On 10/17/2014 1:23 PM, Brian Howie wrote:
In message , John S
writes
On 10/16/2014 3:29 PM, Brian Howie wrote:
In message ,
Lostgallifreyan writes
John S wrote in :

Yes, of course. And, with the free version of EZNEC, one must be
careful
not to exceed the max segments allowed.


I've started reading the manual, I suspect there's little danger of
that.
At least with EZNEC+ 4 onwards, not sure about standard version. I
suspect
like curves built in segments in Sketchup, or the straight bars in the
chains
of the Clifton Suspension Bridge, the amount of fine tuning you'll
get in
using more than about 24 segments for a catenary might be an
exercise in
diminishing returns, and that even just 3 to 5 might be adequate, if
the
deviation from straight is small.

I did a simple sagging 40m dipole on MMANA using the wire editor with
9 wires. I had a 3m sag in the middle . I ran the optimiser for best
match. The impedance worked out at 69 ohm and the gain was 2.06dBi. The
model reported a lobe elevation of about 8 degrees. There was a
vertically polarised component at 90 degrees to the horizontal lobe at
-15dBi.

You'd expect something like this to happen since there is part of the
antenna in the vertical plane.

Brian GM4DIJ

By the way, Brian, do you have data of the non-sagging model for
comparison? Don't do it unless it is fun for you. The data looks just
about the same for a non-sagger anyway. A comparison would show the
small differences.

Super work! Thanks.


I design and build antennas for fun, but mostly VHF and UHF. However my
last one was 5ft screened Rx loop for 472KHz.

The non sagging one was 72 Ohms impedance and a gain of 2.13dBi . The
main lobe was horizontal. Plain vanilla dipole figures and
surprisingly much the same for the sagging one

Above a ground 20m up it has a gain of 7.1dBi and a lobe elevation of
29.6deg and an impedance of 73.6ohm

Now the sagging one 20m up at the ends

82.5ohm impedance, gain 6.23dBi and a lobe elevation of 33.8deg. The
vertical component was 12dB down.

The upshot is that a bit of sag isn't going to impact performance. It is
going to give mechanical problems due to wind sway.

73 Brian GM4DIJ

That was my guess.

The mechanical problems must be handled via another route. There must be
some sag in order to keep the antenna from breaking in high winds.
Somewhere on VK1OD's Web site, he did such an analysis. I think he was
forced to change his call and his site, so you might find it at
http://owenduffy.net/blog/. His stuff is extremely educational and
well worth reading.

73 John KD5YI