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Wednesday 2 March 2016

How you can work the world on 10 Watts?

On the train yesterday I was asked by another passenger and his colleague what all the radio magazines I had (SPRAT and QQ) were all about. (Basically could they borrow one to read as they didn't have anything with them…not going to say no am I?), anyway after 20 minutes or so after thumbing through the latest editions of SPRAT and QQ I was asked if I could explain some more about QRP and who can you speak to with just 10 watts? That is what the magazine said wasn't it? They admitted they used to both be in the Army (city now) but didn't know much about what signals did, it just worked as far as they were concerned. Interestingly they mentioned that they knew their new laptop used 70 Watts.

Fortunately it was not one of the quiet coaches and noone else was in earshot at the time and I happened to have my own laptop with a recording of some sessions in it both me as the DX (SD7B & C5/M1KTA) the later from both sides and as the DXer (TX6G and E30FB, they knew where the Australes and Eritrea were. I did not manage a qso with E3 sadly but I recorded it all the same). (I like to listen to see who I missed or how I could have done it better next time.) So I replayed some of them.

What was interesting is that they immediately picked up the differences in the perceived 'loudness' of some stations and how the signals seemed to change between them, and noise. Some needing repeats some not, they said they remember having to do that operationally. I didn't mention they were probably using some spread spectrum frequency hopping set up with a battle field uwave satellite link etc that would have been too much info.

I pointed out that in my case I was only running between 8W and 10 watts (SSB recordings), several of the stations they could hear were running 400W, some 1KW but most about 100W, they could pick up those I expected were qrp and they could immediately tell the differences. The same in the case of the DX stations we knew were running 400W and the stations we could hear working them.

We scribbled out a sort of world great circle and plotted where the different stations that could be heard were.

They say this hobby is all about self-learning well I thought I learnt a lot myself too as I was explaining this.

So I thought a post about this might be a prudent…

Where in the world can you reach with 10 watts.

“How far can you talk ? (most never think of CW initially)”
is a question that the newcomer to the hobby often asks. It’s
quite a difficult question to answer because of the number of variables involved.

Perhaps a list some of the variables involved

The propagation conditions at the time you transmit
The radiation efficiency of your antenna system
The power matching between the transmitter and the antenna (why spend ages to create a nice clean signal only to use a random long wire and ATU that looses half the power in it)
The quality of the antenna feeder system
The sensitivity of your receiver
Your patience and ability as an operator
THEIR antenna

Ours is a branch of amateur radio (QRP) that has a passionate and devoted following. QRP stands for “Low Power” as far as most of us are concerned however as I am sure we know really QRP stands for “please reduce power” or “ I am reducing power”. Within QRP circles operators attempt to communicate using the least power possible and there are IMHO a very brave group that seem to always try to do this with less that 100mW.

Generally the upper limits on radiated power in QRP circles are:

single side band (ssb) or amplitude modulations (am) or frequency modulation (fm) ie. Speech 10 Watts.

carrier wave (cw) Morse code 5 Watts.

Digital modes (i.e. using a computer, or teleprinter if into RTTY the old way) 5 watts

Any more and it is considered high power or QRO.

Now there used to be a power definition in amateur radio that it was not the radiated power but the input power, watts being volts times amps that was used to indicate how much power an operator was using.

By that calculation that would mean one of my KX3 output signals at 10W ssb whilst using 13.8V and 2 amps actually meant I was using an input power of 27.6 watts! Seems I might have been a QRO operator all along!

There are very definite differences in antenna radiation power gain levels as well so lets assume we are referring to the power level at the output of the transmitter and not using an erp calculation either.

Anyway from my own experience you can certainly communicate with another operator, we would normally say work another operator in Australia on less than 5 Watts when using cw and under favourable conditions (propagation, antenna, patience, skill, THEIR antennas etc) you can work Australia with 10 Watts ssb.

As we know a large part of successful operating is to ensure that your station is operating correctly.
Under QRP conditions you want to be certain that your antenna is working efficiently and that as much as possible of your 10 Watts of RF is being radiated.

You might do this by following good operating practice perhaps:

Putting up your antenna horizontally as high as possible if you are inland and as close to the waters edge and vertically if next to the sea
Keeping your antenna away from obstructions (particularly metal)
Using the best quality antenna feeder and connectors that you can
Keeping all external cable / feeder / antenna joints free from moisture
Probably most contentiously matching the antenna to the transmitter (or use a mono-band antenna)
Reducing noise pick up (use a balanced antenna)

So what do S points really mean to the operator?
I am sure we have all practiced, listened to and worked weak stations.

Assuming you have either a commercial radio with one or you have built a homebrew radio with one it is usually a little easier to explain the levels you see as well as hear. Your receiver will have a signal strength meter. This will be calibrated S1 (usually if a 1uV input signal heard) to S9 (50uV) and then in decibels above S9. (Usually 10, 20 and 30 db). These S-units indicate the strength of the incoming signal. By convention these numbers are interpreted (lifted off an RSGB note):

Faint signals which are barely perceptible

Very weak signals

Weak signals

Fair signals

Fairly good signals

Good signals

Moderately strong signals

Strong signals

Extremely strong signals

For reliable communications anything above S5 is readable. As we know S3 and S4 signals are workable but with some difficulty.
(NB. Not all S-meters are identical in their sensitivity I accept as well.)
Probably teaching granny to suck eggs but we know that it is a logarithmic scale and the difference between each S-point on the scale represents a four - fold change in the strength of the incoming signal . This is a 6 db change in signal level. So lets assume the distant station increases (or decreases) its power by a factor of 4 then your
S-meter should show an increase (decrease) of one S-point.

So lets take an example received signal strength of S4 as the lowest, workable signal level, without being truly difficult, and equate that to a power output of 1 Watt at the distant, transmitting station then we can draw up a simple, albeit rough, table as follows.

S-Units At Receiver    Power Output From Distant Station (Approx)
S9                1012 watts
S8                256 watts
S7                64 watts
S6                16 watts
S5                4 watts
S4                1 watt

We know that S-units are logarithmic and the ability of the human ear to discern level changes is more logarithmic than linear. Then what this crude table might indicate is that if the distant station reduces its power from 1012 Watts to 64 Watts (For simplicity let’s call this a 1KW stations reducing to 60watts) or from, say, 100 Watts to 6 Watts, or from 10 Watts to 0.6 Watts etc. ,then the received signal that was say originally at S9 will drop only two S-points to S7.
(Or two S-points from whatever was the original level)

So, if you are receiving at S9 (extremely strong signals) you will start to receive signals at S7 (moderately strong signals) if the distant station reduces from 100 Watts to 6 Watts.

Returning to the two guys on the train I remember they were able to pick out the differences easily without prompting.

I find that this fact is often overlooked by lots of operators who run at power levels which are many times higher than they need to be for a successful, intelligible contact and always insist on using the linear.

Sadly reality is it is never this simple.
The above is theory, something I am not great on. However, experience says that it passes the test and can be a useful, empirical rule.
We know this already even if we had not thought it out completely, we naturally raise or lower the volume of out voices when talking to others. It shows that pushing out lots of power is not necessarily the answer to long distance communications in every instance. I am positive any QRP fan does not need to be told that making a DX contact at QRP is so much more satisfying than a similar DX contact at higher power levels because you know your station is operating well.

So hopefully this might help explain if you get asked how you can work the world on 10 Watts with a little bit of effort, some smarts and patience.

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