[M1KTA] How you can work the world on 10 Watts?

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M1KTA's QRP ham radio blog

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How you can work the world on 10 Watts?

Posted: 02 Mar 2016 01:47 AM PST
http://m1kta-qrp.blogspot.com/2016/03/how-you-can-work-world-on-10-watts.html

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):

S1
Faint signals which are barely perceptible

S2
Very weak signals

S3
Weak signals

S4
Fair signals

S5
Fairly good signals

S6
Good signals

S7
Moderately strong signals

S8
Strong signals

S9
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.