Topband: 75 ohm RG6 to 50 Ohm radio
k7tjr at msn.com
Wed Sep 28 12:06:23 EDT 2016
Actually there is a reason or two to match the feedline impedances when using these active antennas.
There are amplifiers in most of these systems that rely on their matched impedances to produce their best IMD capability. It may not matter in some areas of the world but in others with high levels of broadcast signals it definitely could. This of course is also dependent on the length of transmission line which can make the mismatch worse or even better under certain circumstances. Then if the mismatch is made better on one band due to mismatch it likely won't be on another band. One other reason that matching matters is the impedance a mismatched line loads the combining circuitry can under some circumstances cause phase and amplitude errors. This can change the originally intended pattern of an array. In the case of the Hi-Z arrays they all are supplied with an amplifier that has a 75 ohm input impedance so the array controllers stay accurate in terms of phase and amplitude. However this amplifier like most post amplifiers partially reflects its output load changes back to its input changing the input impedance. Thus the reason for the 75 to 50 ohm transformer as it keeps the amplifier happy.
Now this may or may not affect some users but as a manufacturer of these arrays it is important to insure that these arrays will work well and do it at every install around the world. We have seen feedlines as short as 3 feet up to the 1000 foot runs I use and one that I heard of at 2000 feet.
So my point is why would one want to possibly give away some performance when it could very easily be done correctly.
One caveat is that the Hi-Z 4 and 4-8Pro have an internal buffer amplifier designed specially to eliminate the amplitude and phasing problems arising from load mismatch but it cannot maintain its IMD capability under some incorrect load conditions. Other models do not include this internal buffering of the output signal.
Lee K7TJR OR
>I suppose I need to ask if worrying about impedance at this point in the
> system is really worth the trouble.
> Once signals have gone through one or two stages of amplification - would
> loss of a db or so in signal strength really matter?
More information about the Topband