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FT8 Power, how much ????

Discussion in 'Working Different Modes' started by K4EM, Nov 12, 2017.

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  1. W6UV

    W6UV Premium Subscriber QRZ Page

    There are many web-based SDR radios across the world available over the Internet that you can use to see what your transmitted signal looks like. There was an article in the January QST that has a list of publicly available SDR receivers.
  2. AA7XP

    AA7XP Ham Member QRZ Page

    I believe FT-8 along with other supported Modes of WSJT-X are intended to be QRP (5 Watts or less) as low power DXing was the purpose within a very narrow bandwidth and I would think more power would splatter the minimal Hz available. Hams police themselves and are expected not to cause malicious interference as license holderS. I am new to WSJT-X Modes and use QRP CW on 40 & 20 meters and do pretty well given band conditions and QRM. As others have already said, power choices are dictated by ones desire to achieve a goal and little else matters. That's why I give up and splurge with 90 Watts and a booming signal to make one contact on a bad night or contesting in poor conditions.
    Last edited: Jan 13, 2018
  3. W4NNF

    W4NNF XML Subscriber QRZ Page

    They were not intended to be QRP at all. Nor was FT8 originally envisaged for use by HF DXers. They were originally intended for weak signal usage--like EME where you'd be running as much power as it takes to get the job done. ;)
    N4UP likes this.
  4. K5TRI

    K5TRI XML Subscriber QRZ Page

    Yes, but the cat on that misinformation is out of the bag and gets repeated now for eternity. Hard to stamp make-believe out.

    Misinformed ham: FT8 and JT65 are low power modes!

    EME op: Sure. (turns on 2m 1.5KW amp and makes EME QSO using JT65)

    Everybody has a right to their own opinion, but not to their own facts.
    WU8Y, N4UP and W4NNF like this.
  5. K5TRI

    K5TRI XML Subscriber QRZ Page

    You could also stop believing and start learning how this all works. Just an idea.
    WU8Y and W4NNF like this.
  6. AA7XP

    AA7XP Ham Member QRZ Page

  7. AA7XP

    AA7XP Ham Member QRZ Page

    I hadn't written to offend anyone and hope to clarify my view points:
    I'm may have misinterpreted WSJT-X January 2018 QST and believed the software was designed for helping QRP stations obtain a larger range for DX contacts. I stand educated in the matter and did not see the article as making any rules for Hams.
    My comment on power and bandwidth was a bit vague and came from 62 years of listening to amateur radio signals and experiencing the affects of very strong signals taking seemingly more KHz width at a receiving station and frequently accompanied by a hissing or overdriving the receiver input modules. I used the term bandwidth out of ignorance and to be brief, in support of experiences of interference and my volunteer mentor's comment in this blog sight, I have the following formula to support "power does not affect bandwidth" but can create spurious radiation only if not adequately filtered in the Final Power Amplifiers. Thanks for your read.

    Formula: Sorry I couldn't copy the formula picture.
    Begin Quote

    C = B log2 (1+ (S / N)) Note: log2 means log squared

    C is the channel capacity in bits per second;
    B is the bandwidth of the channel in hertz (passband bandwidth in case of a modulated signal);
    S is the average received signal power over the bandwidth (in case of a modulated signal, often denoted C, i.e. modulated carrier), measured in watts (or volts squared);
    N is the average noise or interference power over the bandwidth, measured in watts (or volts squared); and
    S/N is the signal-to-noise ratio (SNR) or the carrier-to-noise ratio (CNR) of the communication signal to the Gaussian noise interference expressed as a linear power ratio (not as logarithmic decibels). The bandwidth remains unchanged although the extreme edges of the bandwidth may now spill over into closely localized channels due to overall higher power. In other words, in a practical world, tighter bandwidth filtering may be required. I'm thinking radio as a good example.

    End Quote
    Source: Search: power vs bandwidth
  8. KP4SX

    KP4SX Subscriber QRZ Page

    Your point is valid but that one aspect isn't. There are no 50 Hz filters at the Final Power Amplifier stage to keep problems from earlier stages from bleeding out.
  9. K5TRI

    K5TRI XML Subscriber QRZ Page

    I at least didn't read your post as offensive. Trust me, it takes a lot more than a post on a web forum :)

    The problem with this QRP statement is that it gets repeated and misinterpreted all the time. Sure a QRP station now has a wider range of stations she can work
    because of JT65 or FT8 but that doesn't mean it was designed for those power levels. If you define QRP as 5W output power at the transmitter, am I still running QRP
    by definition if my antenna has 6 db gain and my ERP now increased to 20W? So all these comments of operators running too much power only take into account
    the transmitter itself but ignore the rest of the system.

    For completeness you should also mention the formula itself. Shannon's Theorem describes the available maximum bandwidth in a given channel.
    So As S rises with N remaining at the same level the available bandwidth of the channel increases.

    The keyword here is available. Just because the road you are driving on would perfectly support you going 100 mp/h (let's ignore local traffic laws for a moment and focus on physics) doesn't mean that you have to or will be driving that fast.

    In other words, if you modulate a carrier with 50 Hz bandwidth, the signal can't all over sudden become a 100 Hz signal. Where does the additional information come from?

    What we do see however are indeed signals wider than 50 Hz. The additive information results from artifacts of over driving the input stage on the transmitter side when operators don't understand how to properly adjust their output signal. I tend to hook up a scope from time to time to see what's happening, very few do or even own a scope.
    This is just one example of course. If your neighbor three blocks down the road runs high power, she'll overload your receiver front end and you're toast and go packing
    because not only of that one loud/wide signal but also because of all the IMD products. I have one such neighbor with a nice Acom 1KW amp.

    Now per your comment regarding filtering, that's exactly one of the tools we can apply. Another one would be to reduce the RF gain. It seems everybody who complains
    runs that always at max.

    Edit: The most important thing IMHO to keep in mind is that nobody knows it all. We're all learning.
    Last edited: Jan 15, 2018
  10. AA7XP

    AA7XP Ham Member QRZ Page


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