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RF Transmission loss - RF Cafe Forums
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Starbuck |
Post subject: RF Transmission loss
Posted: Sat Apr 29, 2006 5:32 pm
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Joined: Wed Apr 26, 2006
7:08 pm Posts: 2 |
Hi all, Could someone be kind enough to explian
why in a 75Ohm Transmission system i.e. TV transmission,
the effective impedance of the channel from transmitter
to home antenna is not considered (or seems not
to be) when impedance matching? Wouldn't
the high impedence path from the transmitter top
home antenna mean that a 75 Ohm Load impedence is
mismatched? I ask this because in CATV systems
it (as you all know) is important that the transmission
line (typically a coax) is matched to the source
and load impedance, which is again 75Ohms. If a
mismatch occurs then a VSWR is established weakening
the original signal etc. So the reason I'm
asking this question is because I'm making an analogy
between a transmission line in a CATV network and
the transmission medium of RF terrestrial signals
(air!) So why is no VSWR set up between the
home antenna and the transmitter because surely
looking out from the home antenna the source impedance
of the signal must look huge? Please someone
explain as its driving me nuts!! Sorry if this
is a little basic, but I'm just getting into RF!
Cheers
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Starbuck |
Post subject:
Posted: Mon May 01, 2006 2:18 pm
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Joined: Wed Apr 26, 2006
7:08 pm Posts: 2 |
Hi guys, No probs now!! Did a bit of searching
and got it. Thanks anyway Greg
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Kirt Blattenberger |
Post subject:
Posted: Fri May 05, 2006 8:17 am
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Site Admin |
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Joined: Sun Aug 03, 2003
2:02 pm Posts: 308 Location: Erie, PA
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Greetings Starbuck: Glad to hear you found
an answer. I assume the answer was that indeed the
impedance match is important in all of the RF paths.
If a poor match manages to work, it is only because
the power levels in the system are high enough to
tolerate the excess loss. This topic piqued
my interest because of an installation of a Dish
Network antenna at my home a couple days ago by
a couple of guys who, if pressed, probably could
not even spell "RF." After botching an initial installation
attempt on one area of my roof (and leaving big
lag bolt holes for me to fill) where the view was
blocked by a tree that – I swear – was there when
they started, they decided maybe the chimney-top
mount where my DishTV antenna had been working fine
for three years would be a good spot. So, they took
down the existing antenna and put up their own.
100% expected signal strength was obtained.
The guys then proceeded to hack up the very
nice, and probably very expensive, dual 75 ohm coax
+ ground wire molded cable and replace it with two
separate pieces of RG-6. OK, that's their prerogative,
but it's what came next that really $%^@$ed me off.
They secured the two cables to the roof and down
the side of the house with a standard Arrow staple
gun using standard ½” wire staples! At about 2 foot
intervals, each cable received a nice little dent
in the dielectric. In the part of the run that is
in the sunlight and got soft, it mashed down pretty
well. I guarantee the impedance was nowhere near
75 + j0 ohms by the time it ran the 50 feet or so
to my TV. Last night I went up an yanked
out all the staples and replaced them with the nice
specially-molded dual coax screw-down clamps that
had been there for the DishTV system. I massaged
the cables back into a round cross-section as I
went. The installation looks nice now, and I sleep
well at night again knowing my transmission line
is not writhing in pain up on the roof. Wouldn't
you think the service guys might know a little about
how to treat their equipment? Then again, maybe
they do know, but just don't care.
_________________ - Kirt Blattenberger
RF Cafe Progenitor & Webmaster
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k5dvw |
Post subject:
Posted: Thu May 11, 2006 2:09 pm
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Joined: Mon Sep 19, 2005
1:33 pm Posts: 1 |
I'm not sure I understand the question completely,
but I assume you're asking, if free space impedance
is 377 ohms, why is there no mismatch to a TV receiver
which has 75 ohms input. If that's the question,
then the answer is that the antenna is the impedance
transformer. From a simple perspective... It receives
the energy out of the 377 ohm air and converts it
to something near 75 ohms, which is matched to the
cable. Hopefully that's it. Kirt, to make
you even more angry about your satellite system,
the same dish could have been used on a DISH network
system as DIRECTV. If you're not adding receivers
or anything, they're interchangable. All they had
to do was to realign the existing dish. Ah-ha!
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Posted 11/12/2012
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