My point is that according to EZNEC 3.0, a dipole at 1/2 wavelength above
ground over salt water will have its major lobe at
30 degrees above the horizon and have a gain of 8.36 dBi.
15 degrees it has a gain of 5.62 dBi.,
14 degrees 5.16 dBi,
13 degrees 4.65 dBi,
12 degrees 4.08 dBi,
11 degrees 3.43 dBi,
10 degrees 2.71 dBi.
5 degrees -2.94 dBi
A 1/4 wavelength vertical over salt water will have it's major lobe at
10 degrees above the horizon and a gain of 3.97 dBi.
15 degrees it has a gain of 3.88 dBi
5 degrees 3.68 dBi
So if the radiation angle is above 12 degrees the dipole has the advantage,
below 11 degrees the vertical has the advantage.
No go to W6EL prop and put 11.5 degrees in as your Minimum Radiation Angle
with an SFI of 71 and a K of 1 look at the prediction for the G to KH6 path,
11605 km, mostly over salt water.
At 0700 the probability for 14.1 MHz is 2 D. (2 dB above the noise but not
likely to have propagation)
Click on Advanced and scroll to 0700Z on 14.1 MHz and you will see it takes
7 F layer hops for communication.
Now put 5 degrees in as your Minimum Radiation Angle and run the same
prediction.
At 0700 the probability for 14.1 MHz is 17 C. 17 dB above the noise and
propagation slightly possible.
So how come a dipole with 8.36 dBi gain will have 15 dB less signal strength
than a vertical with only -2.94 dBi gain on this path?
Click on Advanced and scroll to 0700Z on 14.1 MHz and you will see it only
takes 6 F layer hops now. That 1 F layer hop accounts for 15 - 8.36 + 2.94
for 9.58 dB gain.
And if you compare the 30 degree takeoff angle of the dipole to the 10
degrees of the vertical you will see no possibility for propagation with the
dipole and 14 dB above the noise and a slight chance of propagation with the
vertical.
Click on advanced and you will see the vertical still takes 6 F layer hops.
Now drop down to 5 degrees and you have 17 dB above the noise and still a
slight chance of propagation.
And we are down to 5 F layer hops.
The answer is in the F layer hops that cannot be put into EZNEC.
de Paul, W8AEF
ZF2JI/ZF2TA FO8DX/FO8PLA 8Q7AA XZ0A VU7RG TX5C
----- Original Message -----
From: "Steve Hunt" <steve@karinya.net>
To: <towertalk@contesting.com>
Sent: Sunday, April 05, 2009 2:54 PM
Subject: Re: [TowerTalk] EZNEC- needs improvement
> Paul,
>
> I'm not quite sure what point you are making.
>
> The dipole doesn't just "cut off" below some angle. Let's suppose the
> vertical had a 10db advantage over the dipole at 6 degrees, and the
> dipole beat the vertical by 10dB at 23 degrees. If your point is that
> the signal strength on the 6 degrees path is very much stronger (say by
> 20dB) than that on the 23 degree path, the vertical will still only show
> a 10dB advantage because the 6 degree path is dominant for both antennas.
>
> Or have I misunderstood what you are saying?
>
> Steve G3TXQ
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|