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Old 10-Jun-2013, 5:53 PM   #21
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Join Date: Jan 2010
Location: Dallas, TX
Posts: 170
Loss of Gain of S/N, Combining Two Antenna

Originally Posted by Pete Higgins View Post
A compelling advertised feature of the HDB8X is its ability to align the 4-Bay panels to receive broadcasts originating from different directions.

Combining the two antennas resulted in the total loss of 9 LA stations and dramatically reduced the others. ...nearly useless for leveraging its advertised multi-direction feature.
Thanks Pete, for testing this feature. Iím sure that you know this:

There are at least three fundamentally different ways of combining all channels from two antennas. Each involves two or more significant sources of S/N loss.

In all cases, if the antennas point in different directions there is:

- Loss of the stacking gain (up to ~ 3 dB) that the antennas might have (assuming transmission lines matched).

Additional Loss for Each Case:

Case 1. Separate antennas pointing different directions connected to a hybrid combiner (usual splitter reversed) have:

- Loss through the combiner, typically more than 3.5 dB at UHF

Note: When the signals at the combined (input) ports are non-identical, hybrid combiners have 3 dB additional loss above normal circuit losses. Not-identical signals occur when antennas point toward differing stations.

Case 2. If the signals are combined with transmission line techniques that avoid the 3 dB hybrid combiner additional loss, then there is no isolation between input ports.

Without the isolation; The second antenna loads the first, and re-radiates away ~3dB of signal, vice-versa for the first antenna loading the second one.

Case 3. If both antennas have their separate matching preamplifiers prior to combining, then there is isolation between input ports, so:

* No loss thru re-radiation,
* No S/N loss through combiner (single signal input to output)

This is because both signal and noise have been raised above noise floor by preamplifier, so combiner loss in effect just lowers gain of both signal and noise. Not affecting S/N ratio.

But, now there is:

- Twice the noise, compared to one antenna, one preamp, and no combiner.

This is a result of each preamp adding equal noise power above noise floor. Two independent noise sources adds at combiner output.

Depending on preamp noise figure, this Case 3 could be better, or much worse than the first two cases.

Choose your poison.

Each method has some advantages and disadvantages.

Note: Matching two transmission lines is most important when antennas face same direction. When antennas face different directions, phase control of the two signals is usually lost, therefore matching transmission lines typically does not provide gain.

Note: Further in this thread is attempt to add more detail.

Diplexers such as a UVSJ, or a Join-tenna, get around some of the S/N losses mentioned in Cases 1 thru 3 by providing lower-loss frequency dependent isolation.

I suppose one advantage it still has over a conventional 8-Bay ... use it as 2 separate 4-Bays and run separate cables to a coaxial switch?

Last edited by tripelo; 16-Jun-2013 at 7:56 PM. Reason: typo & clarify
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