Verticals and Ground Systems
Submitted By Tom K1JJ, Authored by John N5CQ
There was one email that I thought was quite good...laced with personal experiences and general vertical theory.
If you have any interest at all in putting up a vertical and ground system, read this over.
The author, John, N5CQ gave me permission to post it in it's entirety.
It's long, but worth reading!
I am a Top Band enthusiast with a very poor ground at my QTH. I am
essentially on a limestone ridge. It is a fine location for the HF bands,
as I have nice sloping takeoffs in almost all directions (340 degrees or
so), but the ground conductivity is 1/1,000 of the soil in Dallas (which is
considered average) or 1/100,000 of Corpus Christi (good soil conductivity).
I have to put lots of area of copper strap out to just get a decent
lightning ground, but that's another story.
Here is my take on how to be effective on 160 DX from this kind of QTH. If I
have been too wordy I apologize in advance. Your mileage may vary.
There are two kinds of losses suffered when you have poor ground
conductivity and thus reflections: near field and far field.
NEAR FIELD ISSUES
With a quarter wave vertical antenna, in the near field (< 5 wavelengths
from the antenna) poor ground conductivity will cause a vertical to have
ground current I2R losses - you are burning up power in heating the poor
ground, not radiating a signal. Some people think of this as the ground
working as a "mirror" to create "the other half of the dipole" via the
reflected "image". The poorer the ground conductivity, this less perfect is
this mirror, and the less current can flow through the image and be lost
heating the ground. This is the kind of thing you can affect with buried
ground systems with lots of radials - when more current travels through the
radial system you get more radiation and less I2R loss. If you have good
ground conductivity, you can get "good" performance with 4 buried radials.
If you have an average ground, you will need 100 or more to get the same
performance. If you are over a very low conductivity soil, even a solid
copper disc out several wavelengths from the antenna will still not work as
well as the 4 buried radials over a good ground. Symptoms of this would be
a vertical with a broad SWR curve - it is really a dummy load. As you add
more and more radials, the SWR bandwidth should narrow as the Q of the
The same thing happens to horizontal antennas, except they do not need the
reflected "image" effect. But if they are "close" to the ground (less than
3 wavelengths high) they will still incur losses by being coupled to the
poor conductivity ground, i.e., some of the current will serve to heat the
ground, not radiate a signal. The higher the antenna, the less is this
FAR FIELD ISSUES
Further out from the antenna, the reflections from the ground either add or
subtract from the direct signal from the antenna, depending on the phase
relationships. The primary effect here is the height of the antenna
affecting the "takeoff angle". All else being equal, low (in terms of
number of wavelengths) antennas have higher take off angles, high antennas
have low angles. What works better depends on the "hop geometry": what mode
are we using, how high is the reflecting layer, are we going on 3 hops, or 4
or 10. For example, on 10M at noon at the top of the sunspot cycle,
Europeans are usually much louder on an antenna 30' high at my QTH, since
the hop modes at that time are usually high angle, multiple bounce paths.
On 20M in the early morning, working LP into VU land, for the most part, the
higher the antenna and the lower the angle, the better off you are.
On 160 for DX, you are almost always (0.99999) working in the lower is
better realm. For domestic contests, it's usually another story.
For verticals, there is also a mathematical construct called the "pseudo
Brewster angle". To greatly oversimplify this idea; the better your ground
conductivity, the lower the take off angle, and the poorer your ground, the
higher the take off angle. If you graph the vertical radiation pattern, it
almost looks like all the losses from ground conductivity take a "bite" out
of the low angle radiation instead of being spread out over all angles. So
even though verticals are thought to have inherently "low radiation angles",
in practice a vertical over poor ground suffers selective losses at the low
angles, and ends up with no more net radiation at the desired low angles
than a high horizontal dipole.
PUTTING IT TOGETHER
On 160, for DX, for takeoff angles, higher is better. For reducing ground
losses, higher is better. So we want them high on 160. It turns out that
for really poor soils, it is usually more economically attractive to use
high horizontal antennas than 1/4 wave verticals. What does that mean for
choosing real antennas?
At the W5 DX Bash 2 years ago, Dick Weber, K5IU, did a very well reasoned
and supported presentation on realistic choices for low band antennas on
real lots. He showed for 80M with below average or poor soils, a perfectly
tuned four square array of 1/4 wave verticals mounted on copper discs would
be only about 1 dB better than a 150' horizontal dipole that was "broad
side" to the desired direction. So it was much more cost effective to put up
two high dipoles and switch between them than to build the four square. On
160M the problem is even greater, in that the "pseudo Brewster angle" is
even higher for the verticals, and 300' would be the desired height for the
dipoles. Over good to great ground conductivity soils, IMHO, the situation
reverses and the ground mounted four square is worth the effort.
So what does all this mean about the "sloper" four square array?
There is no free lunch, but it is still a great choice for the "ground
conductivity challenged" 160M DXer. Even though you mount the sloper in a
"lazy V" configuration, and you hope it will have primarily a vertical
pattern (and for the most part, it does) you still have the coupling to the
ground issue and higher is still better.
I know of several people who have given up in frustration trying to set up a
sloper four square array fed with a ComTek box. I have seen great success
from the "K3LR" style array (see Antenna Compendium #5, I think), which is
also simpler to build, and offers an "unidirectional" option.
I have planned a sloper four square array for several years, but I have not
completed it yet, for several reasons. I started out with a single sloper
resonant about 1840 with the high end at about 198' and the low end at about
6'. There is considerable coupling to the ground. I have observed that
several "comparable" 80M sloper arrays work much better when they are moved
from 100' to 135' or so, bringing the lower ends up off the ground and
increasing the included angle between the two legs of the "dipole". This
would imply a 270' height for 160M, and I am not yet ready to tackle the
tower lighting issues involved.
Also, it seemed in theory a great use of resources to "drape" the sloper
array around the tower supporting several other antennas and get double duty
from the support. In practice, the maintenance on even a single sloper has
been a nightmare. I use very high quality Dacron support ropes and "flex
weave" antenna wire, but I am on a windy ridge and I still have 2 or 3
"incidents" per season where either the rope or the antenna breaks and
drapes itself across the elements of the horizontal antennas on the tower.
Sometimes (about 1/3 of the time in my experience) I can successfully
dislodge the sloper without climbing the tower, but more often it is a trip
up in cold weather. Even worse, if the break occurs and the antennas are
rotated before the fallen sloper problem is discovered, then the resulting
tangle of wires and elements can sometimes take three people to free! If I
had four sloper elements on that tower, and thus 8 to 12 such incidents per
season, I would not be able to handle it emotionally or physically!
So I am sticking to the single sloper for the time being, hoping to find a
great deal on the mythical used cellular monopole so I can have a dedicated
sloper support for the array, and if I can find one over 200', I will spring
for the lighting!
BTW, N5TW nearby has had much success with a 160M "sloper four square" array
he put up this fall, but the long term report is not in yet. He is not in
as windy as spot as I am, though.
Maybe I'll hear you on the air and we can talk further.
73 and good DX
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