Tom Rauch W8JI@contesting.com
Wed, 7 Feb 2001 18:44:26 -0500

```Hi Yuri,

>     It appears that some of the "new wisdom"
is coming from the
>     software
> modeling "facts."

Actually it comes from simple math that appears
in antenna engineering books from the 40's and
earlier.

A quad is two 1/4 wl end-loaded dipoles stacked
1/4 wl apart.

Stacking gain for two elements spaced 1/4 wl is
one dB under **perfect** conditions.

>     I just quickly modeled single Quad 1/4
wave/side loop vs. dipole
>     and I
> can get paper advantage of over 2 dB. So no
folklore there.

I can not, no matter how I model it. I see one
dB or less maximum gain over a dipole at the
same mean height.

In freespace, it is about one dB on the button.
At some heights, it is less than 1 dB...and
sometimes approaches zero dB. With more
elements, the advantage is less at any height.

Like any antenna, a quad "gets gain" by forcing
a null in an area of significant radiation. That
area is directly in line with the plane of the
current maximas.

If you place the quad at a height where ground
reflection forces a null straight up, the
horizontally polarized quad element provides
minimal, if any, gain.

If you place it at a height where radiation is
maximum straight up, you get maximum advantage
but at all practical heights that advantage is
less than one dB for the two antennas compared
at the same mean height.

If the model shows more gain than theory
(available since antennas began to be
understood)predicts, the model is flawed or the
comparison is unequal for mean height. Broadside
array gain has been defined for many many years
and the results have never been proven wrong to
this day.

(My 3 el.
> Quad beat 7 el. KLM Log Yagi on almost twice
the boom on 2m.)
>     Quad (circular or delta) loops have larger
aperture than dipole,
>     they
> have fatter vertical lobe and there is
suspicion that they pick up
> some of the "other" polarization too.

I hear "aperture" used all the time to explain
why one antenna is better than another.

Aperture has NOTHING to do with physical size of
an antenna. Aperture relates only to gain and
frequency, not physical size.

A two inch square antenna on 40 meters can have
more "aperture" than a two mile square antenna.

A fatter vertical lobe, for a given azimuthal
beamwidth, means less gain (and more fading when
multipath is involved).

If any antenna picks up two polarizations, gain
is always less than an antenna with the same
pattern width that picks up only one
polarization. If the antenna picks up both
polarizations on a skywave path, or truly
transmits both polarizations, fading is
increased...not decreased.

Quads do not pick up both polarizations because
they are not constructed in a way that lets them
to do that. Both polarizations
are received (or transmitted) only when the
antenna has dual elements or dual radiating
areas with suitable phasing and
polarization, like crossed dipoles in phase
quadrature or a small loop and a small dipole on
the axis of the loop.

Quads pick up one polarization only, although
that polarization might be skewed by feedpoint
placement or feedpoint errors. Even
if it is skewed, it is one polarization. It is
just tilted.

>     Quad loop has lower Q, wider frequency
response, flatter SWR
>     curve, wider
> bandwidth.

So do dummy loads.

>     Quad loops provide more efficient match
and RF transfer between
>     coax and
> antennas.

That applies to UHF and higher where the coax
diameter is large compared to the wavelength ,
but not at VHF and below.

> http://members.aol.com/ve3bmv/Razors.htm
Single Razor would
> consistently beat 6 el. KLM by about 10 dB in
real life tests.

Are you really saying you "razor" has a measured
gain of ten dB over a comparable size yagi?

Either something is wrong with the reference
antenna or the test was seriously flawed. 10 dB
gain over a 5 element yagi would require an
enormous array!!!!!

>     Some notable contesters use(d) loop
antennas to cream competition
>     (W2PV,
> K6UA, VE3BMV, K3ZO, KC1XX, VK3MO) and their
experience confirms the
> benefits of loop antennas. Those who know ..

This sounds like the "science" used in the
infamous "Fractal Run" threads of RRAA! "I
worked "X" stations in "X" minutes so my antenna
must work better than anything else"!

Anyone who deals with statistics knows such data
is meaningless, there are far too many variables
other than gain involved in the results.

>     I am just now "investigating" loops with
vertical polarization for
>     the
> salty beaches, see what comes out. I am
already getting 2 dB from
> single loop over "mighty" dipole.

I modelled a vertical dipole over perfect
ground. The gain for low dipole heights was 7.3
dBi.

I did nothing but convert to a vertically
polarized quad at the exact same mean height,
and gain was 7.83 dBi.

In this case, where everything is equal, the
quad supplied a "roaring" .53 dB gain advantage.

I placed the antennas at the same height over
lossy ground, and they were equal!!!

73, Tom W8JI
w8ji@contesting.com

--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com