Pointer: Using wireless modem over coaxial cable link

A recent thread in the Usenet news group 'comp.os.linux.networking' titled
"USing wireless modem over cable" has some interesting thoughts about the
idea of using a pair of wireless access points and a considerable length
of coaxial cable.

Briefly, a reported one mile (1.6 km) of 75 Ohm Heliax (exact part number
not known - reported size 1.125 inch which I don't see in my catalog). The
original poster never stated if this was pole mounted or buried. The
measured loss of the cable was reported at 75 dB. Given a +20 dBm output,
this should provide -55 dBm at the receiving end. There is no atmospheric
concerns, and no problems with Fresnel zone clearance, trees, interference,
or obstructions to line-of-sight. The resulting signal strength should be
more than adequate for a wide band link, even though 75 Ohm is the wrong
impedance (50 Ohm is the "correct" value).

Problem: This cable is _extremely_ expensive when talking about these
distances. There may also be substantial problems in being able to
place the cable (right-of-way issues). Really, really, really, there MUST
be better solutions than this.

Problem: The cable may act as a ground loop, and raise serious questions
of danger from lightning strikes and other electrical problems. Consult
your insurance carrier, and local jurisdictional building codes or similar
for your area. This could result in cancellation of your insurance, as well
as civil or criminal penalties. You have been warned!

Bottom line: The original poster posted an article early today, with but
four words of text:

Holy Moly it works.

Go to http://groups.google.com and the advanced search page. Select
"Advanced Groups Search". In the "Group" box, put 'comp.os.linux.networking'
and in the "Subject" box, put "USing wireless modem over cable". You can
speed the search up slightly by selecting "Message Dates" "past month".
There were 13 messages in the thread, started Jul 6 2006.

Old guy

On Mon, 17 Jul 2006 14:50:30 -0500, (E-Mail Removed)
(Moe Trin) wrote:

>A recent thread in the Usenet news group 'comp.os.linux.networking' titled
>"USing wireless modem over cable" has some interesting thoughts about the
>idea of using a pair of wireless access points and a considerable length
>of coaxial cable.

http://groups.google.com/group/comp....20265adf1982c7

>Briefly, a reported one mile (1.6 km) of 75 Ohm Heliax (exact part number
>not known - reported size 1.125 inch which I don't see in my catalog).

That's a mighty big and expensive coax cable.

>even though 75 Ohm is the wrong
>impedance (50 Ohm is the "correct" value).

75 ohm coax cable has less loss per foot than 50 ohms. Also, once the
loss gets above something like 20dB, any reflections from the far end
will never be seen by the near end. Therefore, VSWR is not really
that important.

>Tere may also be substantial problems in being able to
>place the cable (right-of-way issues). Really, really, really, there MUST
>be better solutions than this.

A few notes:

1. Comcast was looking into doing Wi-Fi over home CATV cable for a
while. It worked quite nicely but ran into a stupid problem. The
quality of the average home CATV wiring is so poor that chances were
high that sloppy connectors and crappy coax would turn it into a
maintenance nightmare.

2. Laying coax in residential areas is always a problem as anything
that crosses the property line involves considerable bureaucracy.
However, a tolerable rule might be "if they can't see it, they can't
complain about it".

I live on a semi-rural owner maintained marginal road. Many years
ago, when I installed my first bootleg CATV system, I did some
horizontal drilling under the road for the cable. No big deal with
the right equipment.
| http://en.wikipedia.org/wiki/Horizon...ional_drilling
Of course, I didn't do it with such expensive equipments. I just used
a big steel pipe with some teeth ground into the end, a water hose
down the center, a chain drive, and a big motor (1 hp?) and gearbox.
It lasted long enough to drill 3 holes across the road before the pipe
bent, breaking the chain. After drilling, we pounded in a few lengths
of schedule 40 pipe.

3. There is a better way. FTTS (Fiber Through The Sewer):
| http://www.lightreading.com/document...g&doc_id=24713

4. If you don't mind going through the air, there's the G-Line
(Goubou Line):
| http://en.wikipedia.org/wiki/Single-...nsmission_line
Very low loss but must run in a straight line.


>Problem: The cable may act as a ground loop, and raise serious questions
>of danger from lightning strikes and other electrical problems.

Two small capacitors. One to the center conductor and one to the
shield should be sufficient isolation. 50 ohm termination optional.

>This could result in cancellation of your insurance, as well
>as civil or criminal penalties. You have been warned!

Because there is no specific electrical code chapter and verse for
such an installation, even if you do it the right way, it's possible
for the insurance company to claim that it started a fire.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS

On Mon, 17 Jul 2006, in the Usenet newsgroup alt.internet.wireless, in article
<(E-Mail Removed)>, Jeff Liebermann wrote:

>That's a mighty big and expensive coax cable.

No kidding. I had a 5000 foot length of a 1/2 inch 75 Ohm buried in a
simple trench (dug with the angled blade of a a road grader - about 9
inches below grade). I'm pretty sure it was a Times Aluminum foam
type (complete with steel tape armor and flooding compound) - and the
cable and trenching cost nearly US22K in the late 60's. I'd hate to
think what it is now.

>75 ohm coax cable has less loss per foot than 50 ohms. Also, once the
>loss gets above something like 20dB, any reflections from the far end
>will never be seen by the near end. Therefore, VSWR is not really
>that important.

My take was that the VSWR of the cable was still substantially better
than the "normal" VSWR of most antennas. If we really were concerned
about VSWR, a quarter wavelength of 61 Ohm line brings that into reason,
though a stub match would likely be easier to fabricate..

>1. Comcast was looking into doing Wi-Fi over home CATV cable for a
>while. It worked quite nicely but ran into a stupid problem. The
>quality of the average home CATV wiring is so poor that chances were
>high that sloppy connectors and crappy coax would turn it into a
>maintenance nightmare.

I can believe that - the "quality" installation I've seen used something
similar to RG-6 (but with a foil and braid rather than silver inner, copper
outer braid) from the street to the wall jacks, with type F connectors. I
suppose that coax is better than the RG-59 look-alike used from the wall
jack on, it's far from a really decent cable. I don't like single braid
coax above 800 MHz, even though spec-sheets often had imaginative numbers
up to several Gigs. I know that RG-58 was a pretty good antenna at 5 GHz
(it sure didn't act as a coax), and expect RG-59 to be similar. The
connectors? Powdered plastic doggy-doo.

>3. There is a better way. FTTS (Fiber Through The Sewer):

That would be my preference. Dunno what the costs are like.

>4. If you don't mind going through the air, there's the G-Line

>Very low loss but must run in a straight line.

How does that handle birds parking on the line - and catenary suspension?

>Two small capacitors. One to the center conductor and one to the
>shield should be sufficient isolation. 50 ohm termination optional.

Outside/Inside DC Block - but what voltage rating? Actually the concern
I have is lightning. By the time we had our "very nearby" strike (a few
hundred feet from one end of buried cable above), that run had been
decommissioned. However, buried phone lines did have a problem that I
determined was likely due to substantial differential ground levels
at the ends of the cables. While there was lightning protection, the
active stuff attached to the lines failed (as best as I could determine)
from exceeding common move input levels on the chips.

I don't know which would be a better idea - the DC blocks, or powering the
access point at the "ungrounded end" via an isolation transformer, and
locking the box in an insulated cage, with a short fiber link between the
access point and the local computers.

>Because there is no specific electrical code chapter and verse for
>such an installation, even if you do it the right way, it's possible
>for the insurance company to claim that it started a fire.

or that any damage isn't covered because you didn't inform them of this
hidden cable.

Old guy

On Mon, 17 Jul 2006 22:42:09 -0500, (E-Mail Removed)
(Moe Trin) wrote:

>>4. If you don't mind going through the air, there's the G-Line
>
>>Very low loss but must run in a straight line.
>
>How does that handle birds parking on the line - and catenary suspension?

The surface transmission occurs between the conductor and the
dielectric insulation. A bird sitting on the wire would not act as
much of a reflector. The unavoidable cable droop isn't much of a
problem as long as it's gradual. There have been tests using
seriously dropped PG&E power lines which found that it works well
enough with the droop. However, the relatively sharp bend at the
insulators on the pole are a problem. There will be some radiation
(and therefore some loss) at this point. The amount of radiation is
determined by the bend angle.

>>Two small capacitors. One to the center conductor and one to the
>>shield should be sufficient isolation. 50 ohm termination optional.
>
>Outside/Inside DC Block - but what voltage rating?

Dunno. A few thousand volts. Porcelain capacitors should have the
voltage rating and not turn into inductors at 2.4GHz.
http://www.atceramics.com/products/m...capacitors.asp

However, I can see construction problems. Perhaps using the same
trick as the "through the glass" car antennas might be a better idea.
We're only concerned about 2.4GHz, so a frequency selective
transformer or antenna would suffice. Just put a loop antenna,
biquad, or patch antenna at each end. Move the access point close to
the antenna. Instant isolation.

>Actually the concern I have is lightning.

Sorry. That's an act of God and is not covered in the warranty.

>I don't know which would be a better idea - the DC blocks, or powering the
>access point at the "ungrounded end" via an isolation transformer, and
>locking the box in an insulated cage, with a short fiber link between the
>access point and the local computers.

Two helical resonantors in a can acting something like an xformer.
Float one or both connectors.

>or that any damage isn't covered because you didn't inform them of this
>hidden cable.

The insurance company doesn't require disclosure. However, if it can
demonstrate a non-code compliant installation, they can easily deny
the claim. It doesn't matter if it was done correctly. It only
matters that the proper building permits were issued and signed off.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS

On Tue, 18 Jul 2006, in the Usenet newsgroup alt.internet.wireless, in article
<(E-Mail Removed)>, Jeff Liebermann wrote:

>(Moe Trin) wrote:

>There have been tests using seriously dropped PG&E power lines which
>found that it works well enough with the droop. However, the relatively
>sharp bend at the insulators on the pole are a problem.

That's what I expected.

>There will be some radiation (and therefore some loss) at this point. The
>amount of radiation is determined by the bend angle.

I never had a situation where G-Line was going to be a solution, so I didn't
look into it very closely. I knew it wanted dead-straight runs and that if
you wanted to turn a corner you might want to have a pair of launchers
connected by some other form of transmission line to do so.

>>Outside/Inside DC Block - but what voltage rating?
>
>Dunno. A few thousand volts. Porcelain capacitors should have the
>voltage rating and not turn into inductors at 2.4GHz.

I no longer have all of my catalogs, but several vendors used to sell
these - I remember Omni-Spectra being one. Basically, they looked like
a barrel connector (either same sex, or M/F). They were available as
Inside, Outside or Inside/Outside blocks, and the ones I used were 'OK'
(meaning loss and VSWRs not much worse than a connector pair) from mid
VHF (call it 50 MHz) up to the cable/connector maximum. They were rated
at something over 250 volts (DC or peak). A _very_limited_ google search
turned up few hits, and the top one was only talking about 50VDC. Of
course, you could fabricate something on stripline, but as you point out

>However, I can see construction problems. Perhaps using the same
>trick as the "through the glass" car antennas might be a better idea.
>We're only concerned about 2.4GHz, so a frequency selective
>transformer or antenna would suffice. Just put a loop antenna,
>biquad, or patch antenna at each end. Move the access point close to
>the antenna. Instant isolation.

That would probably work.

>>Actually the concern I have is lightning.
>
>Sorry. That's an act of God and is not covered in the warranty.

Me and my lawyers are Atheists (was going to say 'Agnostics' but that
probably wouldn't float).

>>I don't know which would be a better idea - the DC blocks, or powering the
>>access point at the "ungrounded end" via an isolation transformer, and
>>locking the box in an insulated cage, with a short fiber link between the
>>access point and the local computers.
>
>Two helical resonantors in a can acting something like an xformer.
>Float one or both connectors.

That's also a good idea if the Q isn't to high.

>The insurance company doesn't require disclosure. However, if it can
>demonstrate a non-code compliant installation, they can easily deny
>the claim. It doesn't matter if it was done correctly. It only
>matters that the proper building permits were issued and signed off.

Don't I know that. I've still got copies of a Survey Report after a heating
contractor installed heat pumps on a trailer but neglected to install the
duct and insulation where the air outlet entered the distribution ducts.
The result was charring and an eventual fire from the very hot air from
the heating impinging on the wooden frame of the trailer. The government
inspector had signed off the work - and the contractor's insurance carrier
wiggled free (though I doubt the contractor did any more work for NASA).
They also pointing fingers every which way at other things being the cause
of the fire.

Old guy