[Elecraft] Trap dipole fed with ladder line?

[Jeff Burns]

Is there an advantage to using a trap dipole over a simple doublet if they
are both fed with ladder line?

Until just recently I could not figure any good way to get ladder line
anyplace close to my radio. Thinking I would have to use coax I sent for
information on W9INN antennas. I was surprised to see he recommends using
ladder line with his multi-resonate antennas. If ladder line is being used
why not just use a simple doublet antenna? These are the reasons I thought
of:

1. To maintain a typical dipole radiation pattern on the upper bands rather
than breaking up into multiple lobes.

2. To raise the impedance on bands were the antenna is very short.

One more idea is to keep the impedance in a narrower rang. But the idea is
not valid because with changes in the length of the feed line the impedance
changes dramatically.

Now I have found a way to get open wire almost all the way to the radio so I
can consider using a simple doublet. In my case I can put up a wire up to
about 100-ft long. At one end it will be about 35 to 40 ft off the ground.
At the other end it will be 15 to 25 ft off the ground. The broad side of
the wire would be pointing at Europe.

Would there be any advantage to putting up one of W9INN's antennas over a
simple wire? I may be able to put up a second antenna, but it would just be
a vertical or perhaps a short dipole at 90 to the first. Any point in adding
a vertical? I have a K2 with KAT2.

Jeff Burns
AD9T

[Dave Gingrich K9DC]

Post by Jeff Burns
Would there be any advantage to putting up one of W9INN's antennas over a
simple wire?

Hmmm... Helping W9INN make another boat payment comes to mind. But beyond
that, probably not.


================================================
Dave Gingrich, K9DC - Indianapolis, Indiana USA
K2 #2211, K1 #931, QRP-L #2376, ARS #1109,
FPQRP #389, IRLP 4730/5730, k9dc.ampr.org
================================================

[Ignacy Misztal]

W9INN dipole is a dipole with multiple coils. When fed by ladder line,
it makes a lot of sense as opposed to a flattop fed also by a ladder
line.

Flattop with ladderline:
Good efficiency at bands >70% of half-wave frequency; too many lobes at
4 times half-wave frequency. Possibly hard matching for full-wave
frequencies (Impedance as high as 2000 hard to match).

W9INN with coax:
Good efficiency within narrow segments at 50% of half-wave size, poor
efficiency outside those segments. Smaller size than above for lowest
frequency.

W9INN with ladderline:
Good efficiency with much broader segments at 50% of half-wave size,
Good pattern at high frequencies because coils in fact shorten the
antenna.

Summarizing, a flattop for 80m fed by ladderline will work well on
80-20m, with a 4-10db loss on 160m, not too well on 17-10m, and may be
hard to tune on 40, 20 and 10m. W9INN of the same size as 80m dipole
will work with minimal losses on 160-10m and will be easier to tune.

Ignacy, NO9E

[Stuart Rohre]

AT several recent ARRL Field Days, we use doublets of double Zepp
dimensions, center ladder line fed, and do NOT see any undesirable lobing at
four times frequency. IE, it appears that the lobes tend to fill in with
good propagation conditions causing incoming and outgoing signal refractions
of random nature.

Any coil loaded antenna is more lossy than the all wire doublet. Shortening
doublets to 60 per cent of a half wave is still highly efficient in the 90
per centiles per L. B. Cebik, on <cebik.com> antenna web site.
73, Stuart K5KVH

[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

[Ron D'Eau Claire]

OOps... sorry. In the next-to-last paragraph of my last post I meant to
say that the gain off of the ENDS of a 42 foot doublet at 30 feet up on
10 meters can be as much as 18 db below the gain off of the SIDES! Got
it backwards..

Ron AC7AC

[Stuart Rohre]

Jeff, while the reasons you cite are valid, keep in mind that any trap
antenna has more loss if using fixed traps of coils and caps than a straight
wire.

Also, a doublet can be shortened to 60 per cent of a half wave and still be
well up in the 90 per cent efficiency range.

The only advantage other than pattern preservation to trap dipole, is that
it is typically shortened for each band due to trap loading effect.

But by applying the 60 per cent shortening, and feeding with parallel low
loss line, you avoid the losses of LC traps. The use of linear stub
decoupling is another option, but will probably end up longer overall. The
Lattin antenna is one stub decoupled type of multibander. It is on the
G3YCC memorial web site, but has an error in the first 8 foot section. BOTH
sides of the parallel line used as conductor for the 8 foot should join the
feeder at the center insulator, on each side of it.

You will have lowest loss on lowest band with the shortened, parallel line
fed dipole wires. L. B. Cebik shows an 88 foot doublet as a good compromise
length to preserve patterns as well, for the higher bands.

Lumped constant traps are always a loss factor.
73, Stuart K5KVH