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

[maflukey at gmail.com]

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


-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
kr2q at optimum.net
Sent: Friday, December 13, 2013 5:32 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

Interesting comments.

Contributor A says the SWR will be 2:1 while Dan says 1.5:1.

I agree with Dan.

Contributor B says, "What if the impedance at the antenna is only 33
ohms....
and you have a quarter wavelength of 75 coax."  Then...Oh horrors.

Well, don't look now, but if the feedpoint is 33 and the coax is 50, I
calculate that to be the same VSWR as if the feedpoint were 50 and the
coax were 75.  No?  Nobody worries about the former, do they?

How likely is it to get your coax to be in odd multiples of quarter
lambda?  I am guessing, "Not too likely."  
And if it is, well, just add some more coax (if you care about SWR).
Would that not work?

>From what I can find, a mismatch (SWR) is not much of a factor in getting
your signal to the antenna.
See http://www.skyworksinc.com/uploads/documents/vswrreturn.pdf

I don't know if this is accurate or not (I'm no engineer), but I have
found this in several places after doing a Google on SWR and LOSS.  There
is even a calculator out there, that seems to come up with the same
results.  http://www.csgnetwork.com/vswrlosscalc.html

So from what I see 50 to 75 (or 33 to 50) is 1.5:1 with a loss of 0.177
db.  Really?  Really!

>From what the others have said, it might be possible for the 50:75
"mismatch" to matter to you.

Personally, I really like the replies from guys who actually use 75 ohm
coax and have seen "no issue."

I asked for expert input and I do thank those that responded.

Back to running my hospital.  :-)

de Doug KR2Q
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