[Elecraft] Skunks and Antennas (WAS:] WA3WSJ's EPA Skunk WorksSite - Check it out!)

[Ron D'Eau Claire]

Walk, KE7GWZ asked:

The ridge line (on some poles for a touch more height) is quite doable and
could easily feed into where my radio will one day be. However, I'm a little
concerned about the signal going too vertical and having a bad angle on the
horizon.

Should I be considering a vertical antenna if I really want to DX?

--------------------------------

How far above the ground would your ridge wire be?

What bands do you want to work most? 

Contrary to what people are likely to say, world-wide DX does NOT go away
now, but it can get harder to find for many of us urban dwellers. The reason
is that the DX will be on lower frequencies where we are unable to erect
efficient antennas.

First of all, let's consider a good antenna for making lots of reliable QSOs
independent of almost all sun conditions. A horizontal antenna from about
0.1 to 0.4 wavelengths above the ground will provide that out to 1,000 miles
or so. So if your ridge antenna is up 30 feet, it will do a credible job for
short skip on 80 and 40. It will provide good DX on 20 meters and higher
frequencies. 

Where you will find he most DX during the sunspot minimum will be at the
lower frequencies. 160 meters will see regular openings, 80 will see a lot
and they grow less and less frequent as the frequency increases. That
doesn't mean that 20 meters will not have lots of DX, it just isn't as
regular as before.

A funny thing about the way propagation works, the most efficient
frequencies for DX are those very close to the Maximum Usable Frequency
(MUF). During sunspot maxima that is most often up near 30 MHz. At the same
time the losses in the atmosphere caused by RF absorption  by ions created
by solar radiation decrease as frequency increases. That's why you never
hear DX on the standard broadcast band (550 kHz - 1700 kHz) at any point in
the sunspot cycle except at night, and then you can hear DX if you find a
clear channel at any point in the sunspot cycle. It's almost independent of
the sunspots. 160 meters is a lot like that, 80 less, etc. 

So when the sunspots move the MUF up to the higher frequencies, the DX is
more common but it by no means goes away at sunspot minimum. It just moves
to lower frequencies and, because of the ionization produced by sunshine
during daylight hours, DX occurs only at night on the lower frequencies. 

It's interesting to look at the old magazine articles as the short waves
were "discovered" by Hams and the big news wasn't "DX", it was "Daylight
DX!" The ranges worked weren't what was so surprising, it was that DX could
be heard during daylight for the first time in the whole history of radio. 

But, to get a good horizontal DX antenna on 80 meters, you need one about
100 feet above the ground! For 160 meters it needs to be closer to twice
that high! Few of us can even consider such a high antenna. That's where
verticals come in to play. 

A horizontal antenna at ideal height has a gain of about 5 or 6 dB over an
"isotropic" antenna at angles useful for DX. (An isotropic is a fictional
antenna of zero size that is convenient to use for comparing antenna
designs.) So your horizontal at a good height will make your effective
radiated power about 4 times greater. A K2/100 will radiate as much as a 400
watt rig feeding a 0 dBi (the "i" refers to an isotropic radiator) antenna. 

Since few of us can put up horizontal antennas at ideal heights on 80 or
160, we consider verticals. Verticals do not provide the gain a horizontal
antenna has. Indeed, they frequently have about 0 dbi or even a little less.
Although many theoretical antenna models show that a vertical has maximum
gain at the horizon (0 degrees elevation) that is not the case in practice.
That will occur only if there are no ground losses. Vertical induce larger
currents in the  ground that are mostly converted into heat (unless the
station has the good fortune to live on a sheet of metal extending many
thousands of feet in all directions, or perhaps second best is to be on the
salt-water oceans). Almost all radiation from a vertical below about 20
degrees is lost to the ground. Theses are "far field" losses, occurring many
wavelengths from the antenna, so the largest field of radials won't help. 

Bottom line, we live with the fact that a good vertical has about 6 dB less
gain at useful angles than a horizontal at ideal heights: our effective
radiated power is about 1/4 what it would be with that ideal horizontal
antenna. 

Now, there's another loss issue with verticals. If the vertical is not a
dipole (it is not 1/2 wavelength long) it will depend on currents flowing
into the ground for efficient operation. These currents increase rapidly as
the antenna is made shorter. They are fairly large when the vertical is 1/4
wave long, and  grow extremely fast as the length drops below 1/4
wavelength. These are physical, not electrical lengths. That is, on 80
meters a 66 foot tall vertical will show a base impedance of about 35 ohms
and be self-resonant when worked against ground. If you make it shorter, say
33 feet tall, you can use loading coils to bring it back to resonance but
the impedance will be much less than 35 ohms. The ground resistance is in
series with the antenna impedance, so the power is shared between them. For
example, if you have an excellent ground with only 35 ohms resistance, it
will consume 3 dB - half of our RF - leaving only half of what's left to be
radiated. A 35-ohm ground is a good ground indeed. Many radial system are
lucky to be that good. Your ground resistance can be 10 times that in many
installations. 

So, assuming a 35 ohm ground, you now have an antenna that radiates 9 dB or
more less than a horizontal at ideal height. The output from a K2/100 is no
longer equal to 400 watts into an isotropic antenna like it would be into a
horizontal at ideal height, it's now equal to 40 watts into an isotropic
antenna into a typical 1/4 wave vertical. Quite a drop! But it's one we are
often compelled to live with in our urban lots. And on 160 it's often worse
as we must live with an ever-shorter antenna. 

And that's why most Hams mourn the loss of the active sunspots. The DX is
still there but most of us lack the resources or the real estate to reach it
as reliably as we can on the higher frequencies. But it is there and a lot
of Hams can find it with perseverance. It's just harder. And keep in mind
that the higher frequencies will continue to see DX openings. The sunspot
minimum doesn't mean the higher frequencies are dead, they will simply be
open less often than before. But when they are the DX will roll in. 

On my tiny lot, I'll continue to use my 65 foot doublet on 80 meters and up.
On 80 and 40 it will provide me with many contacts out to 1,000 to 2,000
miles just about any evening. When 30, 20 or higher bands are open, I can
work the world. I'll just do a little less "working the world" over the next
few years, but Ham radio will be just as fun! 

Ron AC7AC