Well put Bill. You will find that the vertical
is better for long skip since the angle of
radiation, for a well counterpoised system,
is much lower than the inverted V. Using
a folded dipole will also lower the Q if that's
of any interest. You can actually use some
16 AWG window line (vel. 0.91), 14 AWG
window line (vel.fac. 0.95), or 300 Ohm twin-
lead (vel.0.85) and shorten the antenna. This
is necessary compared to a bare wire dipole
which has a vel. fac. of 0.99 or 1.0 for practical
purposes. BTW, the line losses are going to be
inversely proportional to the vel. fac. as the
dielectric losses increase with closer spacing.
Gary, K5AMH
On 1/11/2016 10:12, Bill Crowell via BVARC wrote:
Michael,
You have several factors with a dipole vs. vertical:
1. Noise tends to be vertically polarized. Thus a vertical antenna is
in the incident wave of noise sources and couples more readily.
2. People tend to think of the dirt and “ground” as if the soil were
the bottom of a metal box. We drive a ground rod into the earth
and proclaim that we are GROUNDED as if this was some virtuous
experience. Soil is, at best, like a big, spongy resistor. We do
our best to provide a reference for the radiator in a vertical to
work against and we do this with ground rods and radials. The
desired effect is to only reduce the RF impedance of whatever
ground we got.
3. The dipole works against itself. The E-field (electric) flows
along the wire and is 180 degrees out of phase at the center where
we put our feed point (in most cases). At 90 degrees is the
M-field (magnetic). These loop through the air back to each-other
as a big bubble of energy. The vertical has a more difficult time
of doing this because of variations in the counterpoise.
4. The dipole is further aided by very high Q or “quality” by virtue
of the wires. The ratio of the wire diameter to the wavelength is
very, very small – especially on 80 meters. Because of this, the
antenna is not as broad-banded as most verticals tend to be.
So, you’ve set up a much better antenna with higher Q and put it 90
degrees out of the plane of the (most) noise sources :)
73!
Bill Crowell, N4HPG
Pearland, TX
Text messaging one-handed since 1982
From: BVARC <[email protected] <mailto:[email protected]>>
on behalf of BVARC <[email protected] <mailto:[email protected]>>
Reply-To: BVARC <[email protected] <mailto:[email protected]>>
Date: Monday, January 11, 2016 at 9:51 AM
To: BVARC <[email protected] <mailto:[email protected]>>
Cc: Michael Rapp <[email protected] <mailto:[email protected]>>
Subject: [BVARC] Why is my dipole quieter than my vertical?
Hey all,
Now that I have multiple antennas up, it is interesting to compare
them. For nearly four years now, my wire vertical (30 radials) has
been my workhorse antenna. This past July I also put up an inverted-V
multi-band dipole, cut for 80 meters and fed with 450 ohm line. The
apex is about 20 feet up.
One thing that I've noticed is that, especially on 20 meters, my
dipole is quieter than my vertical (which is cut for that band). When
I say quieter I mean that when using an A-B switch the volume of the
background noise is quieter on the dipole. I began to wonder is it
just the noise or is it the noise /and/ the signals being attenuated
on the dipole?
With the little equipment and knowledge that I have, doing an
objective test is challenging. I've been using JT65 as it gives
fairly objective signal reports. Being very careful to keep the net
sound card level input the same between antennas, it appears that for
domestic signals on the dipole I can consistently decode down to about
-27 dB whereas on the vertical I seem to be only able to consistently
decode down to -22 dB.
This seems to suggest that the signal-to-noise ratio of the dipole is
better than that of the vertical, which suggests that the dipole is
less sensitive to noise -- unwanted signals -- than the vertical.
Why would this be?
What I've been able to research/Google has been confusing. Some
suggest that it is because most noise sources are vertical polarized.
(Are they?) Others suggest that because half of my vertical antenna
is in the ground (the radials) this does...something... to make it
more susceptible to noise. And others point to the dipole having a
little bit of directivity contributing to its nulling out, probably by
coincidence, some of my noise sources.
73,
--
/*/-=[Michael / KT5MR]-=/*/
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