Sunday, March 15, 2009

Total Daydream

I spent last night making a spread sheet to see how to optimize my vertical array. It is going to cover 160 80 and 40 and pinch hit on 30. The patterns are very predictable figure 8 and cartioid on the lower 2 bands and multi-lobed with gain on 40. I wanted to see how the spacing between elements affected the gain and pattern of the various bands. As you switch between different phasings, it turns out for example the gain goes up for 0 degrees as you move elements apart but the gain goes down at 90 degrees as you move elements apart, and the variations change for each band, so its an interesting exercise in optimization. My verticals will have multiple elements one for each band, and each element will be tuned to resonance at the base. I will chose elements using a coax switch to switch between the output of each vertical. As I played around with the phasing and patterns and mutual coupling between elements, I started thinking about W8JI's probe antenna arrays. I think its very interesting that a guy who has a lot of land in a very quiet area, and who had a bunch of the so called gold standard Beveredges basically has abandoned that antenna for an array of little bitty verticals that sits on a plot of land 70 x 330 ft It means there is hope for us mere mortals with a couple acres. I think Tom's research on this topic is outstanding.

I was reviewing W8JI's website regarding small vertical receiving arrays The feature of this array is the small probe style vertical antenna that is heavily loaded so there is basically no mutual coupling between elements. This allows for nice precise patterns to be acheived.

It occurs to me instead of a FET amplifier at the base of the antenna, and a phasing arrangement and miles and miles of CTV coax, you could put an SDR at the base of each antenna, and bring the signal from the base of each antenna all the way to a digital I/Q signal. If you had an array of 4 antennas therefore you would have 4 streams of sampled digital I/Q data. If you then sent this data back to your computer, in the computer you could build a virtual array and steer the array by adjusting the phase and amplitude of the various probe antenna signals in the computer, and then go ahead and process that signal through the regular SDR software. You could even have the computer optimize the RX and if you had 2 arrays well seperated you could diversity receive as well.

As I was reading Tom's article, he also speaks about his transmitting arrays sending re-transmitted noise into his receiving array. He says if you de-tune the transmitting array that helps to cure that problem. It occurs to me you could sample the output of the transmitting array into an SDR receiver and send that data back to the computer to cancel the noise.

The main problem with this is how do you time signature each bit of data. If you want to manipulate the data you would need to be able to manipulate all the arrays of data from the correct time frame. Maybe you could do something like TCP the data for reassembly back at the ranch (computer).

Like I said its just a day dream (yep I'm spending my Sunday in the OR) but it may be a way to make some really interesting high performance arrays. Some of the A/D converters are beginning to achieve a dynamic range and a frequency range where you could just input strait into the A/D For example:

something like this might be just the ticket. See this article

Yea I know, I need someone to unscrew my head from my neck pipe and check my wiring.