[TowerTalk] 50:75 ohm transformation

[Mike Lamb]

Hello Eric,

I agree with you.  I think the bottom line is that the reason for using
1.5:1 UNUN's is to keep your transceiver foldback circuit from reducing
power and to help keep your lines flat to near 50 ohms so that you can
switch from your dummy load to any of several different antennas without
having to re-tune your amplifier.

73/Mike, N7ML

Eric Gustafson wrote:

> >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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