-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Richards
Sent: Samstag, 6. Dezember 2008 05:11
To: Peter Voelpel
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] vertical antenna ground loss
Hi Peter --
1)
I agree that dipoles at low hight have less gain than the vertical.
That comports with what I said.
2)
I am not considering what a dipole does at 1 wavelength or even at 1/2
wavelength - as that is not realistic or feasible for nearly
all hams on 160, 80, or 40 meters. This discussion is primarily
about a 160 meter Inverted L - which is kinda like a combo
vertical-horizontal antenna and there is no way we are going to be able to
hang a dipole at one wavelength height. Thus, I consider that an
inappropriate comparison or measuring stick in this case.
3)
I respectfully disagree with your claim take off angles are manifested by
the ionosphere.
When researching a big vertical for my back yard, several Extras did
modeling for me comparing the 45 foot vertical with 80, 40, and 20 meter
dipoles at various realistic heights above average ground, using NEC, EZNEC,
and HTFA modeling
programs. These all confirmed what I said, and is the basis for
what I said.
Moreover, I do not believe your statement is correct. I believe the
take off angle is not a function of the ionosphere... it is a function of
the antenna over the ground (i.e. terra firma or the dirt below.) After the
radio wave emanates or radiates from the antenna, then it might interact
with the upper atmosphere, but it has to radiate first, and that is a
function of antenna design, materials, and relationship to objects, such as
good Ol' Earth.
In all the cases studied and modeled, the vertical antenna had a lower
radiation pattern than a low hanging dipole (it is like fairly impossible
for me to get a 1/2 wave 40 meter dipole up 1/2 wave over the ground, so we
used different realistic values, such as
that dipole at 25 feet, 40 feet, and 60 feet, respectively. We
compared the take off angles and radiation patterns with the
angles and patterns for the 45 foot monopole vertical. We
modeled the same sort of setup for 160 meters, 80 meters, 40 meters, and 20
meters.
In all cases the dipole radiated at a slightly higher angle than the
vertical, which is why the vertical is a better choice, sometimes, than the
dipole for low band DXing.
On the higher bands, the patters seemed to reverse for all
dipoles. The upper bands on the vertical radiated at slightly
higher angles and with higher take off patterns than the dipoles on those
bands, favoring the dipoles for DX on those bands. \
It is very often said that such large monopoles tend to be "cloud warmers"
on the upper bands, and good DX performers on the
lower bands. This was born out by the modeling charts. It also
coincided with what I have been reading in the ARRL Antenna Book when
comparing the take off angles and radiation patterns of dipoles and
verticals.
Note: We did not discuss the performance of "theoretical" dipoles hung at
1/2 wavelength heights... that is not realistic at my QTH.
But I really do not think the radiation patters are affected by the
ionosphere - it is way up there in near space, and the antenna radiates down
here on earth according to its design, materials,
and proximity to other objects. The radiation pattern is already
established and a moot point by the time the radio wave reaches the
ionosphere. How can the ionosphere shape the take off angle?
That's my take anyway... Happy trails -- interesting discussion.
============== Richards -K8JHR ================
Hi Richards,
1) As you talked about high bands I described why verticals are preferable
on the low bands (dipoles not high enough) as well.
2) 1/2 wave high dipoles on 40m are quite common, but still not high enough
(30° lobe) for really low angle radiation.
3) I don´t mean lobe forming at the antenna, but the necessary take off
angle for a certain dx path is depending on the ionosphere.
When you try to use the same low band vertical for the high bands then of
course the take off angle is raised and the antenna becomes useless for dx.
Verticals behave the same on low bands and high bands if they are the
same fraction of the wavelength tall.
Of course the ionosphere does not influence the pattern/lobe of an
antenna but the angle which is used for the communication path.
73
Peter
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