Using wireless video on a boat

[Lars Erik]

Hey everyone,

 

I'm doing a shoot for General Electric. The setting is this: there will be filming in one boat, and the client (GE), will be on a boat that's following it. GE wants a link from the main boat to their boat. Shooting on film.

 

My question is this: I know sound people complain when they're using wireless audio on boats, because the signal doesn't have any land to travel by. So the signal breaks up very quickly when the Tx and Rx gets from a certain distance from another, and it's a lot shorter distance on water then on land.

 

Will a video link behave like the audio signal also and break up quicker?

 

I've got the-boxx.tv wireless microwave set, but can easily rent the CanaTrans.

 

Thanks for any info.

 

LE

[Robert Starling SOC]

I know sound people complain when they're using wireless audio on boats, because the signal doesn't have any land to travel by

 

Lars:

 

Maybe the audio guys are complaining but the signal degradation has nothing to do with water or "land to travel by". That would be some sort of urban legend as such or just plain BS. Open travel of a radio wave is referred to as Line Of Sight Propagation.

 

High frequencies such as UHF and VHF are purely line of sight; the more open the space the better. Harder surfaces reflect or bounce RF around in a closed space so they may experience better results indoors from a "scattered" signal that is bouncing around the room. You've probably experienced this with your FM radio when driving between buildings and/or trees and the signal is stuttering which is called "picket fencing". Diversity receivers use multiple antennas so that at least one of the antennas has a good signal at any point in time. But you already know that.

 

The higher the frequency the more that signal or wave is either absorbed or reflected. The lower the frequency, the better able it is to pass through solid or dense objects. As an example, UHF is easily affected by trees, foliage and even fiberglass, wood or steel hulls in your case. Lower frequencies tend for lack of a better word plow right through. Thus the reason the Navy uses ULF / VLF (ultra low / very low) frequency and super long antenna "farms" buried underground to commuicate with submarines under the sea. Lower frequencies from about 3 mhz up to around 50 mhz bounce off the different ionosheric layers around the earth like a game of billiards so to speak. Higher frequencies such as VHF/UHF require a more solid object to bounce off of. In some cases you can bounce off charged electrons created by meteor showers or even the aurora borealis. Sorry to ramble off on a tangent but I hold a world record that has stood for over twenty years bouncing 21 mhz signals around the earth... antennas and signal propagation is something I know a little bit about. On the other hand, I'd be much more pleased if I could do a long Steadicam scene with a perfect lock-off at the end every time.

 

Peter Abraham and I were discussing RF issues this morning and it really boils down to having the best tuned transmitter that is matched specifically to the transmit antenna frequency and a receive antenna tuned for that frequency to match it; anything else is less than optimal on both ends. When you're dealing with frequencies / wavelengths so short that even a millimeter makes a difference it all adds up to the weakest link in the chain. Antenna length in feet for a half wavelength antenna is determined based on 458 divided by the frequency in megahertz.... you'll need to do the remaining math on your own to get down to inches or cm.

 

Hope this helps.

 

Robert Starling

Las Vegas

[Lars Erik]

Hey Robert,

 

wow. Thanks for the great reply. It seems like you know a lot about this. I on the other hand, when my wireless video doesn't work, I just tend to smash the system around until it works. :P Nah, just kidding.

 

So you'll think it'll be ok on the water, yeah?

 

And just one more question. I've always used the 5gHz option on my the-boxx.tv. If I understand you correctly, in dense areas, it's better to use the 2.4gHz? I thought the 2.4gHz was very busy with lots of other signals on it.

 

Thanks again, Robert.

 

LE

[Robert Starling SOC]

So you'll think it'll be ok on the water, yeah?

 

And just one more question. I've always used the 5gHz option on my the-boxx.tv. If I understand you correctly, in dense areas, it's better to use the 2.4gHz? I thought the 2.4gHz was very busy with lots of other signals on it.

 

It should be fine on the water as long as you maintain line of sight. What you won't have on the water is the luxury of a signal that is bouncing back to it from reflective surfaces like walls, ceilings, light stands whatever. If the antenna is inside the boat, unless there is something that reflects the signal to that antenna you'll have poor reception.

 

The 5 ghz -vs- 2.4 is a good question, both are highly subject to absorbtion and deflection and as you state, 2.4 is limited by other interference. Based on what is known about radio wave progagation, given equal parameters and NO OTHER INTERFERENCE, 2.4 would be the best choice but both are incredibly directional and limited in distance no matter what you do.

 

Good luck!

 

Robert

[Dan Coplan]

it really boils down to having the best tuned transmitter that is matched specifically to the transmit antenna frequency and a receive antenna tuned for that frequency to match it

 

Can you educate those of us who don't know on improving the quality of our transmitters/receivers with antennas or other 3rd party tricks?

 

I'm one of those who doesn't know. Personal example: I have a Transvideo Titan microwave system which works great. But I wonder if I can improve it by outfitting it with better antennas or...? And if so, where to get suggested components. I'm also waiting on delivery of a broadcast unit similar to the Modulus. Same question applies. Thanks.

 

Dan

[Robert Starling SOC]

Can you educate those of us who don't know on improving the quality of our transmitters/receivers with antennas or other 3rd party tricks?

 

Hi Dan:

 

There are no "tricks" persea but you can certainly stack the odds in your favor. There are simple solutions and complex solutions; none with a short form answer. I'd like a day or so to write a concise set of suggestions.

 

As I mentioned, Peter Abraham and I were discussing RF transmitters and reception this morning. Peter reminded me to take a look at Radial Larsen; probably the most well respected portable and mobile antenna manufacturer on the planet. Check out their web site http://www.radialllarsen.com/ while I work on this for you.

 

Robert Starling

Las Vegas

[Robert Starling SOC]

Hi Dan:

 

You asked about tips or tricks operators can use to improve transmitted signal quality and reception. I’ll start from the source and work in the logical path.

 

Transmitter Circuitry

For simplicity there are two types; single channel and multi-channel transmitters.

 

In a perfect world a single channel transmitter and all of it’s internal circuitry would be optimized during design and manufacture for the most efficient delivery of that specific frequency to the antenna. But it’s not a perfect world and most manufacturers simply design and build one chassis and either change the frequency with a crystal or more likely synthetically. The result is that there is really only one channel the design is optimized for and the farther you get away from that channel the less efficient it is. For the most part, the same goes for a multi-channel design; though some of the higher end models of either type may modify their componentry for optimization in several segments of the channel range or frequency spectrum…ASK! The unit may be rated at 250 milliwatts output but a 20% loss in efficiency just cost you a good part of your power. The trade-off is the flexibility you gain by having a wide range of channels to choose from in the presence of interference. You now have a few good questions to ask when considering a transmitter and if you get the right tech on the phone you can find out what your most efficient channel to transmit on is. Good luck getting a straight answer on this. No one transmitter will transmit perfectly on every channel unless it is designed with individually tuned circuits for each channel. At the prices I’ve seen, I would be hesitant to believe someone in this industry is doing that with multi-channel transmitters. The loss may not be as high as 20% but it could just as easily be more.

 

Antenna Design

Again for simplicity we’ll just say there are two types; directional and omni-directional. In either case, the antenna can only radiate at its peak efficiency on one specific frequency and it gets worse the farther you get away from the ideal. Some antenna designs introduce gain or a slight boost in transmission but there can be a bit of smoke and mirrors to calculating gain claimed by manufacturers.

 

Directional antennas; usually shaped like a triangle offer gain in one direction and rejection in the opposite direction. Think of the drawings of directional microphones that have “lobes”; the primary sensitivity is in one direction but there a usually other lobes of sensitivity about 45 degs off the rear. For a transmitting antenna, substitute transmission for sensitivity in the above example. Directional antennas have their place but in the case of small reflective spaces, you may actually do better with an omni or several omnis (diversity) so you can take advantage of the “bounce”.

 

Directional antennas work well where you need distance and gain but you also have to be diligent to aiming them properly. The down side of a directional antenna for transmitting is the loss of signal in the coax cable and connectors. Thin UHF cable is VERY lossy so you need to keep the run short or go with cable that is better shielded but also much thicker.

 

As you can see, it’s not that hard to stack the odds in your favor on the transmit end, but it’s just as easy and more likely to lose a great deal of your signal before it ever leaves the antenna.

 

Example: Let say your transmitter design is optimized for the highest channel but you’re using the lowest channel for whatever reason; less power reaches the antenna. Now add an omni directional antenna that is cut for the highest channel and you lose again. Add a directional antenna tuned to the wrong channel with 25’ of thin coax with cheap connectors and you might as well be using two soup cans joined with kite string…sort of.

 

I’ll do reception and interference in another post when I get a few minutes.

 

If this post is too long, just remove it and I’ll post it as a link on my web site or something.

 

Hope this helps!

 

Robert Starling

Las Vegas

[Dan Coplan]

Robert,

 

With regards to transmitting circuitry, I'm understanding that short, thick, omni-directional antennas are the way to go (for what we do). I have 3 different transmitters (Coss, Coherent, Titan) and they all come with their own antennas that seem to meet these qualifications. Is it fair to assume that the manufacturer has supplied the best antenna to accompany their transmitters or are there better antennas that can be purchased from a place like Radiall/Larsen. And if so, what should I look for in their catalog?

 

Great post on transmitting cirucuitry. Looking forward to the accompanying post on receiving.

 

Dan