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Best way to determine real transmit power for FRS radio?

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scootley

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May 13, 2020
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Hi,

I am trying to find FRS radios that have the most actual transmit power within the limits (0.5/2W depending on channel). There is an old thread asking a similar question with a couple of recommendations here:

Seems like the bigger names like Motorola and Midland do not market or publish the transmit power, but the brands like Baofeng and Retevis do advertise it, but I was curious if the advertisements are honest.

So one has to go and look at their FCC filings perhaps, but my main questions regarding this are:
1) Is looking at the FCC filings a reasonable way to determine the actual transmit power of an FRS unit without measuring it myself? (more on this below)
2) Is it actually practical to measure the transmit power myself since the antennas are not detachable? Can't attach an SWR/Watt meter to any SMA connector. Don't want to open the units up. Is there another way without a whole lab?

So take the Retevis RT22 for example. Their own web site claims:
Power 2W/0.5W
But if you look at their FCC filing's grant, it shows:
Frequency RangePower OutputRule Parts
462.5625-462.725 MHz1.08 Watts95B

And the test report from the filing shows(on page 9):
Indicated Frequency (MHz)Test Antenna PolarityERP (W)
462.6375V1.05
Not clear to me where/how the discrepancy between the 1.05W tested vs the 1.08W in the grant. Maybe there's an automatic buffer added by the FCC or something?
So are these FCC filing documents a valid way to determine the real transmit power and validate my doubt that the marketed 2W is simply false?

I know transmit power doesn't correspond directly to range, but short of someone doing a rigorous and well-documented field test to compare units (like where the received signal strength is actually measured), it seems to be the best proxy.

Thanks
 

Project25_MASTR

There's stuff I can't tell you...
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The CCR's will advertise ERP (or even EIRP) instead of true RF power occasionally. 1W is 30 dBm...where that is 32.15 dBm EIRP which is equivalent to 1.6 W and change which is a simple sigfig rounding error to 2W. Now if you could hook it up to a standard Wattmeter a 2W reading could be within the accuracy limits of the meter (depending on how it's rated).
 

scootley

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May 13, 2020
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Thanks. Yeah I figured there may be some shenanigans going on.

The CCR's will advertise ERP (or even EIRP) instead of true RF power occasionally.
Can you help me understand the difference between ERP and true RF power? Is it just that ERP is measured (at least in these FCC tests) by a receiving antenna at a 3m distance vs true RF would be measured by an attached SMA wattmeter or perhaps some other physically attached electrical device (like maybe even a multimeter measuring current draw from the unit's power source or other test points)?
 

mmckenna

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Highly calibrated test equipment, antennas, jumpers, often in an anechoic chamber or test range. To get ERP right, you'd need to have all that, and it's not stuff that 99.98% of hobbyists will even have access to, never mind be able to afford.

If you really wanted to, you could open up the radio, and attach a test cable. But then that would technically violate the Part 95 acceptance.

Best resource for consumers is the FCC OET page, run the FCC ID and see what it says. The labs that do the work are "certified" and are supposed to be truthful in the reports, so it —should— be correct. But, it wouldn't be the first time the CCR's fudged the reports.

The RF power in watts is what the radio puts out the antenna port, without an antenna attached and into a perfect 50Ω load.
ERP is the power out of the radio, feed line losses (if it's not a portable) and antenna gain.

FRS radio very specifically have low gain/non-removable antennas to limit range. The original plan behind the FRS service was to be a short range family type radio service, and being able to reuse the 14 available channels in a reasonable area was the intent.
Well, almost. Most FRS radios have inefficient antennas because consumers will put form W-A-Y over function. Most consumers have zero understanding of the antenna, and will buy the radios based off marketing claims and looks. Radios with small antennas look better. Many years ago I think Icom was selling a nice FRS radio. I forget the model numbers, but they were what most hobbyists that understood radios wanted. It came in two versions, one with a short stubby antenna, and one with a 6 inch long 1/4 wave whip. As you can guess, the short stubby whips sold much better, and slowly the model with the more efficient antenna disappeared.

Looking at the FCC grant documents is your best shot. ERP will tell you more about how far the radio is going to transmit. What you can't get info on, without owning a lot of fancy test gear, is how well the RECEIVER works. If the receiver is half deaf, it doesn't matter how much power the other radio is transmitting with, the other end may not hear it.

But, seriously, don't get hung up over a few tenths of a watt. It really doesn't make that much difference in range. Receiver sensitivity and antenna gain are way more important. If you really want decent range, get your GMRS license and run a 5 watt hand held with an efficient antenna.
 

cmdrwill

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Mar 30, 2005
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The labs that do the work are "certified" and are supposed to be truthful in the reports, so it —should— be correct. But, it wouldn't be the first time the CCR's fudged the reports.
EXACTLY CCR's submit fudge'd reports a LOT......
 
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