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|As the new owner of Arrow Antenna, I want to say a little about gain. Gain is a number that some antenna sellers use more for marketing than for anything else. Some gain figures you will see are beyond what is physically possible for that configuration of antenna. To publish credible gain figures for an antenna requires expensive independent testing which makes the antennas you buy from Arrow Antenna more expensive but doesn't improve their operation in the slightest. In light of this, Arrow Antenna has chosen to mostly stay out of the gain marketing arena. Some of our antennas have a published gain figure, but as the new owner I don't know how those were derived so I hope they are right. There is 20+ years of accumulated information in Al's head (the previous owner) and I have not been able to ask him every question there is to ask yet. The gain of the OSJ146/440 is one of them. |
Lets note a few things about gain:
First of all a db or two of difference in gain is basically irrelevant except in extreme circumstances.
Second, half a wavelength of aluminum in the air whether a ground plane or an open stub j-pole will have virtually the same gain. Half a wavelength is half a wavelength. You can't squeeze more than the physics will allow out of half a wavelength. You may through various construction techniques get closer to the theoretical maximum for a given configuration but there are always tradeoffs. For example, an antenna with smaller diameter elements may have higher Q and thus lower losses, but it would have reduced bandwidth. We have other things to consider when designing antennas besides gain. Bandwidth is one. Angle of radiation is another. Beam width is another. Portability is another. Physical strength to allow antennas to withstand hurricanes is another. All are trade-offs. Increasing one factor decreases another.
So, to get an idea of the gain of an antenna, consult the Antenna Handbook from the ARRL or other similar references. They will give you a good ballpark figure for gain. Any well constructed j-pole will give similar gain. Ground plane, Yagi, etc. the same. Then make your decisions on antenna buying based on the other relevant factors as well such as construction quality, bandwidth, etc.
I also would like to clarify another commonly held misconception about gain. An antenna is not an amplifier. To understand gain correctly, look at it this way. Throw a handful of sand in all directions around you and you are an omni-directional antenna such as a dipole. Throw that same handful of sand on one direction and your are a Yagi or other directional antenna. Same amount of sand. It is just channeled in one direction. The same idea is true for receiving. There is only a limited amount of electromagnetic energy coming by your antenna. The directional antenna has "gain" over a non-directional antenna but at a cost. It can't receive as well in the other directions without being rotated. Remember that up and down are also directions pertaining to gain. We as ham radio operators usually want our radio's transmitted energy and ability to receive other's transmitted energy to be near the ground - especially on VHF and up. This refers to angle of radiation. Ground plane antennas have a higher angle of radiation than a j-pole or vertical dipole because the ground radials are angled up from the vertical causing the RF energy to be angled up as well. Great for if you are trying to transmit out of a valley or trying to skip off the upper atmosphere or maybe try to transmit around the curvature of the Earth. For local communication or from a mountain top, you may want to not radiate as much energy upwards so a j-pole or di-pole (or any Yagi) might be the choice.
The point here is that gain simply decreases one direction to improve another. There is no net increase overall. It is not like adding stages of an amplifier and a few more db of gain in the process. Antenna gain is just a way of specifying gain compared to a reference antenna. Without the reference being stated the gain figure is meaningless. To compare two antennas, the gain has to be compared to the same reference antenna. The reference antenna is usually a dipole or an isotropic radiator. An isotropic radiator is only a theoretical construct and can't be created in reality. So gain in comparison to a dipole (dBd) is meaningful to us humans because we can see and use both of those antennas. Gain in comparison to an isotropic radiator (dBi) is a valid measurement, but if you want to get an idea of how much better that antenna will perform compared to another I prefer to use real world measurements of real world antennas rather than theoretical. The point here is to compare apples to apples. If you are comparing the gain of two antennas, make sure they both reference the same dBi or dBd. If the seller of the antenna doesn't specify which reference is being used, find out.
That's my two bits worth. 73's, Tim Chapman - Arrow Antenna, LLC.