[TowerTalk] DC grounding of parasitic eles with insulators ?

[jimlux]

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)