Hi In the case of a GPS, you really can't increase the aperture (gain = directivity) since you want to cover the entire sky. The filter needed here is far from trivial. The spacing on the "good to bad" signals is pretty tight in this case. A filter can indeed be built, no doubt about that. It's going to be a bit more than I can shoot up in the basement or make on a simple pc board. Bob On Mar 4, 2011, at 8:59 PM, Mike Feher wrote: > Well, as you said John, for FWIW. In this case not much. As said low loss, > so increase in noise temp would be minimal, and, if it makes a difference > between an overloaded front end or a 0.5 dB loss in NF, it will be welcome. > Heck, we use filters in front of most of our Satcom LNBs at 21 GHz with > minimal effect. Been there and done that, as the saying goes. If necessary > the small increase in noise temp can easily be overcome by a slightly larger > aperture, especially since a view of the full sky is not really necessary > and birds below certain elevation angles are typically ignored by software > settings of one's own choosing. And again, I was talking about people who > have done this and can do it again. It would not be an issue for me. Regards > - Mike > > Mike B. Feher, EOZ Inc. > 89 Arnold Blvd. > Howell, NJ, 07731 > 732-886-5960 office > 908-902-3831 cell > > > -----Original Message----- > From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On > Behalf Of J. Forster > Sent: Friday, March 04, 2011 8:47 PM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] GPS Filter > > I'm not so sure. > > A filter ahead of a preamp significantly increases the system Noise > Temperature. > > GPS signals are weak and link margins are small. The receiver preamps are > already very low noise. > > I'd think that a narrow filter might well drive up the systen NF to the > point it'd be useless. > > FWIW, > > -John > > ============= > > >> I used to make some interdigital filters and amplifiers in the early 80's >> for MDS TV reception in the 2.3 GHz range. One can easily fabricate a low >> loss narrow band filter at 1.5 GHz if need be, and as mentioned before, >> antennas should be easy as well. If this really becomes an issue, I am >> sure >> there will be a lot of solutions offered and anyone with some RF >> experience >> will also be able to handle it themselves. Regards - Mike >> >> Mike B. Feher, EOZ Inc. >> 89 Arnold Blvd. >> Howell, NJ, 07731 >> 732-886-5960 office >> 908-902-3831 cell >> >> >> -----Original Message----- >> From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On >> Behalf Of Ziggy >> Sent: Friday, March 04, 2011 7:47 PM >> To: time-nuts@febo.com >> Subject: Re: [time-nuts] GPS Filter >> >> The discussion got me to thinking about how we used to filter out nearby >> interference on amateur TV - namely by using interdigital filters. This >> led to a search for GPS interdigital filters which i did indeed find. >> See Alison Microwave website at >> <http://www.amlant.co.uk/DetailsAD430.htm> for one example of an >> integrated antenna/filter/preamp. (I'm sure these aren't cheap, but I >> haven't asked.) As for retrofitting, you could add a filter after the >> antenna/amplifier assembly but I might be concerned that the amplified >> GPS antenna is pretty wide and may have trouble with a Lightsquared >> transmitter nearby. There are passive antennas though, and there are >> in-line amps - you'd need to add the filter in between. We made these >> ourselves for 439 and 1296 MHz - GPS L1 isn't much above that so with >> some care it should be doable. The tuning can be finicky though :\ >> >> Ziggy >> >> On 03/04/2011 03:31 PM, John Ackermann N8UR wrote: >>> Here's a measurement we did a few years ago on the HP 58535A: >>> >> > http://www.febo.com/pages/hp_gps_splitter/port_1_hp_58535a_two_port_amp.png >>> >>> >>> John >>> ---- >>> On 3/4/2011 1:31 PM, Bob Camp wrote: >>>> Hi >>>> >>>> Ok, now it's pretty obvious the RF world near your GPS will be >>>> changing a *lot* in the near future. Lightsquared and a bunch of >>>> similar outfits will be camping out right next door with very high >>>> power gear. They will be running 1.5KW from somewhere in town. GPS is >>>> running 30 watts from off planet. >>>> >>>> Has anybody tossed the various HP / Symmetricom GPS splitters on a >>>> network analyzer? If so, what do the filters in them look like? >>>> >>>> I probably should corner the market on these things before asking a >>>> question like that. >>>> >>>> The new neighbors will be at 1525 to 1559 MHz. GPS L1 is at 1575.42 >>>> MHz. That's what we are using for timing. L2 is down at 1227.5, >>>> right now it's mainly military use. Obviously these guys are a bigger >>>> deal for civilians than the military. >>>> >>>> So the question is - do the built in splitter filters have any real >>>> rejection 15 to 50 MHz off of center? >>>> >>>> Probably worth checking. It would be a pleasant surprise if they >>>> turned out to be useful. >>>> >>>> Bob >>>> >>>> >>>> >>>> _______________________________________________ >>>> 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. >>> >> >> >> _______________________________________________ >> 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. >> >> > > > > _______________________________________________ > 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.

On Mon, Mar 7, 2011 at 5:01 AM, Bob Camp <lists@rtty.us> wrote: > Hi > > In the case of a GPS, you really can't increase the aperture (gain = directivity) since you want to cover the entire sky. No, in theory it need not be an omnidirectional antenna. One could use a very high gain antenna if it were able to track each satellite. If you had access to the GPS firmware one could build a phased array antenna that was electronically steered. I think it could work but it's not practical because of the very high cost and large size. Would it be possible to build a much simpler version that used a line array phased to null the nearby tower. -- ===== Chris Albertson Redondo Beach, California

Hi Since you are after timing off of the sat's, having antennas that move, either physically or electrically seems like a problem. Any shift in the effective antenna location as you tracked the satellite would be "exciting" to compensate for. There was an early paper published based on doing this (early 80's). I did tear into a Thunderbolt. It certainly looks like there's a filter on the front end. It's roughly similar to the filters in the HP splitters. It might have the same sort of attenuation. Bob On Mar 7, 2011, at 12:39 PM, Chris Albertson wrote: > On Mon, Mar 7, 2011 at 5:01 AM, Bob Camp <lists@rtty.us> wrote: >> Hi >> >> In the case of a GPS, you really can't increase the aperture (gain = directivity) since you want to cover the entire sky. > > > No, in theory it need not be an omnidirectional antenna. One could use > a very high gain antenna if it were able to track each satellite. If > you had access to the GPS firmware one could build a phased array > antenna that was electronically steered. I think it could work but > it's not practical because of the very high cost and large size. > > Would it be possible to build a much simpler version that used a line > array phased to null the nearby tower. > > -- > ===== > Chris Albertson > Redondo Beach, California > > _______________________________________________ > 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.

On Mon, Mar 7, 2011 at 7:34 PM, Bob Camp <lists@rtty.us> wrote: > Hi > > Since you are after timing off of the sat's, having antennas that move, either physically or electrically seems like a problem. Any shift in the effective antenna location as you tracked the satellite would be "exciting" to compensate for. There was an early paper published based on doing this (early 80's). I don't think anyone would build a phased array GPS antenna but if they did the elements of the array would only move within a 1/2 wavelength range. For GPS I think this less than 2 centimeters. but the effective center of the antenna would not have to move. -- ===== Chris Albertson Redondo Beach, California

On 3/7/11 10:27 PM, Chris Albertson wrote: > On Mon, Mar 7, 2011 at 7:34 PM, Bob Camp<lists@rtty.us> wrote: >> Hi >> >> Since you are after timing off of the sat's, having antennas that move, either physically or electrically seems like a problem. Any shift in the effective antenna location as you tracked the satellite would be "exciting" to compensate for. There was an early paper published based on doing this (early 80's). > > I don't think anyone would build a phased array GPS antenna but if > they did the elements of the array would only move within a 1/2 > wavelength range. For GPS I think this less than 2 centimeters. but > the effective center of the antenna would not have to move. > GPS Wavelength is (for L1) is about 20 cm.. longer for L2 and L5, of course. It's not too tough to make sure that the apparent phase center of a phased array doesn't move, and a lot of precision GPS processing already deals with the phase center moving as a function of look angle.