This has been an interesting discussion.
I have two 40M/20M fan dipoles at right angles and they are in a slightly
inverted vee formation. The one that is broadside to NE/SW has the center
up 72'. The one that is broadside NW/SE has its center up 55'.
I also have an 80M OCF that is up 82' in the center and about 65' on each
end.
The comparative results are very interesting in the sense that as Jim Brown
explains the nulls can be very deep but also fairly sharp. I find it
interesting to switch around between these antennas on different bands and
different times of day because the best RX results are often not what you
would expect.
In addition to focusing on the directional peaks and nulls you also have to
think about the elevation angles that come into play. These angles, over
any particular path and band, can change from hour to hour and affect the
results that you get.
I often see the same things Jim does where a signal is stronger off the end
of one antenna than it is off the expected main lobe direction of another
antenna.
-------------------
Wes Attaway (N5WA)
(318) 393-3289 - Shreveport, LA
Computer/Cellphone Forensics
AttawayForensics.com
-------------------
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
jimlux
Sent: Monday, June 15, 2020 4:36 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Inverted Vees
On 6/15/20 1:45 PM, Jim Brown wrote:
> On 6/15/2020 11:18 AM, john@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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|