receiver to a linear polarized antenna on the transmitter results in a loss of gain, which means a loss in range.  We counteract this to a certain extent by selecting a reasonably high gain circular polarized antenna, and then transmit a reasonable amount of power. 


Question
Are 1.2ghz video transmitters legal?  If not, how come I see so many reputable-seeming companies selling them?
Answer
The quick answer is that these systems are NOT legal for use in the USA.  For the long answer, you have to understand the marketing short cut that retailers use when referring to wireless equipment.  When we talk about wireless equipment we say it is '1.2ghz' or '2.4ghz' as a way to describe the general operating frequency.  Most of these systems operate over a range of frequencies.  So a '2.4ghz' video system is actually operating between 2.410gh and 2.480ghz.  Similarly, the equipment marketed as '1.2ghz' is operating over a range of frequencies.  Unfortunately, the range of frequencies used by this equipment starts at 1.080ghz and tops out at 1.200ghz.  In the USA, these frequencies are NOT legal for you and I.  USA sellers of  these 1.2ghz video transmitters are either not aware of the law, or have decided to ignore the law and sell them anyway because the transmitters are gotten very cheaply from Hong Kong and China.