Receiving weak signals with the FT-991 #hamradio

In this blog I would like to spend some time on the use of a number of features of this transceiver from Yaesu which can be quite useful to receive those weak signals that otherwise remain hidden in the noise. Rather than listing the functions of all the buttons as in the manual I want to discuss here an operating practice for receiving those noise QRP stations.

Once you have found a weak station first try to tune it such that the signal becomes maximal, use the fast setting or the channel dial/multi knob only for quickly browsing through the frequency band. Next turn fast off, and tune so that the weak station sounds best. The clarifier button in combination with the clarifier knob can help to adjust the final frequency offset that makes SSB modulation sound like Donald Duck rather than a human voice. (Added: study the difference between clarifier RX and just clarifier, also, eventually turn the clarifier back to zero when you are done with the station that needed it and return DXing with other stations, see comments by PA2QXT at the end of this blog). But now the noise.

Listening to noise is problematic, actually, you will usually fall asleep if you do this late in the evening. Most people give up when they hear a weak voice that is hidden in the noise. There are several types of noise, natural and man made noise. In radio language QRN is for natural noise, QRM is for the man-made noise. Sometimes QSB is used to indicate signal variation like fading originating from changes in the ionosphere. Other forms of QRN are lightning strikes which are clearly audible at all frequencies. You can’t do much against QRN or QSB except to wait when it goes away. High Kp indices with corresponding Ap averages are typical for QSB, this is because the magnetic field is changing rapidly so that signal reports rapidly vary when you listen to a station. You can’t do much here, maybe the advise should be to not give up because sporadic long distance contacts may be possible under these conditions.

The repeating ticking noise that you sometimes hear at many frequencies are usually caused by something in your neighborhood, oftentimes the noise blanker helps to filter the ticks away, so for this we have the NB button. Only use it when the ticks are there, otherwise leave it off. Reason for leaving NB off is that audio may be distorted which is otherwise ok, see the comments from PA2QXT with this blog.

If there is noise of neighboring transmitters or local oscillators then we speak about RFI which stands for radio frequency interference. For this I describe an experiment with a home-built Arduino on youtube. All oscillators in unshielded casings around us may cause some form of RFI. Locate such devices with your SDR like the airspy or tools like the MFJ 805 and change the situation. In particular, make sure that the local noise generators are grounded and that they are within a metal case, or otherwise shielded within at the point where the RFI is generated. It may also be that cable connectors are the problem or that unshielded cables are used. Homeplug PLC systems are really bad, in particular the older ones. If they exist in your neighborhood then ask your neighbors to change to for instance WIFI or a cabled network.

Back to the buttons on the transceiver. My experience is that the NB button usually does not really help against RFI and that you need a better ‘weapon’. PA2QXT reports that the NB does help against the noise of switching power supplies, but also warns that once the NB is on that audio may become distorted because of the NB. The noise of switching power supplies may be suppressed with the NOTCH and the CONT filters that are helpful when your are listening to a station close to a RFI source. With the CONT filter you can suppress the splatter of a station near the frequency where you are listening, the CONT filter is a wide band adjustment to your reception window. The NOTCH filter is good for suppressing a very narrow part of the spectrum within the range you are listening to a weak station caused by an RFI source that comes with a narrow band width. Switching power supplies may appear everywhere in the spectrum usually at every 50 or 100 kHz, sometimes slightly drifting around. If power supplies are really noisy then maybe it is also time to replace them, or alternatively if you can’t do anything against it, to go on a camping trip away from all the QRM. PI4CQ is a club in the Netherlands that does this.

The computer and the screen that you probably want to use near your transceiver is a well known source of RFI because it is full with local oscillators that pollute the spectrum. This used to be the case in particular in the early days, nowadays you can simply ground the metal case of the computer, it takes most of the effect away is my experience. It could also mean that you are sometimes better of without a computer near the antenna or the transceiver. Hard to say, but, rather easy to find out.

There is also noise caused by the receiver itself, if it is the case then we speak about a birdy. If you get a new radio then first learn where all the birdies are, for this you can remove the antenna and just quickly tune through all the bands. My former FRG-7700 receiver had quite a few birdies, in particular between 25 and 30 MHz. The new FT-991 hardly has any. Memorize the birdies so that you don’t waste your time on them. If there are many birdies, or if the problem gets worse over time, then maybe your radio needs maintenance. The NBCONT and NOTCH filters will probably not help against birdies.

Once you have experimented with the NB, NOTCH and CONT then you could continue with the more difficult step, which is to make the signal more audible. To improve the signal to noise ratio you need to experiment with the WIDTH filter, the AGC, DNR and the attenuation knob called RF gain on the transceiver. Normally I first experiment with the AGC response setting and the WIDTH, change them until you’ve found something that works best for your situation. The AGC is the gain feedback, the input into the AGC system is obtained after demodulation and what you adjust is the response time for adjusting the gain at the HF front end amplifier and the intermediate frequency amplifiers of the receiver. Sometimes a fast response time works best, for instance when there is QRN from lightning, but more often a slow response time works better, it is hard to say. Do something that you like best is my advise. The AGC response times can be modified in the menu, for operation you simply choose between off, fast, mid,  slow and auto. AGC off is hardly used because chances are that you enter in the range where non-linearity of the amplifiers makes this cracking sound. AGC at auto is often a good choice although some radio-amateurs like AGC slow better for weak stations.

Next comes the DNR filter which is a digital noise reduction filter that you find on the FT-991. It is a very effective tool and I found that it works best in combination with the RF gain dial on the mid right of the FT-991. What I initially do is to leave the DNR off and turn the RF gain clockwise to maximum gain, and then play with the preamplifiers (IPO/AMP1 and AMP2) until you’ve found the point where the signal sounds best. For signal strength reports you should (I believe) report the signal strength on the S meter that you get with an IPO, the AMP1 and the AMP2 just add a number of dBs per amplifier, this is fine, but AMP1 and AMP2 also increase the noise that comes with the signal. Once you turn the pre-amplifiers on you add a number of dBs that show up in the S meter. The signal strength to report is then the difference between the no-modulation value and the modulated value. All the 59’s that you hear all day are probably exaggerated, my opinion is that the pre-amp added dBs should not go in the signal strength report, but I doubt whether this practice is also carried out by most radio amateurs. We hear you, the signal is ok, so it is a 59. No, this is not the way to do it.

Suppose that AMP1 worked best without RF attenuation on the 20 meter band. A weak station may then still be difficult to hear, but now turn the  DNR to like 1 or 2, that is, hit the square on the display and use the multi-knob to change the DNR filter value. How does this sound? Probably it does sound a bit better but there is still noise, also, it may sound as if someone is speaking through a tube. Next turn up the RF gain attenuation knob (it turns counterclockwise to attenuate more). At some point all the noise disappears, but the voice still remains. The audio gain (AF) can now be turned up, at some point it becomes optimal is my experience. There are more filters on the FT-991, there is also a DNF filter and sometimes this helps to remove some hiss in the received signal if it still persists. I don’t often use to DNF filter, but apparently it has a purpose sometimes. The DNF option could be a last step in this process.

If you ever decide to do PSK, WSPR or any other digital mode then all these filters, including a speech processor (PROC button) or microphone equalizer (MIC-EQ button), should not be used. In this case your computer has to process the signals, not the transceiver. The best results for digital modes were obtained with a pre-amp setting at AMP2 on the FT-991. For the transmission of PSK signals I describe in this article what you should do with the optimal audio input gain.

With the above described operating practice you will see that even a poor signal to noise ratio stations can still be received pretty well. It can take some time to carry it out, but you will see that it really works. Take some time to understand fundamentally how your transceiver really works, the manuals do not help here, and the exam material for your license only partly helped here because there is no common interface for radio receivers.

In particular the noise makes you tired when listening to those weak QRP stations.If you ever have trouble sleeping, then put up a headset and listen to white noise, chances are that you sleep right away. In any case I do recommend a headset because it suppresses the local dBs that you hear. Also, the dynamic range of a headset is oftentimes much better than the computer audio box that I otherwise listen to. When you live with a spouse and children as I do then a headset is certainly recommended.

If I had some experience with different transceivers then I could write a similar manual as in this blog, the filters probably have different names. Previously I owned a FRG-7700, bought it around 1980 and sold it (with a caveat emptor) last year because it was becoming deaf on the amateur bands. Digital filters did not exist in the 80’s, also waterfall displays and the ability to easily store and recall frequencies. The FRG-7700 had a real time clock and I was quite happy with the local oscillator stability, the selectivity of the roofing filters and the birdies were somewhat of an issue, but otherwise it was a really nice receiver. Quickly memorizing a few frequencies is easy on the Ft-991, for this there is the store/recall button, press it shortly and you recall a stored frequency, press it longer and it stores a frequency. This is a first in last out (FILO) type of memory and I find it convenient.

I hope that this blog assists you in your ham operations. Good luck DXing.

Last update: 29-5-2016, PA1EJO Ernst





One thought on “Receiving weak signals with the FT-991 #hamradio

  1. Hoi Ernst!

    Om te beginnen zou ik als ik jou was in mijn wordpressblogje benadrukken dat amateurs die RIT oftewel clarifier niet al maar te pas en te onpas moeten gebruiken want ik maak té vaak mee dat amateurs het kreng per ongeluk nog aan hebben staan als ze middels hun VFO/DDS op mij afstemmen; ik zorg altijd dat ik netjes op de frequentie sta maar zij stemmen dan af op mijn stem en hebben dan niet in de gaten dat ze zelf zwaar naast de frequentie zitten.

    Ook zou ik als ik jou was nog even benadrukken WAAROM je de noise blanker uit moet laten staan als het inschakelen niet noodzakelijk is; namelijk vanwege het feit dat de audio er altijd door vervormt waardoor je de kans loopt iemand ten onrechte een slecht audiorapport te verschaffen, zoals ik zelf ook ooit meegemaakt heb. Gelukkig kwam ik erachter voordat ons QSO voorbij was en met de NB uit klonk Fred PA0MER als een klokje.

    De typische schakelendevoedingsratel krijg je met de notch meestal helaas niet weg; daar helpt juist de NB weer beter tegen. De notch is om echte fluittonen uit te filteren; bijvoorbeeld als er iemand precies op de frequentie waar jij al de hele tijd zit te praten gaat zitten CW’en met een ander station. Zo’n klein hapje uit het signaal is nauwelijks hoorbaar, laat staan storend.

    Tip betreffende de AGC: Wanneer er ergens in Europa onweer in de lucht zit kun je de AGC beter niet te langzaam zetten; je ontvanger heeft dan na iedere klap veel te veel tijd nodig om te herstellen; het beste is om hem helemaal uit te gooien en in plaats daarvan de RF-Gain te gaan gebruiken. Je zult iemand geen signaalrapport kunnen geven maar je zult hem/haar des te beter verstaan.

    Inderdaad gebruik ik ook de preamp voor de 20 meter en hoger omdat de Kenwood TS-480 op die hoge banden een beetje signaalanorexia vertoont; ik smokkel dus op de hoge banden. Op 40/80/160 gebruik ik de preamp vrijwel nooit omdat de meter daar stukken enthousiaster is en de preamp de signaal/ruisverhouding bepaald niet ten goede komt.

    Je hebt gelijk wat digitale modulatiesoorten betreft, en dan in het bijzonder PSK; een speechprocessor is werkelijk de dood in de pot wat jouw leesbaarheid, laat stáán je IMD betreft en als je per ongeluk de notch nog in hebt staan kun je het gevoeglijk schudden aangezien je dan precies de smalbandige signaaltjes die je wilt ontvangen wegnotcht.



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