Jim Brown wrote:
> 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.
>
Although correct I think the statement is a bit misleading. IOW, a given
length of line has a given loss at a specific frequency. If you take
that loss and look up "additional loss" per how ever long the line is
then you have the additional line loss for that line at that SWR. IOW
the resultant additional line loss due to SWR is based on the matched
line loss for that specific line regardless of length.
HOWEVER if we cut the line length in half the matched line loss is now
half of what it had been. You look that figure up based on SWR and it
will not be the same as it was for the line when it was twice as long.
An example taken from the ARRL Antenna Handbook 20th edition (24-10) Fig
14. Given a line loss of 2 db for a matched line then an SWR of 2:1
gives an additional loss very close to 0.3 db. If we cut the line
length in half then the matched loss is now 1 db and the additional loss
due to an SWR of 2:1 is now 0.2 db.
However as to the original statement, the above refers to any line
length that has a 2 db loss when matched and that resultant additional
loss of 0.3 db is good for any line length that has a 2 db loss when
matched. It does not mean for any length of that specific line.
> 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.
>
>
By the same token, if the line is very long most solid state rigs will
operate quite happily with no additional matching.
73
Roger (K8RI)
> 73,
>
> Jim Brown K9YC
>
>
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