Gary asked:
>> I hate to start something here,, but where does the "0.18 dB additional loss
>> come from with any length of line"?
I responded:
>>
>> There's an equation in the ARRL Handbook and in the ARRL Antenna Book, and
>> there are graphical plots of that equation for various values of mismatch and
>> matched loss.
>> On Mon, 20 Apr 2009 18:08:39 -0400, Gary Schafer wrote:
>That is not applicable to this situation:
It's been so long since the original post that we may have forgotten what "this
situation is." As I recall, the original post was concerned with the
additional
loss in 75 ohm hardline over 50 ohm hardline when the impedance of the antenna
was 50 ohms. I could be mistaken about that, but that is the question I was
responding to.:) The worst case ADDITIONAL loss in the line for this degree of
mismatch is 0.18dB, no matter how long the line is. This is shown in the graph
for that degree of mismatch, which is asymtotic to 0.18 dB.
In the 19th Antenna Book, the graph is Fig 14 in chapter 24. In the 2009
Handbook, it's Fig 21-5. This is not new information. In the 1973 version of
the
Handbook, it's Fig 20-8. In the 1966 version, it's Fig 13-9. In the 1949
version
of the ARRL Antenna Book, it's Fig 3-21.
If the line is long enough, the loss of the line will tend to bring the
impedance at the transmitter closer to 75 ohms than 50 ohms (that is, the
circular trace around the Smith chart gets smaller, and for a line of infinite
length goes to the Zo of the line). Depending on the design of the
transmitter's
output stage, that could require a matching network. There are antenna tuners
around that could easily deal with that mismatch with relatively low power
loss.
73,
Jim Brown K9YC
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