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Question by : Ham Radio Dipole questions?
Is there such thing as a full wave dipole?
Why a half wave dipole is more often used? Is it more efficient?
What about vertical antennas? Can i construct one ?/2 ?/4 or ?/8? Will it be efficient for Rx Tx ?
Whats the difference between radials and counterpoise ?
Thats all!
Best answer:
Answer by ?BobB?
it was discovered in the beginning of radio that the 1/4 wave antenna was the most efficient. If you look at the voltage and current curves you can see why this is.
A dipole is two quarter wave radials, also called a HERTZ antenna.
Radials and counterpoise are the same thing.
Think of a hertz antenna in free space. You have one side of the feed point to one element, the other side of the feed point to the radials, counterpoise. Next, using a Smith Chart, you can plot the voltage and current highs and lows (NULLS).
The radials as well as the radiating element needs to be resonate.
additional:
Contrary to the mis information you are receiving here… a dipole is a 75 ohm feed, the Z is adjusted by the angle of the feed. 75 ohms is not a bad match! In fact, it is very good.
A dipole can be mounted up and down or left and right. The only difference is the radiation polarization. Horizontal is for distance, the pattern hugs the ground (sky wave does not care). If your target is mobile, most mobile stations are vertical, so the transmitting antenna is also vertical – there are exceptions (one of my HF mobile antennas is a folded over NVIS antenna using mil surplus antenna pieces). Shunt fed towers are vertical polarized, but again, skywave propagation does not care how your antenna is situated.
A center-fed half-wave dipole is a classic antenna. Most other antennas are based off of it. There is something called a folded dipole antenna which would measure a full wave from feed point to feed point. It does not have any more gain than a half-wave dipole (I don’t think), but it has a higher feed point impedance of 600 ohms, more compatible with balanced transmission line. There is also something called a square loop antenna. Each side is a 1/4 wavelength.
For HF the half-wave dipole easy to make and use. Horizontally, it’s feedpoint impedance is 73 ohms (depending on height above ground). Vertically, impedence is 35 ohms, both a good match for 50 ohm coax.
Take your vertical half-wave dipole, replace the counterpoise (grounded side) with radials a 1/4 wavelength or longer or a ground plane, and you have a vertical 1/4 wave antenna. It is also about 35 ohms. Bend the radials down at a 45 degree angle and the feedpoint impedance rises to 50 ohms. Half-wave vertical antennas don’t need a ground plane but they do need a matching stub or impedance transformer because they have very high feedpoint impedance. This is the basis of operation for a J pole antenna. Eighth wave verticals are too short to resonate, although 5/8 wave antennas have a little more gain than a 1/4 and same impedance.
The ARRL Antenna Book is an excellent book on antenna theory, design and construction. The ARRL Handbook for Radio Communications (2010) has key parts of the ARRL Antenna Book in it along with all other stuff Amateur.
a dipole antenna can be any length, it has two equal radiators center fed. the two radiators behave to the feedline exactly as if it were connected to a load at this point. the equivalent resistance represents the propagation from the antenna. The feed point behaves much like a LCR resonate circuit. At certain combinations of frequency/length there is resonance where the feedpoint is purely resistive. this situation occurs in a dipole antenna with the overall length is approximately 1/2 lamda, or any even multiple of 1/2 lamda. A half-wave dipole in free space has a resistance of about 72 ohms which makes it convenient to match to coax feedline.
the propagation from the antenna is more dependent on RF current distribution on the radiator than resonance per se. the factors that determine how efficient an antenna system works is far too complicated to describe here. generally a dipole antenna has better propagation characteristics for a given power input if it is extended to about 5/8 lamda. one popular wire antenna is the double zepp. here the dipole is 10/8 lamda and the center feed is a higher impedence so a ladder line is used instead of coax.
all antennas behave differently when in proximity to the ground instead of free space. it is necessary to consider the ground as part of the antenna system so as to properly evaluate the overall radiation pattern. This additional part of the antenna system is what is meant by counterpoise. The counterpoise can be just ground, a system of wire radials placed on the ground surface or radial elements elevated on the tower for added benefit. In some cases the counterpoise is placed perpendicular to actual ground to achieve favorable reflection, as in a Serba Curtain antenna, or a Corner Reflector antenna. The spacing of the counterpoise from the dipole is critical to performance, but a counterpoise element itself is not tuned or cut to a special length.
Vertical antennas are not exactly dipoles, so it is important to realize how they are similar or different. The simplest form of vertical uses the counterpoise as a reflective surface, so by symmetry you could think of it as a dipole split in the center with the counterpoise acting as the missing half. so for a 1/4 lamda tower, the base feed is half that of a dipole or 35 ohms resistive. this is the situation for most AM broadcast radio stations, except they are rarely a full quarter wave tall, so they have a “tuning unit” at the base to tune out the non-resonance and couple to the coax feed from the transmitter. The FCC requires that AM towers have 120 radial wires for the counterpoise, they are not cut to a resonate length. At ham HF frequencies it is possible to emulate this for 40 or 80, or even 160 meter bands by running an antenna wire up the side of a tower the proper length. Hams can use less radials and still get good counterpoise results, as few as 3 will “work” but 12 is considered good practice.
At 20m and higher bands, putting the base at ground level is a problem. This puts the radiation center too close to the ground for safety purposes, and reduces the effective height above average terrain HAAT, which becomes increasingly crucial to propagation at higher frequencies. So vertical antennas for 20m and up are elevated on the tower. However putting the feed point more than a foot above ground greatly reduces the effectiveness of the counterpoise. So a counterpoise has to be elevated with the antenna! One popular elevated vertical is the 5/8 lamda which with counterpoise is just the same as the double zepp mentioned above for horizontal antennas. there is some mistaken idea that this antenna does not need a “ground” because it is an “end fed” dipole. not true. what is the case is it uses a true elevated counterpoise whose radial lengths are not tuned so some designs use somewhat stubby but very essential elements. And cutting the feed resistance in half actually helps get it down to 50 ohms for the coax.
What most hams put up for an elevated 1/4 wave antenna is not that at all. Every ham that has ever experimented with a 1/4 wave antenna made a remarkable discovery. If the radials were allowed to “droop” instead of go straight horizontal, the feed point raised from 35 ohms to 50 ohms which is a better match to the coax. However when doing this they found that it became important the the radials themselves be tuned to 1/4 wave. The reason is the radials have stopped being a counterpoise and have become a conical antenna. So the Ham radio 1/4 wave is really a dipole element up and a conical antenna down, Notice that the feed is no longer at the end point of the antenna but rather the center, and with the coax dropping down parallel to the axis of the conical, it is very hard to keep antenna radiation off the coax. This explains why this antenna has a perfect 50 ohm match yet has tons of RF coming back the line.