[Elecraft] Trap dipole fed with ladder line?

[Ron D'Eau Claire]

Something else to consider beyond Ignacy's (NO9E) observations is the
height of the horizontal antenna above ground. As the height of a
horizontal drops well below 1/2 wavelength, it becomes a NVIS or "cloud
warmer" showing perhaps 6 dB or more gain straight up. That's great for
short skip contacts, but there is little  ground wave local contacts and
little low-angle radiation for DX. So if you can't get your horizontal
antenna up 50 feet or so, it might not make sense on 80 meters. You need
to get a horizontal antenna up closer to 100 feet or more to be
efficient for anything beyond short skip on 160 meters .

Of course, that's why verticals are so popular on 80 and 160 meters
among those of us who don't have 100 foot supports in the yard. 

If the horizontal antenna is designed for 40 through 10 meters, it is
easy to establish a length that is efficient across that whole range
without breaking up onto a lot of high-gain lobes pointing every which
way. My doublet is 42 feet long, which fits my space and happens to be
exactly what Moxon recommends for such an antenna. If you can get it up
50 or 60 feet it will be a star performer on 40 through 10. Keep in mind
that there are no "nulls" (meaning no signal)  off of the end of a
real-world doublet. There are areas of more or less "gain", but the half
wave antennas I have modeled show more gain off of the ends than a
vertical does in any direction. That's why verticals got their
undeserved reputation for radiating poorly in all directions. Again,
that assumes the horizontal antenna is up high enough. At lower heights
the dipole pattern of the horizontal breaks down and becomes
omni-directional like a vertical. The vertical shows more gain than the
dipole at useful low angles of radiation. 

Does shortening the antenna reduce its efficiency at 7 MHz? That's not a
simple question because the angle at which radiation is measured is
important. Using 35 degrees, a doublet 66 feet long and 30 feet up has
0.25 dB more gain on 40 meters than a 42 foot doublet. That's about
1/24th of an S-unit difference. It's not going to make any difference at
all on the bands. 

What about directivity? At a height of 60 feet, the difference from the
direction of maximum gain off of the sides and the 'nulls' off the ends
is almost 6 dB or one S-Unit. At 30 feet height the difference drops to
3 dB. In the practical world of on-air operations on the ham bands the
directivity from a horizontal antenna at 1/2 wavelength up or less is
not likely to be detectable. Typical QSB will be much greater than the
difference in signal strengths measured from side to end of the doublet.


The directivity gets serious up on 10 meters. A 42 foot doublet still
only has two main lobes - one off of each side of the antenna, but at 30
feet it shows 9 dbi gain in those lobes! That makes your K2 running at
10 watts output equal to a 100 watt rig into a vertical. That's a lot of
signal from a simple wire. But like all directional antennas, gain in
one direction means less in others. At 10 meters, the signals off of the
sides of the doublet can be as much as 18 dB below the gain in the most
favored directions. That's  3 S-units!  "Gain" in an antenna in one
direction always comes at the expense of "gain" in some other direction.


So now you have a great homebrew (and CHEAP!) doublet for 40 through 10.
I feed mine with open wire line. But what about a vertical for 80 and
160? Make the best ground that you can (elevated counterpoise, ground
mats, radials on the ground - the options and opinions about which is
best are the stuff of endless discussion since Marconi threw up his
first wire). Tie the feeders of your 60-foot high doublet together and
load it as a vertical against ground! The horizontal doublet wire at the
top adds some "top loading" which, as Charles pointed out, raises the
radiation resistance - especially up on 160. You will have an efficient
80 meter vertical and perhaps a star on 160 as well, depending upon how
good of a ground system you can arrange.

Ron AC7AC
K2 # 1289