4391aas@hou2f.UUCP (A.SCHWARZ) (12/19/83)
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Quarter wave mag mount
Asking 12 bucks
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HQ-1 Mini-Quad with W2AU balun.
20-15-10-6 m.
Up only 3 months.
Asking 90 bucks.
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Al Schwarz ND2K
(201) 949-3337
*******************************woodson@ucbvax.ARPA (Chas Woodson) (08/07/85)
I am considering a set of phased verticles for my new QTH. The basic idea is that they would stick out of the top of trees which are about 20 meters high. I am interested in 40,30,20...10. Any recommendations? How would you feed such antennas? Half wave, center fed? Anyone got any suggestions or report of experience? .
jhs%Mitre-Bedford@d3unix.UUCP (08/14/85)
Re: phased verticals. As I have noted previously on the net, I have had excellent results with a single, center-fed vertical on 20-15-10. This one is 32 feet long, which is about as long as I can string up in the trees in my yard. If you are going to use a self-supporting antenna (or at least the top half) you can approach twice the tree height, or around 40 meters of height. This would allow you to use the antenna even on 80 meters as a half-wave vertical. Certainly 40 meters would be within your reach. I have found center feeding with open wire line (or twinlead) to work quite well. Theoretically and practically, it has the advantage that you don't have any losses due to feeding RF currents directly through lossy ground, as you would with any end-feeding scheme. End feeding a half wave is not as bad as for a quarter wave, as the radiation resistance seen at the end of a half wave is quite high, requiring relatively low current for a given power level, and therefore inducing less power loss in the (fixed) ground resistance than the high currents needed to feed a quarter wave would induce. However, center feeding is even better because the entire transmitter output is fed into useful, radiating antenna wire instead of ground. The only losses you are absolutely stuck with are (1) currents induced in the ground and other objects by radiative coupling and (2) losses in the antenna wire and feedline, connectors, tuners, etc. (2) can be reduced to essentially nothing by spending money and by attention to detail and (1) can be reduced to nearly nothing by getting the antenna up away from ground a bit. Even with it nearly touching ground, however, my experience indicates that (1) is a fairly insignificant loss at least at my location. (It would depend on ground conductivity.) It appears to me also that the use of a half-wave or longer radiator results in a better pattern especially for low-angle radiation. Look at the plots of half-wave verticals over ideal lossless ground for some clues as to how to pick up big gains on the horizon or near it. Then compare with the similar curves for quarter waves to see how to lose low-angle power. It looks to me as if a half wave has a significant advantage over a quarter wave in producing radiation at low angles. I have pondered the problem of how to feed an array of such antennas. The best solution I can come up with for multi-band operation is to install a rack full of tuners, one per antenna, and tune them all up for the band of interest so that the tuner inputs look like 50 ohms pure resistive. Then you can switch in 50-ohm coax sections to your heart's content to obtain the desired phasing. If you are rich enough to use automatic antenna tuners, this wouldn't even be especially inconvenient. Of course then you could afford a tower and a stack of monobanders instead! Once you have tamed the impedance problem by using tuners, you still have the problem of how to set up the phasing. The problem is that the antenna spacing is a different fraction of a wavelength on different bands. Maybe you should let your microcomputer model the array in its little mind and select a phasing scheme which it likes and then switch in the proper lengths of coax with a set of relays under its control. Then you never even need to know how it did it, as long as you like the pattern it draws for you on its screen. I suppose while we are dreaming it could also learn how to set up all the tuners for each operating frequency of interest and store all this away for future use. It could either display the settings for you to crank in or else it could tune up the proper settings for you with motors under its control. If you don't like a rack of tuners, there are cheaper ways to phase things, but they may be difficult to figure out for multiple bands. The Sterba Curtain or Bobtailed Curtain are examples of cheap ways to phase radiators on a single band for a single direction. The W8JK array may also give you some ideas. A single pair of radiators in the 8JK configuration will give you about 4 dB of gain and can be fed with a single tuner. You ought to be able to get about 7 dB of gain from a set of four radiators fed according to the 8JK concept. However, I don't see any easy way of getting this much gain in multiple directions without a fully flexible phasing scheme (which always makes me think of the tuners and the coax switches). I would be interested to hear what else you come up with or to fill in details which are not obvious to you from the above. 73, John S., W3IKG