To: <towertalk@contesting.com>
Date: Sunday, July 26, 1998 12:45 PM
Subject: Re: [TowerTalk] Reflections, Conjugate Matching and Jim Reid's
comments
>I work for a small mfr of FM and TV antennas. One
>of the biggest reasons for minimizing
>SWR in the [Reid insert: VHF/UHF TV and FM]
>commercial world is that the reflected
>signal undergoes a time delay going
>from the antenna to the xmtr and back. This can
>cause all sorts of problems from ghosting on TV to
>poor stereo separation on FM.
Hmmm? See my name still remains as part of the subject
line. Otherwise, I would not comment; however, such
time domain problems as discussed above are of
absolutely NO importance in HF amateur radio, not even
SSTV at HF. As discussed before, even at 30 wpm on
CW, a dit lasts about 43 millisecond; all the reflections
on 100 feet of the usual coax line, even with a high vswr,
have reflected back and forth many times until completely
attenuated or radiated, and all is over and done with within
2 or 3 microseconds! The dit is thousands of times longer,
so, the reflection is never noticed by anyone. At lower CW
speeds, obviously the reflection is even more insignificant!
There is no way your ear, nor anything within your HF trans-
ceiver can respond to a 2 or 3 microsecond pulse, even if
it was standing alone. Your headphones or loudspeaker
cannot transduce to sound a 3 microsecond pulse of
current in the voice coil. The pass band of our equipment
is from 250 Hz, narrow CW filter width, to maybe 5 kHz,
AM or wide open SSB or RTTY channel. Passing a microsecond
pulse requires a total system bandwidth of one or more
MHz; that is greater than the band span of any of our
8 HF bands below 10 meters; the widest of which,
is of course the 80/75 meter band which altogether
is 500 kHz wide, or one-half a MHz.
Our next widest HF band below 10 M, is the 15 meter
band, where we have 450 kHz of spectrum.
The 10 meter band is 1.7 MHz wide, so if you used the
entire spectrum of that band, you probably could
pass a fairly faithful replica of a 1 microsecond pulse
of RF energy, though it would still be a bit rounded
from an original perfectly rectangular pulse of energy.
Hope this helps to understand why TV signals need and
entire 6 MHz of spectrum space for the signal and audio;
that's why time delay distortion of energy across an
entire 6 MHz can cause "ghosts"! Same thing happens
to stereo FM transmissions because the FM transmitter
frequency is deviated a good deal more that the audio
frequency it is carrying; so again bandwidths much
greater than amateur HF modulations are encountered,
and again time delay distortion does cause problems.
I think what I have written here is accurate, hope so
anyway, hi.
73, Jim Reid, KH7M
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