[TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

[Matt]

Here are some simple methods I have used for transforming impedance to match
75 ohm cable with 50 ohms:

 

Method #1:  alternating sections of 75 ohm and 50 ohm cable - each cut to

0.082 wavelength.  Example 50 ohm antenna (or down-lead) > .082 L 75 ohm
coax > .082 L 50 ohm coax > 75 ohm coax feed-line (any length).  Simply
reverse the order for the opposite transformation.

 

Method #2:  50 ohm antenna (or down-lead) > 0.141 wavelength of 50 ohm coax
> 0.163 wavelength 75 coax shorted stub > 75 ohm coax feed-line (any

length) from the stub "T" connection.   This method is useful for

grounding a down-lead at the tower base by connecting the shorted end of the
stub to the tower ground.

 

Method #3: same as method #2 except substitute 0.188 wavelength 50 coax for
the shorting stub.

 

All of the above methods are low enough Q to have flat reflection across

any single ham band.   With proper selection can achieve several of the

upper bands with small increase in SWR.  A Smith Chart really helps for
planning in this case.  Smith v3.1 is available in a free version...  

 

http://www.fritz.dellsperger.net/downloads.htm

 

Or if you are working out in the field try iSmith for the iphone.

 

BTW, trying to feed a non-resonant 50 ohm load with a random length of 75
ohm cable without transformation can produce a very poor match at the other
end.

 

Good luck with your cable!

 

Matt

KM5VI