Hi The antenna it's self in these things is a constrained device. Gain and directivity are directly related to each other. If you want to uniformly cover a full hemisphere the gain will be 3 db relative to isotropic. If it is more, you are missing signal somewhere.
Of course what you want is an antenna with gain near the horizon. Throwing away gain straight up as a tradeoff is pretty common. For practical designs that turns out to be a db to a couple of db. More or less you have: 3 db antenna gain +/- 1.5 db + LNA NF of 2.5 db +/- 1 db = 5.5 +/- 2.5 db If you are running down a long / lossy cable, or worse yet through a multi port splitter you can see the noise figure of the receiver get into the act. That's going to happen as the gain of the LNA - loss goes much below 20 db for most receivers. Bob On Dec 16, 2009, at 9:28 PM, Charles P. Steinmetz wrote: > Bob wrote: > >> The Lucent Bullet is rated as a 26 +/-3 db LNA device. >> The Trimble Bullet is rated as 30 +/-3 db at 3.3 volts ( for the III model) >> The Trimble Bullet II is rated as 35 db at 5 volts >> A lot of the simple antennas are up around 45 db at 3.3 volts. >> The less specified parameter is noise figure. The Bullet II has a 2.75 db >> nf, the III at 3.3 db. I haven't seen a number on the Lucent part. >> These days a 1.5 db noise figure is not all that expensive at GPS >> frequencies. >> I suspect that none of that really helps straighten out any of this .... > > As another Time Nut pointed out in an offlist e-mail, like any other radio > system, it's not the LNA that does the serious work, but rather the physical > antenna. In the case of GPS antennas, the LNA primarily compensates for > losses in the feed coax. Higher gain can lead to overload of the LNA, so > it's not necessarily a panacea -- particularly in high-density VHF/UHF > environments. Reportedly, much better performance can be had from larger and > more sophisticated antenna structures. > > I have RF design experience, based on which I wouldn't put any faith in noise > figure specs from any but the most reputable suppliers. I assume Trimble is > one such, and I know that Andrew (the apparent OEM supplier to Lucent) is. > But most others should probably be taken with a large rocks of salt. > > Best regards, > > Charles > > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
