|Subject:||Re: [TowerTalk] Ham radio vs. satellite dishes|
|From:||Jim Lux <email@example.com>|
|Date:||Tue, 11 Jan 2005 10:43:38 -0800|
At 08:13 AM 1/11/2005, Tom Anderson wrote:|
Fellow Tower Talk members:
Typically, a multi-receiver DBS dish setup will use a dual polarization feed, which has a low noise amp and block converter that translates the entire band (Ku-band, around 12GHz) down to around 1 GHz. You wind up with a pair of coaxes from the dish, one carrying one polarization, the other carrying the other. Since there are all channels on the coax all the time, having multi receivers is just a matter of power dividers (usually with some form of line amp that also feeds the power back up to the dish) and feeding the two L-band IF signals to all receivers. The receiver picks the appropriate one. There are also schemes where a relay is used to select which IF to us, depending on commands sent up from the receiver.
It gets a bit more complex if you have a multi feed dish (or multiple dishes) because you want to look at multiple satellites (East, West, and HD, for instance). Then, they put in a relay box of sorts that brings all the dish signals in (2 per dish), and selects the appropriate one for each receiver. Since some sort of switching signal gets fed back up the coax from the receiver, you can get away with a single coax to the receiver. Often, all this switching stuff gets done in a box on the back of the dish, where you'll see a rats' nest of coax and connectors.
Then, there is the issue of the phone line connection for the satellite receiver (needed to do authorizations, etc.). Here is where the notorious PhoneX type RF links were often used, since the satellite receiver is often installed where there is no phone jack.
To look towards your RFI question... I wouldn't expect much RFI from a DBS dish LNB. Everything is operating at frequencies much higher than HF. There's probably some sort of crystal oscillator in the LNB to lock the DRO, but it's pretty low level, and probably around 40 MHz).
Digital hash from the receiver, being carried on the outside of the coax, is another potential issue, as is switcher noise from the DC power supplies. Nothing here any different than any other consumer electronics.
Also I notice most cable companies are going from the old RG59 to RG6 now for home service as they "say" the RG6 gives them better specs on digital signals.
The old RG-59 is inferior to the RG-6 being used these days. The digital 16-VSB signals are much more picky about frequency related effects like group delay, loss, etc. Also, the frequencies being passed are both higher and lower on modern cable systems. no more is it just 50 Mhz to 500 MHz. the "reverse channels" (for cable modems) are down at the bottom, and, as noted above, signals in the 1GHz range are being passed around as well. As a rule, the RG-6 also has more shield layers, which is a "good thing", particularly as the cable gets old, cracked, eaten, etc. Since the stuff costs less than 10cents/ft, I certainly wouldn't be complaining about it.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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