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Author Topic: A question on power output  (Read 3760 times)
KC8KTN
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« on: December 30, 2012, 03:15:03 PM »

when running icom 718 at full power 100 watts also using signal link usb it shows on my antenna tunner at 100 watts. Other times it shows only 10 watts without changing my power output on my  radio. Was just wondering why. Before everyones gets upset i usually   only run radio at 10 on output but it shows nothing on antenna tunner. Does anyone know why . Thanks for any advice.
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AA4PB
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« Reply #1 on: December 30, 2012, 04:15:27 PM »

On PSK31 the power output will decline at the lower and upper ends of the waterfall due to the roll-off of the IF filters in the radio. This might be what you are seeing.

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KC8KTN
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« Reply #2 on: January 04, 2013, 09:46:20 AM »

Thanks for your time...   
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N1ZZZ
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« Reply #3 on: February 07, 2013, 08:54:21 PM »

I tend to agree that your power variation is probably due to where in the waterfall passband you are operating.  The other common change is the amount of audio drive.  If you have adjusted your volume between QSO's your power output will vary.

My SOP for operating soundcard digital modes is to set my radio to maximum power RF. I then vary the drive audio on the computer to get the power out I need without deflecting the ALC meter.  This will ensure a clean signal, especially with Phase shift modes.

I typically run RTTY near 80 watts and PSK between 5-35 watts out depending on what antenna I am using.

73
Jeremy N1ZZZ
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W6UV
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« Reply #4 on: February 08, 2013, 09:01:09 AM »

Before everyones gets upset i usually only run radio at 10 on output but it shows nothing on antenna tunner.

Why would anyone get upset? I frequently run 100 watts (or more) on PSK31 and other digital modes. As long as your signal is clean, then no problem with higher power.
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W5LZ
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« Reply #5 on: February 10, 2013, 11:35:01 AM »

Jerry,
Re-think that just a little bit.  Ever see the screen almost totally 'blank-out' except for one very 'wide' signal on that 'water fall'?  That's a result of power, not a 'dirty' signal.  There can be a lot of reasons why that happens but excess power is one of the most common.
 - Paul
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M0HCN
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« Reply #6 on: February 10, 2013, 12:03:48 PM »

Not really, the numbers don't stack up....

The difference between 10 and 100W is 10dB, with path losses varying by **many** times that, what you are (mostly) describing is rx desense with possibly a little RX overload and a little IMD  thrown in.

Switch in a narrow bandwidth IF filter around the signal you are working and most of it will go away, switch off the AGC and use the RF attenuator to deal with any remaining issues.

Wide RX bandwidth (and associated waterfalls) are nice when conditions allow, but sometimes what is arriving at your first mixer does not allow it and that is seldom really any transmitting parties fault.

73, Dan.
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AA4PB
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« Reply #7 on: February 10, 2013, 12:55:06 PM »

The waterfall "blanking out" is normally just the normal operation of the receiver AGC circuit. It would take a really strong signal to overload the front end of most receivers. With SSB there is usually only one signal in the appox 3kHz bandwidth of the IF filter and the AGC is designed to automatically reduce the overall gain of the receiver so that every signal you tune to produces approx the same audio output level regardless of signal strength. The problem with most PSK31 operation is that the IF passband (approx 3kHz) is filled with PSK31 signals and the AGC adjusts the receiver gain so that the strongest one produces the normal audio output level. That means that all the weaker signals and the noise level decreases as the receiver gain is turned down (just as though you had done it manually with the RF gain control) so the waterfall dims down. It doesn't matter that you have your software set to copy a different, weaker signal because that narrow selectivity occurs after the AGC loop and the IF filter.

Having one signal stronger than the others is very normal. It depends on power output to some degree but also on the antenna used and to a much larger degree, the propagation. I've been accused of running too much power while running QRP at 5 watts output. In one case I was told that PSK31 was designed to be used with indoor antennas and I shouldn't be using an inverted-V hung from my tower on PSK.  Grin

Poor IMD can be caused by non-linearity **anywhere** in the signal chain from the transmit audio to the receive audio. My testing shows that even the receiver AGC action, depending on the decay time setting, can make a big difference in the displayed IMD of a clean signal. A test signal at S9 will show a big increase in IMD when the AGC is switched from slow to fast on an Icom receiver. This could account for stronger signals (even clean ones) displaying poor IMD and appearing to be wide on the waterfall at the receive end.
« Last Edit: February 10, 2013, 01:06:39 PM by AA4PB » Logged
KG6AF
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« Reply #8 on: February 10, 2013, 01:28:24 PM »

Here's a question: why do we usually hear the "you're transmitting too much power" complaint from PSK31 ops, and not from CW ops?  Both modes have modest bandwidths, and both modes are susceptible to the receiver's AGC reducing the gain of multiple signals in the presence of a strong one.  You'd think the CW folks would be fighting mad, too.

But they aren't, because they learned long ago that the way to receive a weak signal in the presence of strong ones is to reduce the receiver bandwidth to eliminate the strong signals before they can capture the AGC loop.  Typically, CW ops use fixed IF filters and passband tuning to knock out all but the signal of interest.  This works equally well for PSK31.

Blaming excess power for one's PSK31 reception problems is a losing battle.  The fact of the matter is, you have no way of knowing whether a signal is strong at your location because of high power, a high-gain antenna, or fortunate propagation.  And unless the person the strong-signal guy is talking to happens to be close to you, you have no idea whether his transmitter power is excessive, i.e., more than needed to sustain communication.

We PSK31 ops should stop complaining about someone else's allegedly high power, and start learning to use rx filtering.  If we want to blame anything on transmitters, it should be the wide signals produced by overdriving transmit audio.  I frequently see PSK31 signals so badly overdriven that they spew garbage over 2-3 kHz, and that garbage can't be filtered out if it's on top of the signal you're trying to receive.  
« Last Edit: February 10, 2013, 01:38:13 PM by KG6AF » Logged
W6UV
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« Reply #9 on: February 10, 2013, 07:31:08 PM »

Jerry,
Re-think that just a little bit.  Ever see the screen almost totally 'blank-out' except for one very 'wide' signal on that 'water fall'?  That's a result of power, not a 'dirty' signal.  There can be a lot of reasons why that happens but excess power is one of the most common.

Nothing to rethink here. That 'wide' signal you're seeing on the waterfall is dirty, and is not the result of power. Someone running high power in PSK31, yet with a clean signal, would have a signal that looks just like a low power signal on the waterfall. Power itself is not the problem.

If your waterfall is blanking out, that just means the AGC circuit in your receiver is doing what is was designed to do and also that you're using way too wide a receiver filter. Try switching in a 200 Hz or 500 Hz CW filter centered around the signal you're working and that should fix the problem nicely (assuming the strong signal is outside the passband of your filter). Using a standard 2.4 or 3 kHz SSB filter for PSK31 is roughly the equivalent of trying to use the same filter when operating in a CW contest -- you're going to have so many CW stations in your passband that copying any one of them will be difficult. That's why CW ops use narrow filters in crowded conditions, such as contests.

So why do I use higher power on PSK31? Because I mainly chase DX on PSK31 and DXing is a competitive activity. If it takes more power to work the DX, then I'll crank up the power. There's nothing illegal about this. I don't use PSK31 as a conversational ragchew mode because too many people are macro jockeys who's idea of a 'conversation' is to send a few long-winded macros telling me intimate details about their computer, etc., that I have no interest in. If I want to regchew, I use Olivia, where most ops don't tend to rely so heavily on macros -- and then crank the power down.
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