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Richard,
With regard to performance. Doing an A/B comparison between the commercial antenna and my version. The range achieved by the commercial antenna was approximately 4 times the range of my version. The range was determined by loss of using one end as a beacon and moving the test unit further away until it could no longer receive valid packets, then move in and out of range several times to confirm consistency. The same was done with my version. Before the test, I monitored the return port on the directional coupler on a Spectrum analyzer while seeping frequency from the sig. gen and noted the SWR on the commercial unit was lower than my version. (Though the measurement was not calibrated I could see the SWR was better but did not determine how much). Previously I designed a Microwave movement detector that operated at 1.2 GHz using Doppler principle. It used a microstrip tank circuit connected to a negative resistance oscillator. The design was published (RF Design magazine Dec 1986). Though there were many technical issues regarding the oscillator (how to adjust impedance in the base for optimal negative resistance looking into emitter, effect of moving the antenna, ground plane under the antenna etc etc), others were able to make the design just as per my prototype and it worked. They could then proceed to"play" with the design to change its performance. I really would have expected that a similar scenario would exist with antenna design (Someone has made one just like what I am wanting to make and is able to share with me the details on what they did that worked, then I would play with the design). I am grateful for the many responses I have received from my postings to this group and am confident there is much good information in what I have received but mixed with people's actual experience is a bunch of theoretical information that makes it hard to pick out what is useful. (ie. some of the information has conflicted and left me not understanding what is actually happening here). If someone has actually made these types of antennas and has some practical experience with them, that would be a fantastic starting point for me. (ie. Low power, UHF, portable). Richard Clark wrote: On Mon, 03 Oct 2005 02:02:13 GMT, David wrote: If this is correct then the electrical length of the sleeve must be modified by the presence of the RG174 coax outer sheath. The sleeve is sort of performing a dual function. Hi David, I think I pointed that out already. The effects of this jacket are variable across many suggestions offered here over the years. As it is difficult to cut the tube once soldered in place, I will need to make heaps of these to get the length right. At least if I had a reasonable starting point it would minimize the number of iterations. Ah yes, the pain of it all. I mentioned that too. You may as well grit your teeth and set to it, there is no shortcut to this all. The other issue is that I read about cutting lengths for elements and not that even an element in free space is trimmed down by about 3% to account for some "end effect" ? Would this also apply to my sleeve ? If so, then the length may be difficult to get right because if I make allowance for the end effect then the choking effect of the sleeve will not work as well. You might want to work this backwards. That is start off with a successful choke section of the sleeve. Let that dictate what follows because its isolation will mediate what I describe. If you have any experience with what is called an Offset Center Fed Dipole, you would find that it offers a closer match to 50 Ohms. You are using the dipole as a variable match by finding the 50 Ohm portion along its full length. This means that the portion above the sleeve's attachment to the drive point may not be the classic length (and certainly not accounted by the 3% of the "end effect" - not even close). That is neither here nor there because at that point you will have simultaneously achieved resonance, a match, and isolation; and yet by no fixed formula pulled out of a hat, nor accredited by an institution of higher learning. And, by the way, it isn't going to pull together in the first pass. I hope you have a stack of tubing, so put on a happy face and get down to it. This is why I am now wanting to understand how these work and have a good starting point for making them. The commercial units used molded construction that was difficult for me to emulate. I have started with RG174 Cable, 5/32 Brass tube all attached to SMA plug and used 1/4" nylon tubing as a radome. It goes together well and looks good but does not perform very well. How do you know? It might be doing the best job you could ask of any design. Your statement requires FAR MORE qualification than a rather subjective toss-off. In other words: 1. To what parameters? 2. Compared to what? 3. By what measure? 73's Richard Clark, KB7QHC |
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