Does anyone have any of those Elliptical or helix designs modeled somewhere?
Was lookin at that Wade site Holl_ands pointed to, specifically WH-14-69 series, and saw they claim 470 - 806 Mhz, 18.7, 19.7, and 20.7 "dBic" with only 12 loops. Is that even possible over such a wide bandwidth?
Nuther question, is how do they get it to a 75 ohm impedance?
Lastly, what the heck is dBic? (c-> circular maybe??)
As you guessed, dBic is relative to the gain of an isotropic antenna with circular polarization:
In 2007, Hasan-Murtaza reported that he had built an Axial Mode Helical Antenna and gave
some dimensions, but I couldn't find any construction details or pictures...and he is no longer active:
Otherwise, I have not seen anyone post a Helical, Cubical-Quad or Quad-Yagi designed for either
Hi-VHF or UHF TV Band, nor will searching reveal any designs on this forum. Suggest start by
looking at Amateur Radio designs and then carefully RESCALE to TV Band. Here's some I posted earlier:
These articles discuss how the final turn acts as an "tapered transmission line", slowly changing
from the impedance of the antenna to the desired impedance at the connector.
If you have an SWR Meter (readily available for Amateur Radio Bands....but NOT TV Bands)
the shape of the final turn can be "tuned" to minimize the SWR [For TV, use a Network Analyzer]:
Helical antennas have adequate bandwidth to cover either the Hi-VHF Band (1.24 High/Los Ratio)
and (perhaps just barely) the NEW UHF Band (1.48 High/Low Ratio). The fol. MS Thesis
addressed the small Spiro-Helical Antenna (a slinky wrapped around a mailing tube) compared
to the conventional Helical Antenna, see for example Figure 4.1 on page27:
Bandwidth can be expanded by using different winding ratios for the front and back halfs or by
slowly varying the winding ratio, such as in this Tapered Helical Antenna design:
BTW: 4nec2 includes a BUILD/HELIX option in Geometry Editor....or use NEC's "GH" Helix/Spiral
statement by reading the NEC-2 Guide (see pg 20). Click on HELP/NEC-USER-MANUAL in 4nec2.
Analysis of Helical antennas, including an example 4nec2 file, can also be found at www.cebik.com
[Free Registration Required].