I'm looking at another antenna for 20M. I've got a partially folded
indoor attic antenna which gets out fairly well, but it certainly is
directional.
The ARRL handbook has had this design in it since they invented
radio. It's often used at VHF, but seems a good candidate for this QTH.
It's a 1/4 wave element over 4 1/4 wave radials at 45 degrees off
horizontal, drooping to earth. My installation would be at ground level,
with the base of the radiating element about 12.5 feet of the earth.
Mechanically, it would be easy to put together, but I hate to waste
the time if it's known to be a poor performer. I tried to use the EZNEC
4 demo, but 55 segments aren't enough to create a reasonable model of
the physical layout, and I'm not smart enough to try a simplification.
So, has anyone done an analysis of this antenna? Would the
performance be similar to a low mounted vertical dipole, or would the 4
radials offer a bit better performance so close to earth?
If there's an literature I missed, please point me toward it. Most of
the antenna manufacturers refer to regular flat, on the ground radial
systems, and except for some CB literature which has no technical basis
for its recommendations, there's not any analytical data I can find. The
best thing I found was at web.wt.net/~nm5k/acompari.htm for 10m
applications, at 40 ft elevation.

73,
W1KF
Steve

Hello Steve,

Yes, a quarter wave radiator fed over 4 quarter wave radials is a good
antenna, it performs almost equally as a vertical dipole. The main
advantage over the vertical dipole is that you can mount it on top of a
mast and you do not need a balun.

Because of the 4 quarter-wave radials, you will get a good virtual
ground avoiding common mode current into you 50 Ohm coaxial feeder and
or the mast. You do not need any further matching. Make sure that the
ends of the radials do not touch ground. If the ends are close to
ground, probably you have to shorten them a bit to account for the
capacitive effect caused by the ground. I would recommend you to higher
the feed point of the antenna.

When you want more gain and are able to do the construction work, you
may extend to close to a 5/8 wave, you only need a series coil to do
the matching. Using a halve wave radiator (fed over the 4 radials) is
also possible, but the impedance is in the kOhm range and therefore
matching is somewhat more difficult.

I hope this will help you.

Wim
PA3DJS

ORIGINAL MESSAGE:

On Sat, 30 Sep 2006 17:39:16 -0400, Steve Reinhardt
wrote:

Mechanically, it would be easy to put together, but I hate to waste
the time if it's known to be a poor performer.

------------ REPLY FOLLOWS ------------

A vertical as you describe is a good transmitting antenna but is
somewhat more susceptible to picking up local man made noise than is a
horizontal dipole. Local noise such as from motors, arcing, etc,
starts out with all polarizations but the horizontal component is
quickly attenuated by travel over the earth's surface. The vertical
component is not, and therefore is received by a vertically polarized
antenna. Because vertically polarized signals are not attenuated when
using groundwave mode, AM broadcast stations use it exclusively, or
very nearly so. They also use enough power to overcome the noise
pickup issue.

If you live in a relatively quiet environment, you may not notice the
difference, and the vertical will have a very low angle of radiation,
good for DX, and will probably outperform the horizontal in that
respect.

The best of both worlds is to have both antennas available and be able
to switch between them as needed. Highly recommended.

Bill, W6WRT

Steve Reinhardt wrote:
I'm looking at another antenna for 20M. I've got a partially folded
indoor attic antenna which gets out fairly well, but it certainly is
directional.
The ARRL handbook has had this design in it since they invented radio.
It's often used at VHF, but seems a good candidate for this QTH. It's a
1/4 wave element over 4 1/4 wave radials at 45 degrees off horizontal,
drooping to earth. My installation would be at ground level, with the
base of the radiating element about 12.5 feet of the earth.
Mechanically, it would be easy to put together, but I hate to waste
the time if it's known to be a poor performer. I tried to use the EZNEC
4 demo, but 55 segments aren't enough to create a reasonable model of
the physical layout, and I'm not smart enough to try a simplification.
So, has anyone done an analysis of this antenna? Would the performance
be similar to a low mounted vertical dipole, or would the 4 radials
offer a bit better performance so close to earth?
If there's an literature I missed, please point me toward it. Most of
the antenna manufacturers refer to regular flat, on the ground radial
systems, and except for some CB literature which has no technical basis
for its recommendations, there's not any analytical data I can find. The
best thing I found was at web.wt.net/~nm5k/acompari.htm for 10m
applications, at 40 ft elevation.

73,
W1KF
Steve

Why not look at the shortened vertical Dipole. Theres some construction
details in the 2006 Handbook.

Another good antenna is the Skeleton slot. Fed with open wire feedline
makes a good multi band low angle radiator.


Placing any vertical antenna high as the cb'ers do is just a waste of
good RF energy. You split the main lobes and get a lot of high stuff.
Which may be good for local stuff.

4 radials closes to earth might work if you have good earth
conductivity. Otherwise you will be losing a lot of your power heating
the earth.

A vertical on the ground needs 60 radials to be competitive with a
good dipole at 40 and above feet.

A vertical fed with high power works because its take off angle is
close to ideal as a general purpose DX antenna. I can thrash guys on 20
meters running low beams with a good vertical! Some just dont understand
how circuit gain is affected by takeoff angle. Its the difference
between the positive and negative gain on both ends this can amount to
10 to 20 db at low angles. Something guys with stacked yagis know of
very well.

If you soil conductivity around you is poor there no advantage to using
a longer vertical. Your take off angle is determined by the brewster
angle. The longer you make a vertical the better the ground system you
must have and further out it must be to prvent losses. In restricted
neighbourhoods its a waste of time!


Just operating in a urban environment with a vertical your signal will
be down 6 db over that same vertical in a open area. Urban areas have a
built in loss factor, this has been documented by the ITU. I sit on the
edge of urban sprawl and can kill stations with low yagis with a good
ground mounted vertical just because i am in the clear.


But suck it and see, try it. It does not cost much. Vertical antennas
are not inherently noisy in a quite location. Its just when you use a
vertical antenna in urban areas most of the major is intersecting and
passing directly through the noise source, so it seems noisier. Try a
vertical antenna in the country and compare it to a dipole they have
about the same signal to noise ratio.

My 2 Bits worth

Greg

Hello Steve

About 20m band antenna height.

The height of your antenna depends on what you want to do.

For "local" daily traffic (ground wave propagation [not confusing
surface wave propagation]) you want as much as possible signal strength
in the horizontal plane (low elevation). In that case height gives
stronger signal. No matter the polarization, over land (for very small
elevation angles) the ground reflected wave is opposite to the direct
wave (elevation (pseudo) Brewster Angle). More height gives some
more phase difference between direct ray and the ray that arrives after
ground reflection. The "two ray propagation model" is based on the
direct wave and ground reflected wave (for very small elevation). So
the CB people aren't that crazy.

When the height equals many wavelengths, you get many lobes in the
elevation radiation pattern, but for local traffic this is not
important.

For sky wave propagation the situation is different (as also mentioned
by Greg). If you don't want lobes in the elevation radiation pattern,
your height is limited to about halve a wavelength. Further raising the
antenna gives lobes, but the elevation of the first lobe decreases.

About losses. You have near field and far field losses. If you place
your antenna about a wavelength (or more) above ground, you have far
field loss only. However placing your quarter wave with some radials
close to ground will bring the ground within the (strong) reactive
field zone. This increases the ground loss (you will notice this by an
increase in the real part of the input impedance). You can reduce the
near field at the ground by adding more radials (as suggested
indirectly by Greg).

In my previous posting I suggested to higher the base of the antenna.
This is also to reduce ground losses. A quarter wave vertical has low
input impedance, causing relative high current in the radials. When
the radials are close to (or in ) the ground, losses do occur.

I did some simulation on a quarter wave vertical (20m band) fed over 4
radials (horizontal), placed at various heights above ground (15m thick
dielectric layer representing average ground, very long run times, IE3D
3D planar simulator). The results were bad (Radiation efficiencies
below 10%). More height gives lower ground dissipation. I hoped to get
better results with a halve wave radiator (high impedance, hence
reducing the current in the radials significantly). The simulation
showed a significant increase in radiation efficiency (above 54%).

A halve wave radiator with 4 horizontal quarter wave radials (radials 2
m above average ground) has a first lobe at 17 degrees and about 4 dBi
in that direction).

So steve, I would recommend you to extend (if possible) the radiator to
a halve wave (you need impedance match at the base), you can run your
radials in the horizontal plane, radials should not touch ground.
Extending a 5/8 wave does decrease the input impedance (hence
increasing the currents in the radials) and thereby increasing the
ground loss.

If you cannot extend the radiator, then increase the base significantly
so that the ends of the radials are several meters above ground, and /
or use more radials.

I hope this will not give confusion.

Best regards,


Wim
PA3DJS