Topband: Noise and reception

[Tom Rauch]

Hi David,

> It does not bother me that the horizontally-polarized *ground wave*
> does not exist/ is not supported by the earth. I am trying to receive
> horizontally-polarized *skywave* signals, and to the maximum extent
> possible, reject all vertically-polarized signals, including noise.

Most DX signals, if not nearly all signals at a modest or greater 
distances, are vertically polarized when near earth. That is why our 
"noise" is substantially vertically polarized, and signals are no 
different. This is why Flags, Pennants, Beverages, and arrays of 
small verticals work so well, and have evolved into the antennas of 
choice.

This is why large horizontally polarized antenna systems are rarely 
found, and why they have not fell into wide favor. Simply put, low 
horizontally polarized antennas are much more difficult to make 
work.    

The reason for this is really pretty simple, the earth itself is a huge 
filter. This is the reason we can not measure wave angle near earth 
with a small loop or rod antenna. 

For example, we can not simply take a low rod or loop antenna and 
"null" a signal to see the arriving polarization or wave angle. The 
earth not only modifies the pattern of the measurement antenna, it 
"modifies the signal" also. 

> There is a very big difference between the idealized (?) elevation
> pattern published by K6STI and the one you describe, which is that of
> a 'cloud warmer'. I can't help believing that my antenna must be
> accomplishing this, at least to some degree. 

I don't think I described a pattern, and certainly can't find where I 
gave any advice one way or another about NOT installing an 
antenna. Certainly if someone has limited resources, they should 
try as many things as possible.

My transgression appears to have been stating low horizontal 
antennas have a tendency to NOT want to work in the horizontally 
polarized mode. I also said we have to be extraordinarily careful to 
make sure the antenna is properly balanced (and that means the 
feedline and surrounding structures are not acting like an antenna), 
and that may be difficult or nearly impossible to do in most cases.

If you see a technical reason why that is not true, point it out.

For example, a  Beverage is a low horizontal wire "forced" to 
respond to vertically polarized signals by the presence of the lossy 
earth below the antenna. The efficiency is terrible. Virtually ALL of 
the feedpoint resistance is in the form of loss resistance, radiation 
resistance is around one ohm because we have forced the antenna 
to radiate in a mode contrary to the pattern of each section!

A lot of 80/160 prop is
> apparently at high angles FWIW. 

I wonder where that idea comes from? How can we know that or 
prove that in a hands-on test?

At the moment, I have a dipole at 300 plus feet and a dipole at ~80 
feet among other antennas. The dipole at 80 feet is a real dog 
virtually 100% of the time at any distance over 200 miles unless 
the band is in a strong peak. While I can make long distance 
contacts on it, it is perhaps 20 or more dB down from the other 
antennas when distances are over 1000 miles.

At my station an omni-vertical with a good ground system always 
beats the high dipole off the ends of the dipole by a large amount, 
although they are at times similar in performance broadside to the 
high dipole.

The low dipole always, unless I am working inside a few hundred 
mile radius, is substantially poorer than the high dipole or vertical.

That's true both for receive and transmit, DX signals that are 
perfectly readable on the vertical and high dipole just "disappear" 
on the low dipole. My direction vertical arrays and Beverages 
**aways** hear things the dipoles do not, when it is some 
reasonable distance away. 

Logic tells me if the signal was horizontally polarized and at 
modest to high angles, the low dipole would be competitive a large 
percentage of the time...yet it is never competitive when working 
distances beyond a few hundred miles.

I am not claiming in less-than-ideal cases this may not change 
around. For example, if I had several dB more loss in my vertical or 
other structures making the vertical have a null both dipoles would 
certainly appear much better against the weaker vertical. If I only 
tested during the peak, when I can hear signals "well" on the low 
dipole, the low dipole would certainly look much better. 

This is how it stacks up in a nearly ideal installation where only the 
antenna differences matter. Nothing indicates the wave angle is 
either high or predominantly horizontal.

Also, K6STI said the antenna worked
> about as well at 10 feet and 50 feet, i.e. that there must not be very
> much change in the shape of the elevation plot as height is varied (?)
> He actually addressed signal strength vs. height, however, if I
> remember right. Is 10 feet a "few feet above earth" or not? 

The effect was there even at 30 feet when I experimented with low 
antennas that were horizontally polarized.

> Another thought- if I take a half wave dipole or a full wave (large)
> loop and put it very close to earth, 5-10 feet, it does not go deaf.
> It is a cloud warmer, of course, and makes a good NVIS antenna.

Any antenna is better than no antenna, so "good" is a very 
subjective description. 

One clue about how well a low dipole works is to look at the 
feedpoint resistance. Without loss, feedpoint resistance should 
constantly decrease as the antenna is brought close to earth. At 
ten feet of height, a 160-m dipole should have 3 ohms of feedpoint 
resistance if the earth is lossless. The actual impedance of such a 
low dipole is typically 50 ohms or more, and more than 15dB loss 
is common. I would call that a "good" cloud warmer, but it is 
definitely a good sod warmer!

With horizontally polarized antennas, reducing height below .1 wl 
causes an ever-increasing reduction in efficiency. A dipole at ten 
feet on 160 (.02 wavelength) is generally a very lossy system.

It is safe to say we find very few few big signals, even at close 
distances, from dipoles 10 feet high on 160, five feet high on 80, 
2.5 feet high on 40...(you should get the idea).

We would never consider placing a dipole in front of a nearly 
lossless screen reflector at .02wl spacing, everything tells us it is a 
bad idea because losses get so high. With that in mind, why would 
we think it would work when the nearly lossless screen is replaced 
with something as lossy as dirt?
73, Tom W8JI
W8JI@contesting.com