[TowerTalk] Quads vs. YAGI (gain)

[Tom Rauch]

 One or more people have mentioned their "quad" was not as
directive as their
> SteppIR.
> Since they did not mention how many elements their quad
was, I assume it was
> a 2-element,
> in which case it's not surprising it didn't have great
directivity.

It's pretty easy to analyze a quad.

A quad is really just two 1/4 wl long elements stacked 1/4
wl apart. The two non-fed sides simply serve to end-load the
1/4 wl radiators.

The shorter width elements reduce "collinear" gain and cause
a slight increase in beamwidth for horizontally polarized
quads. Thus all things equal, they have wider beamwidth
because current is not "spread" over as broad a frontal area
as a full size dipole in a Yagi.

The stacking gain of two 1/4 spaced elements at absolute
optimum conditions is in a graph on my web page at:

http://www.w8ji.com/stacking_broadside_collinear.htm

It is as approximately 1dB. This gain is from a slight
vertical nulling. Now if you mount a quad at a height where
the ground forces a null straight up, the 1/4 wl stacking
gain of a quad goes to zero!!!

If you mount a quad at a height where the ground causes a
peak straight up, stacking gain is at a maximum. The same is
true of freespace. The reason for this is because in order
to have gain, we must have energy to remove from a null
area. If the null area has zero field, stacking to force a
null in a null can't add gain.

If you model a quad and compare it to a dipole, you'll see
all of this is true. If you accurately measure one, it will
behave the same.

Now as the quad is made longer and longer, the multiple
elements force a null where the quad "dipole stacking" has a
null. The longer the quad, the less of that 1dB potential
gain that can be obtained even at a sweet height.

This doesn't mean quads don't have advantages. They are less
susceptible to corona and are easier to make multi-band. The
fact is, no antenna is a magic bullet. They all have pros
and cons. The exception is the occasional "new invention"
antenna we read about, at least until someone honest
measures it.

73 Tom