[opendtv] Re: VHF Stations Seek Solutions for Reception Problems, by Doug Lung

Dan Grimes wrote:

> And how about the tripole CP receive antenna:

Or more than that, I suppose.

> How does the helix antenna achieve broad spectrum?  Is the
> (apparent) rotational speed constant, therefore a fixed radians
> per change in z axis for any wavelength?  Or is it
> wavelength/frequency dependant also?

The wavefront propagates at the speed of light, no matter what the
frequency. Therefore, if the frequency is lower, it will take a longer
distance along the Z axis to rotate, say, 90 degrees. So yes, the
construction of the helix will depend on wavelength, just as a yagi
does. That's why it seems to me more practical to for CP antennas to
feed orthogonal yagis or log-periodics with variable delay lines, than
to mess with the mechanically-constrained helical antenna? May be more
expensive though.

I've seen (in the literature) different ways of fudging a wider
bandwidth from these helical antennas. One is to vary the diameter and
pitch along the axis. Obviously, that reduces gain. Another is to
increase the gauge of the helical conductor. Another is to install two
or four separate helicals intertwined (bifilar or quadrifilar helical
antennas), and then add delay lines between them. Another is to use
different dialectric material inside the helix.

> A 1982 IEEE paper for analog about a late '70s installation of
> a CP transmitting antenna:


> Did CP transmissions for UHF ever catch on, or are they
> still mostly H polarized?  If CP is common, I would think CP
> antennas would be commonplace.
> The article states that the power requirement doubles to power
> each polarized plane for the same ERP.  Any changes there?

I think they are saying that you need to double the power if you intend
to obtain the same coverage when using single polarity receive antennas.
That makes sense. If you're using CP receive antennas, then you wouldn't
need to add any power, ideally.

By the way, these are all axial mode helixes we're assuming here. There
are also normal mode, like those often used in AM radios, which behave
like thick monopoles but require less length.

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