>According to literature from Polyphaser, if the ground systems are
>separated
>by more than 75 feet, they don't "see each other", so it doesn't make sense
I think they are exagerating the 75 feet. Remember that key word
"business".
I think they make a good product and I use some, but I also think some of
their information is a bit misleading.
It is true when looking at rapid rise times they (house and tower) will see
less of each other the farther apart they are located, BUT they still do see
each other. Typical rise times equate to a frequency containing the major
portion of the energy of roughly one MHz. Also if the cables tying the tower
to the station ground consist of a series of ground rods and side branches
it will bleed off or "sink" a lot of the energy into the ground before
reaching the house/station. Therefore to me it makes good sense to tie them
together.
I've mentioned a number of times how often the big tower/array gets hit. It
averages 3 verified hits a year. Tht is hits some one has seen. How many
times it's actually been hit I don't know. I do know there are no
unprotected coax connectors up there that have any plating left on them. and
you can no loger get a PL-259 to screw onto the unused ports of the remote
antenna switch. Next time there will be protection over the connectors.
>to bond them. I believe the reason is that the inductance of the connecting
>wire or strap is too great at that length to allow the systems to float to
>the same potential.
It's not so much the inductance but rather the wave length. If they are far
enough apart the rise and fall times are are such that even were the path
strictly resistive the delta T makes them appear as seperate entities for
all practical purposes. That distance is one whale of a lot farther than 75
feet. OTOH with a major strike, not one of the typical ones, the voltage
difference across a few feet can be substantial. So, depending on how you
view it, or the impression you wish to create, you could make the same
statement for 10 feet as you can certainly get enough voltage difference to
wipe out solid state equipment.
>I hadn't read that tidbit when I installed my first tower system, so I laid
>265 feet of 1/0 wire at the bottom of the trench to connect the tower
>ground
"To me" I'd say 250 feet is approaching where I *might* consider not tying
them together, but more than likely I'd push to at least 300 feet. Even then
I'd most likely make it a run consisting of a network to bleed off the
charge which insulated coax in PVC conduit will not. From the base of my
tower to the entrance groundging bulkhead is over 75 feet. I have two bare
copper cables (one on each side) paralleling the conduit run from the tower
to the house. Those cables tie to the grounding plate which is the single
point ground for everything. There are runs perpendicular to those cables
that run out into the yard as well. Most of the back yard is one big
grounding network with 32 or 33 ground rods CadWelded(TM) to well over 600
feet of bare copper.
>to the single point ground at the house. I'm pretty sure that was
>unnecessary (I like to think of it as a really long horizontal ground rod.)
>When I installed my new tower this year, which is about 225 feet from the
>shack, I omitted the bonding wire. I was convinced not only by Polyphaser's
>argument, but also by the fact that the outer conductors of the two 1-5/8"
Are those outter conductors bare?
>runs of heliax between the tower and shack would undoubtedly present a much
>lower impedance path back to the shack than any wire or strap I could lay
>in
Impedance is only a part of the equation. I think we rely far too much on
just the "impedance" of that connection when for longer, well designed runs
the majority of the energy is disipated into the ground.
Do those outer conductors have the current carrying capacity of the wire or
strap?
Do they provide a way to bleed off the charge?
>the trench. I'm hoping the tower ground system is robust enough to
>dissipate
>any surges that may occur.
It will (In most cases) if the system is well designed and covers a wide
enough area.
Just remember a coax with an insulated jacket is a conductor straight into
your house while the bare copper is a path to ground that also goes to the
house.
>73, Dick WC1M
73
Roger (K8RI)
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