Re: [PLUG] [OT] SW radio question

[Rich Freeman via plug <plug@lists.phillylinux.org>]

On Thu, Oct 20, 2022 at 7:16 PM Steve Litt via plug
<plug@lists.phillylinux.org> wrote:
>
> Keith C. Perry via plug said on Thu, 20 Oct 2022 17:36:40 -0400 (EDT)
>
> >If I can also throw out a recommendation for the hams among us, I
> >highly recommend the  MFJ-971 manual tuner, which requires NO POWER
> >(unless you want to backlight the screen and even then its very
> >littel).  You can't go wrong pairing this with a [QRP] transceiver to
> >work regionally (i.e. inside the skip zone) so you know what is going
> >on around you.
>
> I can't help wondering: If the whole grid went down, so there are very
> few operating transmitters in your area, with no wifi, microwaves and
> the like, I wonder just how far 5 watts could go at 5 or 10Mhz on a
> winter night. I don't think Marconi used much power to transmit static
> clicks across the Atlantic.
>

It really depends on your desired reliability/etc and use case.

As an amateur you can definitely get some long-range contacts with
fairly little power.  The issue is that your coverage is going to be
sporadic and very intermittent.  You'll hear about somebody making
some contact halfway around the world on low power in a park, but the
reason they talk about it is because it is noteworthy.

Now, you might be able to reliably reach a random person somewhere
with a particular setup (ie, throw a dart at a map and there is where
you're contact will be), but that isn't the same as reliably reaching
a particular contact.

Getting to Marconi, some searching online suggests that marine
installations (like on the Titanic) could have transmitters in the 5kW
range (3x larger than allowed for amateur use, and that was for CW,
though I'm not sure how much of that power ended up in the intended
signal vs harmonics/etc).  The transatlantic stations had 200-300kW
outputs, and their antennas were fairly sophisticated - I couldn't
find anything online about their gain but I'm sure it was significant.
They weren't designed to reach random places all over the world - they
were designed to reliably transmit signals to specific stations across
the Atlantic.  They didn't pass traffic only when conditions were
good/etc - these stations were set up to get the signal every day.

I did a bit of searching and even modern HF commercial radios such as
used for contacting aircraft (yes, HF is still used today on aircraft
over the ocean) have fairly elaborate antenna farms.

I'll also note that there is a surprising lack of info online about
these installations, such as info on power or gain and so on.  There
is some info, especially for things like frequencies and coverage
areas since obviously aircraft need to be able to reach them.

If your goal is local communications then you still need to think
about antenna/band/etc.  You don't necessarily need a ton of power,
but you do need to consider where your antenna is and its design to
have a strong ground wave or use NVIS.  I use a horizontal antenna and
on all but the lowest band it isn't very reliable for local
communications.  I can talk to Europe far more reliably than
Philadelphia on something like 10m.

I guess this is a long way of saying, "it depends."  Propagation is
complicated.  The best setup really depends on who you want to be able
to talk to, and how reliable it needs to be.  A QRP rig is very likely
to reach SOMEBODY.  However, whether it reaches a particular person or
a particular area depends a lot more on the details.  If you want to
ensure that two particular points have reliable communications under
all conditions than the requirements are going to be much higher than
if you want intermittent communications.

I guess to use an IT analogy, think about WiFi.  Everybody has a WiFi
AP at home, and they are really cheap.  However, if you look at a
typical commercial WiFi install in an office they can cost tens of
thousands to install.  The difference is that a professional install
is going to map and plan and ensure that you have solid coverage
throughout the desired area, that APs don't interfere with each other,
and that equipment is easily maintained/managed, documented, and so
on.

(BTW, none of this is to discourage anybody from prepping or exploring
the hobby or whatever.  I'm just trying to dispel the notion that two
people can have 5W QRP rigs 100 miles apart and be guaranteed to hear
each other because somebody was talking about that contact they made
with Europe using FT8.)

-- 
Rich
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