Dipole vs Sharkfin vs Polarized Diversity (Diversity Fin)

February 16, 2012 at 2:38 pm

Diversity Fin from RFVenue

A new antenna called the “Diversity Fin” (polarization diversity), made by the company RF Venue, has been getting enough attention to warrant a comparison with two other popular antenna designs, the dipole and the “Shark Fin” (log periodic).

For the purpose of this comparison, the assumption is that these antennas will be used on diversity receivers. Dipoles and shark fin antennas were used in pairs (a pair of dipoles was compared to a pair of shark-fins), and these were compared to one diversity-fin since this antenna is actually two antennas (A and B) built into a single device.

It should be mentioned that the diversity fin is a combination of a shark-fin and a dipole in a single device, arranged so that the shark-fin has a vertical polarization and the dipole has a horizontal polarization. The two antennas in the diversity-fin are polarized 90 degrees from each other.

Starting with the simple “whip” antenna (not used in this comparison) there are pros and cons to each of these antenna types:

Simple whip


The whip antenna is simple, cheap, and it is convenient in that it mounts directly to the receiver chassis, which acts as the antenna’s “ground plane”, which is required for a whip antenna. The need for a ground plane is the reason it is not practical to position a whip antenna on a mast away from the receiver with an extension cable. The whip antenna is omni-directional around its axis. For wireless microphone use, the whip antenna is usually cut to ¼ wavelength, which, for commonly used UHF frequencies, is typically about 3 to 5 inches.

Dipole


A dipole antenna, basically, combines two ¼ wavelength whip antennas, pointing opposite directions (up and down, for example), with a single cable connector between them. Compared to the simple whip antenna, the dipole has the advantage of not requiring a “ground plane”, which allows it to be mounted strategically away with an extension cable. The dipole antenna is omni directional around its axis.

Log Periodic (shark-fin)


This antenna is often laid out on a thin flat circuit board, with its antenna beams created as traces on the circuit board. Since the beams get smaller towards the front of the antenna, it takes on a triangular shape resembling a shark’s fin. The log periodic antenna is directional, much like a shotgun microphone. When the antenna is pointing towards the transmitter, the received signal is increased, and when pointing away from the antenna, the signal is decreased. Generally, pointing anywhere from directly at the transmitter to less than a right angle away from the transmitter does not cause an appreciable amount of signal attenuation. However, pointing away from the transmitter at angles of 90 to 180 degrees causes up to approximately 8dB of attenuation, which will have a significant affect on reducing the range of useful reception.

Polarization Diversity (Diversity Fin)


This antenna is two antennas: a shark fin with a dipole mounted on it, and has connectors for the A and B inputs of a diversity receiver. The practical advantage of this antenna design is that it is a single device for a diversity receiver. While it can be said that it combines the advantages of the two antenna types (the directionality of the shark fin with the omni-directionality of the dipole) it also combines the disadvantages of the two antenna types (the rear rejection of the shark fin and the lack of gain of the dipole). This antenna design is probably best used with what is mischaracterized as “true diversity” systems, where the receiver selects only the antennal with the best signal. However, when used with “antenna-diversity” receivers such as the popular portable Lectrosonics portable receivers that always combine the two antennas, inverting the B antenna when it improves combined signal strength, the Diversity Fin antenna can reduce the combined signal strength at the receiver. In practical applications in the real world, multipath reflections often make the polarization of received signals rather arbitrary, so this disadvantage may not be a factor very often. However, this antenna design guarantees that the combined signal of the two antennas will never be as strong as two individual antennas polarized the same (both vertical, for example) and happen to be in the ideal position for a transmitted signal.

Practical Comparison Test

In the end, while the theory may be interesting to some, the only thing that matters is how the antennas work in actual field use, and what matters with wireless microphone performance is long range and stability. Comparing the performance of three antenna types (dipole, shark-fin, and polarization diversity) using a Lectrosonics SMa transmitter and Venue receiver, the results of walk-tests were consistent with the theory. The environment involved a lot of multipath reflections. The receiver was inside a steel rod-reinforced concrete building, and the transmitter was outside, gradually moving away. There was not a huge difference between the maximum range any of them. They all went about 250 feet without dropouts (very respectable given the obstacles of the building), with the Diversity Fin going about 20 feet further than the dipole, and the shark-fin going about 20 further than the Diversity Fin. Keep in mind that this comparison was with the directional antennas pointing toward the transmitter. When they were pointing the other direction, the order of performance was completely reversed, with the dipole being the winner.

My Opinion

Each antenna could be the best choice, depending on the application. For the majority of the situations that I have worked in production, my preference is for the dipole (specifically, the folding dipole by Lectrosonics). It is the least expensive, the most compact, performs more than adequately for most situations, and they cannot be pointed in the wrong direction when positioned vertically. The shark-fin is the preferred choice when the wind is not strong enough to turn them, the bulkiness is not a factor, when the price is within budget, when the extra gain is needed, and when its certain they can be pointed in the right direction. The choice might favor the new polarized diversity antenna design (Diversity Fin) when its lower price (compared to two separate antennas) is enough of a factor, when having a one-piece antenna is important, when it is certain it can be pointed towards the transmitters, and particularly when used with “true diversity” receivers such as is common from manufacturers Shure, Sennheiser, and Zaxcom (which does not include most portable diversity receivers made by Lectrosonics).