[TowerTalk] Mismatch Loss and Tuners

[Steve Thomas]

Hello all,

I'll comment at various places in the message.

Steve N6ST

> -----Original Message-----
> From: Eric Gustafson [mailto:n7cl@sparx.mmsi.com]
> Sent: Friday, September 04, 1998 6:58 PM
> To: towertalk@contesting.com
> Cc: steve@n6st.org; sbest@cushcraft.com; w2du@journey.com
> Subject: Re: [TowerTalk] Mismatch Loss and Tuners
>
>
>
> >From: "Steve Thomas" <steve@n6st.org>
> >Date: Thu, 3 Sep 1998 21:57:01 -0700
> >
> >Amen Steve! It is also interesting to contemplate what happens on
> >the receive side. The 20:1 SWR doesn't do any good for the
> >receive signal either.
> >
> >73, Steve Thomas, N6ST
> >
>
> Steve(s),
>
> Nobody has been advocating operating at a 20:1 SWR at the end of
> a lossy transmission line.  The 20:1 SWR was mentioned by someone
> (I forget who) merely to illustrate the point that the SWR in and
> of itself is not what causes the losses.  It is the loss in the
> line.

I realize that the 20:1 SWR example is merely an extreme case for
illustration purposes. You are correct that SWR doesn't (precisely) cause
loss, it causes reflection.  The magnitude and phase of the reflection
depends on the impedances at the mismatched junction.  I know this from many
measurements with professional quality equipment (particularly an HP 8510B
Vector Network Analyzer among others).

If the reflected signal doesn't see another mismatch on its way back to the
source, it will continue traveling until it hits a matched load, where it
will be absorbed. If it sees another mismatch, a portion will be
re-reflected and again travel toward the load (an antenna in the discussions
that have occurred on TowerTalk.) Line loss takes its toll every time the
signal passes through and eventually the signal will disappear into the
noise.
>
> He offered an example of a broadcast station operating an
> antenna at a 20:1 SWR successfully.  At 600 kHz (or wherever
> they were in the BC band), the losses in the airline they were
> using probably amounted to very nearly zero.  A reasonable guess
> might be on the order of .001 dB/100ft.

Think about the receive signal. When it hits a mismatch, some portion will
be reflected back to the antenna, to be re-radiated and forever lost.
>
> So for that particular case, would you rather have your receiver
> tied to a tuner that looses 0.5 dB plus the 0.001 dB (we'll
> assume 100 ft distance here) airline and a 20:1 mismatched
> antenna or to 100 ft of 1dB/100ft coax and the same antenna with
> a perfect 1:1 match?

I'll take the 1dB/100 ft case any time with a matched antenna system. If
I've lost 7.4 dB due to reflection at the antenna/feedline junction, I'll
never get it back no matter how low the loss of the feedline is.
>
> I'll bet that you couldn't tell the difference.

If the signal is large, you're right, it wouldn't make a significant
difference in the ability to copy a signal. But if, on the other hand, I'm
trying to work a weak signal on 160 meters where 1/f noise dominates the
receiver, losing 7.4 dB could make the difference between being able to copy
the signal and just hearing noise.
>
> For the above example:
>
> Coax & perfect match:  Total system loss = 1.0 dB
>
>
Tuner with airline and 20:1 mismatch: Total system loss = 0.51 dB

plus 7.4 dB in reflected loss at the point of the 20:1 SWR.
So, give me 1.0 dB loss instead of 7.91 dB loss any time!

>
> Most Ham uses of tuner and mismatched antennas don't involve a
> mismatch anywhere near that extreme.  And while the line
> typically has more loss than that cited above, it is usually low
> enough that combined with a lower mismatch ratio than 20:1, it
> will not usually make the increased system losses _significant_.
>
> Note that in the above example, the mismatch caused the line to
> exhibit losses 10 times higher than would have been the case if
> the line were driving a matched load.  But this was still not
> enough to be _significant_.  This is particularly true when
> operating 1/2 wave long or longer antennas via open line (and
> even with coax most of the time) on the lower 3 bands.
>
> 73, Eric  N7CL
>

73, Steve N6ST


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