Originally Posted by MikeySoft
Do you mean the end phase wires should go straight to the other side, making a long X? Then the crossing will not be at right angle and the two phases may interfere with each other?
I meant a long spiral (or double helix) instead of a short one. But mclapp says the short one works better, so it is moot at least for that design.
I should let you know that I do not plan to use a reflector. This is because I need bi-directional antennas. I believe the gain of the antenna will be 1/2 (3 db) lower, correct?
Might be more than 3dB as the reflector may narrow the beam (simulating an end fired array) as well as fold it.
I am also thinking of building two antennas, a 4 bay UHF antenna, and a two bay VHF antenna. Come Feb 2009, I will have two VHF stations (7 and 11) in almost opposite directions. I will also have one UHF station about 138 degrees behind the other UHF stations I am interested in. I plan to have one side of my UHF antenna pointed about 21 degrease to the right on one station which will put the other side pointed about 21degrees to the left of the other stations. The antenna will need a beam width of at least 42 degrees. Is that feasible?
I don't remember what the beam width was on mclapps design and it takes a long time to rerun the simulation to check it. The beam may be that wide without the reflector but there is probably some gain loss that far off center.
It is a good idea to attach your TV fool results (before and after transition if different) along with notes about what stations you care about and don't when asking specific questions about your location. You may be better off with a different beam pattern. Best results in your situation may be achieved with a pattern that has a butterfly shaped radiation pattern with a null in the directly forward and backward direction.
Look, for example, at the link I posted recently for the army antenna manual. The look at figure 4-37 where it shows how the beam pattern for the V antenna is formed from the patterns of the individual wires. The pattern of a single wire looks close to what you want suggesting that some form of end fed long wire might work (you would have to do some research). You can probably also get that kind of pattern by taking an existing 8-bay (or side by side 2-bay) design and reversing the phase of one of the antennas. That will probably produce a null front and back and strong side lobes either side. You have to get the impedance right though, you can't just connect a phasing line between two antennas that were designed to produce 300 ohms individually or you will get 150 ohms.
But you can put a balun on each (with one connected backwards) and run a short coax to a good splitter/ combiner followed by a preamp so you don't add more coax weirdness. At least in theory. The two antennas must point in the same direction (between your stations); antennas which point in different directions tend to combine very poorly though they might work if the normal beam patterns overlap. Best to simulate this including the feed lines (adjusting for velocity factor) because there can be some weird transmission line effects.
Look at hdtvprimer's antenna pages.http://www.hdtvprimer.com/ANTENNAS/ganging.htmlhttp://www.hdtvprimer.com/ANTENNAS/silver.htmlhttp://www.hdtvprimer.com/ANTENNAS/16bay.html
(look at the "incorrect" pattern).
And before you try this, make sure the signals actually come from where you think they do by using a highly directional antenna and find the best direction for each and that the strongest signal doesn't come from a reflection or diffraction at an odd angle.
You situation may be a great application for a custom antenna but it can be tricky to do the design work. It illustrates the shortcomings of the mass market one size fits all approach. In many cases, if you aren't too close to the antennas, one weird antenna might serve many people in the same community. Thus there is a potential significant benefit to having a local antenna producer. Luckily, your case isn't too far from stock designs.
I also think there is potential for having a computerized system where you punch in your location and it looks at where the signals are and runs a custom antenna generator program using something like a genetic optimizer then feeds a CNC wire bender to make elements for a custom antenna using stock parts. I posted a link here, with text along the lines of "check out this weird antenna", to an example of where NASA did something similar only they were trying to duplicate the performance of an existing antenna in a smaller package. But you can optimize whatever qualities you want. Signal to noise on specific channels in specific directions given specific signal strengths is easy to make a figure of merit from. Some of the results might be very weird. Imagine, for example, having four "flower pots" at the corner of a 20"x20" square with weird metal "shrubs" growing out of them with elements sized, positioned, and shaped such that they not only exhibited significant gain on the right channels in the right direction but do so in a manner that the combined signals don't trample each other (the hard part). If you have never seen a CNC wire bender in action , there is a video of one onyoutube
I had a somewhat similar idea, on a smaller scale, when I started working on these antennas. The idea was to optimize an antenna for my city. Turns out when I look at the post transition configuration that there isn't as much room for optimization as I originally thought, post transition requirements are somewhat similar to the one-size-fits-all solution, but there is still some room for optimization.
There is a significant risk that wind blowing trees, annual changes in foliage, or new construction will change the needed antenna pattern.
You situation is also a potential application for a rotator. Reportedly, though people aren't terribly happy with rotators on digital stations because they are hard to tune. And I have yet to see a rotator controller with a smart antenna inteface so it knows what station you are trying to watch. Rotators can be pretty sloppy about their aim, as well.
But a rotator (at least a good one) lets you use maximum possible gain in the direction of the station you are watching (assuming you are only watching one).