Topband: Ground screen ????

Donald Chester k4kyv at hotmail.com
Sun Mar 6 12:43:34 EST 2016


> Joshua M. Arritt jarritt at vt.edu
Wed Mar 2 11:02:46 EST 2016 wrote:
> 
Galvanized Fe material is problematic from an R standpoint as well (vs. 
Cu).  This results in a "lossy" ground plane.   That condition has real 
impact the effectiveness of the antenna system, when compared to an 
all-Cu (or at least a lower-R metal) plane.
>

I'd think  that kind of loss would be minimal, and should be the least of worries.  What about AM broadcast towers, nearly all of which are built out of galvanised steel, or in some cases ungalvanised steel, merely painted to retard rust and to meet FAA requirements? I recall a story about an AM BC station in Puerto Rico turning to galvanised barbed fencing wire for radials, after their copper wire system had been destroyed for the second or third time by copper thieves, and the FCC did not hesitate to give them the OK.  I also remember seeing an AM BC tower under construction in Ethiopia back in the late 60s using steel fence wire for radials.  The climate there is so dry that the steel  wire would probably last more years than pure copper does in moist, acidic soil as we commonly have in New England and elsewhere. The obvious reason steel was used there was to cut costs.

> Immediately perceived consequence of the poor inter-section bonds is 
walking VSWR, but other factors (including rad efficiency) are also 
walking around.
>

That might be problematic, but measures could be taken to make sure the pieces are well bonded electrically.  If the sheets are securely bolted together, that might be  sufficient in itself.  Again, galvanised steel tower sections  like Rohn 45G are used as AM BC towers merely bolted together;  I have never heard any mention of a necessity to further bond them electrically.  The zinc coating with real hot-dipped galvanising makes pretty good electrical contact between sections, provided they are securely fastened.

> Bonding of dissimilar metals (Cu <-> Fe) requires careful method 
> consideration to hold corrosion at bay.

Direct contact between copper and  zinc must be avoided at all costs, since those two metals are highly reactive with each other and corrode at the first contact with moisture, producing a diode rectifier at every  joint or connection.  OTOH, brass is OK.  It is often used as a go-between for bonding zinc with copper, for example, ground clamps used to attach a copper wire to a galvanised steel ground rod.

Don k4kyv


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