Towertalk
[Top] [All Lists]

## [TowerTalk] (the) Actual Capacitor for Shunt-Fed Tower

 To: [TowerTalk] (the) Actual Capacitor for Shunt-Fed Tower W8JI@contesting.com (Tom Rauch) Fri, 8 Feb 2002 19:51:15 -0500
 ```> I found a couple large air variables (600-800pF range) for my 80m/160m > inverted L's, *HOWEVER* I understand that you could do the same thing > with a length of coax, like RG8, if you had a rough guess how much > capacitance to put in series with your gamma. Consult your coax table > for the "pF/foot", but I think it runs roughly 10-20pF/ft (or > whatever), so it would just be a matter of coiling up 30-40' (or > whatever you think you might need) and and trimming to match and as > they say "Bob's your Uncle". Coax stub capacitors certainly can work in some applications, but we have to be careful how we use them! Coax makes a notoriously low-Q capacitor, and using capacitance times length in feet is only reliable when the cable is a tiny fraction of a wavelength long. The coax is really a stub, and not a capacitor. You would have to use a program like TLA, a Smith Chart, or do longhand calculations in order to know the correct capacitance. For example, 40 feet of RG-8 coax actually looks like 1500 picofarads on 1.85Mhz, yet if we use capacitance-per-foot the expected value would be more like 1100 pF or so. The series resistance of that "capacitor" is about 1/2 ohm, so Q is only about 140. In comparison a typical air-variable has a Q of over 2500! On 80 meters, the same length coax looks almost like a short circuit, and has a Q in the single digits! Another factor is the voltage across the open end of the coax is much higher than the voltage that would appear across a conventional capacitor. This is why antennas that use coax for loading have power handling problems, and are less efficient than antennas using conventional capacitors. 73, Tom W8JI W8JI@contesting.com ________________________________________________________________________ Where do you get ICE bandpass filters & beverage matching boxes? The same place that pays for the hosting of this list: The eHam Store. Order online at http://store.eham.net. ----------------------------------------------------------------------- ----- FAQ on WWW: http://www.contesting.com/FAQ/towertalk Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com ```
 Current Thread [TowerTalk] (the) Actual Capacitor for Shunt-Fed Tower, Mike & Coreen Smith [TowerTalk] (the) Actual Capacitor for Shunt-Fed Tower, Tom Rauch <= [TowerTalk] (the) Actual Capacitor for Shunt-Fed Tower, Don Havlicek [TowerTalk] (the) Actual Capacitor for Shunt-Fed Tower, Pete Smith