[TowerTalk] Hardline ?

[Gary Schafer]

> -----Original Message-----
> From: towertalk-bounces at contesting.com [mailto:towertalk-
> bounces at contesting.com] On Behalf Of Bob Nielsen
> Sent: Monday, April 20, 2009 9:30 PM
> To: towertalk at contesting.com Talk
> Subject: Re: [TowerTalk] Hardline ?
> 
> 
> On Apr 20, 2009, at 3:08 PM, Gary Schafer wrote:
> >
> > I hate to start something here,, but where does the "0.18 dB
> > additional loss
> > come from with any length of line"?
> > Mismatch in and of itself does not necessarily represent loss.
> 
> True, but VSWR can introduce additional loss over and above the
> insertion loss experienced with a perfectly matched transmission.
> There's a figure in the ARRL Handbook (Figure 21.5 in the 2007
> edition) which gives some values.  From the curve, a VSWR of 1.5
> introduces an additional loss of 0.18 dB when the insertion loss is
> around 10 dB.  Unless the loss is pretty high to begin with, this
> additional loss can usually be ignored.
> 
> Bob, N7XY

Yes Bob, that is exactly right. 

My point was that you can not express any additional loss due to mismatch
unless you know what the matched loss of the line is in the first place. 

Just to say that "with 1.5:1 vswr the line has 0.18dB additional loss due to
mismatch regardless of line length" is incorrect and meaningless. 

If for example our matched line (perfect theoretical line) had zero dB of
loss and we then introduce a 1.5:1 swr load to it, there would still be zero
dB of total loss. The mismatch by itself would not introduce any loss to the
system.

Mismatched loss comes from the reflected power traveling up and down a lossy
line. As you stated the additional mismatch loss amount is directly in
relation to the matched loss of the line.

On another note, if a section of half wave 75 ohm line is used on a 50 ohm
system there will be zero reflected power indicated at the transmitter end
of the line if a 50 ohm swr bridge is used. However there will still be a
1.5:1 swr on that 75 ohm line section and it will suffer the additional loss
due to mismatch over what it would if it was terminated in 75 ohms. That
additional mismatch loss will be proportionate to the matched loss of the
line as discussed above.

73
Gary  K4FMX