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October 3rd 05, 04:46 PM
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On Mon, 03 Oct 2005 00:13:54 -0400, TRABEM wrote:
On Mon, 03 Oct 2005 01:09:33 GMT, wrote: I put your tirade on the end of this as it's not relevant to the orininal question. The original post (by A*s*i*m*o*v) mentioned back to back diodes as protection, so my reply was quite relevant. If it was about overload yes. It was about bandwidth and noise. Not sure what ruffled your feathers so badly Allison. But, whatever it was, I am sorry if I did something that didn't meet with your approval. You really came in with and attitude. but completely missed the real subject of the question. I also worked professionally as an avionics tech, and was in the business when the first Motorola commercial repeaters using synthesizers were put up...and they wondered why the other repeaters on the same tower couldn't hear well. That only took 'em about 10 years to figure out. Then, about 10 years after that, they started using gasfets, and it took them another 10 years to figure out they didn't do well in high rf fields. By then, it was the 1980's.....but at least they got it right eventually. We are talking presynth. I had my First Class before that. My first commercial UHF repeater was built from U44 components then later Motrac as they were current radios For 1971-2 that was pretty good. The motrac front ends were 6 cavitys a mixer then IF selectivity, crunchproof. LO was always a crystal for least phase noise. Anyone that read papers by Rhode understood this in the early to mid 70s. Maybe you designed some of those early nightmarish radios yourself? I take that as insult.  Gads no, I thought they were crap too.I'm not all that thrilled with the ARC308C or worse 324 and the put them in planes. I know that as I have a 308 as backup. Gag, Ick, ick. but at 1500 to 2k for a new one I'll keep it till it quits. Anyway, I wish you luck. And hope you realize the importance of keeping the front end clean, which sometimes means dumping the gasfet and using a bipolar preamp instead. Your being pendantic again. I'm not big on gasfets untill I get above 1GHz. At 6m a u310 bipolar fet in common base gets me about 12db. With a good IP and noise figure. Just enough to get past the image stripping filter before the Level-7 DBM. That makes for a nice crunchproof frontend that with sub 3db noise figure. If I could get that noise figure without the fet I'd not use it but I need the gain. I've tried mrf571 in a norton push pull configuration, hard to overload but the 100ma standing current was hard to take for portable ops! If your interested, ops here are QRP weak signal 6m SSB and 2M ssb and occasionally CW using solar power and battery. Power consumption is a consideration as well as perfomance. Overload performance with a KW 800ft away and in band is well challenging. The challenge is to get enough gain to ovecome the mixer noise without having a watt of power at the mixer. Actually, I hope the original poster (A*s*i*m*o*v) got the message about introducing gross non linear back to back diodes across the antenna terminals. That is true. However think about it for more than a second. Diodes or not hes talking about gain, bandwidth and noise so even without the diodes hes already running wide and risking overload at the next stage. Without selectivity you get cascade overload. The classic problem of weak reciever fix of stacking gain before it and watching the input overload threshold wander in to the -40dbm (or worse) region. Bad way to fix the problem. Allison KB1GMX |
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