Friday, June 14, 2019

Extra bandwidth FT8 experiment... June 2019

In a comment on a recent post, Bas PE4BAS in the Netherlands suggested I try increasing the bandwidth on FT8 to see if I could hear more signals.

For FT8, I normally have the IF bandwidth set at 2.4 kHz which is pretty standard on most rigs for SSB.

I waited until 28 MHz was wide open and I increased the bandwidth to 6 kHz. The results are shown below...

For anyone not familiar with the WSJT-X programme, this is the waterfall display.

The green horizontal lines are the 15 second time stamps. The numbers at the top show the audio frequency in Hz going from 200 Hz to 3000 Hz. The oldest signals are at the bottom and the newest are at the top.

The signals to examine here are those at about 2300 Hz and above.

From from the bottom up, you can see how I was listening with the 6 kHz filter and hearing signals from 2400 to 2600 Hz. As soon as I put in the 2.4 kHz filter, those signals pretty much disappeared. Once I opened up the filter again, they reappeared.

Some thoughts....

1) Increase your bandwidth...... It would seem to be worth using wider filters if possible. By using a 'normal' 2.4 kHz filter on SSB, you may be missing out on some signals.

2) Beware the edges..... If the band is quiet then it seems that the obvious place to be in somewhere in the middle of the audio pass band. If however the band is really busy like above then it may be worth spreading out and getting away from the crowd. However it's worth remembering that going on the example above, some people may not hear you.

3) Correct Frequency..... If you're using an older rig then make sure you are on the correct frequency. Just because your radio say 28.174.00 MHz, it's doesn't mean it's right. If you're using a 2.4 kHz filter and are slightly off frequency by 200-300 Hz then you could be missing out on even more signals.

Addendum : 
I found this guideline on the WSJT-X Help page...

Bandwidth and Frequency Setting
If your transceiver offers more than one bandwidth setting in USB mode, it may be advantageous to choose the widest one possible, up to about 5 kHz. This choice has the desirable effect of allowing the Wide Graph (waterfall and 2D spectrum) to display the conventional JT65 and JT9 sub-bands simultaneously on most HF bands. Further details are provided in the Basic Operating Tutorial. A wider displayed bandwidth may also be helpful at VHF and above, where FT8, JT4, JT65, and QRA64 signals may be found over much wider ranges of frequencies.

If you have only a standard SSB filter you won’t be able to display more than about 2.7 kHz bandwidth. Depending on the exact dial frequency setting, on HF bands you can display the full sub-band generally used for one mode.

Of course, you might prefer to concentrate on one mode at a time, setting your dial frequency to (say) 14.074 for FT8, 14.076 for JT65, or 14.078 for JT9. Present conventions have the nominal JT9 dial frequency 2 kHz higher than the JT65 dial frequency on most bands, and the FT8 frequency 2 kHz lower.

1 comment:

PE4BAS, Bas said...

Good post John, I hope this will wake up some people using digimodes and especially FT8. 73, Bas