[TowerTalk] quads

[Tom Rauch]

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

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