Topband: ET3YOTA Lowband Report
Greetings - I thought I would share a few comments from the recently concluded visit with friends at the Ethiopian Amateur Radio Society and our ET3YOTA (Youth On The Air) celebration. It’s been awhile since I’ve been back here, so it was good to reconnect with everyone. Bob, W9XY, and I traveled together to Addis. It was his first visit to Africa. The goal for the week was to spend a lot of time on the air, not just ourselves, but also the club members. It was a chance for us to work with them, to refine operator skills, and jointly work on some station projects. This is a very enthusiastic group. They love to get on the air and operate! Specific to the lowbands, there were three challenges going into this. First and foremost was getting permission from the university and security office to stay overnight. The gates are closed from 8 PM to 6 AM. Second would be to install an effective transmit antenna. There is a Butternut HF2V there now, without the 160 meter adaptor. We were hoping to do something better. And the third item was how to deal with noise in the city. We were granted permission to say for three consecutive nights, both Bob and I along with a couple of club members. I brought along an 18 meter Spider Pole, with the idea that we would put it on the 4th floor metal roof and try to string some sort of vertical/inverted L, using a trap so it would work on both 80 and 160 meters. It’s like what I use at XW4ZW except in Laos, I have a rope between two tall towers to support the antenna. And there I have to lay out a counterpoise. The Spider Pole went up without much trouble. The trap near the top makes it bend a bit. I had to add about 15 feet of wire to the bottom of the antenna, at the base of the spider pole, to get it to resonate near our operating frequencies. We used a piece of string to hold that section of wire off the metal roof. The segment of wire coming off the top of the pole slopped back down to the metal roof, finishing a mere 15 feet above it. The SWR was close, but we ran it through the station tuner to improve the match. This is one of the ugliest antennas I have ever built, but boy did it work! After a few CQ’s on Topband, we were told half of Europe was calling. And guys in the US, as far west as W5, reported hearing us. I looked at Bob and starting laughing, pointing to that antenna and saying, you have to be kidding, that!? The city noise. It was as bad as anticipated, solid S9 plus 20 on Topband and S9 on 80 meters. I was hoping, like I saw during Z81Z, that the noise might subside some by early morning. That did not happen. It was steady. To those who spent a lot of time calling to no avail, we feel your pain too. We didn’t come here to be alligators. Our sense of accomplishment comes from putting stations in the log. I did have an MFJ noise cancelling unit and played with it a bit, but it didn’t offer any relief. Part of that is probably me, not having an effective “noise” antenna, and having a lack of time to really play with it. Remember our days were very busy and we had only three nights to operate. I do think it will be possible to devise a better receive system to combat some of the noise. W9XY and I both agreed that even if we could knock the noise down a few S units, it would have opened up another layer or two of stations. On Topband we heard a lot of people right at the noise, often getting a character or two before they would drop back down. We were both listening. I should mention that W9XY is a frequent winner or top finisher of CW pileup competitors, so he’s no slouch picking out calls. For those wondering about FT8, yes it would be very effective in this environment. I am not anti-FT8, and I sure don’t want to start another debate about it. Simply, it doesn’t interest me. I don’t derive any satisfaction making QSO’s with it. If the choice was FT8 only or stay home, I would choose the latter. I like the challenge this situation provides and developing solutions to overcome it. It’s also a good learning experience for these young engineers at the university. Members of the club are active on FT8, so perhaps they will try. The final numbers are 92 QSO’s on Topband with 24 DXCC entities. As I mentioned, we heard many calling. I didn’t know what to expect going into this. It would have been nice to log more stations, but I’m good with the number given the situation. On 80 meters we had 433 QSO’s with 44 entities. Maybe the signals were louder, but for whatever reason we had much better luck on this band, even working some western US, who I know were not using a remote station in another part of the country. All in all I was very pleased with the results for three nights of operating. We took occasional breaks to let the club members, spending the night with us, operate 40 SSB. Some were also working on WSPR and QRP transceiver kits obtained during their participation in the 2018 YOTA conference this past Augus
Re: Topband: 160 Antenna
OK, I ran the random wire, It was a convenient length, from the eve of the house just outside the shack with a like ten foot length of RG-58 from the tuner, and out with the wire to a tree almost due south 75 feet away. No ground system, wire connected to center conductor of the coax. braid connected to nothing wire end, and to the tuner at the tuner end. wire about 10 feet above ground. It actually hears better than the center pin only vertical set up by an average of about 10 db. And by at least 20 db when the 40 vertical ran fully connected. Now how to improve some? I would assume to make it longer, try to get to at least 1/4 wave,, but then it won't be straight at all. Might make tuner happy if it was longer. the tuner was able to get the swr dead flat BUT the "L" was maxed out. so I assume some more wire over the only 75 feet it has now might help that issue. Now since it is all horizontal, not like an inverted L, I cant really add wire to the braid to make a radial. wonder what I should do to make it better using the braid. Thoughts? Joe WB9SBD Sig The Original Rolling Ball Clock Idle Tyme Idle-Tyme.com http://www.idle-tyme.com On 12/17/2018 8:31 AM, Joe wrote: OK, as users of this band, we all have probably done this at least once in your radio lifetime. You want to get on 160, but do not have an actual 160 antenna. So you connect the largest antenna ya have, usually a 80 meter dipole, but you just push the connector in just so only the center pin is touching, and load the whole thing up like a top capacity hat, vertical, or end fed long wire. Hey it works. I'm thinking of doing something similar, because a full sized 1/4 wave elevated vertical for 40 meters, works as well as a cannenna does when trying to use it on 160. But I never thought of what might be the best way to do this. The antenna as stated is a full sized 1/4 wave elevated Vertical, The base of the vertical is 10 feet above the ground with sloping radials that act as guy wires also to hold the base in place. At the base of the antenna right at the feedpoint, is a large multi turn coax choke. ( Ya know the so many turns on a PVC pipe thing ) The feedline is then ran through the air for about 60 feet to the eve of the house where it runs along the eve of the house on 2 sides and finally into the shack. Total length is about 100 feet. Now I am trying to decide without actually trying to make up connectors or whatever, what might be the best way to use this on 160. 1- As described above just the center pin, touching. I guess with the braid floating the braid gets capacitivly coupled to the power and does the radiating and receiving. YES? NO? BUT I can see the RF actually also going and using the existing vertical because of the touching center pin. BUT, the braid signal, I'm assuming the RF is not getting past the Coax coil and using the radials. 2- Apply power to only the braid? similiar to #1 but backwards. again no power to the radials probably?, and only cap coupled to the vertical. 3- short the center and shield together and run it that way. Anyone have any thoughts of the best configuration any thoughts? Or how would a end fed random wire like 1/4 wave long about 10 feet up work better? Joe WB9SBD _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: Inverted L improvement question
If your inverted L is any good at all it will suck as a receiving antenna. This is one of the key things to accept about medium wave but many casual 160 m. operators can't wrap their heads around it. A flame throwing tx antenna will probably have a completely unacceptable noise level on receive. Tx/rx reciprocity works on HF but not as well on medum wave. Separate rx antenna(s) are mandatory.A significant irritant on 160 are the operators with poor antennas that hear great, therefore they expect to be heard equally well, and can't be made to believe they are piss weak when they transmit. I'll pass along one idea I got from a friend of mine regarding your tree holding inverted L. Since the tree is probably a substantial support, I'd lower the L (this is assuming you have a pulley on a rope over the branch, through the pulley another rope attached to an insulator through which the top of the inverted L transfers from vertical to horizontal, all to facilitate raising and lowering) and bolt three more copper wires to the current wire, near the point at the insulator, with one wire continuing on through the insulator. The 3 new wires should be long enough to drape down to the ground. Now pull it back up and spread your four wires so that near ground, each one is in the corner of a square with 6 to 12 feet on a side, each wire attached to an insulator which in turn is attached to a rope that proceeds on to an anchor stake of some sort. Next, run a ring of wire around the square you have made so all four vertical wires are bonded to each other, the square wire being around two feet off the ground. Bring in your feedline and connect it, or your matching network if you have one out there, to the ground system and the square ring wire. I'd use awg 14 7 strand hard drawn bare copper wire for all of this. What you will have done is a wire simulation of a free standing tower insulated from ground with a 6 to 12 foot face around 100 feet tall. You should have a very flat impedance curve with this which will greatly simplify covering the band with minimal matching network adjustment, and at much less cost than what a 100 foot 12 foot wide skirt fed tower would cost. If I had a tree like what you must have, this would take me all of two seconds to decide to do. Rob K5UJ _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Topband: choke/bleeder resistor on RXvertical?
Since verticals are know to be "noisy" on receive, and a fix is a rf choke or bleeder resistor to ground, anyone try that on short verticals used for receive only to quiet some noise? _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: choke/bleeder resistor on RXvertical?
The choke bleeds off static charges that accumulate on the vertical. While I have witnessed noise from huge static charging to a 32 foot vertical mounted on the roof of the engineering building at Texas Tech in West Texas, the choke does not bleed off “noise”. Noise is radio frequency emissions from noise sources which can be local or distant. Chuck W5PR On Wed, Dec 19, 2018 at 12:19 PM Jamie WW3S wrote: > Since verticals are know to be "noisy" on receive, and a fix is a rf choke > or bleeder resistor to ground, anyone try that on short verticals used for > receive only to quiet some noise? > _ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: choke/bleeder resistor on RXvertical?
At least for the Hi-Z verticals - you can measure a DC voltage when they are active on the antenna against ground. You would not want to short that to ground with a choke!! By having a voltage there - I think you can assume any static DC charges will be quickly dealt with. Essentially a resistor to ground is already there. Tree N6TR On Wed, Dec 19, 2018 at 10:30 AM Chuck Dietz wrote: > The choke bleeds off static charges that accumulate on the vertical. While > I have witnessed noise from huge static charging to a 32 foot vertical > mounted on the roof of the engineering building at Texas Tech in West > Texas, the choke does not bleed off “noise”. Noise is radio frequency > emissions from noise sources which can be local or distant. > > Chuck W5PR > > On Wed, Dec 19, 2018 at 12:19 PM Jamie WW3S wrote: > > > Since verticals are know to be "noisy" on receive, and a fix is a rf > choke > > or bleeder resistor to ground, anyone try that on short verticals used > for > > receive only to quiet some noise? > > _ > > Searchable Archives: http://www.contesting.com/_topband - Topband > > Reflector > > > _ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: choke/bleeder resistor on RXvertical?
thanks, makes sense.thats why I asked - Original Message - From: "Chuck Dietz" To: "Jamie WW3S" Cc: "Topband" Sent: Wednesday, December 19, 2018 1:29:40 PM Subject: Re: Topband: choke/bleeder resistor on RXvertical? The choke bleeds off static charges that accumulate on the vertical. While I have witnessed noise from huge static charging to a 32 foot vertical mounted on the roof of the engineering building at Texas Tech in West Texas, the choke does not bleed off “noise”. Noise is radio frequency emissions from noise sources which can be local or distant. Chuck W5PR On Wed, Dec 19, 2018 at 12:19 PM Jamie WW3S < w...@zoominternet.net > wrote: Since verticals are know to be "noisy" on receive, and a fix is a rf choke or bleeder resistor to ground, anyone try that on short verticals used for receive only to quiet some noise? _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: choke/bleeder resistor on RXvertical?
ALL my antennas have Ohmite OX or OY resistors from the antenna to ground. >From 56k to a megohm or three. Doesn't everybody? :-) Ditto at dipole feedpoints. 73, Mike www.w0btu.com On Wed, Dec 19, 2018, 12:19 PM Jamie WW3S wrote: > Since verticals are know to be "noisy" on receive, and a fix is a rf choke > or bleeder resistor to ground, anyone try that on short verticals used for > receive only to quiet some noise? > _ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Topband: FT8 - How it really works
While sitting around being bored and recovering from a gall bladder operation, I decided to do some experiments with FT8. First thing I did was upgrade the software to WSJT-X v2.0. I hope this post doesn't turn into another FT8 bashing session. My only goal was to understand how this mode works and what it can do and what it cannot do. The official description of FT8's signal reporting cannot be correct. It is obviously not a signal to noise number and it is not an S meter reading. What is it? That was the first question to answer. It's obviously not an S/N number because how do you give a report of -1 dB for a signal that is S9+40 dB on a quiet band. I was unable to find any info on how the signal report was calculated so I tried to correlate those reports to observations. I think I have figured out a method that results in very close to the same number that FT8 reports. Here is the experiment. I set up my main VFO to USB 2500 Hz bandwidth and set the second VFO to CW at about 150 Hz bandwidth. I look for a station calling CQ and tune the second VFO to him and measure his signal strength. I also look at the S meter for the signal level on the main VFO. I also look at the signal report calculated by the software. For stations calling CQ that report is calculated by the software in my computer. The FT8 report is usually very close to the difference in signal levels (VFO1 - VFO2). For example if the main VFO reads S9+10 and the second VFO reads S9, the FT8 number will be -10 dB. Note that the FT8 says that -24 dB is the lowest it can decode. With VFO1 = S9+10, that's about S7 for the smallest signal it can decode. Observations agree. Those numbers will vary a little depending on how your S meter is calibrated. In order to decode a weak signal, all those close USA stations will have to go silent. The official documentation for FT8 says it will decode signals 24 dB below the noise floor. That is not a correct statement most of the time. That statement should be that FT8 will decode signals 24 dB below the sum total of everything in a 2500 Hz bandwidth. If the total of all signals on the band are below the noise floor, it would be interesting to know if FT8 will decode any of them. I haven't observed that yet in a real situation. I did however try to simulate that condition by adding enough noise to the signals such that all the signals were below the noise. The software did continue to decode signals. All the reports were -24 dB. This was a very crude test because I don't know how exactly much the signals were below the noise. This should be of benefit to those people that have S9+ noise on the bands they operate. They should be able to decode the strongest signals on the band. The (VFO1 - VFO2) test just described should always result in a number equal to or less than zero. I notice sometimes the software will report a small positive number. That seems to happen more often when the bandwidth is set to something less than 2500 Hz and there are very few signals on the band. I think this may be related to the fact that FT8 does all its calculations using audio signals and the receiver S meter is operating on RF. Audio shaping in the receiver will affect the FT8 calculations. Audio processing in your computer sound card may be a factor too. This becomes really apparent when the radio is set to CW and the audio peaking filter is turned on. With SSB bandwidth and flat audio response, S meter readings are a good indication of what will be decoded. It should decode signals down to 24 dB below whatever your S meter reads. I also narrowed the bandwidth of VFO1 and chopped out a bunch of signals. I got S7 on VFO1. Then a station calling CQ also measured S7 on VFO2. The FT8 report was 0 dB. Agrees.` That test brings up a possibility. If you can narrow VFO1 to a very narrow bandwidth hopefully containing only a very weak signal, then you may be able to decode it. A strong signal in the passband of VFO1 will kill the decode. It works. I decreased the bandwidth of VFO1 to 200 Hz and it decoded an S2 signal. I had VFO1 in USB mode with that bandwidth. My receiver will go to zero bandwidth in USB mode. I put VFO1 into CW mode at 100 Hz bandwidth and it decoded a signal that was moving the meter between S0 and S1. That signal would have also been easy copy if it was CW instead of FT8. I was using a good receiving antenna on 160 meters immediately after sunset. While this seems to work for weak signals it is a non-starter for normal operation. How do you tune around with a very narrow bandwidth looking for a station calling CQ or any other station that might be DX? It's not like CW, unless you learn to copy FT8 by ear. You can't find him with a wide bandwidth because the software won't decode him. He is only there when the bandwidth is very narrow. Given the number of USA stations on FT8 that band
Re: Topband: FT8 - How it really works
On 2018-12-19 4:28 PM, K4SAV wrote: > The official documentation for FT8 says it will decode signals 24 dB > below the noise floor. That is not a correct statement most of the > time. No, that is a correct statement. Signal reports in WSJT-X for FT8, JT65 and JT9 are *all* measured *with regard to the noise in 2500 Hz*. Note that the tone filters in WSJT-X are on the order of less than 12 Hz or so wide so the SNR *for an individual tone in the DSP filter bandwidth* at 0 dB is -23 dB relative to the *total noise in 2500 Hz bandwidth*. The actual filter bandwidth will change from mode to mode due to the differences in keying rated and tone spacing ... the actual SNR limit is shown in section 17.2.7 of the WSJT_X 2.0 User Guide. CW operators understand this from experience ... a quality 200 Hz filter will have ~12 dB less noise than a 2800 Hz filter. Thus a CW signal with a 200 Hz filter will have 12 dB better SNR than the same CW signal with a 2800 Hz filter (excluding any "processing gain" from the ear- brain filter). With FT8, JT65, JT9, etc. coding (forward error correction) provides some additional SNR (called "coding gain") but the *measurement* is based on strength of the individual tone to total noise. Thus, the lowest accurate report is -24 dB although some signals will be decoded at levels below that. 73, ... Joe, W4TV On 2018-12-19 4:28 PM, K4SAV wrote: While sitting around being bored and recovering from a gall bladder operation, I decided to do some experiments with FT8. First thing I did was upgrade the software to WSJT-X v2.0. I hope this post doesn't turn into another FT8 bashing session. My only goal was to understand how this mode works and what it can do and what it cannot do. The official description of FT8's signal reporting cannot be correct. It is obviously not a signal to noise number and it is not an S meter reading. What is it? That was the first question to answer. It's obviously not an S/N number because how do you give a report of -1 dB for a signal that is S9+40 dB on a quiet band. I was unable to find any info on how the signal report was calculated so I tried to correlate those reports to observations. I think I have figured out a method that results in very close to the same number that FT8 reports. Here is the experiment. I set up my main VFO to USB 2500 Hz bandwidth and set the second VFO to CW at about 150 Hz bandwidth. I look for a station calling CQ and tune the second VFO to him and measure his signal strength. I also look at the S meter for the signal level on the main VFO. I also look at the signal report calculated by the software. For stations calling CQ that report is calculated by the software in my computer. The FT8 report is usually very close to the difference in signal levels (VFO1 - VFO2). For example if the main VFO reads S9+10 and the second VFO reads S9, the FT8 number will be -10 dB. Note that the FT8 says that -24 dB is the lowest it can decode. With VFO1 = S9+10, that's about S7 for the smallest signal it can decode. Observations agree. Those numbers will vary a little depending on how your S meter is calibrated. In order to decode a weak signal, all those close USA stations will have to go silent. The official documentation for FT8 says it will decode signals 24 dB below the noise floor. That is not a correct statement most of the time. That statement should be that FT8 will decode signals 24 dB below the sum total of everything in a 2500 Hz bandwidth. If the total of all signals on the band are below the noise floor, it would be interesting to know if FT8 will decode any of them. I haven't observed that yet in a real situation. I did however try to simulate that condition by adding enough noise to the signals such that all the signals were below the noise. The software did continue to decode signals. All the reports were -24 dB. This was a very crude test because I don't know how exactly much the signals were below the noise. This should be of benefit to those people that have S9+ noise on the bands they operate. They should be able to decode the strongest signals on the band. The (VFO1 - VFO2) test just described should always result in a number equal to or less than zero. I notice sometimes the software will report a small positive number. That seems to happen more often when the bandwidth is set to something less than 2500 Hz and there are very few signals on the band. I think this may be related to the fact that FT8 does all its calculations using audio signals and the receiver S meter is operating on RF. Audio shaping in the receiver will affect the FT8 calculations. Audio processing in your computer sound card may be a factor too. This becomes really apparent when the radio is set to CW and the audio peaking filter is turned on. With SSB bandwidth and flat audio response, S meter readings are a good indication of what will be decoded. It shou
Re: Topband: FT8 - How it really works
Joe, thanks for the information. I am not exactly sure what all that means. My conclusions were based on observed data. It seems pretty obvious to me that a signal that is more than 50 dB above the noise floor should not receive a S/N number of -1 dB, which is what FT8 gives. I don't know how the information you provided can make a calculation like that. I judge that a signal reading S9+40 dB on the S meter should be more than 50 dB above the noise floor when I can tune of to a spot where there are no signals and the S meter reads about S2 or S3 in SSB mode or less than S1 in a narrow bandwidth. Is the definition of "noise floor" being changed for FT8? Jerry, K4SAV On 12/19/2018 7:27 PM, Joe Subich, W4TV wrote: On 2018-12-19 4:28 PM, K4SAV wrote: > The official documentation for FT8 says it will decode signals 24 dB > below the noise floor. That is not a correct statement most of the > time. No, that is a correct statement. Signal reports in WSJT-X for FT8, JT65 and JT9 are *all* measured *with regard to the noise in 2500 Hz*. Note that the tone filters in WSJT-X are on the order of less than 12 Hz or so wide so the SNR *for an individual tone in the DSP filter bandwidth* at 0 dB is -23 dB relative to the *total noise in 2500 Hz bandwidth*. The actual filter bandwidth will change from mode to mode due to the differences in keying rated and tone spacing ... the actual SNR limit is shown in section 17.2.7 of the WSJT_X 2.0 User Guide. CW operators understand this from experience ... a quality 200 Hz filter will have ~12 dB less noise than a 2800 Hz filter. Thus a CW signal with a 200 Hz filter will have 12 dB better SNR than the same CW signal with a 2800 Hz filter (excluding any "processing gain" from the ear- brain filter). With FT8, JT65, JT9, etc. coding (forward error correction) provides some additional SNR (called "coding gain") but the *measurement* is based on strength of the individual tone to total noise. Thus, the lowest accurate report is -24 dB although some signals will be decoded at levels below that. 73, ... Joe, W4TV On 2018-12-19 4:28 PM, K4SAV wrote: While sitting around being bored and recovering from a gall bladder operation, I decided to do some experiments with FT8. First thing I did was upgrade the software to WSJT-X v2.0. I hope this post doesn't turn into another FT8 bashing session. My only goal was to understand how this mode works and what it can do and what it cannot do. The official description of FT8's signal reporting cannot be correct. It is obviously not a signal to noise number and it is not an S meter reading. What is it? That was the first question to answer. It's obviously not an S/N number because how do you give a report of -1 dB for a signal that is S9+40 dB on a quiet band. I was unable to find any info on how the signal report was calculated so I tried to correlate those reports to observations. I think I have figured out a method that results in very close to the same number that FT8 reports. Here is the experiment. I set up my main VFO to USB 2500 Hz bandwidth and set the second VFO to CW at about 150 Hz bandwidth. I look for a station calling CQ and tune the second VFO to him and measure his signal strength. I also look at the S meter for the signal level on the main VFO. I also look at the signal report calculated by the software. For stations calling CQ that report is calculated by the software in my computer. The FT8 report is usually very close to the difference in signal levels (VFO1 - VFO2). For example if the main VFO reads S9+10 and the second VFO reads S9, the FT8 number will be -10 dB. Note that the FT8 says that -24 dB is the lowest it can decode. With VFO1 = S9+10, that's about S7 for the smallest signal it can decode. Observations agree. Those numbers will vary a little depending on how your S meter is calibrated. In order to decode a weak signal, all those close USA stations will have to go silent. The official documentation for FT8 says it will decode signals 24 dB below the noise floor. That is not a correct statement most of the time. That statement should be that FT8 will decode signals 24 dB below the sum total of everything in a 2500 Hz bandwidth. If the total of all signals on the band are below the noise floor, it would be interesting to know if FT8 will decode any of them. I haven't observed that yet in a real situation. I did however try to simulate that condition by adding enough noise to the signals such that all the signals were below the noise. The software did continue to decode signals. All the reports were -24 dB. This was a very crude test because I don't know how exactly much the signals were below the noise. This should be of benefit to those people that have S9+ noise on the bands they operate. They should be able to decode the strongest signals on the band. The (VFO1 - VFO2) test just described should always result in a n
Re: Topband: FT8 - How it really works
Is the definition of "noise floor" being changed for FT8? WSJT-X (and WSJT before that) defines noise as the integrated value of noise (noise power) across the 2500 Hz (approximately based on the receiver filter) receive bandwidth. 73, ... Joe, W4TV On 2018-12-19 9:57 PM, K4SAV wrote: Joe, thanks for the information. I am not exactly sure what all that means. My conclusions were based on observed data. It seems pretty obvious to me that a signal that is more than 50 dB above the noise floor should not receive a S/N number of -1 dB, which is what FT8 gives. I don't know how the information you provided can make a calculation like that. I judge that a signal reading S9+40 dB on the S meter should be more than 50 dB above the noise floor when I can tune of to a spot where there are no signals and the S meter reads about S2 or S3 in SSB mode or less than S1 in a narrow bandwidth. Is the definition of "noise floor" being changed for FT8? Jerry, K4SAV On 12/19/2018 7:27 PM, Joe Subich, W4TV wrote: On 2018-12-19 4:28 PM, K4SAV wrote: > The official documentation for FT8 says it will decode signals 24 dB > below the noise floor. That is not a correct statement most of the > time. No, that is a correct statement. Signal reports in WSJT-X for FT8, JT65 and JT9 are *all* measured *with regard to the noise in 2500 Hz*. Note that the tone filters in WSJT-X are on the order of less than 12 Hz or so wide so the SNR *for an individual tone in the DSP filter bandwidth* at 0 dB is -23 dB relative to the *total noise in 2500 Hz bandwidth*. The actual filter bandwidth will change from mode to mode due to the differences in keying rated and tone spacing ... the actual SNR limit is shown in section 17.2.7 of the WSJT_X 2.0 User Guide. CW operators understand this from experience ... a quality 200 Hz filter will have ~12 dB less noise than a 2800 Hz filter. Thus a CW signal with a 200 Hz filter will have 12 dB better SNR than the same CW signal with a 2800 Hz filter (excluding any "processing gain" from the ear- brain filter). With FT8, JT65, JT9, etc. coding (forward error correction) provides some additional SNR (called "coding gain") but the *measurement* is based on strength of the individual tone to total noise. Thus, the lowest accurate report is -24 dB although some signals will be decoded at levels below that. 73, ... Joe, W4TV On 2018-12-19 4:28 PM, K4SAV wrote: While sitting around being bored and recovering from a gall bladder operation, I decided to do some experiments with FT8. First thing I did was upgrade the software to WSJT-X v2.0. I hope this post doesn't turn into another FT8 bashing session. My only goal was to understand how this mode works and what it can do and what it cannot do. The official description of FT8's signal reporting cannot be correct. It is obviously not a signal to noise number and it is not an S meter reading. What is it? That was the first question to answer. It's obviously not an S/N number because how do you give a report of -1 dB for a signal that is S9+40 dB on a quiet band. I was unable to find any info on how the signal report was calculated so I tried to correlate those reports to observations. I think I have figured out a method that results in very close to the same number that FT8 reports. Here is the experiment. I set up my main VFO to USB 2500 Hz bandwidth and set the second VFO to CW at about 150 Hz bandwidth. I look for a station calling CQ and tune the second VFO to him and measure his signal strength. I also look at the S meter for the signal level on the main VFO. I also look at the signal report calculated by the software. For stations calling CQ that report is calculated by the software in my computer. The FT8 report is usually very close to the difference in signal levels (VFO1 - VFO2). For example if the main VFO reads S9+10 and the second VFO reads S9, the FT8 number will be -10 dB. Note that the FT8 says that -24 dB is the lowest it can decode. With VFO1 = S9+10, that's about S7 for the smallest signal it can decode. Observations agree. Those numbers will vary a little depending on how your S meter is calibrated. In order to decode a weak signal, all those close USA stations will have to go silent. The official documentation for FT8 says it will decode signals 24 dB below the noise floor. That is not a correct statement most of the time. That statement should be that FT8 will decode signals 24 dB below the sum total of everything in a 2500 Hz bandwidth. If the total of all signals on the band are below the noise floor, it would be interesting to know if FT8 will decode any of them. I haven't observed that yet in a real situation. I did however try to simulate that condition by adding enough noise to the signals such that all the signals were below the noise. The software did continue to decode signals. All the reports were -24 dB. This was a very crude test
Re: Topband: FT8 - How it really works
That would be my definition of noise power also. That would not help explain the numbers produced by FT8. It's curious that my VFO1 - VFO2 measurement produces numbers very close to what FT8 reports. I have no information as to why that should be, only measurements that produce those results. Jerry, K4SAV On 12/19/2018 9:57 PM, Joe Subich, W4TV wrote: Is the definition of "noise floor" being changed for FT8? WSJT-X (and WSJT before that) defines noise as the integrated value of noise (noise power) across the 2500 Hz (approximately based on the receiver filter) receive bandwidth. 73, ... Joe, W4TV On 2018-12-19 9:57 PM, K4SAV wrote: Joe, thanks for the information. I am not exactly sure what all that means. My conclusions were based on observed data. It seems pretty obvious to me that a signal that is more than 50 dB above the noise floor should not receive a S/N number of -1 dB, which is what FT8 gives. I don't know how the information you provided can make a calculation like that. I judge that a signal reading S9+40 dB on the S meter should be more than 50 dB above the noise floor when I can tune of to a spot where there are no signals and the S meter reads about S2 or S3 in SSB mode or less than S1 in a narrow bandwidth. Is the definition of "noise floor" being changed for FT8? Jerry, K4SAV On 12/19/2018 7:27 PM, Joe Subich, W4TV wrote: On 2018-12-19 4:28 PM, K4SAV wrote: > The official documentation for FT8 says it will decode signals 24 dB > below the noise floor. That is not a correct statement most of the > time. No, that is a correct statement. Signal reports in WSJT-X for FT8, JT65 and JT9 are *all* measured *with regard to the noise in 2500 Hz*. Note that the tone filters in WSJT-X are on the order of less than 12 Hz or so wide so the SNR *for an individual tone in the DSP filter bandwidth* at 0 dB is -23 dB relative to the *total noise in 2500 Hz bandwidth*. The actual filter bandwidth will change from mode to mode due to the differences in keying rated and tone spacing ... the actual SNR limit is shown in section 17.2.7 of the WSJT_X 2.0 User Guide. CW operators understand this from experience ... a quality 200 Hz filter will have ~12 dB less noise than a 2800 Hz filter. Thus a CW signal with a 200 Hz filter will have 12 dB better SNR than the same CW signal with a 2800 Hz filter (excluding any "processing gain" from the ear- brain filter). With FT8, JT65, JT9, etc. coding (forward error correction) provides some additional SNR (called "coding gain") but the *measurement* is based on strength of the individual tone to total noise. Thus, the lowest accurate report is -24 dB although some signals will be decoded at levels below that. 73, ... Joe, W4TV On 2018-12-19 4:28 PM, K4SAV wrote: While sitting around being bored and recovering from a gall bladder operation, I decided to do some experiments with FT8. First thing I did was upgrade the software to WSJT-X v2.0. I hope this post doesn't turn into another FT8 bashing session. My only goal was to understand how this mode works and what it can do and what it cannot do. The official description of FT8's signal reporting cannot be correct. It is obviously not a signal to noise number and it is not an S meter reading. What is it? That was the first question to answer. It's obviously not an S/N number because how do you give a report of -1 dB for a signal that is S9+40 dB on a quiet band. I was unable to find any info on how the signal report was calculated so I tried to correlate those reports to observations. I think I have figured out a method that results in very close to the same number that FT8 reports. Here is the experiment. I set up my main VFO to USB 2500 Hz bandwidth and set the second VFO to CW at about 150 Hz bandwidth. I look for a station calling CQ and tune the second VFO to him and measure his signal strength. I also look at the S meter for the signal level on the main VFO. I also look at the signal report calculated by the software. For stations calling CQ that report is calculated by the software in my computer. The FT8 report is usually very close to the difference in signal levels (VFO1 - VFO2). For example if the main VFO reads S9+10 and the second VFO reads S9, the FT8 number will be -10 dB. Note that the FT8 says that -24 dB is the lowest it can decode. With VFO1 = S9+10, that's about S7 for the smallest signal it can decode. Observations agree. Those numbers will vary a little depending on how your S meter is calibrated. In order to decode a weak signal, all those close USA stations will have to go silent. The official documentation for FT8 says it will decode signals 24 dB below the noise floor. That is not a correct statement most of the time. That statement should be that FT8 will decode signals 24 dB below the sum total of everything in a 2500 Hz bandwidth. If the total of all signals on the band are below the
