RF strength vs antenna gain

[jkowtko]

Continuing on the topic of antenna distribution systems and what produces "solid" signal strength ...

Our AKG WMS450 receivers measure RF strength on a meter that shows the range of -100 to -60.

I would like to know how that scale compares to the 0db reference for isotropic antennas, and all other gains and losses incurred in an antenna distribution system.

IF these are on the same scale, and the receiver is simply stating that anything stronger than -60 is considered "solid" signal, then my issue is not signal strength so much as antenna placement to always pick up a clear signal. And, if this is the case, then losing 5 or 10, or even 20db of antenna signal strength by extending with coax, pales in comparison to the advantage of placing the antennas in better positions.

But I don't know that it is the case. I couldn't find any info out there that explains it. I did check out the specs from a few of the wireless though ... seems they have different ways of measuring it:

AKG WMS450:
- Rx shows RF bar from -100 to -60, indicates "field strength", no indication of units of measurement. Squelch from -100 to -80.
- Tx is rated at 50mW

Sennheiser G3/100:
- Rx shows RF bar from 10 to 40, specs indicate dbu(micro)V units. Squelch from 0 to 25 dbuV. Sensitivity is 2.5 uV
- Tx is rated at 30mW

Shure SLX:
- No RF meter, specs state receiver sensitivity at –105 dBm. No squelch.
- Tx rated at 30mW

Shure ULX:
- RF meter has no scale. Specs say sensitivity -95 to -30 dBm, -20 dBm recommended. RF sensitivity 1.26 uV. Has squelch but no indication of scale.
- Tx is rated at 20-30 mW

So, my initial take from this information:

1) the scale of the AKG and Shure ULX are the same -- dBm -- and that -100 dBm equates to around 1.25 uV.

2) a "strong" signal should be around -20 dBm, and if that matches the AKG scale, then the AKG RF meter topping out at -60 dBm is a very conservative high end, and the signal should always be much higher than -60.

Is my above interpretation correct? I could be way off ...

And this still doesn't answer the question -- how does antenna signal strength relative to the 0db isotropic calibration point relate to my RF levels. I'm sure it has to do with the Tx transmission power and Tx/Rx distance ... I just don't know the formulas to calculate this ...

Thanks. John

 

[Chris15]

Here's where we get to delve into the fun of decibels. dBs are a ratio, nothing more, nothing less.
dBi for antenna gain is referenced to a fictional antenna that radiates energy equally in all directions. It cannot be practically achieved. A standard dipole has a gain of 2.2dBi.

Conversions between dBm and voltages require the characteristic impedance to be known, in this stuff it should be 50 ohms.

0dBm = 1 milliwatt of power. 30dBu is equivalent. 1W = 30dBm, 60dBu.

Let's consider a transmit antenna because I think it's easier to visualise.
If I have an RF signal that is 10mW at the feedpoint of the antenna (so we're ignoring cable losses for now) and then I have 10dBm at that point. If I feed that into a 10dBi antenna, then at the end of the antenna I get a signal power of 20dBm. If I'd started with a Watt of power, then I'd have 40dBm at the output of the antenna.

• Antenna gain (in dBi) + input (dBm) = output(dBm)

Ok so if we take this across to a radio mic, we've got 10mW of transmitter power being fed into a dipole. So the output at 0m is then 12.2 dBm.

What happens now is that the signal propagates through space, and as with all EMR, it obeys the inverse square law. Let's make it easy and we'll use the free space path loss, this ignores reflections and other real worldness, ie. you will always have more loss than the calculated free space loss, so you go off and calculate the free space loss. Because this is 2010, you use one of the online calculators to do so (google and find one you like).

Free space path loss is frequency dependent. So for our example we'll use 600 MHz. For 100m, the FSL is 68.1 dB. So you have a received signal of -55.9 dBm. Let's put that into an antenna, say a dipole, gives you -53.7 dBm, then you get to cable losses... With some interpolation off a spec sheet, we get a loss for RG6 of about 15 dB/ 100 m. Say we have a 20m cable, there's 3dB of loss. Add 1dB for the impedance mismatch. Our signal is now down to -57.7 dBm. Add a 4 way split to the mix and there's say 7dB of loss, now at -64.7 dBm.

When you then go and take the absorption of radio waves by things like walking water tanks err humans, then the path loss becomes more than the FSL.

Now just to complicate things, there is another scale used in radio land, dBuV/m, which is referenced to 1 microvolt per metre field strength. Regulatory authorities like that one, I don't know enough to be certain on it's conversions so I shalln't speak to it... You can also get dBuV which is used for some things also.

Hopefully there's enough there to work out what the next questions are...

 

[jkowtko]

Okay, just getting back to this with a bit of time to think through --

* Our transmitters are 50mW max. I found an on-line calculator that equates 50mW to 17dBm

* These are bodypack transmitters with simple 1/4 wave antennas. Can we assume 0dBi? So, leaving the transmitter, 17dBm

* FSL for a 10m distance would be 48dB (I couldn't find a calculator to compute this, but am assuming a log-based ratio, so a 1/10 reduction in distance gives 20db back.

* that puts us at -30dBm at the receiving antennas located in the receiver rack. Sounds about right.

So I have 30-40db of headroom to get clean signal out of the receivers using the receiver-mounted 1/4 whip antennas. Therefore the biggest problem I face is momentary loss of signal strength due to actor movement and poor vantage point of the whips.

If I do nothing more than to try to move the antennas into better positions a bit closer to and on opposite corners of the stage, I can afford to lose 20-30db of signal strength in cabling in order to get the antennas into a position where they will pick up a strong signal more consistently. A 20m RG-6 cable with a 4, or even 8-way splitter will still get me within 20db of loss. And I may pick up another 5-10db or so of strength by cutting the FSL distance in half.

How do those numbers look to you?

Thanks. John

 

[SHARYNF]

The problem with the calculations is that it does not factor in the noise floor. So if you had 0 noise then what you are saying would probably work, but as the noise floor rises in the system the room a significant reduction in the signal level starts to dramatically get reduced.

These are the areas where the high cost wireless units start to take effect. It is a combination of how low the noise floor is in both the transmitter and receiver and how well does the system reduce interference.

Again not to beat a dead horse but in my experience with the less expensive units, placing them on the stage reduces most of the issues, just having a shelf mounting the receivers on the shelf and then if you really need to be able to see what is going on then a cheap remote camera can be used


This way, you get around all the antenna design and cable loss issues. There really is a reason why there are 600 dollars units and 6000 dollar units especially when the distance from transmitter is large AND you are trying to run a lot of channels and you want to place them all in a single rack.

Just for an example look at Shure's chart comparison

http://www.shure.com/stellent/group...s/web_resource/us_pro_wireless_comp_chart.pdf

Sharyn

 

[FMEng]

So I have 30-40db of headroom to get clean signal out of the receivers using the receiver-mounted 1/4 whip antennas. Therefore the biggest problem I face is momentary loss of signal strength due to actor movement and poor vantage point of the whips.

The answer to that is to take maximum advantage of diversity. There will always be signal nulls (holes). Extra fade margin helps, but won't always be enough to overcome a null. Assuming that you have diversity receivers, spacing the antennas widely apart improves the chances that one antenna will have signal when the other doesn't.

Some diversity in antenna polarization can help, too. You can have a null in one polarization, but not another. One antenna of the pair should be tilted 45 to 90 degrees off from its mate.

Generally speaking, there will be fewer nulls near the transmitters than in the far field. That's another reason that putting the receivers near the stage helps, besides just reducing distance losses.

 

[jkowtko]

Okay Sharyne, you win

For lack of time to experiment with anything else, I purchased a snake and moved the receiver rack around to the side of the stage. Haven't heard any noticable RF noise since.

It's not ideal in this theater because of the flex stage arrangement, i.e. we'll have to potentially find a new location for the receiver rack for each show depending on how the set is built -- but it is one headache that's off the top of my list for now

I would still like to experiment with consumer UHF antenna equipment when I get the time. If anyone out there has actually tried it can you please share your experiences ...?

(Or, if anyones has any used distro equipment they would like to dump, I'd be interested if it will cover 500-680)

Thanks. John