[TowerTalk] Hi-Tech Station Cost

[Tom Rauch]

Hi Tom and all,

>  Lindsay made his models around 400 MHz where the
>  matching efficiency to his quads was better than to his
>  Yagi's.  Today's concensus is closer to 1 dB.  See the
>  chapter on loop antennas in the W2PV Yagi Antenna Book.
>   de  N4KG

Let me try this again this way.

A quad element is two current maxima's fed in phase separated by 
a certain distance. The two current maxima's occur because the 
quad element is a bent dipole over a bent dipole.

Look at any gain chart for broadside spacing and you'll see with 1/4 
wl separation gain is under two dB.

Look at any stacking gain chart for yagi's (a multi-element quad is 
really two "bent-element yagi's" separated 1/4 wl), and you will see 
the stacking gain decreases with yagi length and is almost ZERO 
for long yagi's.

Since we all know an antenna gets gain by forcing a null in one or 
more directions, look at where the quad forces a null. Because 
spacing is 90 degrees, and phase is 180 degrees, in the far-field 
the fields above and below the element are 90 degrees out of 
phase. There is a very shallow null in line with two current 
maximas, with the minimum nulling at right angles to a line through 
the current maximas.

If we put a conventional quad element over ground, at a mean 
height of 1/2, 1, 1-1/2, and so on wavelengths, the stacking 
distance will be trying to force a null where ground reflection 
already forces a null, and gain from using a quad will be minimized.

(If you add a null to a null, there is no gain increase. The deeper 
null has control, and that is the null caused by ground reflections)

Same thing if we make the antenna long. The more elements we 
add, the less important a null straight up becomes because the 
end-fire elements null the same area where the quad has a 
radiation minima.

If you model the antenna, you will see it works this way. It's simple 
basic vector addition of fields. There is no magic. The gain 
advantage isn't fixed at X dB, it CAN'T be. It is maximum for a 
single element in freespace (under 2 dB), and decreases with more 
elements or when the antenna is placed over earth at certain 
heights.        

73, Tom W8JI
w8ji@contesting.com

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