A recurring theme that I have noticed in this and other threads on the
quad versus the yagi is the anecdotal claim that quads are quieter for
receiving than the equivalent Yagi. There could be a good physical
reason for this - the parasitic quad array may have a cleaner cross-pol
pattern than the equivalent yagi.
Has anyone looked closely at the difference in vertically polarized
component of a horizontally polarized quad antenna's pattern versus that of
a horizontally polarized yagi? This can make a big difference if the
dominant
source of a user's noise floor is due to local sources which are propagated
via vertically polarized surface waves. At W6UE in Pasadena, our horizontal
160 meter dipole is a much quieter 80 meter receiving antenna than our
80 meter inverted-vee at the same height. In many cases the SNR
improvement is upwards of 10dB. I believe this is due to the fact that most
of our noise is from local sources which are vertically polarized. The
station is
sitting smack in the middle of an urban QRN jungle. Although the dipole
pattern does respond to vertically polarized signals (especially off the
ends),
it does so to a much lesser extent than the inverted-vee which exhibits much
more vertical polarization. Perhaps the quads reputation for quietness can
be explained by comparing its cross-pol (vertical) polarization pattern with
that of an equivalent yagi. EZNEC displays both total field as well as H
and V components of the radiation pattern, so it should be easy enough to
verify with computer modeling. For those out there uncomfortable
with computer modeling, horizontally polarized quads and yagis could
be compared on ground-wave paths (5 to 10 miles) where the dominant
polarization mode is vertical. This sort of thing might explain why a quad
is said to hear better even though its total-field forward gain and F/B
model
out to be the same as the equivalent long-boom yagi. This would also
suggest that the advantages of the quad would tend to disappear in rural
locations (that lacked power line noise) where the dominant source of
noise is propagated from distant sources via skywave that exhibit
random polarization.
73 de Mike, W4EF...............................
----- Original Message -----
From: "Guy Olinger, K2AV" <k2av@contesting.com>
To: <towertalk@contesting.com>
Sent: Tuesday, November 19, 2002 9:45 PM
Subject: Re: [Towertalk] Quad vs Yagi
> While real world construction can modify results, and there are
> well-known noise performance and ease of construction differences
> between quads and yagis, comparing the two in idealized circumstances
> in models does give a sense of some interesting playoffs between the
> two.
>
> These have not thus far been mentioned in this thread. Speaking only
> of horizontally polarized configurations,
>
> The optimal two and three element quad runs ahead of an optimal yagi
> with an equal number of elements. A *wide-spaced* optimal four element
> yagi (does not include most commercial 4 element yagis which have
> shorter booms) and an optimal four element quad are nearly equal to
> one another. This is slightly in favor of the yagi at optimal five
> wide-spaced elements, and the yagi is clearly and increasingly in the
> lead after that.
>
> There appear to be two factors involved in this.
>
> 1) As the feed impedance of the quad and yagi go down with the
> additional elements, the losses in the quad's #12 copper wire
> increase, whereas the typical tubing elements of the yagi stand the
> higher current better. Remember the loss is I squared R. This
> difference if not present if the quad loop conductor is 3/8" copper
> tubing.
>
> 2) In the low element count configurations, the quad loops have a
> natural reduction of radiation to the vertical due to effectively a
> pair of radiating conductors per quad loop, whereas the dipole
> elements of the yagi do not. As the antennas lengthen with more
> elements, the vertical radiation is significantly reduced in either by
> the horizontal gain. Reducing the vertical radiation from -15 to -17
> db in a long quad or yagi doesn't do much to increase horizontal gain.
> Reducing it from 0 to -2db DOES make a notable difference, as in the
> single element comparison.
>
> The additional power gain is less and less a result of the loop's
> reduction of vertical radiation as elements are added, and at a point
> in the lengthening, you can actually increase the gain of a long quad
> by substituting a yagi element for a loop director (5 or 6 element
> configurations, depending on height).
>
> This last is the derivation of the quagi. Given the spacings of long
> beams, though, I would put THREE quad elements (ref, de, dir1) in a
> quagi before switching to yagi elements.
>
> The highest gain configurations I have derived for 5+ element beams
> seem to have Ref, dir1 through second to last dir in near equal
> spacings, the DE approx halfway between Ref and dir 1, with the dir
> spacing slightly reduced between the forward most directors.
>
> 73, Guy.
>
>
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