Rabbit73, looks like you have a good collection of equipment for portable testing.
Having a near field view over open water helps with signal reception.
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Originally Posted by rabbit73
]... how is it possible for you to make a valid comparison of the performance of the two systems?...
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That depends on the accuracy one may be looking for and the test environment. If looking for accuracy in the one dB or less range, it may be very difficult.
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... Have the two antennas on a sliding carriage that moves each antenna into the same location, or what? ...
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That could be useful. Depending on the variability of the received signals and the length of time the test systems are evaluated.
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... how can you possibly get a comparison like you would on a lab grade antenna range? ...
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Probably not practical for most.
As you know, a lab grade antenna range is implemented in such a way to minimize all reflections that could be received in the main beam or sidelobes of the antenna under evaluation.
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... On a range, you would use a constant strength test signal and place each antenna, in turn, in the same location...
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Test ranges are usually setup to evaluate antennas loaded by a specified impedance. They do not usually evaluate antenna preamp combinations.
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...The closest I got to the ideal was to switch the two antenna locations, but I really wanted to be able to slide them sideways...
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If the signal variability is not too great, and the accuracy requirements are not too stringent, in most cases that probably works OK.
General comment:
All measurements have limits in accuracy. For most TV reception purposes, the measurements taken with less than optimum conditions may be satisfactory.
In comparative testing, if there variables that one cannot control. Some form of averaging to wash out the variability can be helpful.
In the case of systems receiving signals that vary with time, then time averaging can reduce the errors. If long enough time and enough samples are averaged for each system under test, then errors in measurement can be reduced.
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Originally Posted by eclipsme
...would splitting 1 feed to the 2 systems work? Then reverse the feeds to compensate for output differences of the splitter and average the results?...
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If accuracy requirements are not too stringent, then what you seem to be suggesting can help with comparison of two preamps or two receiving systems.
However, the preamps may behave slightly different when connected to an actual receiving antenna with different transmission line and no splitter. In many case, this difference could be small enough to not matter to the individual performing the test.
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