[HCARC] Phasing Antennas Such As A Vertical

[Gary J - N5BAA]

Kerry,

Yes it does answer my question and reading your response makes me miss the responses I used to get here on the Reflector.  I have this 20 foot section of tower that seems to be an orphan child.  Yes, it does fit on my crank up tower as the 4th section which would take the tower to about 72 feet vs 56 feet, but it is too small to really do anything with, in it’s dimensions.  Using it on the crank up tower would mean either mounting a  rotator on top of a tubular mast or mounting the rotator on a plate on the outside of the tower section.  Since I already have a rotator plate and bearing on the third section, I keep trying to think of things to do with this section with or without a mast added on top of it – it has the fittings for one.  I have thought of making a Field Day Tower trailer with it, of using it for a support for an inverted L (gets me up 30+ feet), as a support for dipoles, etc.  If I were to conveniently erect it near the 53 foot vertical I am buying from Dr Jim, then maybe it could serve multiple uses.  I have read where others have used two of the 53 foot HyGain HyTower AV-18HT antennas as a phased array – that drove the question asked to start this line of questions.  Since I don’t have any tall trees (who does in this part of Texas??) anything that will provide height is a valuable thing.  Thanks for responding.

73,

Gary J
N5BAA
HCARC Secretary 2013/14

From: SARA SANDSTROM 
Sent: Monday, January 27, 2014 4:58 PM
To: Gary and Arlene Johnson 
Cc: H - Reflector 
Subject: RE:Phasing Antennas Such As A Vertical


Gary,



I never saw a response to your query on phasing antennas.  The answer to your question is no - the antennas don't have to be identical.  When you talk about phasing, you are really talking about driven elements.  If you had two or mmore verticals each fed with a transmission line from your transmitter, they would be a phased array.  If you have one driven element and reflectors or directors that aren't driven, then you have a parasitic array.  The directors and reflectors are called parasitic elements because they get their RF by "robbing" it from the radiation from the driven element - they are parasites!  The parasitic elements don't have to be connected to the driven element, but if your're using quarter wave elements they all have to be over a large ground plane.  Many Yagis were built using half wave elements with non-metallic booms. If you have radials for your driven element you need radials for your parastitic elements.  And, yes, you do need radials for your driven element.  



The beauty of a phased array is that if you have, for instance, an arrangement of phasing lines of different lengths which can be switched in and out by RF relays, you can change the pattern and main beam direction of the phased array.  You can't do that with a parasitic array, you have to physically rotate the array.  For low frequency antennas a phased array is the way to go.  



The driven elemnents don't have to be identical.  One special case is the antenna array used with a radio compass.  I don't know if anyone still uses a radio compass but they were on many aircraft during WW II and after.  It could determine not only the bearing to an LF navaid  but also its sense.  It was pretty easy to determine the bearing with a 180 degree ambiguity with a loop antenna.  With a radio compass, a short vertical "sense" antenna was added which was used to eliminate the 180 degree ambiguity.  It's hard to find to more dissimilar antennas than a small loop and a short vertical but in the radio compass they were used in a single array.  So, no, the antennas don't have to be identical.



The way a parasitic array works is by the parasitic elements receiving RF from the driven element and re-radiating it.  The phase of the re-radiation is determined by the effective length of the parasitic element.  You can change a parasitic element from a director to a reflector just by changing its length.  The antenna pattern is set by the relative phase of the re-radiation of the parasitic element to the radiation from the driven element.  You need to have the right phase delay for the re-radiation combined with the right phase delay of the radiation from the driven element to the parasitic element.  Thats why every time you change one dimension in a yagi, a type of parasitic array, you have to change every other dimension. 



A phased array is much easier to calculate and build because you control the phase through the length of the phasing lines and the distance between the elements in the phased array.  Once you've calculated it, thats it - there is no fooling around.  It is easier to calculate the antenna pattern if all the elements are identical, but that is only for mathematical convenience.



At a wavelength of 80 meters, most towers have very small diameters in terms of wavelength.  A tower that is 3 feet on a side (that is a big tower) has a diameter of about 1 % of a wavelength.



Does that answer your questions?



Kerry  

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