[TowerTalk] Radial Connection to tower

[Jim Lux]

On 12/13/15 10:56 AM, Richard (Rick) Karlquist wrote:
> You can estimate 20 nH per inch for these hook up
> wires. On 80 meters, think of that as as about
> a half ohm of inductive reactance per inch,
> or 6 ohms of inductive reactance per foot.
> Thinking of it that way, you can see why a single
> 10 inch wire in series with your ground connection
> isn't a great idea. You might try modeling it
> and see if it rises to the "noticable" level.

I use 1 uH/meter, which works out to about 25 nH/inch.. I doubt the 
difference is important.

There's also the whole thing with series L (which can be "tuned out") 
and the series R (which is plain old loss).

Putting strap, ribbon, or multiple parallel wires (that are close to 
each other) reduces the R but doesn't change the L very much. The 
magnetic fields couple between parallel wires or "filaments" within a 
wide flat strip, so the L stays pretty much the same.

Strip has lower AC resistance (more surface area) than a round wire of 
the same cross section.

So if you had multiple wires going out to the ring in different places, 
you'll reduce both L and R (the magnetic field from the wires won't 
interact).


This is why a single big wire works just fine for lightning protection 
(AC loss isn't a big deal compared to the L, as long as the wire doesn't 
melt).

But if the grounding system is part of the radiator (e.g. AM Broadcast), 
then you want that big flat conductor to minimize AC resistance loss. 
The fact that it's an OK lightning grounding conductor too is just icing 
on the cake.

>
> I have a 230 foot diameter ground screen with
> wires spaced 3 feet apart in each direction.
> When I first completed this, I placed a 40
> meter vertical at the center junction, such
> that there were four wires emanating from the
> shield of the coax connector. These 4 wires
> run 3 feet until they reached the next junction,
> etc. The driving impedance of this was
> 70 ohms, despite being over a gigantic ground
> screen. After adding 30 foot radials to tie it into
> the ground screen 30 feet away from the vertical,
> the impedance steadily decreased with the
> number of radials until it reached the
> theoretical value of 36 ohms. This is the
> kind of weird stuff that happens if you deviate
> from the classical radial paradigm.
>