On Mon,12/12/2016 1:55 PM, Catherine James wrote:
Jim Brown wrote:
Cathy James wrote:
It all depends what you are trying to do. For solid regional coverage, a low
dipole gives excellent NVIS coverage. For DX, it is less than ideal.
That's an urban myth. See my tutorial on this.
http://k9yc.com/VertOrHorizontal-Slides.pdf A low dipole is much less
efficient than a high one. Especially study slides 18-22. Then fast
forward to slides 60-66.
A FAR better choice for something suspended between trees is a Tee vertical,
where a flat top wire provides top loading for the vertical
section, which does the radiation. Because antenna current splits equally
left and right into the top wires, radiation from the top cancels, and you end
up with a nice vertical radiator and a nice low angle of radiation.
That's great for DX, but lousy for regional coverage. And here in northern New
England, our ground conductivity is awful, which makes verticals challenging.
No matter how much you improve the near field with an excellent radial system,
the far field will still have lousy conductivity and that will push the
radiation angle higher.
Lousy soil does NOT vary the vertical angle, it simply attenuates the
signal. In the near field, it burns transmitter power by warming the
worms. In the far field, it degrades ground wave and weakens the first
reflection that creates the vertical pattern.
AND -- there's another point that virtually EVERYONE who has commented in this
thread seem ignorant of. ANY distortion mechanism produces BOTH harmonics and
INTERMOD. On SSB, it shows up as splatter, and on CW it shows up as clicks.
I'm still in the process of learning about intermod and how to minimize it .
It is simply the result of non-linearity in the signal path. To minimize
IMD, minimize the non-linearity.
the rise and fall of ANY rectangular wave consists of an infinite number of
harmonics that excite IM.
Yes, in Fourier analysis terms, the definition of a square wave is essentially
the sum of a sine wave and all of its odd harmonics.
But CW is not a square wave, it is a non-periodic rectangular wave.
Yes, we can filter the harmonics, but we can't filter the IM.
That's why I'm trying to understand better how good design suppresses IMD. It's easy to
say "you need a linear response", but real world systems will never be
perfectly linear, and there's often a serious tradeoff between linearity and efficiency.
Of course NOTHING is perfect, but part of good design is optimizing
parameters to minimize the bad stuff. And yes, there's a tradeoff -- ALL
designs involve tradeoffs. That's what engineering is all about.
Clearly you did much better than the New England ops I've spoke to! I'm not
sure how much of that is regional (e.g., how long did New England have good
paths to anywhere outside of NE?) vs. time of day.
Of course. One of the many things that hams learn that makes them better
operators is the study of propagation. 10M and 6M are quite similar in
their behavior. Both bands have Sporadic E, F2, and transequatorial
propagation. Paying attention over the years teaches us when those modes
are likely to be active. 6M also has multiple tropospheric propagation
modes and meteor scatter. We don't know much about WHY sporadic E
happens, and we almost need a crystal ball to know WHEN it will happen.
The ARRL Handbook and ARRL Antenna Book have good chapters on
propagation, and the ON4UN book has several good chapters on low band
propagation (160, 80, 40).
Contests for 160M, 10M, and 6M are usually 48 hour contests so that we
can take advantage of band conditions. A successful operation on a 10M
or VHF contest means staying close to the radio for all daylight hours
and into the evening. For 160 contests, it means being on the air from
an hour or two before sunset to an hour or so after sunrise. This is
propagation enhancement on all of the lower bands for an hour or two
either side of sunrise and sunset, and propagation also varies a lot
through the night. And from night to night. Running legal limit to
vertical, I can routinely work 800 miles on 160 at least two hours
before sunset. To a 120 ft high horizontal dipole, I can't even get a
QRZ? I eventually took the horizontal dipole down. :)
I now need only VT and SC to finish it QRP. I've been at that for four years.
Well, I'm in Vermont, so we can try some time with a sked. My RF noise is
pretty low here, so you may get lucky!
Let's correspond off list on that.
73, Jim K9YC
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