A vertical antenna is the type to use when You want to have a good DX-capability
under 10 MHz.
This type of antenna is especially suitable for DX-EXPEDITIONS !
If You are located nearby the equator, most probably You will need some sort of LOW NOISE receiving antenna in addition.
If You want superior performance, consider a MULTI ELEMENT PHASED VERTICAL
This antenna is a major undertaking. Though effective, it is quite large and You must have the grounds for it. The direct spacing between the elements is in order of 0.1 to 0.5 wavelength.
As the research of SM6EHY reveals, the necessary radial system can
up to 2.5 wavelengths long in every
direction (see RADIALS)
and is effective all it's way. (It can also be made as short as 0.1 wavelength
For 80 m this will be close to 200 meters and for 160 m it will be 400 meters in diameter (660 Ft and 1320 Ft respectively) !
The common way of testing a recently put up antenna is to make some
comparison with the regular stations on the band. Most often they are located
not too far away. At this distance the vertical is NOT a good performer
The comparison antenna to use under these sessions must be located at some distance away from the vertical; understand wavelengths ! This is to minimize the interaction (coupling to) the comparison antenna.
This vertical is placed 'upside down'; the groundplane is orientated upward. Most commonly used is association with a normal type vertical in a repeater setup; minimum interaction due to that the antennas are close to 180° out of phase. This type is used primarily on frequencies higher than 30 MHz.
The 'hot' radiating part of the antenna is placed close to ground, resulting that coupling to nearby metal objects is one degrading aspect. The other is that dangerously high levels of RF-voltage is near the reach of people and animals.
For lower frequencies this type minimizes the groundloss, due to that the nearest part to ground has maximum RF-voltage and therefor maximum impedance. To calculate groundloss, this high impedance is relating to the impedance of the ground, that mostly is less than the end impedance. Most often the groundloss in this case is neglectable.
The height of the element is made shorter by means of linear loading; part of the element is folded back upon itself. The folded part is preferably located halfway from the ends. At the far end we have extremely high RF-voltages (and high impedance) which makes the coupling to the acient parts bothersome. At the feed end we have high current (and low impedance) and this part must be made 'full size' for the 'antenna surface' to be as large as possible (efficiency). The parts of the element that carries the highest current has to be made straight in order for the antenna halfs to be as far apart as possible and to minimize loss.
(Element center is at left; end at right.)
This type of vertical is essencially used nowdays on Low Frequencies (LF) and Extra Low Frequencies (ELF). The most common problem associated with LF is the (too) low feed impedance. Under most circumstances the antenna height cannot be made 'full size (1/4-wave)', which happens to be some some 500 meters up to several km's !
The 1st degree of multiple tuning is the 'folded marconi', which transform the feed impedance up by a factor of 4, depending upon the individual wire diameter and spacing.
Multiple tuned vertical feeding arrangements.
The tuning coils above near ground are used only to equalize
the antenna current between the UN-feed folded wires. As a rule of thumb,
the resistive step up ratio is the square of the number of wires used;
the 3 wire example will have a ratio of 9; the 4 wire example will have
a ratio of 16 etc.
At the top of the feed wire we have another coil, just in order for the RF-impedance to rach its maximum at the top, just as in the single tuned folded element. In order to have highest efficiency one of the "guy-wires" shall be made feeder, not the supporting tower that is screened under the "hat" of wires.
After some studies of efficiency, it is obvious that if the radiator is RF-wise longer than a 1/4-wave, we will get an inductive feed reactance, that is more efficient than a loading coil at the high current feed point. To cancel the reactive component at the feed, we just have to use some capacitors, that by nature is almost lossless, if we don't obtain any corona around it. The added benefit is that the resistive part becomes slightly higher and in turn enhances the radiation efficiency. "The more conducting material we put up higher in the air, the more effective will the antenna be" - is a good motto to remember (be aware: NOT to generate any corona at the high impedance point !).
Some recent made installations for the 10 to 20 kHz-band is made up by one 200 m high tower, and up to some 200 tuned folded wires, ofcourse with a huge ground system. This antenna is some 0.01 wavelength high, and with the arrangement above the feed impedance is raised up by some (200)2 = 40,000 ! We can be quite sure about that the net feed impedance is close to the feeder-systems 50 ohm (!) and the net efficiency is quite high.
For the newly opened amateur radio allocation of 136 kHz, this means that a 20 m high supporting tower, with the above feed method, will be equally efficient...
© Björn Waller, SM6EHY 1998/-99