[TowerTalk] Inverted Vees

[jimlux]

On 6/15/20 1:45 PM, Jim Brown wrote:
> On 6/15/2020 11:18 AM, john at kk9a.com wrote:
>> Also an inverted V does not have the big nulls that a flat dipole has 
>> making the inverted V's orientation is less critical.
> 
> Calling the nulls "big" is not accurate -- the nulls ARE deep, but they 
> are also relatively narrow in angle. i have dipoles at right angles to 
> each other for 80 and 40 at 120 ft. On any given signal, I rarely see 
> more than 10 dB difference between them, and I work a lot of east coast 
> stations on 80M on the antenna that's off the end to them. And I've 
> worked a lot of JAs on the antenna that's broadside to the east coast 
> and EU.
> 
>

I've been looking at this for analyzing deviations from perfect cross 
polarization for antenna imperfections.

for an idealized half wave dipole, the 10dB down point is at about 67 
degrees off broadside.
For a 1/4 wavelength (half length) dipole, it's at about 70.5 degrees, 
and for an infinitesimal dipole it's at 71.5 degrees

For 20 dB it's 82.7, 84, and 84.3.


That is, if you are 23 degrees off the end, your "depth of null" is 
10dB. To be 20 dB down, you've got to be about +/- 7 degrees.

The null is sharper for the smaller antenna, which seems somewhat 
counter intuitive.

These kinds of things are handy to know with people who seem to think 
that a 17 cm monopole sticking out of a 10x10x30 cm box somehow has 30 
dB nulls, or that a S-band patch on the side of the same box has no 
radiation to the back (because the same patch on an infinite ground 
plane has a radiation pattern all on one side of the plane).

My general response to radiation patterns from objects comparable to 
wavelength is "there are probably a couple of nulls, but they're 
narrow(single digit degrees), not real deep (30dB would be deep) and you 
can't predict where they will be without some time modeling"


One thing you also need to be aware of when working with nulls is that 
the slope of the gain vs angle is steep. For a half wave, at 10 degrees 
off the null (-18dB relative to broadside) the slope is 1 dB/degree