[TowerTalk] 50:75 ohm transformation

[Eric Gustafson]

>From: w2xx@cloud9.net
>Date: Wed, 10 Feb 1999 21:28:51 -0500
>
>jd wrote:
>
>> I use 3/4 inch CATV and a run of about 180 feet to my
>> tri-bander.  The worst case I have is 2.2 to 1 on 15 cw.  I
>> look at it as 2-2.5 to 1 is negligable on HF, whats it going
>> to do, reduce your delivered to the antenna power a couple of
>> watts?  The other end will never know..
>
>Um, well...a 2:1 SWR will eat up about 10% of your power, so
>2.2:1 will be marginally worse.  Still, 10% of 1500 watts is not
>pretty.  many of us would kill to find another 10% of efficiency
>in our systems.
>
>To each his own...
>
>J.P. W2XX


Hi J.P.,

I think that analysis is a bit misleading.  

The matched loss per 100 ft. in 3/4 inch 75 ohm CATV line at 30
MHz is 0.35 dB.  We have been discussing 200ish foot runs.  So I
will use 200 feet.  Assuming the 50 ohm pigtail at the antenna
end is short and matched, we'll ignore the losses there.

The CATV line is operating at an SWR of 1.5:1 not 2:1.  So, the
matched loss in the CATV line is 0.7 dB.  The mismatched loss in
the CATV line is 0.75 dB.  The loss _due to mismatch_ is
therefore 0.05 dB.

The 2:1 SWR is what the 50 ohm jumper at the radio end of the run
is seeing.  So the total loss has to include this cable.  How
long is it?  I'll use 25 feet for the sake of argument here.  The
matched loss at 30 MHz for 25 feet of RG-213 is 0.31 dB.  The 2:1
mismatched loss for this jumper is 0.4 dB.  So again the
increased loss due to mismatch is 0.05 dB for the jumper.

And the total additional loss for both cables due to the mismatch
from not using UNUNs is 0.1 dB.  This is not a 10% efficiency
decrease it is a 1% efficiency decrease.

The difference is actually smaller than this since I don't think
anyone can buy or build lossless UNUNs.  I don't know what the
losses in a typical UNUN are.  But I bet they are comparable to
the 0.05 dB they would need to be in order not to make the system
worse rather than better.  Anyone have some real world measured
loss numbers for this component?

73, Eric  N7CL

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