[TowerTalk] Flat top vs Inv Vee

[w8ji.tom]

Hi Dennis,

> I agree with Bill.  However, when I got EZNEC, the first thing I did was
> model inverted vees vs. dipoles.  The difference was startling, in favor
of
> the flat-top dipole.  I used the same center height for the inv-vee as
the
> flat-top height.  As the inverted-vee center gets higher than the dipole,
> the performance levels out, and bringing the ends up helps also. 
Different
> heights and frequencies will give vastly different results, but for the
> typical 80 meter antenna up 40 or 50 feet, the difference is substantial.

I don't know what quantity "startling" is. Perhaps the problem is my fault,
because I call a change "insignificant" while others correctly define the
very same change as "startling" or "substantial". 

Here's what EZnec sez for two wave angles:

Flat dipole at 30 feet, 8.4 dBi at 90 degrees and 2.6 dBi at 25 degrees.

Dipole with 30 ft apex and ends 3 feet off the ground is 7.5 dBi straight
up and 1.3 dBi at 25 degrees.

Pattern shapes are almost exactly the same! Bringing the ends to within
three feet of earth results in about 1 dB change, the smallest amount
perceptible to the average human in a controlled test. One dB is a few
hundred percent smaller change than the addition of a small counterpoise
system would make to the either antenna!

IMO, any worry about such minor "loss" is silly when compared to the
inconvenience. This is especially true since people never even bother
adding five or ten 120 foot wires on the ground below the antenna even
though that causes much greater improvement in field strength than any
change caused by making the dipole "flat" or "Vee'd".

At higher heights?

Flat dipole at 120 feet, -6.9 dBi straight up and 7.7 dBi at 25 degrees.

90 degree V angle 120 ft high apex dipole, -4.5 dBi straight up and 5.9 dBi
at 25 degrees.

90 degree V angle 145 ft high apex dipole, -7.3 dBi straight up and 6.3 dBi
at 25 degrees.

While this tiny change might be called "substantial", none of it would keep
me awake at night or cause me to erect a second tower just so the dipole
would be "flat"! While the flat dipole might look better and impress more
visitors, I would simply add 20 feet of apex height to the 120 ft high
dipole and save the few thousand bucks required to install a second tower.
This is one case where money spent on the antenna results in a VERY poor
return on investment.   

If someone's flat dipole is "smoking" a droopy dipole at the same height,
something else has to be at work. It could be ground conductivity,
propagation at the moment, rigs, operators, power, processing, feedlines,
other conductors around the antenna, or any number of things that would
amount to more than one dB or so of loss caused by "Veeing" the antenna 90
degrees (or less).

73 Tom

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