On 5/11/2022 6:51 PM, Richard wrote:
Jim —
I just finished building a half-wave 40-meter dipole and have it nicely
tuned for the middle of the SSB segment. Out of necessity, the tuning
process was carried out with the antenna horizontal.
Feedpoint Z of horizontal antennas is slightly affected by height and
surrounding objects, so ideally should be tune at the rigged height,
then lowered and re-adjusted if necessary.
*QUESTION 1:* Should I ever hang this antenna as an inverted V, would
that result in a significant change to the location of the sweet spot?
Slightly.
When we finally got the leg lengths to the point where the SSB sweet
spot was damn near perfect, we had folded back 30 inches of excess leg
wire on each leg. The excess was not twisted around the legs but instead
laid parallel to the leg wires and Gorilla taped in place in several places.
Remember that SWR is NOT a measure of antenna performance, it only tells
us approximately where it is resonant. That's because the feedpoint
impedance varies a bit with height, thanks to the reflection from the
ground coupling back to the antenna.
Folding it back as you have done is perfectly good technique -- the
antenna ends at the fold (that is, where it goes through the insulator.
We routinely do that when rigging new wires.
*QUESTION B:* I’d like to cut off 12 of the 30 inches of excess wire,
leaving 18 inches in case I or some future owner wants to retune down to
the CW segment. Will shortening the excess wire from 30 to 18 inches
influence the sweet spot?
SWR matters two ways. First, the match at the transmitter must be good
enough that the transmitter can put full power into it. Additional loss
due to SWR doesn't matter much until the SWR gets pretty high unless
you're using small diameter coax or it's a long length of coax. As long
as you have an antenna tuner (really an adjustable network to match the
transmission line to the rig), any SWR below about 2.5:1 is plenty good
enough unless the line is long enough, or small enough diameter, to have
a lot of loss. That's why smart hams use big coax like RG8 or RG11 to
feed dipoles. Low dipoles are closer to 50 ohms, high dipoles are closer
to 75 ohms. Low would be much less than a quarter wave, high would be
much more than a quarter wave. That feedpoint Z also depends on ground
conductivity.
SO -- since the full width of 40M is a small as a percentage of the
frequency, so a half-wave dipole cut for 7150 will be just fine to cover
from 7,000 - 7,300 kHz. Ditto for all the bands above 40M until you hit
10M, but even there, unless you work FM, which is at the top of the
band, a dipole tuned to about 28.45 MHz will be "good enough" for where
all the action is.
BTW -- as study all this stuff and learn to use the tools, there is
antenna modeling software that will let you learn by modeling.
Here's a study I did several years ago on the effect of antenna height
both as text and slides.
http://k9yc.com/AntennaPlanning.pdf
http://k9yc.com/VertOrHorizontal-Slides.pdf
73, Jim K9YC
Looking forward to your input.
Cheers.
Richard
That said, what's your question? OF COURSE there are LOTS of folks
here who know a LOT about half wave dipoles. I have six of them in the
air for different bands and directions.
73, Jim K9YC
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