I'm Not An Engineer...But Need One's Advice

[AndyWaldrop]

A friend of mine has a 12 year old Nautel FM transmitter. I believe it is either a 10 or 11 KW transmitter with a 50 watt Nautel exciter (an NE50, I believe). The exciter is set at 20 watts output and it appears as though that is the power the exciter/transmitter was tested at before shipping to the customer (my friend).

For some reason, though, my friend has NEVER been able to get 11 KW out of the transmitter. It only needs to put-out 10 KW to achieve the licensed ERP. Problem is...running the transmitter at 10 KW output is running the transmitter at full-tilt boogie. The Reflected Power is 50 watts or less.

Would increasing the output of the exciter achieve what I'm trying to achieve...or would that mess something up?

Thanks, in advance, for your comments/suggestions.

 

[Matt]

It depends on the amount of undistorted headroom that the exciter has. As you increase output power of the exciter, you increase the THD - Total Harmonic Distortion on the signal. You also cause heat generation in the circuits of the exciter. Too much THZ and the signal becomes un-listenable. Too much heat generation (as in that not dissipated by convection or forced away by fan) and you damage the exciter components. How much is too much? Keep your receipts that you paid for the transmitter handy, and review them before attempting to turn up the power knob. You'll quickly find your margin of comfort in this matter.

Later . . . .

 

[Kmagrill]

Non-IBOC FM amplifiers are non linear. They gain efficiency by operating in a mode known as class C. Because class C operation relies on the flywheel effect of resonant circuits, THD isn't much of an issue, but there is a small chance that you could drive the first amp into producing spurious emissions if you significantly overdrive the transmitter input. It would probably take more than just a few extra watts to do that, though. Of more concern is how much power the input circuit is rated to handle. Driving it beyond its suggested rating can cause overheating of the stage or even a failure of the first amplifier section. If the transmitter requires more drive than it should for a given power output, then that tells you that something is wrong. Somewhere, you're going to find a bad amplifier or something negatively affecting the efficiency.

If you don't have factory test data at your needed power level, then the wise course of action is to call Nautel and ask them what the suggested ratings are for a station on your channel and power level. You can also inquire what the transmitter's maximum safe input power level is.

 

[oldiesstation]

Why would he want to try for 11kw when 10k is the licensed tpo and he thinks that's a stretch?I;m sure that's a 10kw xmtr,but hey they all have some headroom.If he's concerned about headroom how about adding another bay or get a 15 or 20kw and run it (loafing) at 10kw.It requires 20 watts input.Sounds like he's just having a slow day

 

[Bengalsfan]

For some reason, though, my friend has NEVER been able to get 11 KW out of the transmitter. It only needs to put-out 10 KW to achieve the licensed ERP. Problem is...running the transmitter at 10 KW output is running the transmitter at full-tilt boogie. The Reflected Power is 50 watts or less.

If it's a 12 year old model, chances are it's an FM series transmitter. They didn't make that model in an 11kW output, only 10. So chances are, you have a 10 kW FM series Nautel transmitter. If it's a 10kW transmitter, then indeed it's running at it's maximum, rated, power and trying to run it up to 11kW is gonna burn up amps in the transmitter. However the bigger question is if it only needs 10kW TPO, why do you need to run it to 11kW? Running it up to 11kW on a regular basis is not advisable and, according to you, not needed so why do it? You said yourself that you only need 10kW to get your licensed ERP. I would tell your "friend" to leave it at 10 kW or risk running afoul of the FCC rules regarding running over your licensed power.

Would increasing the output of the exciter achieve what I'm trying to achieve...or would that mess something up?

What you are trying to achieve is running the station at higher than licensed power. yes, it would mess things up greatly for your "friend" if caught. Not to mention the added stress you will put on the components of the transmitter causing them to fail sooner. My advice to you and your "friend" is to leave the transmitter at the power it needs to achieve your licensed ERP.

 

[BobOnTheJob]

I don't think the transmitter is running Full Tilt Boogie in terms of a car with the gas pedal pushed to the floor. Think of a car that has been directed to go 60mph by the cruise control. It may be capable of greater speed but the fact that the cruise control is regulating it to 60 is actually reducing the stress level on the engine. Same concept applies to the transmitter. Nautel has great tech support people who'll talk to a person regardless of his/her technical expertise or lack thereof. I'd trust their opinion on your question if you're looking for peace of mind. Without their input, I would touch nothing...if it ain't broke, I wouldn't fix it. And a HEARTY AMEN to everything Bengalsfan posted above. Not only is operating above 10.5KW illegal, it will provide no noticeable improvement in signal strength at the receiver. To double the strength of the received signal, you have to QUADRUPLE the transmitter power. So even if it were legal, making a substantial improvement in signal strength would require 40KW of transmitter power. Bottom line, play by the rules & enjoy your friend's reliable transmitter.

 

[Kmagrill]

If I understand the issue correctly, the owner is trying to discover if they can have any headroom to adjust the transmitter controls down from the maximum power setting. It's not that they necessarily want ot get 11kW out of a 10kW transmitter, but that they want to know if driving the input harder will produce more gain at the output.

As a general rule, most transmitters are capable of delivering 5-10% over their rated power. In this case, 10.5kW or 11kW should have been possible when the unit was new. It may still be possible to get that and, theoretically, they can run at maximum output indefinately, however, I suspect that this leads to early failures in a lot of rigs because componants do change over time and as the tuning and impedances drift from ideal, the opportunities for failures increase with time and power.

Again, this is a good point to have a brief conversation with Nautel about the normal operating parameters and limits of your transmitter. If the transmitter is at max, then you have three choices:

1. Learn to live with it.
2. Change the antenna. Adding a bay or two will give you more gain and allow you to reduce the TPO.
3. Change the feedline. Depending upon the length of the run, going to more efficient feedline will buy you a few percent to play with. The longer the run, the more you gain by going to better feedline.


Except for some peculiar multipath situations, varying the power output of a station by 10% has no measurable effect in coverage. I'm always amazed by owners that run over legal power. Unless they're running 200% over legal, they increase their utility costs and risk FCC fines, but gain nothing. Even doubling the power only increases the coverage radius by a small amount. The reverse also applies. Running the station at 95% power should have no effect on coverage.

 

[gtomas]

Even doubling the power only increases the coverage radius by a small amount. The reverse also applies.

Exactly. Small variations in power will have negligible effects on reception and coverage. You really have to increase or decrease power tenfold to notice significant change in coverage (all other things being equal). The dependence is logarithmic, not linear.


Regards,
Goran Tomas

 

[ncradioeng]

I have an Nautel FM5 about the same age running at 5300 watts to make licensed ERP - the power meter on it goes to 6 kW. The NE-50 exciter runs at 26 watts. The IPA in the transmitter wants to see a certain amount of input from the exciter - it won't come on if it is too high or low and this input level is dependent on the maximum required output from the transmitter.

 

[fm-engineer]

This reminds me of the GM's and PD's that always want their transmitter running at 104.5 percent of licensed TPO to stay below the 105% tolerance level. I gave up trying to explain It makes no difference at all in reception.

I have an Nautel FM5 about the same age running at 5300 watts to make licensed ERP - the power meter on it goes to 6 kW. The NE-50 exciter runs at 26 watts. The IPA in the transmitter wants to see a certain amount of input from the exciter - it won't come on if it is too high or low and this input level is dependent on the maximum required output from the transmitter.

 

[fm-engineer]

At 10 kW TPO, the extra 1kW will make no difference at all in reception, and it would also be illegal.

A friend of mine has a 12 year old Nautel FM transmitter. I believe it is either a 10 or 11 KW transmitter with a 50 watt Nautel exciter (an NE50, I believe). The exciter is set at 20 watts output and it appears as though that is the power the exciter/transmitter was tested at before shipping to the customer (my friend).

For some reason, though, my friend has NEVER been able to get 11 KW out of the transmitter. It only needs to put-out 10 KW to achieve the licensed ERP. Problem is...running the transmitter at 10 KW output is running the transmitter at full-tilt boogie. The Reflected Power is 50 watts or less.

Would increasing the output of the exciter achieve what I'm trying to achieve...or would that mess something up?

Thanks, in advance, for your comments/suggestions.

 

[AndyWaldrop]

I thank everyone for their input. We'll just continue to run balls-to-the-wall...10KW...and hope the transmitter doesn't mind being run at full throttle (it just won't go higher that 10KW on the power meter on the transmitter or on the Bird meter).

I was thinking that particular transmitter was an 11KW, but, I guess not.

P.S. We have no desire to run above the licensed power. As everyone has pointed out, and as we already knew, running at even 1 KW more would make no noticable difference whatsoever.

Thanks, again.