REALLY nicely done, Doug. An excellent analogy. And I would wager heavily on
the correctness of your prediction. ;-)
Bill / W5WVO
DOUGLAS ZWIEBEL wrote:
Hi all:
I knew this would be a tough one for many (most). Try this on for
size....
Imagine that you walk into a very large office space (the band),
filled with desks and a hundred people working at those desks. You
sit down at a desk in the middle of the room.
At first, nobody is talking (everyone is tuning the band). You sit
down at a desk in the middle of the room and you strike up a
conversation with the person next to you (one qso on the band). You
can easily hear them.
Now assume that five other "deskmates" (hams) within the room (band)
also start talking and that they are randomly distributed about the
room (the band). Your conversation will likely continue without
issue. But maybe, ONE of the other conversations if taking place only
5 desks (kc) away form yours. It's a bit annoying. What to do?
Well, you would ask your conversation-mate to talk louder (run more
power), move to another desk (qsy), or you could put up some walls
around the four desks adjacent to you (make a big cubicle). This
helps to shut out some of the adjacent conversation (you narrow a
conventional IF filter).
If another conversation strikes up only 3 desks away from you, you
might compensate by making the cubicle even smaller - say around just
your two desks (narrow the conventional IF filter even more).
Now assume that everyone in the room is communicating verbally with
another person. A few people are whispering to each other (qrpers),
some are at normal conversation levels, many are shouting (KW), and
some are screaming at the top of their lungs (you know who they are).
You have already build the smallest cubicle possible, but the other 99
conversations are still audible. You can not actually understand the
other conversations, but the "noise" (IMD) is clearly there and making
it hard or impossible for you to hear the person you are talking with.
What to do?
One solution is to make a smaller room with the big room (add a
roofing filter). You might build a sound-insulated, concrete room
that is one-tenth of the original size, but housed within the big
room. This must mean that there are fewer other conversations within
ear-shot because there are fewer desks in the "new" sub-room.
So...lets suppose that you now have your smaller room built within the
larger room. You now discover that there are NO other conversations
that are so loud that you can hear them. In fact, it is so quiet now,
that you can even take down your cubicle walls (open up - widen - the
conventional IF filter) and still hear your own conversation without
any other noise (IMD).
[By the way, noise here = IMD squeaks, pops, bleeps, boops, etc., not
static crashes or ignition noise, etc]
Let's go back to the beginning. If you are having your solitary
conversation in the original GIANT room and then build a walled-off
section (sub-room), you WON'T notice any difference because it was
quiet to start with and making your private room (smaller part of the
big room) had no impact. It is still just as quiet after the new
walls are put up.
In other words, you can NOT hear the difference that a roofing filter
makes unless there are lots of other LOUD signals within the first
(typically broad) IF. I can guarantee you that Wayne and Eric will
get "complaints" that the roofing doesn't work because folks just
don't understand it's purpose. They will say (and I'll take bets on
this), "I don't hear any difference whether I use the wide or narrow
roofing filter." And they'll be correct.
Hope this analogy makes some sense for you.
de Doug KR2Q
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
