At 11:04 AM 7/10/2003 -0700, Michael Tope wrote:
>This brings up an interesting question, Jim. The gas
>tube ground blocks I mentioned are basically the same
>thing with a gas tube used in addition or in place of
>the air-gap. Since the aggregate RF voltage swing on
>a cable drop is pretty low, I wonder how much additional
>protection is really afforded by the gas tube? IOW, how
>do the breakdown and clamping voltages compare
>between a simple air-gap and the gas tube.
>
>At 40 volts/mil or whatever air gives, it may be a
>manufucturing issue to get the breakdown voltage on
>the ground blocks down to the level of a gas tube
>(CATV ground blocks are really low-cost devices).
>
>73 de Mike, W4EF..........
I've seen the transient suppressors that use a standard gas tube spark gap
with a probe to the center conductor of the coax. I think they just make
the dimensions such that the mismatch isn't all that bad. Then, they can
just buy a standard gas tube from Victoreen or whoever, just like the ones
in a plugstrip surge suppressor. They come in a variety of voltages,
typically corresponding to the various UL or ANSI test levels (330V, 440V,
etc.)
The other approach is to make a gas/air dielectric coaxial connector,
perhaps with a small bump in the outer conductor to set the breakdown
voltage. Air won't breakdown below 320 Volts, even with a very small gap.
I think the big variability in the device would be the temperature (and
hence the density) of the air, but, on the other hand, if you just make the
gap small, the breakdown voltage would be set by the minimum sparking
voltage for air, which is pressure independent.
Air breaks down in a uniform field at around 3100 V/mm, so it would be a
mighty small gap (0.1mm)
I found one data sheet, for an Edco CATV-DF, that gives the following data:
Frequency 0 to 2GHz
Insertion Loss <1.0db 0 to 1GHz Typ; <2.0db 1 to 2GHz Typ
Characteristic Impedance 75 Ohms
Connectors Female F-Type
Firing Voltage 180V
Surge Current 5000 Amps
Response Time 9100ns (10kV/?s)
GDT Life :600 Times @ 500 Amp pulse
Size (W x H x L) 2.5 in. x 1.25 in. x 3.875 in
It's a gas discharge tube based device
http://www.digitaltele.com/edco/catvDF.htm
But that's a pretty bulky device.
There is also the Cable Innovations CLPS-3009, which uses a Sidactor as the
suppresion device. This is used on a CATV system for pole mounting.
http://www.cableinnovations.com/CLPS-3009.htm
They claim 140V-220V triggering, in 1 nanosecond, and 3000 Amps for a 8/20
pulse.
They also have a "drop line suppressor" which looks more like what the
cable company would be installing:
http://www.cableinnovations.com/dlps-15dp.htm
Lots of specs, including return loss, but only goes from 5-1000 MHz..
They have a device specifically for cable modems..it has a directional
coupler built in. http://www.cableinnovations.com/diu15d.htm of interest
is that one of the claims is that it "eliminates the use of a ground block"
Seems to me that this is the sort of thing you want. The Cable Company has
an interest in good suppression (cooking the cable box that they own and
you rent with a transient is an expensive proposition.. service calls are
expensive, etc.) They also are very interested in low purchase cost, but
have the advantage that they're going to be buying the things in 10,000
unit lots, unlike hams, who buy one piece every 10 years, and then gripe
and grumble about it.
A bit of googling turns up a lot of similar devices, and, frankly, they all
look very inexpensive to manufacture. There's also a lot of inexpensive to
manufacture, but expensive looking stuff intended for the end user consumer
market (to be sold along-side speaker wire with directional power flow
arrows for best fidelity, no doubt)
Sidactors are made by Teccor, who make all sorts of clever 4 and 5 layer
semiconductor devices (probably every light dimmer in the US uses a Teccor
Quadrac). These little puppies have very low "off state" capacitance, so
they work well in RF applications.
http://www.teccor.com/web/menuitems/products/sidactor.htm
Lots of fascinating information about how to use these devices in all sorts
of applications. http://www.teccor.com/web/PDF%20Files/Telecom/3_rd_2.pdf -
take a look at page 26 and they have reference designs for protecting CATV
equipment. Interestingly, they talk about using an inductor in series to
compensate the capacitance of the device (how clever), and then design the
inductor to saturate when the sidactor breaks down and the transient
current is flowing (even more clever).
They're also designed with fuses that will not open when shunting a
lightning impulse, but will open if there is an inadvertent short of power
line to CATV drop. A lot more to this surge suppressor design than at
first glance, eh?
Jim, W6RMK
|