Job announcement questions re: radio installation

[RolnCode3]

I was looking at a communications install technician job announcement for an agency here in CA. I admit I'm not really qualified for the job. But, they have two questions on the exam that seem to be trick questions. But I could be wrong.

1. How many amps would a 15 watt radio draw in the transmit mode with 12 volt being supplied to the radio?

2. Within fifteen feet of distance, what gauge wire would you use to supply 12 volts of power to a 15 watt radio in a vehicle?

In my mind, RF radiation and DC power draw are only loosely correlated. There is no direct equation to go from one to the other because radios are not all the same efficiency. Question one seems to be asking 15/12 = 1.25 amps, although having read spec sheets, a 15 watt (RF) radio would probably be about 8-10 amps? Similarly, question two seems to be asking what gauge wire would be required for 15' @ 1.25 amps, which is probably 16 gauge or so. But an 8 amp radio would probably be 12 or 10 gauge. (those are just rough guesses without looking at a wiring chart)

Am I totally wrong? Is there a direct connection between RF radiation and DC power draw?

 

[W2NJS]

On question 1 you would have to assume that the radio draws a TOTAL of 15 watts on transmit, that's all.
On question 2 I'd have to look up the current carrying capacity of different wire gauges. I don't think those are "trick" questions but are simply unusual and obscure, and maybe written by someone whose knowledge of electrical theory is not too deep.

 

[Thayne]

You are mostly correct on your assumptions; the relationship between DC current draw and RF output can vary greatly. I used to work on some 900 MHZ paging transmitter installations and to get 400 watts RF out some of them had 30AMP 208/3phase circuits to feed them. Lots of pancake fans inside to keep it cool also (Which crapped out pretty regularly)

The hardest thing about the job you want is getting hired. As long as you work hard & don't be a jerk then your co-workers will help you learn as much as you want.

 

[kb2vxa]

Trick question the first most certainly is and designed to lead you into disaster! It would stump the hell out of me if it were serious because maybe the transmitter puts out 15W of RF but draws more than twice that from the DC supply. Not only is the final no more than 50% efficient at VHF and UHF but you have oscillators, multipliers, buffers and IPAs to power as well. The question is designed to weed out those who try to fake it, BSing their way into a job they're unqualified for.

While doing the math (15 divided by 12) will give you amps it's not a correct answer, (there can be no correct answer) you're already set up for the classic fail by the first one so even using the wire table will give the wrong answer for the second.

Boss man sayeth:
"Congratulations stupid, you've been double whammed and you're outta here!"

"I admit I'm not really qualified for the job."

Good thing you asked first, you've just saved yourself from looking like a fool in front of a prospective employer. Word gets around on The Old Boy Network so miserably failing one by bluffing can easily set you up to fail them all, in other words they'll see you coming.

"The hardest thing about the job you want is getting hired."

Honesty is the best policy so admitting up front you don't know everything but are willing to learn just might land you a job at least on a trial basis. You might suggest that, presented honestly and humbly the boss just might give you a chance to learn the COMPANY way. I landed a job as maintenance electrician that way, I knew the basics and learned a whole lot along the way.

"As long as you work hard & don't be a jerk then your co-workers will help you learn as much as you want."

That's usually the case but there's a dark side of the (electrical) Force. Never try to cross job classifications in a union shop or they'll tell you "that's not my job" and walk away. Don't blame them for avoiding a rules violation. Then there was the guy about to retire and I was his trainee/replacement. Rather than making a smooth transition he did quite the opposite, he sabotaged me at every turn including setting me up for a fall, literally. When I quit I told the boss that no job is worth risking life and limb for and he told me I was only the last in a long line of trainees who got fed up and quit. This guy was a real dork to say the least. Bottom line here is as long as you don't BE the dork and play your cards as dealt (No cheating!) and do it wisely you'll come out a winner every time.

 

[RolnCode3]

Trick question the first most certainly is and designed to lead you into disaster! It would stump the hell out of me if it were serious because maybe the transmitter puts out 15W of RF but draws more than twice that from the DC supply. Not only is the final no more than 50% efficient at VHF and UHF but you have oscillators, multipliers, buffers and IPAs to power as well. The question is designed to weed out those who try to fake it, BSing their way into a job they're unqualified for.

While doing the math (15 divided by 12) will give you amps it's not a correct answer, (there can be no correct answer) you're already set up for the classic fail by the first one so even using the wire table will give the wrong answer for the second.

Boss man sayeth:
"Congratulations stupid, you've been double whammed and you're outta here!"

"I admit I'm not really qualified for the job."

Good thing you asked first, you've just saved yourself from looking like a fool in front of a prospective employer. Word gets around on The Old Boy Network so miserably failing one by bluffing can easily set you up to fail them all, in other words they'll see you coming.

OK, well first: I have no professional communications installation training. Just 12V DC training. I would have no problem putting my answer on the sheet and letting them toss the application. That's what they are there for.

Clearly I suspected that 15/12 was not the right answer or I would not have posted. I thought I laid out my suspicion fairly clearly and I hope I don't come across as "stupid". Just not an expert. I didn't say it, but I am actually not going to apply for this job, because as I said, I don't think I'm qualified. But the question was something I had wondered about before.