...most efficient for me at a 25 ft. height for dx purposes...
...again I had to lower this from 40 FT. down to about 23 ft. for best efficiency...
I also would question the use of the word "efficient" here. Do you perhaps mean "effective"?
How are you measuring efficiency?
With horizontal polarization the angle of radiation drops as the antenna height is increased
because of the interaction with the ground reflection. Increasing the height from 25' to
40' often makes a big improvement in low angle radiation for working DX, though there may
be specific stations or paths that rely on a higher angle that don't work as well.
If this really is the case, then I'd suggest that you have significant common mode current
on your coax that is responsible for the radiation. In that case, the vertical radiation pattern
from the coax acting as an antenna would raise the angle of radiation as the coax is made
longer. A good friend used to work DX on 40m with a dipole in the rafters of his garage, about
12' off the ground. One day he plugged in a 20m dipole to an unused port on his antenna switch
and the SWR went haywire. Turns out that high common mode currents on the outside of the
coax were responsible for most of his radiation, and he had changed the "antenna" when he
connected the shield of the new feedline. Things calmed down after he added a balun to
the antenna, but he could no longer work any DX, because the antenna is too low without
the help of the vertically polarized radiation from the feedline.
Certainly what is now called a "Windom" (an Off-Center Fed Dipole: the original "Windom"
had a single wire feeder) is renowned for common mode currents, even in spite of the addition
of a choke to try to reduce them.
That's not to say that, in your specific situation, there might not be some reason why you get
better results at a lower level due to local buildings, terrain, power lines, etc. But that certainly
isn't the experience of most hams, so one tends to look for other explanations. (And, of course,
it depends on what you consider to be "DX".)
As I understand it,cob webs are somewhat omni directional compared to a longwire or dipole type antennas..
Slightly more omnidirectional and slightly lower gain than a dipole. A little more loss, too, because
the radiation resistance is much lower - about 12 ohms, requiring a step-up transformer of some sort.
I wouldn't expect any difference in noise other than that due to being able to null out
noise sources off the ends of a dipole, or otherwise due to differences in the pattern.
If you have common mode current, you may be picking up noise on the outside of the
coax as it passes by all the electronics in the house, in which case the antenna with
the best common mode rejection (improved by adding an effective balun) probably will
have the lowest noise.
Over the years I have noticed that antennas fed against earth ground at my location
tend to have a higher noise level than those that are balanced (even a dipole fed
without a balun.) This appears to be due to a high level of electrical noise coming in
on the power ground lead, possibly from an arcing connection that I can hear on a
clear night from 1/4 mile away. But when the antenna is isolated from ground, the
noise level is often S1 or S2 with most of the antennas that I've used.
If you are only using the Windom on 40m and 30m, then a trap dipole may give you the
best performance, or two dipoles (one for each band) on a common feedpoint. Or you
may be able to add 30m wires in parallel with the 40m wires on your current antenna.
If vertical polarization works better in your situation then a half square or "T" antenna of
some sort might worth trying.
It may also be practical to string a 30m dipole around the outside of the 20m Hex frame,
somewhat like a Cob Web but a bit larger. I'd have to do some modeling to see how
much impact that has on the 20m pattern.
I used to operate mostly on 40m, and had good results even with antennas at relatively
low heights. I learned a lot when I started using 30m, because the antennas that worked
well for me on 40m were no longer effective due to the high angles of radiation. (It gets
reflected back to Earth on 40m, but not on 30m.) I ended up putting up a homebrew
vertical, though getting the 30m antenna up to 40' would have helped as well.
