The Horizontal End
This is the simplest and cheapest antenna of all. It will require a very good ATU to tune it up though! Very high voltages may be present at the feedpoint, dependant on the exact length and frequency in use. You will probably need to use a so called kilowatt ATU, when using 100 watts, to avoid arcs on the ATU tuning capacitors. Severe interference problems are common. The antenna is obviously unbalanced with respect to earth, so vertically polarised radiation will occur. This radiation will be either off the vertical feed wire connecting to the end of the horizontal span, or from the earth lead if the wire comes straight into an upstairs “shack”. RF in the shack problems often occur, RF feedback, RF burns to the operator if any metal is touched, and general EMC problems normally restrict these antennas to QRP operations. There is an obvious radiation hazard when using this type of antenna. The local field strengths produced can be way above the maximum permitted safety levels.
Any long wire can be fed at it’s centre with a tuned open wire feeder. This should maintain balance to prevent vertically polarised radiation off the feeder and subsequent interference problems. Whenever a twin feeder system is used though, the antenna itself must always be exactly electrically balanced and symmetrical, any slight lack of symmetry will cause feeder radiation, with the dreaded vertical polarisation. It will require a high quality proper balanced ATU i.e. not an unbalanced one with a toroidal balun transformer added on the output! (The balun will only work efficiently over a limited range of impedances) As with any long wire antenna, the polar diagram will contain many deep nulls, in which directions communications will be difficult. This more than negates the few dBs of gain that doublets have on the higher frequency bands.
Note Large spaced open wire feeder should not be used in the shack, or anywhere where people or animals may be able to come into close proximity with it. The fields will not cancel in line with the feeder (they only cancel broadside to the feeder), unless the distance is many times the wire spacing. Unless the small spacing twin is used, the RF radiation problems in the shack can be almost as bad as those with end fed wires.
This antenna is very cheap to make, but because it is only resonant on one band, it requires a very good ATU to tune it up on all the other bands. It is really too long to give consistent results in all directions on the higher bands, the polar diagram becomes very “petal” shaped with many deep nulls. It is far better to convert it into a doublet antenna and use open wire feeder all the way to a proper balanced ATU, rather than to use co-ax cable for part of the feeder. The SWR on this antenna is very high on all the bands (except 14 MHz, where it is only about 2:1 at resonance), so the use of co-ax can cause very high losses. The ribbon feeder normally supplied with commercial G5RVs is very poor mechanically, if it swings in the wind it breaks quite quickly.
Vee Beams and Rhombics
Great, if you have got the necessary real estate! Radial terminated Vee beams and/or Rhombics, with switching to beam to any part of the globe. Shack in the middle of the 10 acre site, which is a Polynesian island, with only local girls for company! Ahh, this is paradise, but would you bother going on the air? Dream on!!
Too big and low gain for the size. Note the CobWebb has a “gain” of 7 dBi.
Trap dipoles use tuned circuits to isolate sections of a dipole, such that electrically it looks like a half wave dipole (with a low impedance feedpoint) on each band. Four pairs of traps would be required for a 5 band dipole. If the inverted “V” configuration is used, then the feed impedance should be about 50 ohms. However the traps act as loading coils on the lower frequency bands, so the antenna becomes shorter than normal. This should reduce the radiation resistance of the antenna on the lower bands. The radiation from an antenna is determined by the product of current and length. If the length is reduced then, for the radiated power to remain the same, the current must be increased. The increased current for the same power must mean that the voltage will reduce. The shorter length should therefore cause the feed impedance to be much reduced. This should cause the SWR to increase, if it does not the traps must be lossy!
This is in effect half of a trap dipole, fed against ground. They can give a good match, but being on the ground the signals will be attenuated by surrounding objects, particularly on the higher frequencies. If your ground has poor conductivity results will be very poor. Verticals can be elevated, using either radials or extra traps. They can then work very well, as long as you don’t have any neighbours. In an urban environment the EMC problems can be chronic.
All vertical antennas suffer from another problem. They require good conductivity soil for many wavelengths around them to compete with horizontals. Even verticals that do not use a ground connection i.e. vertical dipoles or ground planes etc. can still be as much as 8 dB down on a horizontal dipole antenna, due to reflection losses. The difference between sea water and very poor ground is up to 9 dB for a vertical antenna, but only 1 dB for a horizontal.
The Magnetic Loop
This is really just a short dipole that is bent and end loaded with a variable capacitor. There is no magic involved in the way it works, a standard “electromagnetic wave” is produced from them, as per any other antenna. It is not producing magnetic waves with special immunity to “Electric Field Interference”. It does have a confined electric field so that near field “capacitive” coupling to surrounding conductors will be reduced. There will be some reduction in performance on the lower bands, a well designed 5 metre circumference loop will normally work over the 14 to 28 MHz bands with a 50% efficiency on 14 MHz.
To keep the efficiency this high, the loop will have to be made out of large diameter copper or aluminium, NOT co-ax!! The resultant high “Q” will mean that the loop will have to be continuously re-tuned, as the receiver is tuned round the band. The remote control system that is needed to do this can be quite expensive, also the tuning capacitor will have to stand many thousands of volts, even when only running 100 watts. For high power operation a vacuum variable capacitor is normally used, which is very expensive.
A loop standing on the ground in the vertical plane will radiate a vertically polarised signal. The ground will absorb/reflect the horizontal component straight upwards. The vertical component at low angles to the horizon i.e. that required for DX operation, will have a sharp null in the response broadside to the plane of the loop. This can be useful for reducing interference, but it does mean that the loop will need a rotation system.
To prevent low angle signals from being absorbed by surrounding objects i.e. shrubs, fences, trees, houses and the myriad of conductors around the QTH, it is obviously far better if the loop antenna is mounted up in the air. If it is mounted about 30 feet high then the horizontal radiation will be able to be used for low angle DX operation. The loop can then also be mounted in the horizontal plane to eliminate the vertically polarised radiation, and so reduce EMC problems.
The small diameter loops made from co-ax or flat bar give very poor performance. They don’t need retuning as often as you tune round the band, because they are low “Q”, the problem is that most of your transmitter power will be dissipated as heat!
The main advantage of the loop antenna is it’s continuous frequency coverage over the entire spectrum, by remote controlled re-tuning. This is ideal for military users etc. but against this is the cost, especially if you only wish to transmit on the amateur bands.
These antennas are a very big compromise. They generally only cover the 14, 21 and 28 MHz bands, because of interaction effects with 18 and 24 MHz. A reflector resonance for 28 MHz will act like a director resonance for 24 MHz, a reflector resonance for 24 MHz will look like a director resonance on 21 MHz etc. etc. The result is a very expensive trap dipole! The gain that they are supposed to have is only over a very narrow bandwidth. They are very difficult to set up, most people just give up and feed them with an ATU. They need to be rotated, not because of a good front to back ratio, but because of the nulls off the ends of the dipoles! They are often rated for high power use, presumably what this means is they don’t actually catch fire when used with QRO, because they certainly get very hot!
Broad Band Verticals
These antennas use most of the applied power to produce a low SWR reading. The fraction of power that is radiated can still produce some DX QSOs on the higher frequency bands though, it’s amazing what you can do with QRP on HF! If the guy with a 100% efficient antenna is getting a 59 plus 20 dB report, then the guy with the 1% efficiency will still get S9!! On the lower frequency bands these antennas are very good dummy loads!
It always amazes me that if a 100 watt rig was only putting out 10 watts, most amateurs would be very upset, yet they will use a 10% efficient antenna and be quite happy. The reason is, of coarse, they can’t measure the radiation efficiency but they can measure the power/SWR. Many radio amateurs seem to think that a low SWR means that all the power is being radiated, so you cannot really call the producers of these antennas “Con Artists”, they are merely producing what the radio amateurs say they want! They are even quite good for lack of TVI and general EMC problems, as they radiate such a small amount of the applied power!!! Caveat Emptor!
Broad Band Terminated
These antennas also use most of the available power to produce a low SWR reading on the lower frequency bands. On the higher bands they only use about half of the power for this purpose, so about half of the power is actually radiated! Unfortunately, on these higher frequencies, the antenna will have lots of lobes and deep nulls as per a standard doublet antenna. The military have used this type of antenna in the past, because of its ease of matching over a wide continuous frequency band, but it really is just a waste of power to use it on the amateur bands.
Crossed Field Antennas
These antennas don’t work! A little knowledge is a dangerous thing! The co-ax feeds radiate a little power, because they don’t have proper baluns on them! The metal bits radiate a little power because they are metal and have a small amount of RF current flowing through them! One guy on the internet has even offered a $10,000 dollar reward to anybody who can demonstrate one working! The so called “antenna design experts” who invented and patented the CFA have not claimed their reward!!! A short length of wire and a parallel tuned circuit ATU will work much better! It's a classic case of somebody inventing a theory and then trying to make the facts fit the theory!!
These antennas work very well. The dipoles need to be spread out so that the high impedance ends of the shorter ones are not affected by the longer elements. They can be arranged “maypole” style round a central support, so that they can act as guy wires as well as antenna elements. When arranged in this way the feed impedance will be about 50 ohms so they can be fed with 50 ohm co-ax, via a choke balun. This system works very well when out portable, the only problem is the vertically polarised radiation off the ends. This radiation fills in the nulls off the ends of each dipole, so that no rotation is needed but it can cause the dreaded interference problems.
These antennas really need to be horizontally polarised for minimum EMC problems, but they then need a 75 ohm feed. They can be double gamma “T” matched to 50 ohm co-ax, as long as an effective choke balun is used. This is in fact what the G3TPW CobWebb antenna is! The dipoles are each bent into squares, so that they can be supported by a single horizontal fibre glass cross, rather than having separate supports for all ten dipole ends. Bending the dipoles into squares also eliminates the dipole end nulls, resulting in an omni-directional radiation pattern. Another major effect is that the electric field of the square dipole will not couple into the ground, or any other nearby conductors so losses are much reduced.
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