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Old 21-Jun-2013, 8:48 PM   #38
Pete Higgins
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Join Date: Apr 2013
Location: California, 58 miles @112 degrees from Mt. Wilson
Posts: 83
HDB8X multi-directional config.

tripelo & all,

A gentleman on the digital home forum (holl_ands) identified the combiner picture that 329049 posted as a “Magic Tee” and posted several links for me to bone up on Magic Tee theory & design. 329049’s would basically be the T2 portion of the Mini-Circuits diagram that you posted and appears to be missing the port-to-port (2 X R) 150 ohm mis-match dissipation resistor (RINT). According to what I read, the 75 ohms on each input port should transform to 37.5 ohms @ the combiner’s output, necessitating the output impedance matching auto transformer T1 (also missing from 329049’s combiner). The only way I could tell if my model has a resistor across the windings would be to de-solder the metal cover, since a 150 ohm resistor in parallel with a couple of inches of wire is going to appear as a short to an ohm meter. Based on my continuity measurements, I can tell that mine doesn’t have a grounded auto transformer on the output side but maybe if I’m lucky its designed something like this:

I wanted to show how I have the HDB8X configured for multi-directional reception. At my location, I really need the extra gain of an 8-Bay vs. a 4-Bay but when the signals aren’t faded this configuration seems to work very well –without a rotor. When the signals are faded, even my CM-4228 & amplified 91XG tend to drop in & out.

As you can see from my TV Fool report, KVCR, RF channel 26 @ -15.6 dBm (and an FM station on 99.9 @ -13.4) would severely overload my pre-amps., but all the rest of my San Diego (~168 deg.) and LA (~292 deg.) stations are 1 & 2 edge @ -85 dBm and below. That’s why I added the 55 dB NF-471 notch filter that rejects off-air UHF TV channels 24 to 29 and the Antennas Direct 20 dB FM Band reject filter to the AP-2870 LA panel amp.

A couple of things to note are that:

1. I am powering both amps. through the two panel signal combiner that came with the HDB8X.
2. The NF-471 completely removes RF channel 26 @ -15.6 dBm with the LA panel pointed right at that station and the amplified & unfiltered San Diego panel that’s rotated ~123 deg. is also completely rejecting KVCR. I wasn’t expecting this. This panel does bring in RF channel 30 (KPBS @ -96.3 dBm) from San Diego with an SNR in the mid 20’s.
Attached Images
File Type: jpg MagicT.jpg (9.5 KB, 7141 views)
File Type: jpg HDP8X Multi-Setup -3.jpg (61.5 KB, 2122 views)

Last edited by Pete Higgins; 24-Jun-2013 at 4:58 AM. Reason: Add Title
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