actually, when all the variables are taken into account a null, or even series
of nulls, usually have very little real effect on ham radio beyond random
fading
and sometimes weird results in A/B antenna comparisons or station to station
signal comparisons. because our normal style of operation has us
changing beam heading, path length, the time of day we operate, and frequency
relatively often we don't notice most of the effects of nulls in antenna
patterns.
all that agility makes most of those effects relatively fleeting observations,
like why did that guy with the low tribander work that dx guy before I did with
my
high up monobander?
Aug 1, 2013 02:02:19 PM, jim@audiosystemsgroup.com wrote:
On 8/1/2013 7:09 AM, Cqtestk4xs@aol.com wrote:
> BIP/BOP makes a difference.
Phase is a continuously valued function, it has the units of degrees,
and it increases linearly with frequency. BIP/BOP (both in phase, both
out of phase) is a simple-minded view of things, and confuses PHASE with
POLARITY. Polarity is what we're talking about when we reverse the wires
feeding a circuit element. The difference is VERY important.
Interestingly, the pro audio world figured this stuff out almost 40
years ago, thanks to the prodding and teaching of the late Dick Heyser,
a really sharp engineer working in communications at JPL, and whose
hobby was audio. He wrote and taught prolifically, and his work is well
worthy of study.
To understand what's going on with our antennas, we must first
understand that there are many sources of phase shift in a system. One
is the transmission lines feeding the antennas, which varies linearly
with frequency, AND is a function of the source and terminating
impedances. There's a discussion of this in the ON4UN book -- look for
the discussion of the "Christman feed" to a 2-element vertical array.
Another important source of phase shift is in the radiation pattern of
the antenna, and that will be different for every antenna. There is
also the contribution of the ground reflection.
As an example of how the difference between phase and polarity matters
-- let's say that we produce a 180 degree phase difference by reversing
the feed to one element of an array. That will result in a 180 degree
difference at EVERY frequency. But if we do it with a length of coax, or
by space between elements of the array, or with a reactive network (L
and C), the phase change will vary with frequency.
The radiation from ANY array is FAR more complex than adding the
magnitudes of the components. It's the COMPLEX (magnitude and phase)
addition that occurs, and depending on all of those phase relationships,
can put peaks and nulls in the pattern (both in the horizontal and
vertical plane). Combine this with wildly varying propagation, which
causes signals to arrive at varying vertical angles, and even to take
skewed paths, and it's obvious that a null can do quite a lot of damage. :)
73, Jim K9YC
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