It is not just the quality of the ground, but rather the proximity to
the ground. Ground losses are present for a low horizontal antenna
regardless of the quality of the ground.
For a vertical antenna, the ground quality is what matters, hence the
need for radials.
Model a dipole in free space and note the maximum and average gain of
the antenna, then model the same antenna at 0.05 wavelength above
perfect ground and compare the gain. Changing the ground quality from
perfect to normal to poor will also influence the loss.
At low heights the radiation pattern will totally change from free
space, with most of the radiation going straight up. Not necessarily a
bad thing on 160 and 80 metres.
One of the tricks for tropical shortwave broadcasters is to bury a
radial under a dipole and then optimize the height of the dipole to get
the desired radiation pattern. Usually these stations are looking for a
coverage area within a few hundred miles of the station, and a radiation
pattern directly up is what is desired.
Darrell Bellerive
On 08/03/2013 08:17 AM, k6jek wrote:
How big are the ground losses? Are they the difference between an
EZNEC prediction over perfect ground and what it says over average
ground assuming average is what you have? Is it the difference
between the model's prediction of impedance over perfect ground and
what you actually measure as Bob Orr said in his book years ago, for
example, the model says the impedance should be 35 +jx over perfect
ground and you measure 50 +jx, at the feed point, that's 15 ohms of
loss?
Jon
On Aug 3, 2013, at 7:46 AM, Darrell Bellerive wrote:
The other factor that I don't think has been mentioned is ground
loss of the transmitted signal. For horizontal antennas, antenna
height is the major factor, and for vertical antennas, radials.
The impact of ground loss on a low horizontal antenna on the 160
and 80 metre bands will be significant compared to feedline
losses. A dipole at 25 to 30 feet above ground for 160 metres is
only 0.05 of a wavelength high. As a horizontal antenna is lowered
below 1/4 wavelength above ground, ground losses increase
significantly.
Remember though that lots of hams make lots of contacts with low
dipoles, and any antenna is better than no antenna. So like Bob
has stated, striving to eliminate that last dB of feedline, tuner,
or balun loss may be insignificant compared to other factors. We
need to be mindful of our complete system of transmitter power,
losses, propagation, etc. and the impact each has on our
transmitted signal.
73, Darrell VA7TO
Darrell Bellerive
On 08/03/2013 06:37 AM, Bob McGraw - K4TAX wrote:
Steve et al:
I'm not saying that loss does or does not change with the vinyl
type window line between wet and dry. I do agree with your
results in that loss does increase with a wet line as opposed to
a dry line. I also agree that loss is greater per unit at 28
MHz vs. the same length of line at 1.8 MHz or 3.8 MHz regardless
if the line is wet or dry.
My point, with today's receivers, in most all cases the
atmospheric noise and man made noise will mask any receiver
internal noise and will easily overtake any loss in the
transmission line. However, the loss in the transmission line
will affect the NF of the receiver, which on HF is of little
significance. In many cases, we worry about 2 or 3 dB loss in
the transmission line but run the attenuator of 10 dB to 20 dB
at the input of the receiver. Now on transmit, that point makes
a different in the power arriving at the antenna. Again,
typically less than 1 S unit on the other end. To that point,
most of the time I run the Argonaut VI at 10 watts and can work
about any station I hear, regardless of line loss.
True open wire line, by definition, is two conductors supported
only at the source end and the termination end, drawn taught,
and without any spacers. This of course is a real challenge to
make work reliably in practice unless one uses large conductors
and spaced at 6" to 18" and used at lower frequencies and
typically with very high power in the near megawatt range. We
used this feed line approach in some of the commercial SW
stations to which I attended. Some of these feed lines were each
several thousand feet in length. All of this is far beyond the
scope of most ham installations.
I would like to see more data on dry line vs. wet line from
natural cause as opposed to "wetted" line. I use the vinyl
covered line with 66% of the web spacers removed. {Remove 2,
leave 1, remove 2, leave 1.} I see little change from wet to dry
on HF.
73 Bob, K4TAX
----- Original Message ----- From: "Steve Hunt"
<steve@karinya.net> To: "Discussion of Ten-Tec Equipment"
<tentec@contesting.com> Cc: "Phil Sussman"
<psussman@pactor.com>; "Bob McGraw - K4TAX" <RMcGraw@Blomand.net>
Sent: Saturday, August 03, 2013 8:01 AM Subject: Re: [TenTec] OT:
Openwire/Window Line and Bad Wx
We're talking here about reported changes in loss that - if
true - would be equivalent to a 5dB change between dry and wet
on a 100ft of ladderline feeding a doublet on 10m.
Are you folks trying to tell me that 5dB makes "little to no
difference"?
Steve G3TXQ
On 03/08/2013 13:27, Phil Sussman wrote:
Bob is right! In the end, propagation will dictate. External
conditions have more of an effect than the subtle
differences over which we have control.
Sure we can increase efficiency, yet the results are subtle.
It all depends upon whether the band is open, eh?
Well said, Bob!
73 de Phil - N8PS
------
Quoting Bob McGraw - K4TAX <RMcGraw@Blomand.net>:
As I said in my closing remark in an earlier post:
"I realize that we'd like to eak out every dB we can, but
in the end, it makes little to no difference on HF."
If one can match the load, using what ever means and
equipment, then energy will be transferred. On receiving,
atomospheric and man made noise will overtake any losses
in the antenna system and will over ride most all receiver
noise.
73 Bob, K4TAX
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