On 10/28/19 12:40 PM, K9MA wrote:
Does anyone know of a modern NEC-designed yagi that does NOT use
isolated elements? Perhaps there is some software that can accurately
model the effect of the boom, but I suspect it's way out of the price
range of the designers of amateur antennas. I know NEC4 is better than
NEC3, but still has problems with very closely spaced wires.
I'd expect the water to pretty effectively bleed off rain static, but I
don't know about snow. This will be my first winter with isolated elements.
73,
Scott K9MA
SM5BSZ comments WRT to high gain VHF/UHF Yagis
There is a distinct advantage in using isolated elements mounted through
the boom tube so this mounting is most popular among amateurs. Elements
in contact with the boom tube requires very good electrical contact
since the currents are very high and a fraction of an ohm will cause
significant ohmic losses. An insulated element is much less affected by
the boom tube because there is a substantial magnetic field inside the
boom tube that makes the inductance change much smaller.
Jim W6MRK comment:
This, however, is when the boom is a significant fraction of a
wavelength (e.g. a 2.5cm diameter boom is 1/30 th of a 70 cm
wavelength). With a HF Yagi, a 10cm diameter boom (4") is a tiny
fraction (1/200th) of a 20m wavelength.
The problem with NEC (in general) is that it doesn't model the current
"around" a conductor. If you wanted to allow for this, you'd have to
model the boom as a mesh of wires to allow for the circumferential
current. And that might start getting into numerical precision issues
although with NEC4, this is very much less an issue than with NEC2 (the
newer version of NEC has a more sophisticated model of the current along
a segment that deals better with very short segments)
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