CharredPC said:
sidha:
Sounds like you're doing the neat stuff I want to try! Any chance you could document the mods you're doing, or giving a few helpful links/pictures? Until I get my WineGard ordered, I'll be sitting here with my new Twinhan card (FedEx now says "On FedEx vehicle for delivery!" Can't wait!) and an old already-installed DTV dish; perfect opportunity to experiment. I have a long background in small electronics repair/modification, and am using a small homebuilt (made it yesterday) VHF/UHF antenna of my own design that outperforms the huge standard OTA roof arial- pics available on request by those that want to laugh
First you will need to acquire a donor lnbf, working or dead. Can you dissect it to the point of scavenging the ferro-ceramic DRO puck, and using pieces of it, change the physical dimensions of the recipient DRO puck -- without inflicting unintentional damage to the delicate surroundings? You won't get very far by adjusting the local oscillator frequency of an lnbf using the fine tune set-screw -- that might be part of it's design. It's only good for a few MHz either way; the LO needs to be lowered 500 MHz from 11250 to 10750 MHz. You might be able to sufficiently drop it (on some lnbf's) with the fine tune screw and allow reception of an uppermost transponder on a Ku satellite -- to prove it works. On (only) the first lnbf I tried, I was able to do just that. It's also necessary to remove the small teflon dielectric wedge (that may or may not be there) from the throat of the feedhorn to convert from circular to linear polarization. If you're familiar with C-band feedhorns and the need to insert a teflon dielectric plate to receive circular transmissions from international satellites like NSS-806, you'll know what I mean. (BTW, I receive digital FTA C-band on that bird using an 5' 10" mesh dish with 17 degree lnb.)
How it all began:
All I had were my Primestar dishes with their Ku lnb's, and I needed more of them for co-feeds. I also wanted to experiment (like the person who began this thread) using 18" dish hardware on the lower power Ku linear band. It was time to think about buying a couple lnbf's, but what to do with all my DBS lnbf's incompatible for serious FTA use? So I thought about attempting a conversion experiment with my growing pile of possibilities. Now I will never need to buy a Ku lnbf nor will I run out of free replacements. And, of course, the experience is priceless.
I removed the outer plastic shell and feed cap from an DTV lnbf and pulled the teflon wedge from the throat of the feed. Then I removed the silicone sealant covering the head of the LO frequency adjust set-screw. I reinstalled the lnbf on the dish and blind scanned on T-5/A-5 with my Pansat. After turning the set-screw a turn or two in either direction, it was able to load the highest transponder at 12.177 V. I had lowered the LO by 23 MHz, extending reception below the lower limit of 12.2 for the DTV lnbf. Of course the IF frequency was the result of an uncalibrated LO so the Pansat computed the frequency as much lower. If you don't have a way to measure the DRO frequency, a blind search receiver is the next best choice to locate what's available in the IF range.
But, only one out of ten of my various DN and DTV lnbf's was built so conveniently for that experiment. It even had screws that held the back plate in place against an O ring. The other lnbf's had press-fit silicon sealed plates that needed to be carefully pried off from the semi-permanent seal. And they did not have removable dielectric wedges, but instead had a shaped form that was molded as part of the cast metal feedhorn throat. I removed all of it by using a file, after removing the entire circuit board to clear the work area. It's not that difficult to do. As far as the ceramic puck (looks like an aspirin tablet), it requires more mass to lower the local oscillator frequency. To increase mass means to add more puck material from the donor lnbf puck. I used a side-cutter to cleave a piece from the puck (inside a plastic bag because it's brittle material that scatters like glass and you don't want to breathe the dust). Using epoxy or super glue, attach a fragment of the donor puck to the top of the other puck. Adding slivers and fragments will alter the dimensions and thus lower the frequency as needed. Just remember to retract the adjusting screw to prevent it from crushing the modified puck due to any change in height. In my case I needed to increase the mass of the puck about 30%; some small fragments were added around the circumference to better maintain the low profile. I replaced the back plate and rescanned for results, adding or removing ceramic material until it was close enough to use the set-screw for fine tuning.
I didn't find any need to modify the front-end bandpass. The lnbf microstrip bandpass, antenna and waveguide are designed for 12.2 - 12.7 instead of 11.7 - 12.2, but my quality readings are comparable using either a standard Ku or DSS modded lnbf on my Primestar. And that's saying something, because the scaler rings on the DSS lnbf are not designed to fully illuminate the wide 1 meter surface of such a (short vertical axis) Ku dish.
There is no standard DBS lnbf to use as a model when it comes to converting from circular to linear polarization, but adjusting the size of the ceramic puck is about the same in each case. I could show you some internal pictures of my lnbf's if you enjoy getting involved with odd projects like this one. There are plenty of surplus lnbf's available. DN installers around this area accumulated piles of dishes and lnb's they removed when upgrading their customers to the super dish. Driving in the country, I still see 18" dishes and Dish 500's left laying on the ground beside poles that now have the super dish. Lots of Primestar poles in the ground with DTV or DN retrofits. But, unfortunately, most of the Primestars (Channel Master built) are long gone in some dump.
I never seem to be able to find any of the remaining survivors if I'm trying too hard.
-sidha
