Re: [TowerTalk] Ham radio vs. satellite dishes

[Jim Lux <jimlux@earthlink.net>]

At 08:13 AM 1/11/2005, Tom Anderson wrote:

> Fellow Tower Talk members:
>
> I'm considering changing from cable tv to one of the dish systems.  Has 
> anyone had any problems with RFI, etc. to dish systems vs. cable and HF or 
> VHF transmissions?  I use a 10 element log periodic at 50 ft. with a 
> Collins 30-L-1 and a Kenwood TS 430S that I usually load to maybe 500 watts.
>
> Some of the dish systems, that "advertise" giving you four digital boxes, 
> actually give you only two and the other two sets are controlled via some 
> type of RF link.

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.
>
> 73 de Tom, WW5L

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. 


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