[Lowfer] LF Lop Antennas

[ToddRoberts2001 at aol.com]

In a message dated 8/25/2008 10:39:24 A.M. Eastern Daylight Time,  
kc0vsj at yahoo.com writes:

A good page wint a nice colleciton of other links at the bottom. One thing  I 
am trying to get a handle on is the relative Q of various types of capacitors 
 - Air variable, Poly variable as in what Doug Hendricks has,  mica, mylar,  
polystyrene etc.   Does anybody have a resource on this?  So far  I have seen 
air variable as in the 1500' range and mica at 1200. 
 
 
 
Crystal radio set builders are always interested in the highest Q  
capacitors. There doesn't seem to be very much information published on the  actual Q of 
different types of capacitors. The best way to determine the best  kind of 
capacitor is to resonate them with an actual coil on a good Q meter  and look 
for highest Q readings. The Hewlett-Packard 4342A Q meter is one of the  best 
for this.
 
There is no set Q with any type of capacitor as different batches and dates  
from factory production can be different. Polystyrene caps are usually better  
than fixed Mica. Some of the newer ceramic chip capacitors have extremely 
high  Qs. Also the type of insulation used in a variable capacitor can affect Q. 
The  more common air-variables with phenolic insulation are worse than the 
military  types with ceramic insulation. Also some types of air variables are 
silver  plated which increases Q. The consensus is that the silver plated 500pF 
air  variable used in the URM-25 signal generator is one of the best. Crystal 
set  builders nickname it the Holy Grail Cap.
 
Check with some crystal radio set groups to see what capacitors they are  
using.
 
73 Todd WD4NGG
 



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