[TowerTalk] lightning tolerance

[K8RI]

On 7/8/2013 10:28 AM, Jim Lux wrote:
> I wonder if some of the time and effort put into building elaborate
> grounding systems for towers and such might not be better invested in
> making the actual signal paths more lightning tolerant.
>
> For instance, it makes no difference whether the tower rises 1kV, 10kV
> or 30kV relative to the shack: they're all enough to cause potential
> problems.  So, is it worth it to put in more ground system than is
> necessary to make sure the tower isn't physically damaged?
>

The more mitigation at the tower the less is needed at the shack.

> For control lines, particularly for slow speed things like rotor
> controllers, I think it wouldn't be too hard to build bullet proof
> galvanic isolation or transient suppression.  Enough L in series and C
> in parallel, along with appropriate over voltage transient suppression.

With my grounding system I've never had a control problen, but that's 
just one installation.

>
>
> it's the broadband RF signals that will be the challenge. but what is
> the real magnitude of it? You could probably help a lot (particularly on
> higher bands) with a DC block between equipment and feedline; but since
> 160m isn't that far from where the energy peak is for lightning, I don't
> know that this is the silver bullet.  A DC block (and a DC ground for
> the center conductor, on the antenna side of the interface) would sure
> solve the electrostatic charging thing, though.

A simple choke to ground will eliminate static build up as will a 1 mef 
resistor

>
> I think the "common mode" problem can be fairly easily solved with
> galvanic isolation techniques (transformers, capacitors, etc.), it's the
> differential mode that is going to be challenging: e.g. the voltage
> between center conductor and shield.

Grounding of the shield at the top and bottom of the tower as well as 
the entrance panel with the capacitance of the coax should drop any 
voltage spike from the antenna to manageable levels.

>
> A kilowatt into 50 ohms is a bit more than 220 Volts, and you've already
> got to stand that.  If you put transient suppression that clamps at 750
> or 1000 Volts, you've already limited some of the problem.
>
> The receiver front end is a challenge. The traditional approach is to
> put some sort of low voltage clamp (back to back zeners, for instance)
> across the input that limits the voltage to less than what the first
> mixer can take.

It needs to have very low capacitance or it will soak up signal.

>
> Or, to put a narrow band filter.  Maybe the long term solution to HF rig
> lightning protection is to put in a good filter that passes the HF band
> of interest, but is a short at other frequencies (including the
> lightning transient).  For "one frequency at a time" operation, that
> filter could be quite high Q, and acts as a preselector and could be
> tunable.
>
> But more folks are doing waterfall and pan displays, so I think a
> "practical" solution would need to pass an entire ham band.

I still think the old, disconnect everything along with a good ground is 
the best approach.  It's certainly the safest

73

Roger (K8RI)


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