Note:  This page is intended to document only how I put my system together and a few other miscellaneous items.  It should go without saying that there are probably better ways to do these things, that your mileage may vary, blah blah blah...

A while ago, I bought a used HP Z3801A GPS receiver with the intent of providing a handy time/frequency reference.  As with most projects, this one is ongoing, with incremental progress dependent on available time and current whim.  Nevertheless, it didn't take too long to get it up and running, following the "make it work, then make it look good..." mantra.

Click here for the plots from my receiver, on a web page created by GPSCon.  Look at the bottom of this page for recent news/info on the receiver's operation - or lack of it.  For a look at the receiver status of others, go to the Web Plots page at the K8CU website.

Uses of a frequency/time standard:
 

"What does this 'Signal Strength' (SS) reading really mean?" On the status screen of both the HP SatSat program as well as GPSCon (see below) a signal strength indication is given.  All the Z3801 manual really says is that this value should be above 20-25 for reliable tracking, and it should be higher than this for best accuracy.  In digging around the Motorola Oncore GPS documentation (the receiver used in the Z3801) I determined that this is an indication of "Signal to Noise Density ratio" (S/No) and is shown in db.  What does this mean, then?  This explanation requires a bit of communications theory - so I'll keep the explanation as simple as practical:  Take a relatively weak CW carrier as an example.  If your receiver has a bandpass filter 1 KHz wide, let's say that the strength of the that CW signal is the same as total energy in the noise everywhere else in that 1 KHz bandpass.  In this case, one has a 0 db Signal to Noise ratio, for the noise and signal are of the same total strength.  It would make sense, then, if you were to cut your bandpass filter down to 100 Hz bandwidth, you'd still be intercepting all of your original CW signal, but you'd be intercepting only 1/10th as much noise.  In this case, the CW signal is now 10 db stronger than the noise.  If you take it the other way, let's say that you widen the bandwidth to 10000 Hz (10 KHz) - the signal/noise ratio is now -10 db.  The GPS signal consists of several different signals, all modulated atop the same carrier.  If all you want to do is recover the GPS receiver's carrier (but no information) you could (theoretically) use an extremely narrow bandwidth to detect the carrier.  For receiving the data, you need wider bandwidth - a bandwidth proportional to the data rate.  So, to keep lock on the GPS signal, a C/No ratio of 20-25 db  is the minimum required.  This represents a pretty "ratty" signal, however, and the amount of noise present makes it difficult to recover the data stream and timing with great accuracy.  As you might expect, increasing the the C/No will reduce the amount of noise - and improve the accuracy of the recovered timing.  How much signal is "enough?"  As it turns out, by the time one gets up to a C/No of 45-50 db, enough of the uncertainty (noise) is gone and relatively little improvement may be had (comparatively speaking) by a further increase - at least for the "typical" L1-only (non-differential) GPS receiver.  As it turns out, most receivers don't go through a lot of pains to obtain really good C/No values, so it is more reasonable to take a "sliding average" to get a picture of how well a particular satellite is being received.  (I'm a bit skeptical about the receiver's reporting, though:  I have a bit of difficulty believing that this parameter is, in fact,  accurate when it reports a C/No of >200 db...) An interesting article that appeared in GPSWorld that describes C/No as well as effects of interference on GPS systems may be found here.

Why does one need a frequency/time standard?

• It has been said that the man with two watches doesn't know what time it is.  I now know don't know what time it is with much greater accuracy than ever before!
• A frequency standard is handy for making sure that everything else is on frequency.  At this location (Utah) reception of WWV on 10 MHz (the most convenient frequency for calibrating against) is very sporadic.  It is often difficult to get a good zero beat owing to the constantly changing path and the usual presence of WWVH.  True, most equipment will not hold accuracy great enough for this to be a real problem, but so what...
• A time standard is handy for certain newer operating modes.  One such example is K1JT's WSJT program which can benefit from having the computer synchronized to a "universal" standard.

The receiver:

The receiver itself needs several things to operate:

• An antenna.  This one is pretty obvious:  Because we are receiving satellite signal that are relatively weak, this antenna should be placed in a location that has a clear view of the sky.  Owing to the inclination of the GPS constellation being about 55 degrees (more or less) at this latitude one needs a northern view clear to only about 70 degrees or so - but the views in the directions of the other cardinal points should extend to the horizon, if possible.
• Power supply.  These receivers conveniently require a "48 volt" telephone-type supply (although some of them may have been supplied with the option for a "30 volt" input)  Actually, a "48 volt" supply typically operates around 54 volts because they often consists of 24 lead-acid cells in series maintained at "float" voltage.  When first powered up, the nominal current consumption is about 0.6 amps, dropping to 0.3-0.4 amps as the crystal oscillator's oven(s) attain operating temperature.
• RS-422 interface.  The RS-422 interface is a robust, high-speed interface scheme based on differential signaling.  For the 1pps and 10 MHz outputs (on the DB-25 connector) it makes perfect sense - but it is somewhat awkward to interface with the standard PC serial port.  For this reason, many '3801 users opt to make some internal modifications to invoke the internal (but unused, by default) RS-232 driver circuitry.  Keep in mind that - if you wish the absolute minimum propagation delay on the 1 pps output, RS-422 is the preferred choice as RS-232 has, by definition, a comparatively slow slew rate.
• To make use of the receiver as an absolute time reference, one needs a program to interrogate the receiver to discern the current time.  Typically, one interrogates the receiver for the time and the receiver responds with the time that it will be precisely at the leading edge of the next 1 pps output pulse.

The "PBJ" (Peanut Butter Jar) antenna:

Generally, an amplified (powered) antenna is employed with a GPS receiver such as this with the receiver (usually) located at some convenient distance - say, in the ham shack.  Owing to the very high frequency of the L1 GPS frequency (centered on 1575.42 MHz) typical coaxial cabling has high losses.  Because these losses directly contribute to receiver noise figure (and thus, overall sensitivity) it doesn't take very much coax loss before the GPS signal from an unamplified antenna disappears into the thermal noise.  Having an antenna with a built-in preamplifier allows the receive system to tolerate significant cable losses without degradation of the actual signal - a factor that also translates to being able to use inexpensive, smaller-diameter coaxial cable.
 

View of the top side of the "PBJ" antenna.
Click on the image for a larger version.

While a wide variety of amplified antennas are commercially available, I decided to "roll my own" GPS antenna.  Starting from the article "An Inexpensive External GPS Antenna" (QST, October 2002, Pp. 36-39) I constructed a turnstile antenna, but I departed from the article on several points:

• The article suggested a plastic cream cheese container as a radome (weatherproof cover.)  Typically, these are made from polyethylene - and not of a sort that is likely to be stable when exposed to UV (Ultraviolet radiation) from the sun - not to mention heat, cold, and moisture for extended periods.  Instead, I chose to use a glass jar (specifically, one formerly containing Adam's [tm] Peanut Butter - hence the name "PBJ") but its diameter wouldn't accommodate the suggested 4 inch diameter metal disk.   Instead, a disk approximately 3-3/8" diameter (of double-sided printed circuit board material) was used.
• Just to be on the safe side, it may be worth soldering small pieces of copper foil or wire at various points along the edge of the board to electrically bond both sides.  This is in addition to the bonding achieved by soldering the shield of the coax to both sides of the board.
• Small pieces of circuit board were soldered to shield the input circuitry from the output circuitry.  This is probably not strictly necessary, but it doesn't hurt...
• The article describes the turnstile elements being supported/fed by a piece of transmission line constructed of circuit board material.  Because I already had it onhand, I used a length of UT-141 semirigid PTFE coaxial cable.
• Because I planned to put the antenna on the roof - and because the GPS receiver is in the ham shack (in the basement) I wanted to add an amplifier to overcome coax losses.

A close-up view of the bottom of the "PBJ" GPS antenna.  (The antenna was turned upside-down for this picture.)
Click on the image for a larger view.

The upper picture shows how the two turnstile conductors (comprising 4 elements, actually...) are mounted atop the short length of UT-141 coax.  While not obvious from the picture, the bottom end of the UT-141 coax goes through the circuit-board disk with the shield soldered on both sides of the board.

On the bottom side of the board is a simple preamplifier (see the next picture) constructed using a Mini-Circuits MAR-6 MMIC (see the schematic, below.)  This amplifier has a moderately low noise figure (3-4 db) and reasonable gain (13-15 db or so) at this frequency - enough to overcome reasonable coax losses.  This amplifier is powered from the 5 volts superimposed on the coaxial cable:  The DC is first decoupled with a coil with the RF being blocked by a chip capacitor, then the DC is bypassed with an electrolytic capacitor.  Finally, DC is re-injected into the circuit via another choke.  Note that there is no blocking capacitor in the input of the MMIC, so a small amount of DC (about 1.5 volts) is actually present on the antenna - but this isn't any sort of problem.

The output of the amplifier is fed to another length of UT-141 coax which is soldered directly to an "N" connector (refer again to the top picture.)  This piece of coax is used to support the antenna and its circuit board, and the entire assembly is "captured" by the glass jar.  If preferred, one could use a BNC or even an "F" type connector (if one used RG-6 coax, for example) but in any case, one must assure that the connector is adequately waterproofed.

The "PBJ" antenna as installed.
Click on the image for a larger view.

The third picture shows the PBJ antenna installed on the roof.  In this installation, the coax (about 60' of RG-11) is fed up through the center of the pipe to which another antenna is mounted.  The antenna is secured partly by the weight of the coax pulling down, and partly by the black nylon wire ties which tie to a mast clamp below the GPS antenna.  It is worth mentioning that while the antenna mast to the right of the PBJ antenna (due south) somewhat blocks the view to the south, it doesn't appear to have any significant effect on the received signals.

As may be seen from the top picture, some holes were drilled in the "lid" (on the bottom) to allow ventilation.  After the picture was taken, small pieces of nylon window screening were glued into place (to help keep out insects) and all of the pieces were sprayed with clear lacquer to protect against the possible effects of moisture and condensation.  In this case, ventilation is important:  When placing a piece of equipment outside, one must either hermetically seal it (usually not practical) or one must provide ventilation to avoid accumulation of moisture due to repeated condensation. A thin layer of very light oil or grease was smeared over the metal lid's threads to prevent any accumulation of rust from "seizing" the lid onto the jar making it difficult or impossible to remove the lid.  Finally, electrical tape was applied at the joint between the lid and the jar itself to prevent accumulation of moisture in the lid's threads.

How well does it work?  Surprisingly well.  The length of RG-11 (I could have used RG-8 type but I had the length of used RG-11 laying around - and the impedance mismatch is, in this application, inconsequential...) offers about 6 db or so of loss at this frequency.  Even so, the signal strength peaks at about 180-190 for satellites that are between 60 and 90 degrees elevation and at 50-60 for those satellites that have gained enough elevation to clear ground effects and clutter.  RG-6 TV-type coaxial cable will also work well - provided you can somehow put the appropriate connectors on it...

Some eyebrows may be raised concerning the use of the MAR-6 as the amplifier.  True, there are more modern, lower-noise MMICs around - and if you have one, by all means, use it!  The MAR-6 (also known as the MSA-0685) is relatively noisy at this frequency - a noise figure of 3-4 db or so - but because the current generation of GPS satellites have such strong signals, a full-sized antenna that is in the clear gets plenty of margin.  Also, owing partly to aperture, a full-sized turnstile antenna such as this is likely to intercept a little more signal than a dielectrically-loaded (i.e. smaller) patch antenna - something that can help make up a bit of lost signal.
 

The schematic of the "PBJ" antenna.
Click on the image for a larger view.

In looking at the track map and statistics, the tracking ability of the receiver/antenna system appears to be limited only by horizon visibility.  To the southeast and west/northwest, visibility is somewhat limited by some tall trees - but in other directions (due south, due east, southwest) only mountains (elevations of <7 degrees, typically) limit the view.  I have seen the GPS receiver track satellites as low as 4 degrees - but this is rare because there are usually enough satellites at higher elevations and the receiver rarely has to try to track them...

Comparison of the homebrew antenna with a commercially-made antenna:

For several weeks (ending 3/10/04) I used a commercially-made GPS antenna instead of the homebrew.  The antenna used was a Magellan OEM-type of antenna (the round, white one.)  The results were observed to be as follows:

• On the commercial antenna, signal strength was slightly higher at lower elevation angles, but noticeably lower overhead.
• The commercial antenna was much more affected by snow than the homebrew one.  This is no doubt due to the fact that even when covered with snow, the sides of the large, glass radome of the homebrew antenna were relatively clear.
• The commercial antenna seemed to be unaffected by local 2 meter/70cm operation.  This definitely indicates that some sort of simple highpass filtering would be warranted on the homebrew antenna.
• The homebrew antenna is much less-affected by birds.  For some reason, quails and magpies like to sit on the antenna housing.  For the same reason that snow doesn't bother the homebrew antenna much, it's mostly unaffected by this, whereas a bird pretty much covers up the commercial antenna.
• The mast supporting the DF array (see the picture) caused blocking of the signal using the commercial antenna, but has no noticeable effect on the homebrew antenna.  This is likely because the aperture of the homebrew antenna's turnstile is much larger than that of the dielectrically-loaded patch antenna on the commercial antenna.

Putting this antenna on the roof:

The best location for any GPS antenna is one with a clear view of the eastern, western and southern horizons (or northern horizon, if you are down 'unda, or both if you are nearing the equator.)  In North America and Europe, for example, the inclination of the GPS constellation causes the minimum elevation to be fairly high.  In most cases, therefore, the preferred location for mounting the antenna is on the roof.

What are the mounting options?  A few come to mind:

• A Chimney mount.  These either anchor into the chimney or, using bands, wrap around it.  These work nicely - if you have a chimney.  (I don't...)
• A tripod on the roof.  These are available at many home improvement and electronics-type stores and work well.  The "gotcha" with this is that one must often drill a hole through the roofing material into the sheathing.  This is something that is best avoided - especially if your roofing is somewhat old and is likely to crack upon the tightening of the lag bolts.  Clearly, one must waterproof the final installation - something done with either tar or a roofing sealant (often called "Gorilla Snot" owing to its black color and it's tendency to stick to everything - and not come off.)
• Most houses have a vent pipe, an existing TV mast, a satellite TV dish mount on them:  The small size and weight of the typical powered GPS antenna means that even of attached to a 1 or 2 foot length of pipe, the wind load will be minimal.

Unfortunately, none of these are viable options on my roof::

• At my house, all of the vent pipes (both of them...) are on the northern slope of the roof - and a few things on the roof (the "swamp" or evaporative cooler, for example) blocks a portion of the sky toward the south.  Not only that, the only "free" vent pipe (the other one has a UHF/VHF vertical attached to it) is of such a diameter to make it awkward to even attach anything to it.  Finally, were I to use this "other" vent pipe, it would be within a few feet of the UHF/VHF antenna - and placing a GPS antenna near a transmitting antenna is something to avoid wherever possible.
• I have no chimney (I mentioned that...)
• My roof is metal.  While this makes a great groundplane (I use it with my MedFER beacon, for example) it has a problem with a "penetrating mount" (e.g. one that uses lagbolts.)  This metal roof is "floating" on about 1 inch of foam.  This is done not only to improve the insulation value, but it also make for a level, uniform surface for the metal of the roof, with the foam deforming to the slight variations in the roof.  Furthermore, since my roof was originally gravel - and the original roof is still present (albeit with all of the loose gravel removed) the surface is especially rough.  With this "floating" roof, bolting to the underlying sheathing is rather difficult, owing largely to the compressibility of the underlying foam.  Finally, I'm reluctant to do anything to risk the claimed 100 year warranty of the roof...

These ballast mounts are also available for the smaller direct-to-home satellite dishes - although one may have to mail order it, as they aren't always found at local sources.  For the GPS antenna, one of these types of mounts - along with the requisite number of cement blocks - is overkill and you are likely to lose your shingles before the mount blows away...

The GPS antenna on its ballast mount.  See text for more details.
Click on the image for a larger view.

You could also make your own mount from a metal plate sized to accommodate two side-by-side cement blocks.  Using angle brackets and a piece of pipe, one can make a suitable mount for small antennas.  One would also use one or two pieces of metal as a "strut" not only to stabilize the pipe but to make the pipe sit vertical to accommodate the pitch of your roof.  Under this plate, one would put the protective mat, and atop the plate, the two cement blocks:  Two are recommended as they form a "square" footprint, aiding in mechanical stability.

A few comments:

• If you install a ballast-type mount, you must put down something between the mount and the roofing to protect it.  Commonly, rubber roofing membrane is used (a suitable scrap piece is often available for the asking from a roofer.)  Alternatively, you can just go to your home improvement store an buy some doormats...  This mat protects the original roofing from scuffing and it helps to distribute the pressure somewhat.
• The company I work for has deployed hundreds of these types of mounts to support transmit/receive Ku-band satellite dishes, in sizes ranging from 1.2 to 2.4 meters.  When properly installed, there has never been a problem with the mount.  In those areas hit by hurricanes, it wasn't uncommon to see a roof stripped bare of its covering - except around the still-surviving roof mount.
• Size the mount appropriately.  For the GPS antenna, even a tiny mount using, say, 2 cement blocks, will probably suffice.  Even for larger mounts, the weight is distributed over such a large area as to be only a fraction of that cause by a person walking on the roof.  It should go without saying, however, that with larger mounts involving hundreds of pounds of ballast, the "point pressure" may be low but the overall weight may require one to consider placement of the ballast mount over a portion that is well-supported - especially considering snow load, where applicable.  (That is, avoid mid-spans of rafters, etc.)  For a small mount, this is inconsequential.
• It is preferred to place the cement blocks so that the openings are sideways and not straight up.  This is done to prevent accumulation of snow and ice - which can eventually break the block apart - as well as preventing the accumulation of debris such as leaves, etc. which can also trap moisture.

Needless to say, these are suggestions:  It is up to you to do the appropriate research and take appropriate safety measures!

The power supply:

I recently replaced the simple voltage-doubling unregulated transformer-type supply with a cast-off "48 volt" 2 amp switching-type supply from a telephone switch.  A simple modification was made to raise its voltage from 50 to 56 volts (the addition of a parallel resistor which was done to keep the Z3801 happy.)  The supply was mounted in a metal box and "brute-force" L/C EMI/RFI filtering was added on both the input and output to keep its noise out of my receivers.  I am still evaluating the efficacy of this supply as compared to the old one.  (It does run cooler...)

There is one "gotcha" about using switching supplies:  The Z3801A itself has a switching-type supply - and like typical switching supplies, it has its own start-up issues:

• You can't "soft start" many switching supplies - that is, (relatively) slowly increase voltage from zero.  Because a switching supply is a power conversion device, it will try to draw a constant amount of power from its source.  In plain english, its input current will increase as voltage goes down.  Sometimes, for whatever reason, they won't "bootstrap" themselves and start running.
• There is also another potential problem that compounds the issue:  Input filtering capacitors.  These capacitors (like all capacitors) present a (momentary) dead short when fully discharged.

The solution to this problem?  Actually, the fix is a very old one:  Delayed start.  If you power up the supply - then connect it to the GPS receiver - it may do just fine.  This "delay" may be done with an old-fashioned relay and delay circuit, or one could take a solid-state approach and use a transistor (bipolar or MOSFET) switch.

Power supply RFI:

As mentioned before, the Z3801 uses a switching supply to generate its internal voltages.  Actually, it uses several...

One of my "sub-hobbies" is LF/VLF listening.  With winter arriving - and the summer static subsiding - I decided to test my LF receive setup once again and was greeted with a noise floor about 30 db higher than I was expecting.

Presuming that the antenna itself was having some problems, I checked further and no, there wasn't a problem with the antenna at all - but that it was an external noise source.  It was now time to start shutting things down and/or disconnecting things.

The "Ah Ha!" moment came when I disconnected the antenna from the GPS receiver and the noise dropped 15-20 db.  Shutting the receiver off (something that I hate to do...) returned the LF receive antenna system to its expected performance levels...

The Fix:
 

Schematic of the power supply input filter.
Click on the image for a larger version.

At this point, I knew what I had to do:  Add a power supply filter in the Z3801.

Removing the top cover, the obvious place to mount it was against the back panel - next to the power connector.  I carefully cut a piece of circuit board material to the maximum size that would fit in the space and mounted it with two screws.  (Note:  When drilling holes in the Z3801 case, turn the receiver upside-down so that all filings will not fall onto the circuit board.  Then, before righting it again, de-burr the holes with a larger drill bit and vacuum all the filings from the tabletop and the lip of the chassis.  Failure to do this will likely result in hair-pulling intermittents or overall failure of the receiver!)

Then, the power connector was removed from the chassis and disconnected from the input on the power supply board.  I then carefully "un-crimped" the "board-end" of the internal power cable and soldered longer wires to it - although I could have simply cut and spliced.  Doing this gave me a long enough run of wire to go from the power supply input to the (to be added) filter.

Effective filtering at VLF/LF/MF frequencies isn't as straightforward as it might seem - especially when higher currents are involved.  Too often, I have seen hams simply slap ferrite beads on QRM sources - only to be disappointed whendoing so wasn't particularly effective.  Furthermore, simple ferrite beads and snap-on chokes only have effects above several MHz anyway and will do absolutely nothing at VLF/LF and MF frequencies.

The Filter:

L1 was salvaged from a computer power supply.  Typically, these chokes come in two flavors:  A bifilar-wound toroidal choke, or one that looks like a small transformer with two distinctly separate windings side-by-side.  In either case, both windings are extremely tightly coupled together and any common-mode energy passing through them is suppressed significantly.  If you have a means to do so, make certain that this choke has quite high inductance.  The choke that I used measured about 1.5 mH (that's Millihenries) per winding and thus has enough inductance to offer significant reactance at LF frequencies (nearly 100 ohms at 10 KHz.)  If you don't care about frequencies that low, then a choke with lower inductance may be used.  Remember:  We are trying to keep LF energy from the switching supply inside the GPS receiver from being conducted onto the power cable!

As mentioned before, capacitors C3-C7 re-assert common-mode signals and ground them out.  By this time, LF energy is mostly quashed by L1 and is then routed to "ground."  Both electrolytic and ceramic capacitors are used to help guarantee that both low and high frequencies are bypassed.

L2 can be either a choke similar to that of L1, or it may be two smaller inductors.  Their primary purpose is to keep HF out of the receiver and thus, the inductance of each winding (or inductor) may be in the 10-100 uH range (or higher, if you wish.)  Finally, C8 is used to make any energy entering (or exiting, for that matter) the GPS receiver's power supply common-mode.  Resist the temptation to connect a capacitor to ground at this point because any RF energy entering on the power supply lead will find it as a low-impedance path to the chassis:  Let L2 do its job and choke it out, first!

Finally, make certain that all capacitors that you use are rated for the voltage:  Ratings of at least 100 VDC are recommended on all capacitors.  If you don't see a voltage rating plainly printed on the component, don't use it!

Results:

The results of adding the filter were good:  No detectable noise on LF, MF, or HF emanates from the GPS reciever anymore.
 

The RS-422 interface:

One popular scheme is to utilize the Z3801's built-in RS-232 circuitry.  Although installed, it is not connected by default - a process that takes some minor surgery to complete.  Because this procedure may be found on the K8CU page (see below) it is not detailed here.

There are also numerous schemes to convert RS-422 levels to RS-232 levels - also found on the K8CU page.  If you are in a real hurry, however, you can do a faux conversion using just a single (optional) resistor:

• Connect RS-232 ground (pin 5 on a DE-9 connector) to pin 7 of the Z3801's DB-25 connector
• Connect RS-232 receive data (pin 2 on the DE-9) to pin 2 of the Z3801's connector
• Connect RS-232 transmit data (pin 3 on the DE-9) to pin 3 of the Z3801's connector via a 470 ohm resistor.  This limits the current from the RS-232 driver into the RS-422 receiver.  It is probably not necessary, but it doesn't hurt...

• Keep the cable very short!  The voltage swing on the RS-422 is definitely not within RS-232 spec.  The capacitance of long cables can degrade the data and cause errors.
• It may not work very well - or at all for you!  Because of the minimal voltage swing, it may not even cross the threshold of the RS-232 receiver in your computer in a reliable manner and is thus subject to noise.
• Use it only as a temporary measure (e.g. a "kludge") if you haven't a means to convert the receiver immediately.
• Do expect the occasional communications errors.  This scheme is, by no means, robust - for all of the reasons above.
• Reread the above!

Because of the differential nature of this scheme, it is possible to run RS-422 signals for miles - the distance being limited mostly by the high frequency response of the wire pair limiting the data rate.  A note here:  Even though RS-422 doesn't use the "ground" wire for signaling, per se, one is strongly recommended to limit the swing of the common mode signals on the wire pair to a level that the receivers can handle.

Another important reason for using RS-422 over RS-232 is that '232 has in inherently slow slew rate - that is, it can't swing very fast.  While RS-422 is capable of carrying signaling rates of several megabaud (for short distances) this simply isn't the case with RS-232.  What this also means is that if you want to use the GPS receiver for absolute timing accuracy with the 1PPS output, you don't want to use the RS-232 output unless you take into account the "slowness" of the RS-232 hardware - and the likelihood that this reference is likely to drift around a bit as thresholds in the RS-232 hardware change with time and temperature.  This isn't likely to be too much of a problem for the vast majority of users, as the difference is only likely to be a fraction of a microsecond, anyway...

Jamming myself... Several years ago, I was hiking with a group of hams, and we happened to be using a simplex frequency of 146.52 MHz.  I also happened to have my GPS receiver with me (back then, a Magellan GPS-4000 - non-XL version) turned on.  I couldn't help but notice that it didn't seem to be working very well at all:  It was constantly losing lock on all satellites.  When we stopped for a break, I happened to let my HT (a Yeasu FT-530) do it's timed auto power-off:  Since our group was all together in one place, there was no need to leave the radio on.  I then noticed that my GPS receiver was working normally.  Putting two-and-two together, I surmised (and later verified) that the '530 was, in fact, jamming the GPS receiver.  Here's how: At 146.52, the FT-530 uses low-side injection for its local oscillator.  With an IF of 15.25 MHz, this places the LO at 131.27 MHz - the 12th harmonic of which is 1575.24 MHz - almost exactly on the L1 GPS frequency of 1575.42 MHz.  Apparently, when the GPS receiver and the HT are within a few feet of each other, there's enough LO energy present to "jam" the GPS receiver!  The cure for this problem?  We now use a 147 MHz simplex frequency - which isn't as busy, anyway...

Use as a time reference:

It would be nice to be able to, say, drive a clock and supply GPS information to a host computer.  Being that there is only one serial port on the Z3801, one would have to multiplex both systems in a manner that the Z3801 can support.  I'm currently working on a PIC-based controller that will do the following:

• Interrogate the Z3801 for time and synchronize with the 1pps signal
• Make this time/date information available in a format of my choosing.
• Buffer transmit/receive queries from a remote computer - such as one running GPSCon - and multiplex these queries with those from the PIC itself.  Of course, the PIC could "eavesdrop" on such communications and use any data that it might need anyway...

Additional projects:

One problem that I need to address is receiver jamming.  When I operate 2 meter SSB, for example, the receiver loses lock on all satellites.  Why is this?  There are several possible explanations:

• The preamplifier.  The amplifier itself is a MAR-6 connected to the turnstile.  This amplifier is intrinsically extremely wideband and it is entirely possible that the 1.5 GHz turnstile is picking up enough energy from the 2 meter antenna (about 20 feet away) to be overloaded.
• The GPS receiver itself may be being overloaded.  Even if the MAR-6 isn't being overloaded, it is entirely possible that the GPS receiver itself is.  The MAR-6 amplifier could be amplifying enough 2 meter energy to cause this to happen.
• Weak harmonics of the 2 meter signal.  The 11th harmonic of 2 meter SSB operations (around 144.200-144.300) land "kinda close" to the L1 GPS frequency.

As it turns out, I answered the question (at least partially) by substituting the homebrew antenna with a commercial one.  Because the commercial antenna was unaffected by the presence of a 2 meter transmitter, the "theory" that weak harmonics are the cuplrit was discounted.  It could still be that the amplifier and/or the receiver itself are being overloaded by the presence of the local 2 meter signal.

Status of the GPS receiver at KA7OEI: - 3/08: Not much to report, really:  The "PBJ" antenna and the Z3801 continue to work.  Note that the computer connected to the Z3801 is not used very much, so the graphs on this web page are not updated vary often. - 7/04: I recently had some trouble with the Oncore GPS module:  After a power cycle, it would never lock and making it load defaults resulted in it believing that it was in an impossible location (e.g. at "596 degrees west and east at an altitude of 13k miles or so) and due to "bad geometry" the survey would never complete.  I remedied this by building an interface to power the Oncore module and connect it to the computer and, using the Motorola "WinOncore" program was able to reset the receiver and successfully test it.  Upon reinstallation, it again exhibited some strange behavior - but I was able to work around it.  (It may be heat-related:  Further investigation is warranted.)  - 3/04:  For several weeks (until 3/10/04) I have been using an OEM Magellan GPS antenna to compare it with the homebrew unit.  The results of this testing are in the text, above.  - 9/03:  On about 9/27/03, I fixed the GPS antenna.  For some weeks prior to this, I'd intermittently lose all GPS signals.  The problem got worse - to the point of total loss, but I didn't have time to look at it until the 27th.  The problem turned out to be a solder tack joint had failed where the UT-141 coax from the turnstile connected to the input of the MAR-6.  This change was done as part of the "permanentizing" of the receiver installation.  Much work remains to be done...

What if it is the second situation?  A simple bandpass filter stuck inline (a relatively tightly-coupled 1/4 wave cavity filter) would take care of the problem.  The "awkardness" here is that I'd have to feed the DC (for the amplifier) around the filter.  (I'll probably just build a GaAsFET preamp and avoid the problem...)

What about the last one - weak harmonics of the 2 meter signal?  Because I don't think that this is likely to be too serious a problem, I haven't given it much thought.  In reality, the occasional nature of the SSB operation - and the fact that the receiver seems to recover nicely from "holdover" (resulting from loss of GPS lock) may make it so that I'll just live with the problem...

Now, I wonder how things will work when I fire up my 23cm transverter???

This page will be updated as time goes on - check back later...

For more info on the Z3801A and other GPS receivers used as time/frequency standards, go to K8CU's page at:
http://www.realhamradio.com/GPS_Frequency_Standard.htm
 

Any comments or questions?  Send an email!