A question for Mr Plod - techincal pursuits
- From: Bob Bob <bob3bob3@xxxxxxx>
- Date: Sat, 11 Mar 2006 09:55:37 -0600
I see so many posts from you on what I would call non technical issues that I never feel like wading through what you have said to see if you actually talk about any.
What are your interests in the area?
I have a number of projects going;
- I am doing some major NEC modelling of a "new" 144MHz antenna system I am putting in place here. This is not a super duper high gain/directive approach but one that is a moderate project but with high expectations. I am concentrating more on reducing ground reflections and upward skewing of the radiation pattern. I am following mast and coax decoupling issues and even modelling in the surrounding metalwork. The vertical will probably end up being a EDZ with a centre hairpin style feed and end fed 5/8 elements. The horizontal will probably be a wide spaced 3 el quad. Their interaction is also being modeled.
- The older radios here are undergoing some realignment. At the same time I am replacing the older 3SK frontend MOSFETS with BF981's and their lower noise figure. In the case of my old IC202 I am taking the front end BPF out of cct as it contributes to about 3dB loss. (Hoping the high IP3 performance of the DG MOSFET will negate any overload sistautions. GaAs FETS are really bad for this. When you lower the gate current for best noise performance you get real bad intermodulation problems) The PIN diodes also contribute some major loss but I dont want to put a relay in just yet. I actually use this radio more for PLI noise tracking whilst walking down the street. The mods are still worth doing though.
- I am setting up a general purpose low phase noise "beacon" on 144MHz that transmits from the car. Maybe 100mW or so) There are lots of experiments I want to do with this. The basic idea is that I drive the car around whilst it transmits and my home station records the results;
* The CW signal being very narrow will be a good candidate for FFT processing. I may be able to modulate the signal withn something of a very low data rate (say 1-2Hz). The net result is that it will be able to be heard further into the noise, maybe down to -160dBm or so. I'll be able to do propagation studies of sorts. (eg send slow GPS data back to the base)
* The doppler shift one gets on 144MHz when talking narrow bandwidths is not trivial. In the order of 10-11Hz at 100kph. Wondering if this can be tracked to determine the car stopping and starting at traffic lights etc. You'd also be able to see the density of oncoming traffic and their speed by the higher shift. Obviously the driection of travel relative to the base station modifies all this.
* Maybe modulating the TX CW signal through those doppler antennas one uses for foxhunting would yield directional info. If there is a GPS on board it would be easy to phase lock to that. The net result would be to be able to determine what direction the car is from the base station and which direction it is facing. Lots of DSP/computer processing on that but it should be do-able.
- Look into DSB I/Q synchronous radios and where they stand signal to noise wise compared to SSB. ie using these detection techniques is there any S/N gain of using sync DSB over SSB even though it uses twice the bandwidth.
- Further on DSB I/Q radios setup a system on 144Mhz in that "newish" 100kHz bandwidth allocation for data modes. (Might also try this on 900MHz ISM) Probably end up being a TCP/IP packet system with Verterbi or Turbocode FEC on the base signal. Should be able to get 256kbps with that kind of system.
Any discussion from you or others on the above is most welcome.
Cheers Bob VK2YQA (Who also works in radio/tech professionally)
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