Topband: Hybrid Coupler

[Tom Rauch]

> A good source for high current mica transmitting capacitors is Nebraska
> Surplus Sales 800-244-4567 or http://www.surplusales.com    The have a
> wide selection of broadcast type G1, G2, or G3 series. Unforntunately the
> prices are not for most hams with a typical 300pf 6KV 3.9 amp. unit priced
> at $89.00.  They also sell the HT-57 series which are ceramic dooknobs
> rather than mica porcelain and epoxy.  A 500pf 15kv 7.0amp HT-37 is priced
> at $36 but the choice in values is limited. 

Brand new HT57s cost about $15-20 from HEC, if they will sell to 
you. The problem with higher value capacitors is they have high 
ESR, and heat. They also have high capacitance drift with 
temperature.  

Another source for transmitting capacitors are PA manufacturers 
like Ameritron. They use massive quantities in manufacturing.

Modern chip capacitors are more stable with temperature, handle 
more current, and have lower ESR so the RF doesn't heat them. A 
small chip cap less than 3/8 inch square will handle ten times or 
more the current of an HT-57 with no heating and no noticeable drift 
in capacitance with temperature. 

When I need low voltage capacitors (under 2 kV), I always use HV 
chip caps.

Mica caps are good, like those in old transmitters, but stay away 
from doorknobs in capacitance critical applications unless you 
absolutely need the voltage rating.


73, Tom W8JI
w8ji@contesting.com



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