So.... concensus is, 50dB gain antenna is too much gain, unless feed line is too long, reception is poor, or there are other circumstances extra gain is desired?
--------------------------------------- (Mr.) Taka Kamiya KB4EMF / ex JF2DKG On Thursday, November 21, 2019, 3:00:14 PM EST, Bob kb8tq <kb...@n1k.org> wrote: Hi That is indeed the gotcha. Once you get past a certain amount of gain in the preamp, the C/N levels don’t change enough to notice. Looking today vs looking tomorrow is unlikely to be of any help if you are after a fraction of a db. About the only way to check would be to fast switch an attenuator in and out of the signal path. Watch things for a minute at one setting and then do the same at another setting. Run for a while and log all the deltas. If you see a degradation of more than a few tenths of a db, you are getting towards the minimum gain point. Indeed there are some receivers that have an AGC built in. *IF* your receiver has one and *IF* you can get at it, that would be a great way to work this out. Indeed anybody who makes it past both of those constraints has a pretty unique device. ==== Simple answer for a 50 db antenna is to put an attenuator in after the DC has been eliminated from the circuit. It’s not idea, but it’s the best you can do. Running a great big splitter is one great way to come up with attenuation ….. Bob > On Nov 21, 2019, at 10:29 AM, John Ackermann N8UR <j...@febo.com> wrote: > > Bob, this is a great summary, thanks! > > One related question, especially with mixed systems -- how do you tell > if you have optimum signal level at the receiver? > > Most show some sort of SNR or Cn value. What should we look for? What > are the indication of *too much* signal? One issue in particular is how > to handle a modern GPS that expects modest antenna gain when it's > plugged into a system with a 50dB gain antenna at the top. > > Thanks! > John > ---- > > On 11/21/19 8:00 AM, Bob kb8tq wrote: >> Hi >> >> Way back in time, the first gear out there to use what we now look at as >> “normal” antennas >> was survey gear. For various reasons they decided on a 12V power supply and >> 40 to 50 db >> of gain in the preamp mounted in the antenna. They also got into L1 / L2 >> pretty quickly. >> >> A bit later the cell phone (and later broadcast) guys got into this. In a >> location with a lot of >> RF (like a cell site) having a lot of gain at the antenna didn’t work all >> that well. IMD issues >> got into the act pretty quickly. In addition, front end filtering was >> required to reduce overload >> issues. The focus was on L1 only so filtering was relatively easy. >> >> There is a whole separate set of antennas that put a big chunk of the RF >> portion of the radio >> in the antenna. Those still survive here and there. I have one of them and >> probably a couple >> of dozen of the more “normal” antennas. >> >> As time marched on, supplying 12V to antennas became a bit less popular. >> Most of the cell >> guys went over to a 5V antenna supply. The net result was 12V 50 db survey >> antennas that did >> L1/L2 and much smaller 5V 25 db antennas for “timing”. The timing antennas >> didn’t do L1/L2 so >> not going to work for survey. The survey antennas had way to much gain and >> no filtering so >> not going to work for a cell site. >> >> Indeed things did and do get crossed up in various pro and basement systems. >> With care and >> the right set of circumstances things may work. In other cases the result >> can be an ongoing set >> of systems issues over an entire network of stations. >> >> Prices for a good new survey antenna are up in the many thousands of dollars >> range. They have >> very stable phase centers and (usually) test results to allow correction of >> any residual phase >> issues. This is part of what lets you get into the “couple of mm” range on a >> survey. >> >> For timing, you have to dig a bit and answer a few questions. Is your >> concern how close you >> are to BIH? If so you will need to know all the delays in your system. This >> includes the delays >> in the antenna filters and the preamp. Is your concern (or measure) the ADEV >> at 1 second? >> If so the delays are not a concern. Your antenna choice may be a bit >> different depending on >> this focus. >> >> Bob >> >> >> >>> On Nov 21, 2019, at 1:25 AM, Taka Kamiya via time-nuts >>> <time-nuts@lists.febo.com> wrote: >>> >>> I have been looking antennas. Prices seem to range less than 30 dollars to >>> more than 500 dollars. Some are 20db gain and some are 40 db gain. Some >>> are specified as marine use only. Some are specified as timing use. Some >>> doesn't say anything at all. Power supplies are different. >>> Other than obvious, antenna is an antenna, isn't it? It captures L1 >>> signal, amplify it and send it down the coax. What makes one more costly >>> than others? What makes one timing antenna and one navigation antenna? It >>> doesn't make sense to me. >>> >>> I did some simple experiment with 26db, 40db, and magnetic stick on type. >>> I didn't really see significant difference. Signal level itself even >>> wasn't all that different. I have nearly a clear sky view 360 degrees >>> above 30 degrees above horizon. In some directions, clear view to horizon. >>> My feed is Timewave type. So It may not be the best but nearly ideal. >>> >>> Can someone shed light on this topic? (of course, I know some antenna has >>> integrated receiver. I am not talking about those) >>> >>> --------------------------------------- >>> (Mr.) Taka Kamiya >>> KB4EMF / ex JF2DKG >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@lists.febo.com >>> To unsubscribe, go to >>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.