> Has anyone got any views on the suitability or not of using metal film
> resistors to Terminate Beverages. 73 Clive GM3POI
The only problem you will have is the resistors will fail during
lightning storms much more often than true carbon composition
resistors fail. There is absolutely NO electrical or performance
difference other than the life of the component.
Metal film and carbon film resistors have a very high mortality rate
when hung on antennas!
There are very few sources of carbon composition resistors left,
most are custom orders.
NOTE!
Many resistors "sold" as carbon comps are really carbon films, and
are unreliable in high energy absorbing applications. You MUST be
sure you are getting a true carbon composition resistor. You can
tell this by cutting a part open and looking at the inside. carbon
comps have a very thick solid carbon core, while carbon films have
a thin deposit of carbon over a ceramic or other insulating core.
You might have some success protecting film or oxide resistors
with a spark gap and a neon bulb. Perhaps you rarely get lightning
hits nearby.
I have over 30 Beverages, life of components is important. Two-watt
unprotected carbon comps take nearby lightning hits over and over
without failure, while metal and carbon film resistors (no matter how
I try to protect them) have a *very* high failure rate.
Don was speaking of the surge current rating, not the power
dissipation. While we CAN get more dissipation by series or
parallel connecting resistors, it still does not allow the component
to survive high surge impacts. The problem is the cross-sectional
area of the conductors in a metal oxide or carbon film resistor is
very very small. It can take dozens or hundreds of parallel
connected two-watt metal film or carbon film resistors to give the
same cross sectional area of one two-watt carbon composition
resistor!
With a dual five-foot long, one-inch diameter ground rod system,
my antennas require about 470 ohms of resistance. Each ground
rod is about 80ohms resistance in my soil, and it has been
between 50 and 150 ohms-per-rod for a five-foot rod in the last five
places I have lived. You must of course space the rods several feet
apart.
73, Tom W8JI
W8JI@contesting.com
________________________________________________________________________
Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
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