Split-level Combining

by **BroadcastDoc** » Wed Nov 21, 2007 12:40 pm

Anyone here using it? If so, how has the experience been? Seems like that type of system would be kind of finicky!

by **tmay** » Wed Nov 21, 2007 2:47 pm

Not running Split-level per se, but I am running a combined low-level transmitter. RF for the HD and Analog gets combined after the exciters, split and fed to two linear transmitters and combined at the outputs with a 3dB hybrid. It's a Harris box and has been running rock solid. Very redundant and reliable.

by **BroadcastDoc** » Wed Nov 21, 2007 3:03 pm

I forgot that you were doing that!

How closely do the individual outputs have to track, and do changes in the characteristics in one or the other of the boxes cause problems with the HD carriers? I imagine that since the transmitters are combined the effects are additive, so while the overall power output may rise or fall, the carrier ratios stay pretty constant.

by **K9EZ** » Wed Nov 21, 2007 4:14 pm

tmay wrote:Not running Split-level per se, but I am running a combined low-level transmitter. RF for the HD and Analog gets combined after the exciters, split and fed to two linear transmitters and combined at the outputs with a 3dB hybrid. It's a Harris box and has been running rock solid. Very redundant and reliable.

Of course the standard reply is.... PICTURES PLEASE!!!

by **tmay** » Wed Nov 21, 2007 4:58 pm

The ratio is always stable. It's set by a series of pads in the low power combiner network and the APC on both exciters is controlled by the transmitter to fine tweak things.

Things do tend to get interesting during heavy icing on the antenna. The 3dB hybrid arrangement tends to make the system a bit fussy with reflected power. Typically what happens is one transmitter sees more reflected than the other because of the 90 degree difference in phase, causing a difference in foldback power between the two units. The imbalance excess gets dumped into the reject load. It doesn't screw up the power ratios, but I think it may mess with the re-growth of the IBOC carriers. Now that I finally have a golden eagle, I can hopefully get a snapshot of the spectrum during heavy ice to see what it's actually doing.

I'll post some pictures of this rig soon, they're on a different 'puter.

by **BroadcastDoc** » Wed Nov 21, 2007 5:04 pm

But at least that extra heat inside the building from the reject load comes when it's cold outside!!

by **tmay** » Mon Nov 26, 2007 1:41 am

Here's a photo of the low-level combined hybrid transmitter for WUWM. Sorry about the awful white balance. For those wondering, this is a first-generation IBOC installation. The importer lives at the transmitter and the AES for the HD-2 comes over the STL. This will probably change to EXGINE at some point down the road.

Additional photos and descriptions at: http://nomad.dynalias.com/jalbum/WUWM_Transmitter/

by **BroadcastDoc** » Mon Nov 26, 2007 10:36 am

Awesome pics! Glad to see that the transmitters are doing more than just heating the building, which is what they were doing when I first visited them. You had just installed it all and were waiting for an STL or something...

by **rockmanac** » Mon Nov 26, 2007 12:22 pm

What is WUWM running on their HD2?

-A

by **K9EZ** » Mon Nov 26, 2007 12:29 pm

rockmanac wrote:What is WUWM running on their HD2?

-A

Triple A

-K

by **tmay** » Mon Nov 26, 2007 3:15 pm

BroadcastDoc wrote:Awesome pics! Glad to see that the transmitters are doing more than just heating the building, which is what they were doing when I first visited them. You had just installed it all and were waiting for an STL or something...

Yeah... We were waiting on STL and the new FM antenna. The old one was such crap that I didn't want to burn up a perfectly new transmitter. You probably visited during the winter before the spring when the antenna went up. I had to keep that transmitter running to heat the building. That thing gives off plenty of heat too. We end up running the HVAC economizer even on the coldest winter days. That room can easily get up to 90 degrees in a real hurry if the HVAC fails.

by **K9EZ** » Mon Nov 26, 2007 3:37 pm

tmay wrote:
BroadcastDoc wrote:Awesome pics! Glad to see that the transmitters are doing more than just heating the building, which is what they were doing when I first visited them. You had just installed it all and were waiting for an STL or something...

Yeah... We were waiting on STL and the new FM antenna. The old one was such crap that I didn't want to burn up a perfectly new transmitter. You probably visited during the winter before the spring when the antenna went up. I had to keep that transmitter running to heat the building. That thing gives off plenty of heat too. We end up running the HVAC economizer even on the coldest winter days. That room can easily get up to 90 degrees in a real hurry if the HVAC fails.

LOL I had an AC fail here in North Carolina..... the room got to 135 degrees and I still kept the transmitters on.

Also, the Continental 316HDs have a stack temp of 185 degrees. WMIL and WQBW have them.

by **tmay** » Mon Nov 26, 2007 9:17 pm

K9EZ wrote:LOL I had an AC fail here in North Carolina..... the room got to 135 degrees and I still kept the transmitters on.

Ouch, poor transmitter.

We didn't use to have air cond. before we rebuilt the site. Our Z5 was the main TX and it burned up a controller card after 3 days in 110+ ambient. I put our backup CSI 5000 on the air, it lasted about a day before the filament rheostat on the PA burned open, causing the VSWR on the IPA to go sky high and eventually causing the IPA tube to shatter and destroy the socket. (Just my luck that the overload relay on the IPA wasn't working.) I tried to drive the exciter into the PA, but no go, and I ended up putting the exciter into the antenna and driving to Quincy in the middle of the night for a new controller board. Ever since, I've been a little anal about keeping the site cooled.

Split-level Combining

Split-level Combining

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