An important point worth mentioning here is that the life of gas discharge
tubes is reduced every time they conduct. Exactly how much depends on the
arc current, which is explained in the Bournes GDT data sheets, but their
life can be significantly extended by the following simple and inexpensive
modification.
After I read that data sheet, I started paralleling 33k Ohmite OX resistors
across the 90v GDTs (between each Beverage wire and ground) to prolong
their life. Those resistors provide a DC path to ground, which constantly
drains static charges to ground. This eliminates many small discharges
through them from the inevitable static charges that would otherwise build
up, thus prolonging their useful life.
These Beverages have a surge impedance of about 450 ohms. Therefore, 33k
(some use 47k, 10 times the surge Z) resistors will have a minimal effect
on received signal strength, RDF, or the required matching transformer
impedance ratio.
For a higher impedance antenna, the drain resistors should have a
correspondingly higher value (and perhaps a TVS diode should also be in
parallel with the GDTs and resistors?).
Some well-meaning hams have recommended much higher value resistors as a
static drain, even at a transmitting antenna feedpoint where GDTs are
seldom practical. However, if the resistance value chosen is as low as
practical, then the GDT's life will be maximized.
73, Mike
W0BTU
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
|