The N2MO station has an external GPS antenna on the gable end of the
building.  It's connected to the polyphaser arrestor with FSJ4-50
superflex.

The antenna mounting pipe has a #2 ground wire  (33.6 mm/2)  the
polyphaser has it's own #2 ground wire.  Both connect to an 8'  x 5/8"
(2.4m x 16mm) driven ground rod.  The jacket of the superflex is
grounded with the factoryt Andrew kit as well

Even with the GPS antenna lower in elevation then the HF beam and other
antenna (with similar protection)  I have concerns about leaving it
connected all the time.

73 Martin Flynn
W2RWJ

On 11/26/2014 4:37 PM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:

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On 11/26/14, 1:37 PM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:

Here's what we do at JPL for spaceflight equipment: reradiators.

Antenna with preamp on the roof... long coax to wherever the signal is
needed, amp(maybe), DC bias T (minicircuits has them, as do others, or
build one), and a variable attenuator (in case the system has too much
gain), feeding a passive antenna.  Another passive antenna (or your GPS
receiver or your whatever) is a meter or so away.

The inside antennas can be pretty crude.  I suspect stripping back 1/4
wavelength of coax shield would work.  We use ones that resemble a
hockey puck and that have the required bandwidth (L1,L2, L5).

You could, if you like, build some sort of shielding box with absorber
around the two antennas which would cut down on multipath reflections,
etc.  But that box would require some design so that it doesn't become
the path for the lightning to your gear.

The entire path from roof antenna to reradiator is sacrificial.

Sure, there's all sorts of RF over fiber stuff available. Some is even
designed for GPS signals specifically.

On 11/26/14, 2:00 PM, Martin A Flynn wrote:

AWG #2 seems a tad overkill, the current in a stroke can be carried by
AWG #10 without melting, but maybe you had a lot of it around for other
reasons.  I suspect the coax shield has smaller cross sectional area
than AWG #2 and you'll protect your grounding wire by blowing up the
coax<grin>. (in fact, looking at the data sheet for FSJ4-50, the DC
resistance of the outer conductor is 1 ohm/1000 ft = AWG 10.. it's
actually more resistance than the inner conductor (the inner conductor
is 0.820ohms/kft, and 0.140 inch in diameter, compare to AWG 10 which is
very close to 0.100 inch in diameter).

Hopefully your driven ground rod is bonded to the other system grounds?

I'd worry about multipath from the HF beam and tower (although maybe
you're not using that GPS for time-nuts 1E-20 precision...<grin>)

You CAN (almost) lightening proof your system.  The trick is to give
lightening a low impedence path to grind at very opportunity.

Start with the antenna mast and call.  Use iron pipe for the mast and feed
the antenna cable down the center of the pipe.  Place two large ground
clamps on this pipe and connect a large diameter wire that takes a straight
path to a group rod.    This will go a long way to diverting energy to
ground because high voltage likes to flow on the outside of a conductor
which would be the pipe and not so much the antenna cable.

The ground rod needs to be bonded to the rest of the building ground system.

Then the antenna cable passes through a metal bulkhead with a bulkhead
connector and all this is also grounded.  After this is might be a high
voltage e on the center conductor.  Use an "lightening arrester that is
bolted to the bulkhead.

At this point you are reasonably safe.  Remember that Ethernet is always
gavalically isolated by transformers

On Wed, Nov 26, 2014 at 1:37 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby@kirkbymicrowave.co.uk> wrote:

--

Chris Albertson
Redondo Beach, California

On 11/26/2014 5:14 PM, Jim Lux wrote:

The #2 copper was recycled.  The main RF grounding trapeze is tied to
the grounding electrode system with 1/0, which was also recycled from
another project.

Re the time-nuttery:  Only 1E-14.  Can't afford better (yet).

On 26 November 2014 at 22:14, Chris Albertson albertson.chris@gmail.com wrote:

BUT if the almost is not quite enough, one could damage a lot of
expensive test kit.

I lost Ethernet ports on

• Sun Blade 2000 workstation
• Sun Ultra 60 workstation
• Sun Netra T1 server
• HP Printer
• ADSL modem

All went at the same time. The next time just the modem went.

But I think I will be using an internal antenna.

Dave

On 11/26/14, 2:14 PM, Chris Albertson wrote:

Not so much high voltage, as AC and skin effect.  However, bear in mind
that lightning has a rise time of a microsecond or so: you can think of
it having a fundamental of 300-500 kHz (e.g. the first quarter cycle of
a sinewave), with most of the power below  1MHz.

Skin depth at 1 MHz in copper is 0.065mm.

In iron (using conductivty of 9.6 and relative mu of 1000) skin depth is
0.005 mm

So, steel/iron pipe is a terrible conductor for a lightning impulse,
compared to that nice copper coax next to it, or inside it.

From a electrical code standpoint, a grounded bulkhead connector isn't
compliant: you need one of those clamps that attaches to the shield in a
quasi permanent way. I'm not sure of the entire rationale, but I think
it's because connectors can become disconnected, but bolted connections
less so.

Which won't necessarily stand off a 10 kV lightning impulse. and, of
course, a common mode impulse carried on both wires of a pair might
couple via either capacitance, or more likely, through magnetic fields.

the ethernet galvanic isolation does a nice job dealing with the 10s or
maybe 100V common mode issues, and protects the network if there is an
internal short in a piece of equipment connected to the network.

Of course, with the increased prevalence of Power Over Ethernet, some
implementations of which are, shall we say, sketchy, that PoE system
might be a dandy conductor of transient energy.  For instance, a point
to point microwave network terminal up on a mast running power for the
circuit up the network cable.

Sweet.  I settled on #3 copper for my antenna grounding system on economic grounds and had a "debate" with a residential electrical contractor about bonding the antenna ground to the electrical service ground.  The city inspector passed the system with the bond installed.

I haven't used poly phasers but do hope to at least stop the house from burning down in the event of a lightning strike on the roof mounted antennas.

I also found that the metallic city water supply pipe is by far the best ground I have (at least for 60 cycle AC.) (Using a clamp on meter I can measure an appreciable current thru it, unlike the other two grounds.  I presume the current is being diverted from the utility neutral and is eventually making it's way back to the transformer that powers my house.) I'm glad the water pipe is also bonded to the electrical service ground.  I'm happy that I don't have any appreciable currents flowing thru my antenna ground.    This might be something to carefully consider (or possibly have looked at by a pro) if you have a really low impedance ground.  I've heard anecdotal accounts of sparks occurring when low impedance antenna grounds are connected to an electrical service ground.

My usual disclaimer of I'm not an expert in this field, the preceding is only my opinion,  and don't rely on this applies.

Mark Spencer

On 2014-11-26, at 2:56 PM, Martin A Flynn maflynn@theflynn.org wrote:

On Wed, Nov 26, 2014 at 3:10 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby@kirkbymicrowave.co.uk> wrote:

You can never be 100% but you can be nearly 100%.

The minimum is to follow the electrical code.  That might cost $40 in
materials for a single story house.  They allow bare aluminum grounding
wire which really reduces the cost.

After this minimum you have th think about the probability of a strike.  If
you live in Orlando Florida then it might be 100% and nearly zero in other
places and then you ask what the radio equipment cost.  I paid $18 for  my
Motorola Encore GPS receiver.

The iron pipe mast if it also provides a good path to ground will take
almost all the current.

Chris Albertson
Redondo Beach, California

On Wed, Nov 26, 2014 at 4:03 PM, Jim Lux jimlux@earthlink.net wrote:

Really?  What you care about is the impedance, not the depth of the skin
effect.  Also not the the coax is NOT exposed to the environment.  Tacitly
the coax comes out from the user side of the antenna, so it never sees
daylight.  The current flows in that first .005mm of steel.  Running the
coax down the side of the pipe would be a really bad idea.

--

Chris Albertson
Redondo Beach, California

On 27 Nov 2014 01:14, "Chris Albertson" albertson.chris@gmail.com wrote:

If

my

Chris,
if a GPS receiver was the only item that would get damaged I would not care.

If it damaged 3 signal generators,  three VNAs, a frequency counter, and a
spectrum analyzer, I would care.

My experience with the lighting going down the telephone line and
destroying a few computers, printer and modem would make me unhappy about
an external antenna.

Nobody has answered my other question - whether one gets a better
disciplined oscillator with an external antenna.

Dave.

On 11/26/14, 5:23 PM, Chris Albertson wrote:

Skin effect greatly affects the resistive impedance.

Compare a copper pipe vs a steel pipe of the same diameter.

The copper is basically conductive ring 0.065 mm thick and resistivity
of 1.67 and the steel is .005mm thick and resistivity 9.6

So the resistance ratio is 26:1920.. that is 2 orders of magnitude.

However.. the dominant impedance at lightning frequencies (at least for
copper) is the inductance, which is very weakly dependent on the shape
and cross-sectional size of the conductor: it's close to 1 uH/meter
regardless..

A copper pipe that is 2 cm in diameter and 1 meter long has a resistance
at 1 MHz of 0.004 Ohm

A steel/iron pipe that is 2 cm in diameter and 1 meter long has a
resistance at 1 MHz of 1.43 Ohm

The inductive impedance is about 6.3 ohms.

So the steel has a somewhat higher impedance than the coax inside it.
Yeah, there probably is some shielding effect, but I'm going to guess
that there's some insulating gap between the "bottom of antenna" and
"top of pipe", although that gap may be bridged by the plasma from your
lightning strike.

It's just that I'm not sure I'd trust the "shielding effect" of the
pipe, nor any preferential current distribution from the magnetic
fields.  For all one knows, that pipe might wind up filled with a plasma
of vaporized coax.

I'd just consider the antenna and feedline sacrificial, and worry about
dealing with the transient at the point of entry to the building,
assuming that the coax is carrying all of it.

Hi

You need a good antenna setup to get a good disciplined oscillator. The bigger question is if you need the sort of “better” it gives you. If you are running a simple TCXO based GPSDO, you will have a number of issues to deal with compared to an OCXO or Rb based system. If “better” is an objective, consider the entire system.

Bob

On 27 Nov 2014 03:06, "Bob Camp" kb8tq@n1k.org wrote:

bigger question is if you need the sort of “better” it gives you. If you
are running a simple TCXO based GPSDO, you will have a number of issues to
deal with compared to an OCXO or Rb based system. If “better” is an
objective, consider the entire system.

Bob,
can you explain more about the effect of antenna performance on a GPSDO
system?

Now you have told me it is important,  I would like to know more! My lab
has an East facing window.

The only way I would use an external antenna is if I had optical isolation.

I live in an area where the lightening risk is considered low,  but having
lost equipment twice, I am probably more concerned than others might be.

Dave.

Hi

If you are happy with 1x10^ -9 frequency accuracy, then there are a number of things you can ignore. If you are after < 1x10^-11 frequency accuracy 99.9% of the time you have to do everything right. Different limits apply to different measures of stability.

To reduce multi path you need a clear sky view. To keep the timing solution in the GPS happy you need to let it see >80 % of the sats it should see. In this case stuff below 20 degrees does not count towards the total.

In both cases the exact  numeric impact depends on a bunch of things. It would take a few hundred pages to go into all of it. You ultimately need a site survey to work out the multi path math.

In the extreme case the antenna can not see any sats. The math is simple then. Your stability is just that of your oscillator. That gets us right back to Rb's are better than OCXO's. Both are better than a TCXO.

Best advice is still the same. Get as good a GPSDO as you can afford. Put the antenna up on the roof.

Bob

Sent from my iPhone

Physically what is happening here is that the error bars on the fix your
GPS gets will cycle over a 12  hour period because of the sats all have 12
hour orbital periods.  If you can only see part of the sky the amplitude
of this is greater.

As an example thing of the worst possible case where you antenna can only
see a few degrees of the sky.  Every 12 hours one GPS satellite comes into
view and your GPS gets a decent "fix" but then for 8 hours the GPS sees
nothing and drifts off.  Now think about moving the antenna to a marginally
better place where it always sees at last ONE GPS stiletto but for 8 hours
there are two and for 2hours there are three satellites.  The quality if
the "fix" would still vary but would be better.  In the best case your 12
channel GPS receiver ALWAYS is able to select the 12 BEST places GPS
satellites that are in view.

This is not a great effect as long as there is enough sky that there are
always some in view.

Don
t worry about listening strike on your antenna.  If is FAR MORE likely that
lighting will strike some utility pole within 1/4 mile of your house and
the surge will come in through the AC mains power.  So if you want to fix a
problem fix that one first, then worry about less and less likely things.
This is not to say not to take normal precautions and ground the iron pipe
with a $7 aluminum ground wire just like you would do to an old fashioned
TV antenna.

--

Chris Albertson
Redondo Beach, California

Hi

The 12 hour periodicity of GPS is (in general) less obvious in an ADEV plot than 24 and 48 hour effects. Part of this is due to the lower “floor” at longer tau. Another part of it is due to things like the  ionosphere being at different places at the 12 hour points.

Bob