Topband: The Q

[Tom Rauch]

There are many uses of the word Q. In tuned circuits and related 
components, we mostly consider two uses of the word Q. 
Unloaded Q and loaded Q. Loaded Q is typically the operating Q. 
Unloaded Q is a "test Q".

Q MUST be measured at the operating frequency. Many test 
instruments measure Q at low frequencies, where Q can be 
measured many times higher than it actually is at RF.  

There is also bandwidth, which will not relate directly to component 
Q except in a simple perfect resonant circuit that is not connected 
to ANYTHING that loads the circuit down.

The Q of individual components used to make up a filter often has 
next to nothing to do with the operating Q and bandwidth, because 
the circuit connected to the components lowers the Q by loading 
the circuit. The primary requirement is Q of the individual  
components be several times the operating (loaded) Q you select, 
mostly for efficiency concerns.

> What is the Q that I should use for the parallel LC circuits? I have been
> digging and have found other circuits but nothing that gives me the Q. The
> L and C combos are infinite. I assume a high Q but how high? 200? 100?

200 would be far too much, and probably unobtainable. It would 
also do no good, unless you had a crystal set for a radio. You 
probably want an operating Q of 10 or less, which might give you a 
bandwidth of 30 kHz or so before you need to retune.   

If you are using a resonant small loop, you most likely don't need 
any additional Q. That's because the loop already should have a Q 
much higher than 5.

If you are using a large non-resonant antenna, or an antenna that 
picks up whopping signals out of the 160 meter band, you only 
need enough Q to reduce unwanted signals to a point where 
overload stops.  Beyond that point, additional Q is a total waste of 
time. It will only aggravate you, and complicate your operating.   

Contrary to folklore, additional preselection  Q won't help S/N in 
most installations. Noise window is set by the bandwidth of the 
narrowest radio filter. Additional filters won't noise bandwidth as 
long as the front end of your receiver is not overloading.


73, Tom W8JI
w8ji@contesting.com



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