Posted by
K7TV on
Mar 04, 2018; 10:00pm
URL: http://elecraft.85.s1.nabble.com/KPA1500-and-FT8-tp7638861p7638872.html
Jim, I totally agree about the choice of bandwidth for RTTY, i.e. wider than what one would first assume based on the frequency shift. The same surely applies to PSK and WSJT modes too, although I haven't yet tried the latter. But I don't think PSK31 with its tiny shift requires anything close to 500 Hz. Maybe 150 Hz, which is a lot smaller than 2.5 kHz. When I wrote about using a wide receiver, I was referring to SSB bandwidth for modes like PSK, which is clearly non-optimal. Personally often find it helps to turn off AGC entirely, even if the slow setting may minimize decoding problems caused by pumping AGC. And, as Dave suggests (and I implied in my previous post) I crank down the K3 bandwidth to something related to the width of the signal I want to copy. The purpose of that post of mine was not to suggest I have problems, but to point out that some users of mainly PSK criticize use of high power based on limits of their receiver's selectivity or limits of their understan
ding of how/when to use it, and this leads to myths about higher power being harmful. Again, I have no experience with FT8, but I made the assumption that the discussion of high power for FT8 is mostly a repeat of the discussion of high power for PSK. On the topic of digital myths, it seems many users of digital modes look at a strong signal, see that it is visible over certain frequency range, and judge it to be "broad" as in improperly modulated based on that, even if the signal is properly modulated but just strong. I believe the reason of such a mistake is that these modulation types (like also FM) do not have a definite cutoff in bandwidth; it just tapers off. Which is another way of showing that the filter needs to be wider than what would first be assumed. However, as filter bandwidth is increased, the returns in copyability decline rather quickly, while interference from other stations steadily increases. When I get to trying FT8, I don't expect it to be very differe
nt from PSK31 when it comes to the choice of power etc, but I may want at that time to review advice given in this thread by you and others.
73,
Erik K7TV
-----Original Message-----
From:
[hidden email] [mailto:
[hidden email]] On Behalf Of Jim Brown
Sent: Sunday, March 04, 2018 2:15 PM
To:
[hidden email]
Subject: Re: [Elecraft] KPA1500 and FT8
On 3/4/2018 11:58 AM, Erik Basilier wrote:
> The reason seems to be that they have been taught to use a wide
> receiver in combination with a sound card, so as to get a panoramic
> view including other stations. This lets a strong nearby station
> control the agc, and affects reception of stations on a range of frequencies.
There is a VERY good reason why WSJT developer, K1JT, advises users to use wide IF bandwidth, and it's NOT the panoramic display. The reason is that filters introduce phase shift in the passband, and phase shift is the enemy of good decoding. The same physics, by the way, applies to other digital modes, including RTTY. Serious RTTY contesters have learned that 500 Hz is a good setting. Our ears may hear off-frequency QRM louder than the signal we're trying to detect, but the decoder does just fine.
This is true of both WSJT and other decoders I use for RTTY. Indeed, even though a strong signal may APPEAR to cover much of the display, if the strong signal is CLEAN and you have your receive audio levels to the computer set to that the strong signal doesn't overload the computer, the decoder is likely to copy signals that appear to be covered by it. If you consider AGC pumping a problem, simply turn it off.
FWIW, with both FT8 and RTTY, I keep AGC on Slow, and set WSJT-X for an indicated level as high as 50 dB, using the RF Gain. With this setting, if a strong signal reduces gain in the radio, weak signals will still be strong enough at the computer to be decoded. I mostly use WSJT-X on 6M and 160M, but the few times I've gone to FT8 on HF to work a DX trip, I've more than filled the 36 slots in JTAlert. So I guess it's working. :)
73, Jim K9YC
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