I did live in a fairly, Amateur Radio Operator populated area (Canberra ACT) with quite a few Amateur Radio Operators close by. Some of them with the same transceiver outfit as me. There were of course slight differences. For instance, my antenna arsenal then comprised of a Multiband Vertical (Cushcraft R8), 40m horizontal loop, non-resonant dipole (ZS6BKW), a 1/2λ 80m inverted L, (RX/TX 80m and RX down to 200kHz) and then a few RX only E-Field and H-Field antennas (MiniWip clone and a Hermes Loop). Whereas most had rotary dipoles on 40m and some even had 2 element beam (Yagi/Uda) antennas. Additionally there were 80m dipoles at 10 to 15m height at the centre of the dipole.
And yes, the QTH was on a small suburban block surrounded by houses, which were positioned higher than my Antennas at the back and to the right. To the left they started to climb up gradually and the only real take off was into the NNW at about 280 to 320 degree.
Yes, I've been lucky to be able to get these antennas into the air (I do do dishes and other chores around the house which does give me AR-brownie-points).
Anyway, what I've noticed is that some of my local Amateur Radio colleagues don't seem to hear as well as I do. The complaint is about NOISE or to be a bit more accurate a noisy receiver. I do understand that there are differences in antennas, antenna patters, antenna angle of radiation etc. but, most of the time those stations are being heard a lot better than me. However, when it comes to receiving, I seem to be able to hear station on the end of the contact a lot better. Well, It can't be the radio, as some actually have the same radio, an ICOM IC-7610.
So what could it be?
Maybe it is the way I use my receivers reception improvement arsenal and so I thought I'll share the way I use the tools my receiver provides to improve my listening pleasure.
So lets have a quick look at the way I conquer some of the noise.
Please note, that these are some of the things I do to help me overcome QRN and even some QRM. This is not a blurb about technical specs, even though I might throw some into the mix. This is more a HOWTO get the best SIGNAL to NOISE (SNR) performance out of your system, i.e. the system made up of the receiver/transceiver, the antenna and not to forget the antenna cable. Being able to receive and understand a station that does not move the S-meter up to the S9 point but still has reasonable good recovered audio quality even if the signal strength is as high as the band noise floor.
First up a disclaimer, I do not talk about the "OH NICE SIGNAL 59+20", sounding like John Laws and are > 3 kHz wide, no I'm talking about stations that are in or just above your received noise floor. And second, I refer to Single Side Band (SSB) but will show the effects on a CW signal to make it easier for me to show the effects.
We know that our receivers have BIG EARS. For instance the Minimum Discernible Signal (MDS) of my ICOM IC-7610 starts at about -122dBm and goes to over -140dBm. Basically what that means is, that the radio is capable of hearing the proverbial fly fart.
I would like to see the ambient noise floor at my QTH go that low but according to ITU P.372-12 the noise floor at my QTH should be about -91 dBm at 7 MHz (S6) (Note: an accurate S-Meter is required to confirm this). At the new QTH, here in VK5 land it is slightly better, by about 9dB, but at the old QTH in VK1 land it was around -97dbm.
Which is still a far cry away from -122 or even -140dBm which the IC-7610 is capable of achieving (sorry about the technical digression).
To show you what I'm talking about I've been taking some screen shots which show what can be achieved by applying the following, let's call them receiver-improvement-tools, to improve our receiving pleasure. The setup for the demonstration comprises a SDR-IQ withand the following software SpectraVue Ver. 3.39 and SBSpectrum Ver. 1.31.
To have a steady signal I'll be tuning the radio to our local NDB (ground wave) at this frequency the noise is still high and the signal will be at a constant strength for the duration of the test. This makes it easier to show the effects of the applied countermeasure.
So here is a list of the Tool Set I've been talking about.
- RF-GAIN control
- RX-Attenuator
- AGC
- RX-filter bandwidth
If we apply any or all of the above appropriately we'll be able to dig out signals that are difficult/marginal to hear/understand. What we are doing is what is know in professional circles as the improvement of the Signal to Noise Ratio (SNR). It does not mean that the signal strength will increase, most likely the signal strength will be lowered. However, not only the signal strength will be lower but also the received noise. And, this is the aim of the game, if we apply our toolset appropriately we will reduce the noise level more than the received signal! And as such we will have improved our SNR.
Let's start with our receiver bandwidth (RX-BW), some call it the channel bandwidth, e.g. the frequency span the receiver is listening too. Receivers have a basic RX-BW of about 3 kHz. And most Voice transmissions are with a 2.8kHz transmit bandwidth (TX-BW) (yes, yes I did say most).
Now if we look at those number we can see the we already have 200Hz unused RX-BW and this 200Hz is filling our receiver with noise (RX-BW3000-TX-BW2800=200Hz of noise) Imagine if the transmitted signal would have a TX-BW of 2.4kHz and we would receive the signal with a 3 kHz RX-BW. We would now receive 600Hz of additional noise. To improve the quality of the signal we would need to get rid of the 600Hz of noise.
In the olden (Golden?) day's we added X-Tal filters to limit the ingress of additional noise (a quite expensive exercise let me tell you). However, this is were the new breed of SDR's shine, they use "software" to do the filtering and some of those software filters are exceptionally good. Additionally they are very flexible, with soft or hard skirts etc. You'll be able to adjust the RX-BW quite easily to adapt to the transmitted signal.
So by adjusting the RX-BW to the TX-BW we would improved the SNR by 600Hz.
On a good receiver S-Meter, one would be able to see that the noise floor has dropped and the signal has gone up.
Let's have a look how that would look in real life.
The grey area is the noise that we are receiving. Looking at the bottom CW signal we see that we can see (hear) it but it is quite buried in the noise. Taking away the noise by limiting the RX-BW we are able to see that the signal becomes more darker, almost black. It is now popping out of the noise and listening to the signal it is more audible, e.g. clearer and more intelligible. If we widen the RX-BW again it becomes clearly visible that the signal loose's its intelligibility, as it is fading back into the grey again (picture below).
For me reducing the RX-BW is one of the most effective ways to improve the received signal quality. Even if I receive a signal that does not limit its TX-BW to say 2.8kHz I always run 2.7 or even 2.1kHz RX-BW on SSB. I even go as low as 1.7 or 1.5kHz RX-BW if the going gets tough but that has mostly to do with QRM. If you go above 7.2MHz at night you will find some US Amateur Radio Operators there between the big AM BC stations. And those narrow filters make it possible to hear those station without to much interference from those potent signals.
What we have done is we have limited the receivers SIGNAL+NOISE ingress e.g. we've improved the SNR. The net effect is that the SIGNAL we are interested in, SOUNDS clearer/louder.
Another tool in our receiver-improvement-tool is the Automatic Gain Control, the AGC. Most modern Amateur Radios have the ability to adjust the TIME CONSTANT (ATTACK TIME) of the AGC.
The below picture shows the AGC disabled and then enabling the AGC, the noise disappears and the signal pops up. It is worth while playing around with the AGC time constant. This is a bit of an art form and it is quite different between radios and the style of radio. I have three different settings on my radio for different noise events/modes. I have a slightly faster AGC recovery time for thunderstorm/lightning QRN then I have for quite band conditions on 10m. Also I have different timing setups for different modes (SSB, CW, DIGI).
Switching the AGC off and riding the RF-gain can also improve our listening pleasure. Below you can see me adjusting the RF-gain manually (this is called riding the RF-Gain) to bring the signal out of the noise. However, as you can see from the above picture, a proper adjusted AGC does do a better job than I can do.
The next picture show a combination of AGC and manually adjusting the RF-gain.
I wish it would always be that easy.
Instead of ridding the RF-gain, most radios have the ability to add predefined attenuation. The below picture show similar results as using RX-BW limiting and predefined attenuation switched in and out.
Below is a good view of the AGC in action. As soon as there is no signal, the AGC increases the gain (darker grey), but as soon as the signal shows up the AGC reduces the gain enough for the signal to pop out of the noise.
So, as you can see we have a great arsenal of tools available to BETTER our receivers ability to get the desired signal out of the noise, or to phrase it more appropriately, to ease the HSP between our ears to only work on the SIGNAL and not needing to apply filters to remove the noise (remember the HSP has limited HIPS).
Please remember that I used a CW signal for display purposes only, this will work equally well for SSB and even for some DIGITAL modes.
On 7 MHz (40m) and below I have between 9dB and 24dB attenuation as atmospheric noise levels are high on these bands. Start with whatever you feel comfortable with. Next I'd setup the filter bandwidth to remove the higher pitched noises, the noise above 2.4kHz and then I adjust the filter to cut out the low rumbles, the noise below 100 kHz. My RX filters are set at 2.7 kHz, 2.1 kHz and 1.7 kHz for SSB, low cut at around 100 - 200 Hz for example, my 2.1 kHz filter bandwidth is setup as 200 - 2300 kHz. And last but not least, adjust the AGC to your liking. My preference SSB AGC settings are Fast 1.2, Mid 2.0 and Slow 6.0.
The AGC, RX-gain and RX-attenuation are basically controlling the overall system gain, which does includes the gain from your Antenna System (Antenna, Cable, ATT, LNA etc.).
The RX-gain/attenuation values will be different for a lot of situation it depends on a lot of factors i.e. the mode of operation, what antenna is being used, the frequency of operations, the operator mood, the time of day, band conditions etc. etc.
However, as you get more and more familiar with your radios rx-tool-set you will not only get more and more pleasure out of your receiver (station), you will also manage to finish more contacts with ease.
Remember, you will need to play around with the setting until you find a setting that sounds/feels right to you. The above are setting that work for me and should only be seen as a guide. Your environment, radio, antenna, HPS and HIPS are different.
Let me mention a few more tools that some of the radios provide or are external items that would improve our listening pleasures.
An additional AF filter (Equaliser) can help in reducing noise, Digital Noise Reduction Systems (NR), which most newer radios have built in. It is however, better to reduce the noise before it hits the AF stage of your receiver. Some digital Noise Blankers are quite impressive and then there are the Notch Filters, either the automagic ones or the manual ones (the dual notch filter in the IC-7000 is quite amazing).
There is one (two) more thing(s) I would recommend and that is using a decent speaker or two if your transceiver has two receivers. And lets not forget a good set of headphones, which will bring the recovered signal directly, without any additional noise to the HPS.
There are of course additional measures we could apply. One of those measures is to use a low noise receiving antenna or use antenna diversity.
I worked my best DX with a noisy vertical as a TX antenna and a quieter RX antenna (see above my antenna arsenal).
73 and good DX.
HSP = Human Signal Processor (normally found between the ears)
HIPS = Human Instructions Per Second (it has been said that a man can only do one HIPS)
DX = Normally used to refer to a station on another continent. From the old telegraphy abbreviation for Distance eXchange.
SBSpectrum V.1.31 by Peter Martinez G3PLX
SpectraVue V. 3.39 by RFSPACE Inc.
SDR-IQ by RFSPACE Inc.
DXing
© ¼ ½ ¾ ⅜ ⅝ @ π ω µ Ω ε η λ °
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