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"Broken and Garbled" (Additional data)

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nuckollsr(at)cox.net
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Posted: Sat Jan 06, 2007 10:32 pm Post subject: "Broken and Garbled" (Additional data)

I think it's one of those once-in-a-blue-moon or perhaps in-our-fondest-wishes

kinds of advertising. We see it a lot when manufacturers talk about the wattage

of their audio systems. The $low$ hand-helds have transmitters that run in the

100 mW range. Some $high$ transceivers for the $licensed$ service may be 

allowed

to run more . . . I haven't dug out the rules on this.



But consider the 100 mW case by punching numbers into a path loss calculator

at:



http://tinyurl.com/y9tte2



Use 0.1 watts, 14 km (about 10 mi) and 450 Mhz.

Use 0, 0 and -3 for transmit and receive antenna values

Use 0 for "other losses". Hit Calculate.



We see that this gives us a 4 microvolt signal at the

receiver end. A really good receiver can be barely

useful at about 1/10th this value or 0.4 microvolts.



This means that assuming the receivers in these $20 handhelds

are capable of 0.4 microvolt performance, we can tolerate

20 db of "other" losses over a 10 mi course and still communicate.



It would be interesting to evaluate these radios in the lab

but the ones I have don't have coax connectors where the antennas

go such that one might easily evaluate real transmitter power output

and real receiver performance. Given the less-than-ideal

antennas and poor probability that the power and sensitivity

numbers are equal to the example cited, I doubt that the

radios I've been using can be expected to produce useful

performance at the "advertised" ranges.



One can purchase much more expensive radios that operate

in this service. It's reasonable to expect some

improvements in performance, but it's still difficult to minimize

"other" losses in the communications path.



Most of the radios I've purchased were the $lowest$ I could

find because my path-lengths of interest ran from a few feet

to perhaps 50 yards. I often use them to communicate with

a technician in the cockpit while I'm working in the "hell-hole"

of an airplane. The poorest performing was good for perhaps

100 yards between vehicles on the highway and the last set

I bought are much better. They are usable out to 1 mile if

the two cars can see each other.



Idealized conditions are generally used when you're being

seduced into buying the critters. Real-world performance

and non-idealized path loss conditions are an all together

different matter.



Bob . . .

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cjensen(at)dts9000.com
Guest

Posted: Sun Jan 07, 2007 4:03 am Post subject: "Broken and Garbled" (Additional data)

Since I just happen to be in the market for a half-dozen walkie talkies

for a project, is there any concensus on the best, cheap unit....Cobra

MicroTalk?  Motorola TalkAbout?  Or, are they really all about the same.

I'm buying cheap as after a week, they are a Good-Will donation. 



Chuck Jensen

 

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