[continued from previous message]

by the low-pass filter. The gap between the overlap of the high-pass and low-pass filters is what creates a space where the 15m band gets through.


If you move things around a little, the same can be constructed to make a
15m band-stop filter. Something that lets anything through, except a 15m amateur signal. To make such a gadget would require a low-pass filter that allows everything below 15m combined with a high-pass filter that lets everything above 15m through.


So, if you can construct a high-pass filter and a low-pass filter, you can pretty much create any combination and allow or stop specific frequency
ranges.


If you're wondering why this might be useful, think about a contest. Two
radios in the same shack. One transmitting on 15m and one on 40m. These two bands, one at 21 MHz and one at 7 MHz are third harmonics to each other.
This means essentially that a radio on 40m affects one on 15m and
vice-versa. If you had a set of filters that stopped 15m and passed 40m on
one transceiver and a set of filters that stopped 40m and passed 15m on the other, both of you would be much happier.


You don't need to do contesting to benefit from a filter. If you use an
RTL-SDR dongle, it's affected by nearby strong signals, like say a local
radio or television station. That's fine if that's what you're trying to
hear, but not so much if you're trying to hear something else. Filters can
help to make your life better.


Now, to round this off at a suitable point, you can think of an inductor as device that lets low frequencies through but blocks high frequencies. Similarly, a capacitor is a device that blocks low frequencies but lets
high frequencies through. So, it's fair to think of an inductor as a
low-pass filter and a capacitor as a high-pass filter. The symbol for a capacitor is the letter C (Charlie) and for an inductor it's the letter L (Lima).


You could make a circuit that either directly blocks from a certain
frequency, or one that lets it through, but sends it to ground. This gives
you two designs for a low pass filter one using an inductor or an RL
circuit and one using a capacitor or an RC circuit. Similarly you can
create a high-pass filter using either an inductor or a capacitor. That
gives you four designs for two filters.


Each of these can be combined to create band-pass and band-stop filters.


The maths behind it isn't particularly daunting with basic high-school
maths and if you want to see it happen before your eyes, check out
the "Organic Chemistry Tutor" on YouTube. The play list you're looking for
is cleverly disguised as "Electronic Circuits".


As a direct result, I started hunting for breadboards, but it turns out
that you can simulate these circuits online using any number of simulators.
Of course there's going to be a gap between simulation and reality, but
that's when you get out your soldering iron.


Remember, if you smell chicken, you're holding it wrong.


I'm Onno VK6FLAB
This posting includes a media file: http://podcasts.itmaze.com.au/foundations/20200216.foundations-of-amateur-radio.mp3

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How I care for my connectors

Posted: 08 Feb 2020 08:00 AM PST


Foundations of Amateur Radio


If you've ever found yourself in the position of attempting to screw a
PL259 into an SO239, or an N-type plug into an N-type socket you'll have
likely come across the situation where the thread doesn't quite fit. If it does, you might have issues attempting to undo the connection, even if you didn't particularly do anything strenuous in relation to mating the two in
the first place.


This kind of situation happens to me more than I think is reasonable. It happens on cheap connectors, on expensive ones, on the back of radio gear,
on adaptors, patch leads and the like.


Initially I put this down to cheap vs. expensive, but that really doesn't
add up if you're attempting to connect an expensive plug into an expensive radio.


If you're into machining you'll know about swarf. If not, think metallic
dust. Of course it doesn't have to be metallic, it could be a single grain
of sand, or it could be a slightly damaged thread.


A couple of months ago I went on the hunt for a tap and die set that would solve this issue once and for all. If you're not familiar with the terms, a
tap is like a long bolt with a square head and a die is like a thick washer with holes cut out.


In addition to being hardened, they each have cutting edges, which allows
these two tools to do their job, the job of cutting threads.


Normally you'd use a tap to make a thread into a hole that you've drilled. You'd use a die to make a thread onto a rod that you have. There's lots of technique associated with this, cutting fluids, alignment, pressure and the like. Plenty of relaxing YouTube videos around - which is how I came upon
this idea in the first place.


You can also use a tap or a die to cut across an existing thread and you
can do this with connectors.


A die, threaded over a socket, will clean up the socket threads. Similarly
a tap screwed into a plug will clean up the plug thread. There's a
disclaimer coming for that last point, but stick around.


Trying to find a tap and die to match can be a challenge. The PL259, SO239
and N-type connectors are all 5/8th size threads. They're 24 turns per
inch, and also known as UNEF (Uniform November Echo Foxtrot) threads, or Unified Extra Fine.


So if you start on your hunt, you'll be looking for 5/8th, 24 TPI, UNEF
taps and dies.


I found mine online at $15 or so from a US supplier. Got to me in about a
week.


When they arrived I immediately set about cleaning up all my sockets. This
was amazing, all of a sudden stuff started fitting well. Unfortunately I couldn't use the tap. The centre hole in a standard tap isn't big enough
for the pin of a PL259, let alone an N-type connector, but a friend of a
friend has access to machine tools and made the centre hole bigger. Word of warning, this is hardened steel. A hand-drill won't cut it.


I must mention that this won't allow you to use the tap inside an N-type
plug, but you can use a die on the socket.


I'll also point out that if you need to use a tap wrench or a die holder, you're doing it wrong. We're cleaning up the thread, not making a new one.
If you need extra force the most likely scenario is that you've cross
threaded the tool onto the connector.


Of course if you've got a completely stuffed connector thread, then these
tools can help, but you might want to consider replacing the connector.


My tap and die live in my go-kit right next to the coax adaptors. On my
next field-day I won't be having to deal with poor connections, nor will I
have to worry about unscrewing them after the event.


A tap and die, great simple tools to fix a recurring issue.


I'm Onno VK6FLAB
This posting includes a media file: http://podcasts.itmaze.com.au/foundations/20200209.foundations-of-amateur-radio.mp3

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How I host a weekly amateur radio net for new and returning amateurs

Posted: 01 Feb 2020 08:00 AM PST


Foundations of Amateur Radio


If you've ever had the pleasure or misfortune to hear an on-air net, you
might have considered, however briefly, how that net came to be, how it's
run and what's involved behind the scenes to make it happen.


I host a weekly net called "F-troop". It's been running every week since
the 12th of June in 2011. Since then I've made over 5000 contacts with
stations scattered all over the globe. A typical net has about ten people,
but depending on the weather, what's on TV or if people had a hard Friday
night that number fluctuates. The biggest was about 40, the smallest just
two.


At this point I could tell you that the infrastructure to make this happen,
the preparation, management processes, network and marketing are what take
up the bulk of my week. I mean, there might be a weekly stand-up between stakeholders on a Wednesday, a plan for the content, what to discuss, you
know, the typical.


If I told you that, I'd be lying.


The reality is that F-troop is an organic animal. I generally get to my
radio a couple of minutes before we start, midnight UTC, switch on,
kerplunk the local repeater and wait for the clock to tick over.


I then launch into my opening spiel, something along the lines of: "Hi
folks, it's me, it's F-troop, who's awake?"


After taking a few calls and logging them, I'll circulate through, call for more people, rinse and repeat.


There are two invisible things happening, one required, the other I do
because I'm a computer geek. The required activity is logging. I chose to
log in an online spreadsheet. It's helpful because it makes for a single
place where all contacts are stored and it allows for others to host the
net if I happen to fall off the air, either by being somewhere else, like a holiday every decade or so, or because my radio isn't being cooperative.


The other thing that logging gives you is a memory. I generally recall a person's name from their callsign, but if you listen closely you'll notice
that every now and again I'll extend my babble so I can search for a
callsign and appear not to be suffering from memory loss.


The other thing that happens is that I update the website. I'll be merrily adding articles from emails or discussion as it's happening. If someone mentions a product or a website, a callsign or a project, I'll often be searching for it in real time and adding it as a post to the F-troop
website. That way people who want to refer back at a later time, that
includes me, can search and find the thing that someone showed us.


As simple or as complex as that sounds, depending on your level of
experience, it's really not rocket science. You can do this with pen and
paper. I know, I've done it, standing in a car-park with a notepad, whilst dodging rain showers and preparing for a field-day. It's fun to test your
skill and to get out of your comfort zone every now and again.


I should interrupt this story for a word from our sponsors. Don't have a kitten, we're not talking about advertising, we're talking about repeater
and network operators who graciously give of their time and resources to
link the main F-troop repeater to others around the world. The network of AllStar, Echolink, IRLP and IRN radios that carry F-troop is astonishing to
me. We have regular participants all over Australia, the United States and
the United Kingdom. There have been contacts with stations in Asia and
Europe.


For that to happen I don't do a single thing. Well, technically I let
repeater operators know I exist and when it breaks, but that's pretty much
the sum total of my efforts.


Why am I telling you this?


Last week it broke. My radio was acting up and someone commented on that. I handed over the reigns and let them at it. They were very unsure. I let
them know that F-troop is for beginners. It's expected that people are
going to make a mistake, I know I do, plenty of times.


It occurred to me afterwards that hosting a net can be scary. If you have
no idea what's involved, how to make it happen, what to do, then hosting
must be immensely daunting.


I hope that sharing how I do this will give you the confidence to host your
own net in your own community. Perhaps you can tell me more about it, or
come and visit F-troop. Saturday morning at midnight UTC. If you want I'll
even help you host it.


I'm Onno VK6FLAB
This posting includes a media file: http://podcasts.itmaze.com.au/foundations/20200202.foundations-of-amateur-radio.mp3

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Is this frequency in use and other lies we tell ourselves.

Posted: 25 Jan 2020 08:00 AM PST


Foundations of Amateur Radio


When you switch your radio on to start a radio fishing expedition you join
all the other spectrum users across the planet. To be fair, you'll likely
only become aware of some of those for the time that your radio is switched
on, even if there are thousands around.


One of the ways you can find other users is by ditting out "QRL?" in Morse
or saying "Is this frequency in use?" into your microphone.


This simple courtesy of checking to see if the frequency you're on is
actually being used by someone, is part of your license. You're taught to
do this and it's expected.


What's not clear is what happens next.


It's simple if you hear a callsign, or a "yes", but what if you hear
nothing?


Sometimes nothing means exactly that, there's nobody on the frequency, but that's not always the case. There's plenty of opportunity for the frequency
to be in use and you still not getting a response back.


Let's imagine for a moment that the frequency you're on is in use by two stations talking to each other. You come on frequency, hear nothing and ask
if the frequency is in use. You hear nothing. You try again, still nothing.
You start calling CQ. Moments later, you get an earful from some random station.


Sound familiar? If it doesn't, you'll need to spend more time on-air. I can guarantee that you'll experience this in your amateur adventures, much more than once.


How does this happen? You did everything right.


Imagine two stations, let's call them Amanda VK4FRST and Marc VK3OHM,
having a conversation, a QSO. They're discussing the ins- and outs of the
WIA awards system and having a grand old time.


You turn your radio on, happen to tune to the same frequency as their QSO
and after listening to nothing for a bit, you call "Is this frequency in
use?". You still hear nothing so you try again: "Is this frequency in use? VK6FLAB". Still nothing. You call "Nothing heard." and start calling CQ.


You're on one side of the country, Amanda and Marc are on the other side.
They cannot hear you and you cannot hear them. Then the sun moves a bit and
all of a sudden your CQ is all over their discussion. Unhappy people on
both sides of the country.


There are six paths to consider here. The one between you and Amanda, and
the reverse. Similarly the path between you and Marc and that reverse. If
you ask for frequency in-use, neither Amanda, nor Marc can hear you.
Similarly, you cannot hear either Amanda or Marc. You should also take a
moment to consider the path between Amanda and Marc and vice-versa. They
might have a really great 5 and 9 conversation, or they might be struggling along with a 3 and 2.


I've simplified this, because of course, you calling over the top of a conversation can also disturb the contact under way. Saying that the
frequency is in use makes it worse.


While all this is happening, the sun is moving, the ionosphere is moving, propagation is moving, the whole thing is like the Cat in the Hat balancing
on a beach ball, complete with cake, rake and a fish still in its bowl.


The first thing you need to do when this happens is stop and take a breath. Nobody owns any frequency, so claiming that this is your frequency is not
going to help anyone. If the other station is having a QSO and you're
calling CQ, it's time for you to move, change frequency and QSY.


If you're Amanda or Marc, you can tell your contact that there is some interference and then call the other station that the frequency is in use.
If they change frequency, all good, if they don't, tell your contact to
change frequency.


There's no need for aggravation. There is no ownership. There's no point in getting upset and no mileage in making life hard for the other station. The fact of the matter is that there was what we call in networking, a
collision. It's time to back off and renegotiate.


All this is exactly the same if you're using voice, Morse, FT8, or any
other mode.


Take a breath, renegotiate, move on.


Now, if you're a QRP station like me, it's much more likely that you'll not
be heard most of the time. In that case it's often much quicker to just to move without going through the negotiation process. Of course you can
attempt to make a QRP contact with one of the other stations, but it's considered pretty rude to stick your head between two people who are having
a cup of coffee together and ask them for their autograph, so don't do it
on air either.


If you assume malice from the get-go, you'll find yourself unhappy most of
the time. If you celebrate that all of a sudden there's propagation between VK6, VK4 and VK2 you'll end up much happier with your on-air experiences.


While I'm giving out advice, here's something I learnt during the week.


If you break a toe, tread carefully. Stubbing a broken toe hurts. Really. Badly. In case you're wondering, my new boot is not a fashion accessory.


I'm Onno VK6FLAB
This posting includes a media file: http://podcasts.itmaze.com.au/foundations/20200126.foundations-of-amateur-radio.mp3

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The lessons we teach.

Posted: 18 Jan 2020 08:00 AM PST


Foundations of Amateur Radio


When you become a member of the amateur radio community you become part of
a small group of humans who know and understand certain aspects of life.
That's not to say that others don't share this or that the knowledge is
unique or special, but radio amateurs are required to know this before they receive their license.


In the past I've spoken about how getting a license is like receiving a key that opens the door to the world of radio communications. It's one of the
more accessible ways to grab hold of this key and it's the recipe for life
long learning.


During the week a friend of mine, a newly minted amateur, pointed out that
this represents something that the general population isn't aware of or attaches little in the way of value to. The interconnectedness of radio spectrum is something that radio amateurs take for granted. To us it's
obvious. A transmitter on 3585 kHz is fundamentally the same as one on 92.1 MHz. A key fob on 434 MHz is similar to a computer on 2.45 GHz as is a
laser on 500 THz or an X-ray machine on 30 PHz.


As a radio amateur we're taught that the radio spectrum is a continuous phenomenon and that spectrum is shared among users with specific rules
around interference and interaction.


Another thing we know as radio amateurs is the difference between the front
and the back of a Yagi-antenna. We know about radiation patterns, about the ionosphere and how the sun and sun-spots interact with some of our
activities.


The point is that our knowledge, it's fair to say, specialised knowledge,
even at the lowest level of licensing, exceeds that of the general public.


This is all by way of background because this leads to something that I
learnt during the week.


As amateurs we have a responsibility to be custodians of that knowledge,
that is, to care for it and to ensure its accuracy and to preserve that knowledge.


For some amateurs that means that they want this information to be
exclusive, but for me it means that this information should be shared and nurtured and encouraged in those people who make choices based on incorrect information.


For example, as a radio amateur it's my duty to inform a person who is contemplating breaking the radio spectrum licensing rules that they are
doing so. Not because I'm a regulator, but because I have specialised information that they lack. Importing a radio module that's using a
frequency that's not available in your country is an example of something
that I am compelled to point out.


I know that some amateurs take this compulsion to the next level and become
a de-facto police officer attempting to enforce those restrictions. I understand where that comes from, but I also know that this is not my role
and it's not your role. If you feel strongly enough about a transgression, perceived or real, there are plenty of ways to deal with that. Reporting
the offence to the regulator is one option for example.


Knowing which end is the front of a TV antenna means that you can point out
a mistake to a home-owner about the direction their antenna is pointing at,
but it doesn't mean that you need to climb on their roof to turn it around.


I've said many times before that having an amateur license is a privilege.
It's a gift, even if you worked hard for it, it was given to you, bestowed
on you by the regulator in your country.


It seems to me that having such a gift means that it should be treated as
such. As radio amateurs we're not entitled to a license, nor are we
entitled to transmit. We're granted permission to do so.


I think that it's important to keep that in the back of our minds when we
set out to educate those around us.


As for the education itself. It pays to consider what you take for granted
when you're giving advice. Telling a person about Wi-Fi propagation through
a home is a complex topic. You can make the explanation as hard or as
simple as you want, but don't expect that the person receiving the advice
has the same background information or interest that you have.


I was once told by a statistician about how various statistics worked and
what their background was. I was translating a program from Modula-2 into
WingZ hyper-script. I didn't care about how it worked, just that the
provided code did what it was supposed to and that what I wrote did the
same thing. I had no interest in becoming a professor in statistics,
despite the earnest instruction enforced on me by my employer 30 years ago.


It's been said that you must learn from the mistakes of others. You can't possibly live long enough to make them all yourself.


What and how we teach those around us can be the seed of something bigger.
I may well have become a statistician if the information had been tailored
to my requirements, but that chance was lost 30 years ago.


I think it's a great way to consider what we teach and how.


I'm Onno VK6FLAB
This posting includes a media file: http://podcasts.itmaze.com.au/foundations/20200119.foundations-of-amateur-radio.mp3

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What's in a plan?

Posted: 11 Jan 2020 08:00 AM PST


Foundations of Amateur Radio


As radio amateurs we learn which frequencies we're allowed to transmit on, where stuff lives and who has priority when there's a signal on the
frequency you're operating on and when you need to contact your regulator
if you hear an illegal station on the air.


Some of that information arrives in your brain by way of the education
process that eventually becomes your license after a test. Depending on
which country your license is valid, determines which region of the International Amateur Radio Union your activities fall.


Here in Australia, I'm part of the IARU Region 3, together with the rest of
the Asia - Pacific region. In the Americas you're part of Region 2 and
Europe, Africa, the Middle East and Northern Asia fall into Region 1. As amateur population sizes go, Region 2 and 3 each cover about 40% of all
radio amateurs. Region 1 is about 20%.


Each of these IARU regions has a specific band-plan that is updated
regularly as member countries adapt and negotiate different frequencies for different users. The band-edges might not change that often, but bands come
and go, segments are added and removed as needs change. For example, here
in Australia or VK, the 6m band has been changing because analogue TV has
been changing.


Information about band-plans is not easy to come by. For example if I look
at IARU Region 2, their documentation is pretty sparse. I've never managed
to actually load their website and by the looks of it, neither has the
Internet Archive. Given that Region 2 is all of the Americas and represents pretty much two fifths of all amateurs on planet Earth, that's a big hole.


There is some availability in Region 1 and 3, but those too leave to be desired. There does not appear to be any formal method of archiving or
naming and the transient nature of the Internet all but guarantees that historic information like this is being lost at a high rate.


Even with those limitations in mind, there is plenty of information to be found. Let's look at Australia, for no other reason than that I was able to pull some of the historic information out of the bit-bucket.


You might be surprised to learn that there is much more change under the
hood that far exceeds the band edges and segment changes. The Wireless Institute of Australia publishes the Australian Amateur Band Plan. Using
the Internet Archive I was able to count that between November 2007 and November 2019 there were at least 25 different versions of that band plan published, for example in 2008 alone there were at least five different versions.


I managed to download 11 of those band plans which show the introduction of
the 2200 meter band, the 630 meter band, changes to mode frequencies, DX frequencies, the allocation of emergency frequencies, changes to FM
bandwidth from 6 kHz to 8 kHz on bands below 10m, the formalisation of WSPR frequencies, JT65, FT8 and JT9.


Now I must point out that the information I'm presenting here is
incomplete. There are many more changes, just in VK alone. I'm relying on
the Internet Archive which only sampled the WIA website 162 times between
March 2008 and January 2020. Within those pages there were only 11 copies
of the actual band plan and I've only compared three of them, August 2009, March 2015 and October 2019, and of those only a few changes that stood out.


And this is for Australia alone. This is on the HF bands. I've not even
looked at the veritable feast of changes associated with the VHF and UHF
bands, let alone the cm, mm and higher bands.


Even with this massive disclaimer, my point should be pretty clear. A
band-plan is a living document. It changes regularly. Likely much more
often than you realise.


I'll leave you with one burning question.


When was the last time you got yourself a copy of the band-plan? Seriously, when was it?


I'm Onno VK6FLAB
This posting includes a media file: http://podcasts.itmaze.com.au/foundations/20200112.foundations-of-amateur-radio.mp3

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Where do you start?

Posted: 04 Jan 2020 08:00 AM PST


Foundations of Amateur Radio


So, you've got yourself a license, or it's coming but you're waiting for
the regulator to get the administration done and for your payment to go through. The excitement is building, you're itching to get started and
you've told your family and friends what your new callsign is.


Then the day arrives. Your callsign is allocated, it's paid for and you're allowed to call yourself a licensed radio amateur, a member of the
community, a part of history, the next thing in radio.


Now what?


Where do you start on this adventure of a thousand hobbies in one?


The truth is that you already did. Look behind you. You have a license, a callsign and you've found the community, well, at least some of it. How do
I know that you found the community? Amateur Radio is a secret, known only
to those who bounce into it. That bounce is where you found the community.


Like any community you'll find people you like and people you don't. People
who share your interest and people who are doing things that make your head explode.


That said, is there any sage advice that I can share with you on your
journey?


I do, but, you're not going to like it. In fact until you've been an
amateur for a little while you're possibly even going to hate my advice. I know, hate is a strong word. If it's not all black and white for you,
you're going to think I'm chickening out in giving you advice.


Wanna hear it?


Start somewhere, anywhere.


Let me say that again.


Start somewhere, anywhere.


One of the most fundamental aspects of this hobby is that it's driven by
your personal exploration, your journey, your imagination and your
adventure. It's entirely up to you to decide what you like and what you
don't.


I know that there are those who think that advice should come in the form
of buying a radio, erecting an antenna and getting on-air. For many that's
a journey worth doing. For others that's the beginning of the end of the
hobby for them. If you're unsure which of the thousands of activities you'd like to do, since you don't know what they are, I'll tell you a secret.


Neither do we.


Seriously. There are so many things to do in this hobby that not a day goes
by that I find a new thing to do and look at. A new toy to play with, or a
new adventure to embark on. This morning I realised that the antenna design
and build I've been working on represents roughly 600 million different variations. If I did those manually, taking a generous 10 minutes per
set-up, I'll be here for nearly 35 years, 8 hours a day, trying another
set-up. Clearly my hobby now includes automating antenna modelling.


My point is that there are so many different aspects of life, the universe,
and everything that intersect in some way with the hobby of amateur radio
that there's bound to be several that you can think of right off the top of your head. You might immediately be dismissing them as foolish, but if we
all did that, nothing would ever happen.


If you're looking for ideas, that's a whole different thing. Of course the nearest search engine is a possibility, but I do have to confess, it's a
dogs breakfast. Another is to visit your local club and see what others are
up to. You could watch YouTube videos or listen to podcasts or read
articles. All these are options to get suggestions.


Ultimately, the whole point of this hobby is that you embark on your own adventure, start on your own journey, down your own yellow brick road. If
you think what you're proposing intersects with amateur radio, you're right!


That's not to say that there's no benefit to be had from engaging with
others, far from it, just that you are the chief architect of your destiny. You're in charge.


So get to it. Go do something, anything. While you're at it, document the adventure. One day you'll be glad you did.


I'm Onno VK6FLAB
This posting includes a media file: http://podcasts.itmaze.com.au/foundations/20200105.foundations-of-amateur-radio.mp3

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What does Amateur Radio mean to you?

Posted: 28 Dec 2019 08:00 AM PST


Foundations of Amateur Radio


Over the years I've been asked what the hobby of amateur radio is all
about. My response has evolved over time, but it started with the lure of simple point-to-point communications. The antidote against such an example
is that a mobile phone does that and more. Of course if you're already in
the hobby you know that there is a massive difference between the two, but
if you're an onlooker that is not nearly as obvious.


There are other problems with an answer like that. It doesn't cover the
spirit of the hobby, the intent, the reach, or any of the other aspects of
our pursuits that keep us all coming back for more.


I was asked recently to provide a credit to a fellow amateur for providing inspiration for an episode. Since then I've reflected long and hard about
the nature of inspiration and what causes me to contribute and participate.


The reality is that my inspiration comes from all manner of nooks and
crannies, from articles I read, videos I watch, discussions I have,
activities I participate in, builds I make and emails I exchange. Not to mention friendships, random comments, shower thoughts and flights of fancy.


My understanding of our community of this hobby continues to evolve as I participate and contribute.


I think that underlying all of this is the expansion of my mind, my
interests, my exposure to new things is what makes amateur radio such a massively interesting activity.


When I started I had no inkling that between learning how to solder and
what a Fourier transform is lies this immense field of individual and
community activity. What other hobby has the ability to link astronomy,
moon, camping, community, planning, building, drilling, sound, language, antennas, internet, computing, valves, maths, propagation, mapping,

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