|RF Transmission loss - RF Cafe Forums|
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Post subject: RF Transmission loss
Posted: Sat Apr 29, 2006 5:32 pm
Joined: Wed Apr 26, 2006 7:08 pm
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!
Posted: Mon May 01, 2006 2:18 pm
Joined: Wed Apr 26, 2006 7:08 pm
No probs now!! Did a bit of searching and got it.
Posted: Fri May 05, 2006 8:17 am
Joined: Sun Aug 03, 2003 2:02 pm
Location: Erie, PA
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
Posted: Thu May 11, 2006 2:09 pm
Joined: Mon Sep 19, 2005 1:33 pm
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!