[TowerTalk] Hardline ?

[Roger (K8RI)]

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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