[TowerTalk] re: feedline length

[TexasRF@aol.com]

In answer to k4oj's question about cutting his 1/2" catv hardline:

You can add RG11 jumpers at one or both ends of your 75 ohm hardline to
"tune" the overall length to a multiple of 1/2 wavelength. As you know, 1/2
wavelength is the same as 180 degrees of line length. We want to use 180
degrees because the input impedance is repeated at 180 degree intervals along
the line. If you want a line that is four 1/2 wavelengths long, then the
total line length is 720 degrees. The signal at the input end does not know
that this 720 degrees is made up by one or more different pieces of coax. All
that matters is that the total length equal 720 degrees and all pieces of the
coax are 75 ohms.

As a side note, the loss of the coax will shift your 50 ohm load impedance a
little toward the 75 ohm figure. I mention this only because at VHF or UHF,
where cable losses are higher, someone might try to use this "trick" and
wonder where their 50 ohms went. To understand this interesting
characteristic, consider a coax cable long enough to have 10 db of loss. If
this cable were short circuited at the far end, the reflected power would
measure 10db + 10db for a total of 20db attenuated at the near end of the
cable. This is the same as a vswr of 1.222, implying an impedance closer to
50 /1.222 (40.9 ohms) instead of the expected  zero ohms. In the 20 meter
case, the losses are much lower than 10db and the load impedance is much
higher than zero ohms so the effect will be very much reduced.

This transmission line stuff is great fun, eh!

73 de Gerald, K5GW, Owner/General Manager, Texas Towers


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