Re: antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??

Frank,

I observed by playing with the relative inductor values on the vertical segment and the radial elements that it was possible to move the relative feedpoint. This supports tuning the antenna by either inductor.

Reg's c_poise program predicts a 75 uH loading coil.

I am excited with prospect of coil models.

Thanks - Dan

Frank's wrote:
"Roy Lewallen" <w7el@xxxxxxxxx> wrote in message news:120acppce6jle62@xxxxxxxxxxxxxxxxxxxxx

Frank wrote:

As I understand NEC; large errors can be introduced by junctions of dissimilar wire diameters, and in particular when the wires are at 90 deg. Therefore, when you have designed your "GH" inductors, the rest of the antenna should by constructed of the same diameter wire. This may be difficult since Dan is using two coils of significantly different Qs. I guess you could overcome this problem by varying the conductivity of the inductor to obtain the desired Q. Also, since segmentation tends to be relatively high in a helix, should segment length tapering be applied to those segments adjacent to the helix?

Frank, VE6CB

It's difficult to give an absolute answer to these questions, but some general comments and guidelines should help.

First, the error introduced by NEC-2 when wires of dissimilar diameter are connected is generally small, unless the wires are grossly different. This error can be minimized by making the segments as *long* as possible adjacent to the junction, which of course is contrary to the general principle that more segments are better. Even a small error can cause major changes in the pattern when the dissimilar diameter wires are in a parasitic element. EZNEC and a number of other programs have a built-in method of avoiding this problem for certain antenna types, but plain NEC-2 doesn't. NEC-4 is relatively free of this problem, but it's quite expensive for hobby use.

The Q of an inductor is determined by the inductance and the loss. The loss is a function of the dielectric, wire resistance, and radiation (which isn't really loss, but lowers Q as though it were). NEC type programs automatically account for the radiation, and it's easy to include wire loss. So assuming negligible dielectric loss, the programs should predict Q fairly accurately -- except for proximity affect. Proximity effect could be modeled in NEC by increasing the resistivity of the wires in the coil. EZNEC currently allows only a single wire resistivity for the whole model (although this will probably change in the next version). However, since the overall loss will be dominated by the inductors, the higher resistivity could be specified for the whole model without sacrificing significant accuracy. Alternatively, a number of resistive loads could be inserted in the inductors.

Segment length tapering usually isn't necessary with NEC based programs, unless there's a source near a place where the segment length changes. An average gain check should be run to determine if there's a problem. If there is, segment length tapering is one tool which can be tried in improving the average gain.

Roy Lewallen, W7EL


Thanks for the information Roy, all remarks noted and saved. Will see what I can do to generate some realistic helical models.

Frank VE6CB


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