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Author Topic: Power levels by mode  (Read 4132 times)
N9IZ
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Posts: 16




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« on: July 27, 2011, 11:09:07 PM »

I operate very little due to my work obligations.  I'm interested in digital modes and have logged a few qso's using psk and mfsk so far.  Digital is nice sometimes since I can make a few quick contacts when I have a little time or leave it running quietly and check up on conditions from time to time or while I'm in and out of the shack.  I know that psk is meant to be a low power mode (and also Olivia and Contestia, I think).  But with DM780 there are so many modes to choose from and I'm not sure if all of them are for weak signal.  Seems like older radios used fsk rtty at high power.  If i'm operating rtty or sstv or whatever I may choose should I still be operating low power with no ALC meter deflection?  My radio will handle full duty cycle (if I needed it).  I also have an amp so I can run the radio at lower power and still get the high power out if need be.  Just curious if there's a guide somewhere that distinguishes which digimodes are for low or high power?  Thanks.

Jeff
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KX5JT
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« Reply #1 on: July 28, 2011, 01:25:45 AM »

Most modern digital modes do very well at low power levels.  I have noticed a general attitude that less is more.  PSK operators pride themselves on running qrp to maybe a dozen or two watts.  They often chastize others for running more than that.  However, it's much more important with PSK to run a CLEAN (all modes actually) signal.  This is a function of the audio not the RF power much to the amazement of some operators.  

Other modern modes i.e. OLIVIA and MFSK and relatives do very well with low power.  (Here comes a statement likely to stir up some controversy...) But, they do BETTER with higher power!  Run the least amount you need to for good copy, as is good amateur practice of course.  But yes, running MORE POWER just might allow to make contacts you wouldn't otherwise.  If you do decide it's necessary, make sure your signal is clean.

RTTY and HELLSCHRIEBER are modes that actually benefit from higher power.  RTTY is a continuous duty mode and hard on a transmitter but serious RTTY operators usually do run QRO.  HELLSCHRIEBER mode is a "fuzzy" mode meaning it's more analog in nature and a stronger signal translates into clearer, darker print.


Use the least amount of power necessary for a comfortable contact.  This might be controversial to many, but if your signal is clean, you CAN run more power if needed.  There's nothing wrong doing so.


That's my 2 cents.... let the flames flicker...
« Last Edit: July 28, 2011, 01:32:28 AM by KX5JT » Logged
WB6RQN
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Posts: 484




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« Reply #2 on: July 28, 2011, 08:49:26 AM »

Just curious if there's a guide somewhere that distinguishes which digimodes are for low or high power?  Thanks.

It is not so much a question of which are high-power and which are low-power modes as it is which modes require more link margin for the same quality of copy. The problem is actually surprisingly complex. It helps a lot to understand what the ionosphere does to your signal at different frequencies over different paths.

The first modes to fail, either due to path loss or ionospheric effects, are RTTY and PSK. The MFSK and IFK modes tend to fare much better with either reduced link margin or with ionospheric distortions. If you want to improve your chance to make a solid contact, switch to Olivia, Contestia, MFSK16, DominoEX, or Thor.

That having been said, it turns out that there is a small set of conditions, usually over a polar path, where QRO RTTY actually works better *IFF* you have the right type of decoder.

Digital over HF is one of the last areas where amateur radio can add significantly to our knowledge and understanding.

73 de Brian, WB6RQN/J79BPL
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N9IZ
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Posts: 16




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« Reply #3 on: July 28, 2011, 09:48:15 PM »

Thanks for the thoughts.  I'll continue to run the lower power levels on digimodes, but it's nice to know that I can run more on a few modes should there be poor conditions.

Jeff
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N1ZZZ
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Posts: 161




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« Reply #4 on: July 29, 2011, 06:19:16 PM »

It's more about you ERP, not transmitter power.  If you have a big beam, you can do better with 5w PSK31 than I can do with my portable vertical on a balcony at 30W.  You don't usually need a lot of power for many digital modes, but sometimes you need more if there is a lot of QRN and the station is DX.

I usually run PSK31 at 20W, RTTY at 50-75, Pactor/Clover at 50W, MFSK at 20W.  Those are examples with simple wire antennas that are fairly poor radiators.

73
Jeremy N1ZZZ
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G0GQK
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« Reply #5 on: July 30, 2011, 01:43:16 PM »

The digital modes were intended to be used with low power. Its a pleasure often spoiled when a station using excessive power blacks out the screen with a distorted transmission. Its possible to use RTTY with Airlink Express and you don't need to whack out full power over the PSK operators, 30 watts is quite sufficient.

G0GQK
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KG6MZS
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Posts: 476




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« Reply #6 on: July 30, 2011, 04:51:09 PM »

blacks out the screen with a distorted transmission.

The truly distorted signals I see don't "black out the screen."  Those are simply strong stations that are easily filtered.   The distorted signals I see produce interference across the bandpass that lights up my screen.  I find they are surprisingly rare.
« Last Edit: July 30, 2011, 06:55:52 PM by KG6MZS » Logged
W8JX
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Posts: 6463




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« Reply #7 on: July 30, 2011, 05:13:59 PM »

The digital modes were intended to be used with low power. Its a pleasure often spoiled when a station using excessive power blacks out the screen with a distorted transmission. Its possible to use RTTY with Airlink Express and you don't need to whack out full power over the PSK operators, 30 watts is quite sufficient.

G0GQK

It was not designed for low power but lower power can work. The biggest reason for lower power is radio duty cycle. A lot of the smaller HF rigs would get pretty hot running PSK at 50 watts and more since PSK like RTTY is basically a full key down mode in duty cycle. 
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N8DV
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Posts: 60




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« Reply #8 on: July 31, 2011, 09:37:16 AM »

Depending on band conditions, I use PSK31/63/125: <40 watts; RTTY: 50 to 75 watts; FELD-HELL: 50 watts. OLIVIA: 40 watts. Again, depending on your antenna and band conditions. When conditions are good, I will use <20 watts for most digital modes. Some modes tend to be better in poor conditions. Two years ago, I did an experiment and turned my power down to 5 watts. I made a few contacts. Considering line loss and antenna efficiency, the radiated power most likely was less than that. I can't wait for better conditions to see how far I can get on 5 watts in the digital modes. I hope this helps. 73  Smiley
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W8JI
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« Reply #9 on: July 31, 2011, 04:32:02 PM »

The biggest problem with most digital modes is they are run through the SSB chain of a transmitter and receiver. Dynamic range can get pretty bad when doing that, because of the accumulation of signals throughout the passband and the generally poor dynamic range of receivers within the channel width of the receiver IF system filters. Plus audio injected signals are subject to all sorts of distortion problems.

A constant amplitude but frequency shift modes like RTTY (generated by actually shifting an RF oscillator rather than injecting an audio tone or tones into a voice designed transmitter system) has a lot better dynamic range because the transmitter and receiver is designed to function on that mode. It's the same reason a properly generated 1500 watt CW signal using huge antennas can park a few hundred Hz from a noise floor CW signal and not affect copy.

We really have to limit power based on flaws in the systems, otherwise we ruin other people's fun. Running amplitude varying digital modes through SSB transmitters and receivers is a good example of a flawed RF system where we have to pay extraordinary attention to transmitter and receiver operation.





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KG6MZS
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« Reply #10 on: August 01, 2011, 07:34:39 AM »

Well, yeah, but strong signals can be received from stations transmitting at very moderate power levels.  Is it really the fault of the transmitting station if the distortion is introduced on the RX side of the signal chain?
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W8JX
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« Reply #11 on: August 01, 2011, 04:12:50 PM »

Well, yeah, but strong signals can be received from stations transmitting at very moderate power levels.  Is it really the fault of the transmitting station if the distortion is introduced on the RX side of the signal chain?

Say what??? It is transmitting stations fault if it overloads radio bandpass? A real strong signal will bury a weak one in wide bandpass and this is receivers problem this is why you use a narrow bandpass.
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M0HCN
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« Reply #12 on: August 01, 2011, 04:41:38 PM »

Of course you do,  I do, everyone with any sense uses the narrowest IF filter they have that will fit the signal, but there are those out there who treat an SSB width waterfall as some kind of right (And they moan so when they cannot use the thing because their RX BDR is not up to it (Hint, guys, build yourselves a better radio)).

Then we get the wonders of using an SSB transmitter to transmit audio frequency modulated digi modes, fine if the carrier suppression is up to snuff (and there is no earth loop, and the speech proc is turned off, and the AF level is right...), but there are SSB radios out there with the carrier suppressed to only 40db below PEP! Then we get the phase noise sidebands on that carrier, and that much vaunted 31hz signal now has a spurious mess a Khz or more removed from it, hardly a well behaved radio transmission!

Regards, Dan.
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KG6MZS
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« Reply #13 on: August 02, 2011, 09:55:56 AM »

Well, yeah, but strong signals can be received from stations transmitting at very moderate power levels.  Is it really the fault of the transmitting station if the distortion is introduced on the RX side of the signal chain?

Say what??? It is transmitting stations fault if it overloads radio bandpass? A real strong signal will bury a weak one in wide bandpass and this is receivers problem this is why you use a narrow bandpass.

Say what? is right, you didn't understand what I was saying.  I'm not talking about a strong station, I'm talking about distortion - which can cover the entire bandpass regardless of any filtering.  I've been told that distortion can be introduced anywhere in the signal chain -- including on the RX side.  Is that really the TXer's fault?  A strong signal is not just the result of high power.  I have a pretty good receiver in my radio with very good crystal and DSP filtering and I see very few truly distorted signals that can't be filtered out.  This leads me to believe that a lot of what people complain about is on their end of the equation.

73 de Eric, KG6MZS
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M0HCN
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« Reply #14 on: August 02, 2011, 10:58:54 AM »

Of course it is, within the passband of most radios IP3 degrades massively compared to what happens outside the filter passband (especially the roofing filter passband).
No surprise there and no surprise that if you use SSB bandwith for a narrowband mode the radios IF linearity is sometimes going to get stepped on by a nearby signal that is 60 or 70dB stronger at your aerial, this is not the fault of the 60 or 70dB stronger signal....

This has little to do with transmitter power differences which are usually within less then a 30db range (QRP to full US legal limit), propagation is the great equalizer.

Regards, Dan.
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