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50 mW vs. 100 mW performance with Lectro 411's


Matt

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Hey Everyone,

I read recently on a post that someone was getting BETTER range and performance out of their Lectro 411 systems by setting the wattage down to 50 mW. This seems counter-intuitive. What has your experience been? I'd think that 100 mW would give me more range, but I guess 50 mW could give less "splatter" and therefore less intermod problems.

-Matt

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If I am in a small space indoors with a lot of wireless or close to another mixer reducing to 50mW makes sense.

Always coordinate with other mixers.

Hey Everyone,

I read recently on a post that someone was getting BETTER range and performance out of their Lectro 411 systems by setting the wattage down to 50 mW. This seems counter-intuitive. What has your experience been? I'd think that 100 mW would give me more range, but I guess 50 mW could give less "splatter" and therefore less intermod problems.

-Matt

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If shooting close to subject, lowering the output will also give you more battery time.

Not necessarily when using the Lectro SMQV transmitters. Cutting power output from 250 mW will, indeed, improve battery life. But moving from 100 mW to 50mW has no battery advantage. The lower power setting is available for situations where 100 mW is not permitted. Broadway theaters, for instance, have an agreement to limit transmitters to 50 mW to limit range so the signal from one show doesn't invade the system at another theater.

David

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There are cases where, when using a greater number of frequencies (with less stringent regard to intermodulation), one could get greater performance at lower output power. You are reducing the strength of any potential 2nd, 3rd, or 4th order harmonics which can significantly reduce the effective range of your carrier frequency.

Best,

Wyatt

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Note also that Lectro reports that the SMV's (conversely) do get more battery life out of 50mW than 100mW, but not a lot: 7.25 hours vs. 5 hours (1 AA Lithium). So it might depend on the specific transmitter being used -- SMQV vs. SMV -- and the battery type (Lithium vs. NiMH).

The Senator has pointed out before that there are overseas countries where crews routinely run 10mW and somehow manage to make those work. My experience is, from 50-100 feet away, in an interior, it's not going to be that much of an issue in the real world. Outside, in a moving vehicle, or in a heavy RF area, all bets are off. Too many variables to predict. But I bet 50mW could be made to work under many circumstances.

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The higher power outputs will create more RF bouncing around the room I.E. Axial, oblique, and tangential modes which can cause phase cancellation and null points, which may not be present or problematic at lower power levels due to the absorption of the reflecting surfaces. All the while the lower power setting is still plenty of power to get a strong direct signal.

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I guess 50 mW could give less "splatter"

HUH ?

are you making this stuff up ???

There are many wireless options that use 50 mw or less as the maximum transmitter power.

Part of the "voodoo" of radio frequency stuff is the stuff your questioning.

Some of the things mentioned so far are among the many of factors at work, and, yes, in many cases less is more!

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As wbrock001 correctly states, RF propagation follows an inverse square law, this means that doubling the power gives around 40% increase in range.

One factor to really watch out for is receiver overload, as an example let's say you have two transmitters and two receivers on nearby frequencies, transmitter A is 3 feet from the RX antenna and transmitter B is 50 feet. The signal from transmitter A will be much higher in level than that of TX B. If the level from A is above a limit that the receiver can tolerate (this level will depend very much on the quality of the design, the frequency spacing, performance of filters etc and varies a great deal between different products) then it will produce an effect known as 'blocking', this is where the signal from A will cause the signal from B to appear to get weaker.

You can easily try this by setting up a 2 channel system and putting one TX at some distance where you are not getting a full scale signal indication on your RX and then put the other channel's TX nearer and nearer to the RX antenna for the distant channel until you start to see the indicated level reduce. This effect will be more pronounced if the frequencies are relatively close, if they are significantly separated then the selectivity of the filter in the front end of the RX should reject the unwanted signal much more effectively.

The effect produced in the receiver that causes this 'blocking' is usually 'gain compression', this is when the RF amplifiers in the receiver are out of their linear region (much like clipping a mic input) and this basically turns your receiver into a mixer that means intermodulation products will shoot up, all very undesirable indeed!

There is one other, usually less significant, effect other than receiver blocking that will give similar range reduction where one TX is much nearer than the other, and that is wideband phase-noise from the TX, all transmitters will raise the noise floor to a certain extent over a wide band of frequencies, the effect of this is to make the distant transmitter sound weak and noisy, even if there is a good indicated signal.

I am not saying that this is the reason for the effect noted by the OP, but it is a good candidate. It is very important to be aware of the limitations of your equipment, so do try this test so you have a better idea of where the limit is for your kit. Never run more power than you need, especially if you are working very close to the receiver, and watch out for talent wandering up to you with a 250mW buttock warmer while you are still recording someone else on a different channel in the distance!

Someone also said that it might be something to with reflections, this is a red herring as changing the power level will also change level of any reflections by the same amount, thus the relative phase and amplitude of any reflections will remain the same as you vary TX power.

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  • 1 month later...

Geeez.

In general, to make a difference in any radio system range you need to increase the power by a factor of 10.

50 mW vs. 100 mW is only 3 dB which is in most field applications difficult to measure accurately.

50 mW vs 250 mW is only 7 dB which is still hard to measure accurately in the field.

Most "range" issues are more interference limited than signal limited.

Always keep in mind that your UHF transmitter is in the middle of the TV broadcast band where the transmitters are measured in killoWatts. Lots of kW. So if you're only 6 MHz away from a 100 kW TV transmitter the wireless receiver, no matter how well it is designed, is going to be killed by a much stronger signal. In the perfect world one should use a fixed frequency receiver with lots of pre-selection but that may not be practical.

Also keep in mind that taking a 50 mW transmitter and putting it on a body reduces the radiated RF by as much as 20 dB.

My recommendations are (1) use as much of a high gain antenna for the receiver as you can. This reduces multipath and improves the signal-to-noise and interference that the receiver has to work around. In practice, a really high gain antenna may be physically too big. Check this one out

http://www.kathrein-scala.com/catalog/PR-TV.pdf

(2) get the receive antenna and receivers as close to the talent as possible. There is much less loss in a long mic cable than there is in coax.

Bill Ruck

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Also keep in mind that taking a 50 mW transmitter and putting it on a body reduces the radiated RF by as much as 20 dB.

I have often quoted Larry F's very funny (and true) line, that "a human being is basically a large, walking bag of water. And a large bag of water is not an ideal environment for any kind of RF transmitter."

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The higher power outputs will create more RF bouncing around the room I.E. Axial, oblique, and tangential modes which can cause phase cancellation and null points, which may not be present or problematic at lower power levels due to the absorption of the reflecting surfaces. All the while the lower power setting is still plenty of power to get a strong direct signal.

Hey everyone! I just made the leap to some professional wireless and got some SMQV's. I get the gist that 250mW gives a stronger signal than 50mW but as for the RF Bounce discussed above, I'm totally lost. Is there any material you guys can recommend that teaches about RF principles and such? It seems like good stuff to know.

-Alan

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Hey everyone! I just made the leap to some professional wireless and got some SMQV's. I get the gist that 250mW gives a stronger signal than 50mW but as for the RF Bounce discussed above, I'm totally lost. Is there any material you guys can recommend that teaches about RF principles and such? It seems like good stuff to know.

-Alan

A great place to start; http://www.lectroson...less-guide.html

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There is very definitely a degree of Black Art to RF. What blows my mind is when you do a frequency scan at the beginning of the day, all is well, and then after lunch, suddenly there's a big spike where none used to be. Crap happens. Retune and move on.

I personally think the SMQV's are more useful just in that they'll run twice as long at 100mW than an equivalent SMa (or an SMV at 100mW). I agree with Robert: sometimes, just getting the RX antenna 5 feet closer gives you another 3-4dB in signal strength, especially in office buildings, going down hallways, or having to cut through a large crowd of people. Getting the antenna high helps a lot, too.

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" What blows my mind is when you do a frequency scan at the beginning of the day, all is well, and then after lunch, suddenly there's a big spike where none used to be "

It shouldn't (blow your mind)... that is very normal, as TV broadcasters, (on all the time) are but a small part of the interference picture, which changes from time to time and from place to place

" guess it's not as ridiculous as I thought. "

not ridiculous at all, it has been a common, effective, tried and true technique for decades.

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Run a scan at any major airport over a number of days and that will really blow your mind......

The "random" RF that comes and goes makes you wonder what is going on there; guess the black helicopters need to land and fuel up somewhere - and why not in plain sight......

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It shouldn't (blow your mind)... that is very normal, as TV broadcasters, (on all the time) are but a small part of the interference picture, which changes from time to time and from place to place...

My mind is easily blown! smileyvault-explode.gif

I just finished working on a doco in an industrial factory, and I bet there were at least 150 high-powered machines all over a 2-block square building. I've rarely seen so much spikey interference all over the spectrum (at least in what I could see on blocks 19 and 21). But still, I was able to find a few channels that were clean. I agree with Chris above: the only time I've personally seen this much RF activity was at LAX. I'm told military bases and aircraft carriers are much worse; lots of nasty hash around Radar, which goes from 3MHz to (potentially) 10GHz. If they're sending out jamming frequencies, god help ya...

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