Rick ha escrito:

I am interested in operation primarily on 160, 80, and 40.

I have space for a full-size dipole for 160 but I cannot get it up
very high, maybe 30 feet, higher than that only at great expense and
with great difficulty.

I will likely put up a 160-meter dipole at 30 feet and feed it with
ladder line and a tuner, and use it for NVIS work on 160-40.

For DX, though, I'm wondering if I will be happy with a vertical such
as the Voyager DX from Gap Antennas, or if I should bite the bullet
and go to the expense and difficulty of getting the dipole up 60-80
feet (which still won't be very high, compared with a wavelength, at
160 or 80).

What's your advice?


High Rick,

Easy question, difficult answer.

Basically, a vertical antenna has more low elevation radiation with
respect to dipoles at h0.25*lambda above ground. However, it is, in
most cases, difficult to get a reasonable efficiency in the vertical
case. In addition, the amount of low angle radiation depends on the
type of soil. Many horizontal dipoles operate better than verticals (on
DX).

My practical proof is: when I get more signal strength (not S/N ratio)
out of an antenna when receiving a station, I will generate more signal
strength at the station's receiver. When you are able to do a
simulation, simulate your antenna (with perfect ground) and determine
the BW. Build your antenna and measure the bandwidth. When your
bandwidth has doubled (with respect to simulation), your efficiency is
about 50%. Make sure that your wire thickness in the simulation is
equal to the real antenna.

Regarding horizontal dipoles. Up to h= 0.2*lambda, the radiation
pattern does virtually not change; however the efficiency does. A halve
wave dipole at h=0.1*lambda will put more energy in the earth (and
radiates less) with respect to the same dipole at 0.2*lambda. At low
height, an efficiency of less then 15% is not uncommon. It is because
of the low efficiency (power dissipation in the soil) that most HF
dipoles has reasonable bandwidth....

Above h=0.25*lambda the vertical radiation will reduce gradually in
favour of the radiation at lower elevation.

When you are above bad soil, I would recommend you to try to increase
the height (especially for 160m) of your dipole. This is because of a
vertical antenna in combination with poor soil will probably have a low
efficiency and radiation under low elevation will be suppressed
(because of the relative high pseudo Brewster Angle).

When you can make h=80ft (24.3m), your 160m dipole will still have an
"NVIS" radiation pattern, but with higher efficiency, so finally more
radiated power in any direction (therefore also at low elevation).
Same is valid for 80m, radiaton pattern will flatten a little bit in
favour of lower elevation. At 40m, radiation at high elevation will
become less, enhancing radiation at lower elevation. At 20m your will
get multiple lobes in the elevation radiation pattern.

When you go vertical, your height will be limited too. Probably the
height will be far below a quarter wavelength (for 80 and 160, unless
you use a kite or balloon....). This will result in a low radiation
resistance in combination with a high feed current. This feed current
must be drawn out of a ground network. Mostly, the ground network will
dissipate lots of the RF power. Of course, the situation is better when
you live in an area with heavy, wet, mineral rich ground (like me).

Maybe for 40m you can get a (top loaded) vertical into a halve wave
resonance. The input impedance will be very high (in the kOhm range),
resulting in low feed current. This will reduce ground losses
significantly. Another option to force the antenna into halve wave
resonance is to add inductance in the middle of the wire. The
disadvantage of relative short forced halve wave radiators is the very
high end-fed impedance (10 kOhm). Mostly this requires a dedicated
tuner.

Top-loading the antenna (L, T, square, or multi wire cap) will give
more current in the top of the antenna. This increases the radiation
resistance (maximum factor 4) and decreases the losses in the ground
system (better overall efficiency). It also eases matching.

Regarding the ground system. Try to get as much as metal connected to
ground from where you feed the (vertical) radiator. More smaller
ground rods do better then one very long one. Ground your
(asymmetrical) tuner to the ground system and add a common mode coax
choke between the tuner and Transceiver. There can be RF voltage on the
tuner with respect to the transceiver's ground. If the VSWR of the
radiator itself is not that bad, you may put the choke between the
tuner and the radiator.

Another option is a floating ground (I am planning to do this for the
next JOTA for 40m with a halve wave radiator). Two or three ground
wires of 0.25*lambda are completely above the ground (some 6 feet).
Depending on your back yard, this may be a problem. For a multi-band
antenna this will require multiple wires.

A complete other option is using a (side-fed) loop.

The Voyager DX antenna.
I do not have experience with that antenna, however it is also used
with a counterpoise kit (I saw on several reviews). If this kit is
really necessary to operate the antenna, then it is a more or less
asymmetrical antenna that needs ground. The bandwidth of 90 kHz at
160m is rather high. It may be the result of poor efficiency.

In brief:
#If you have very good ground, you may try a vertical monopole antenna.
#In case of very bad ground and no opportunity to make a good ground
system, use the horizontal dipoles (as high as possible) and or use a
vertical polarized radiator that has high radiation resistance at the
feed point (halve wave characteristic prefered).

Best Regards,

Wim
PA3DJS