[TowerTalk] coax as capacitor

[i4jmy@iol.it]

> > On Wed, 25 Jul 2001  Pete Smith <n4zr@contesting.com> writes:
> > 
>   Is it feasible to use coaxial cable as the series capacitor in a 
> > shunt feed
> > system, simply pruning the length of cable as appropriate?  Or is 
> > there
> > some reason why not?
> > 
> > 73, Pete N4ZR
> 
> 
> Voltage is always HIGHER at the open end than at the connection 
> end, again that amount varies with length and peaks at 1/4 wl. This 
> effect can be dramatic as you approach 1/4 wl. That's why gamma 
> capacitors on yagi's seem to arc and fail at lower than expected 
> power.

This effect is even more evident in verticals, I mean in shunt fed 
verticals and expecially in those (short) ones loaded with a capacitive 
hat and using an omega.
 
> Q decreases and loss increases as you approach 1/4 wl, and Q 
> can be much lower than conventional well-designed lumped 
> components produce. That is because the line operates with a near-
> infinite SWR.  This is why you can't use a coaxial capacitor in a 
> high-efficiency small loop antenna, and why a high-Q capacitor has 
> to be "boxy shaped" with multiple plates. (Multilayer ceramic chips 
> have Q's in the ten's of thousands while single layer ceramics have 
> Q's in the hundreds.) Coaxial stubs can have Q's in the 20-50 
> range when used as capacitors.

As well as for the small loops, this is also valid with all the other 
antennas with high Q (small BW) and overall efficiency may get 
substantially compromised.


73,
Mauri I4JMY

> 
> Voltage at the open end also increases over the voltage you might 
> normally expect. 
> 73, Tom W8JI
> W8JI@contesting.com 
> 




-----
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com