Survivor Antennas

About Gain, understanding the differences. 

The future of Antenna Technology

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Understanding differences in Gain

            Since measuring the gain that day some field experiments have been tried.  And some things were noted that should be reported for the users to fully understand this design and how it functions. I'd like to have done this with drawings but I don't have the facilities here to do that at this time.
              Imagine a wave traveling though space, spreading out as it travels. As it travels it gets larger and more spread out, thus spreading its energy out more and more, and as it spreads out it becomes weaker.  Now imagine this weak wave hitting a conductor like your antenna.  Now, based on size and design, this wave induces a small electrical current into your antenna.  Some antennas, like a beam have multiple elements, and each of its elements receive a small amount of signal and because of their spacing, they 'give' that signal current to the next element and the next until it is fed off of the 'driven' element.  Since having multiple elements to feed from, it gets a stronger signal to the feed line.  Similar, the 5/8 wave vertical concentrates it receiving and transmitting wave closer to a near parallel wave front of the element itself.  It was found years ago that 5/8 wave length has the peculiar characteristic of concentrating its radiation pattern lower and more parallel to itself than other lengths of verticals.  These radiation patterns you see are actually phase angles of radiation, or in simpler terms, directions that the antenna will accept a wave to and from itself efficiently.  Now suppose an antenna is on the back side of a hill, with the last foot or two extending above the hill.  Now this antenna, we'll call A has 3 dbi of gain and the Survivor antenna (B) has the same 3 dbi of gain.  The difference being that the Survivor is not extended above the hill at all and is receiving its signal via 'back scatter'.  Back scatter is the same as looking up a hill from the back side that someone is shining a light on.  You can see faintly the light shining on the hill on the other side, same thing.  So as you can see, antenna A is still receiving some of its signal via a more direct wave, the Survivor is not.  Both have the same amount of gain, but one is getting more signal, in this case A is.  Therefore, A will render a stronger signal than B.  This has been noticed in some situations when testing our 2 meter antenna mobile against a 5/8 λ.  As you can see, on VHF/UHF operation, location is an important factor but this also occurs at HF frequencies to. 
         In the world of antennas, every installation is different.  Height, orientation, coax length and loss, ground conductivity, and reflection and absorption from nearby objects.  All of these factors affect signal strength and thus, what is interpreted as gain.  A good example is the ground mounted vertical that uses the earth ground radial system to complete the design.  It can be shown that there is considerable differences in apparent gain from one that has 4 radials to one that has 120 radials.  Here we have the same antenna, at the same height, but with different ground radials or ground conductivity.  There is more than 3 dbd of difference in signal strength between the two antennas.  Similar experience happens when dipoles are placed at different heights above the ground.  So what we often call 'gain' can actually vary a lot even in simple designs such as dipoles. (decoupling hasn't even been considered in these examples but it too can make a difference in 'gain', es using a balun feed.)
            Another thing to consider is polarization, most people are not aware that there is a 20 db difference in signal strength from vertical to horizontal polarization.  Indeed this 20 db difference in the very reason we can double the channels broadcasting from satellites as they use the same frequency twice, once in each polarization and separate them on the ground with either a vertical element or horizontal one.  One last thing, the 2 dbd gain I claim is only a 57% percent increase in signal which translates to about 1/4th of an S unit.  Another way to say it is that for 100 watts input, the effective radiated power is 157 watts.  There is more physics involved than I am mentioning here but I intended for this to be a simple explanation.   The bottom line is this, the Survivor design has gain, compared to a simple 1/4 wave vertical ground plane antenna or dipole.  And it will get you on the air with dipole equivalent performance.  And that is no small thing considering their size.

 Please feel free to contact me for comments or questions on the contact page.

Tom Brent

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