Topband: Voltage rating of matching capacitor(s)

Thu Jul 30 07:57:29 PDT 2009

Hello,

For a shunt fed tower system I installed at EA5ELT, I made a capacitor with two cardboard pieces taken from the back of a pair of picture frames. Better to use two plastic cutting boards for the kitchen, but this was at hand, and it took me less than an hour to make the cap. The size of the boards was A4 (297mm x 210mm or 8-1/2" x 11-1/2", +/-). I covered one surface of each board with adhesive aluminium tape 2" wide, with minimum overlap between stripes. The adhesive side is not a good conductor because of the glue, so I had to put a stripe, conductive face down, to run across all the stripes and secure it tightly with tape to ensure more electrical contact between all the stripes.

To install the connection cables to the plates y used a No. 10 insulated cable made of many thin wires, I stripped about 150mm (6")  insulation, and spreaded all the wires in a fanned layer and sandpapered them carefully on both sides of the fanned layer.

This fanned wires are set on top of the aluminium tape of each board covering as much area as possible and a couple of aluminium tape pieces are used to hold the wires in place, is important to massage down with the finger nails the tape between the wires to ensure tightness between the wires and the aluminium tape stripes, you have to maximize the contact area. After this, install a good layer or two of insulating tape or a combination of it with layers thin plastic film from the kitchen to provide insulation. Depending on the capacitance needed and voltage, you can install the plates with aluminium face to face, or use the board thickness as additional insulation but giving up pF's. A glass from a picture frame can be used as board and provides good insulation and has a dielectric constant than can provide more capacitance.

The cap had about 1300 pF when the plates were slided to be completely face to face, and with a rock on top of the plates exerting pressure, the meter reading was 2200 pF. About 4 pF / cm2 or about 25 pF a square inch. Keep in mind that this figure depends on plate separation and the dielectric constants of materials used.

We used this cap for our 160m TX shunt fed tower using maximum power without any problem. Some times cap specs have gotten in our way while in a proyect, but there are many ways to make a capacitor, variable or fixed, for little money and time.

73

David  
HK1KXA
EC5KXA

> Date: Wed, 29 Jul 2009 08:53:28 -0700
> To: k8gg at voyager.net; n4zr at contesting.com
> From: pbaldock at verizon.net
> CC: topband at contesting.com
> Subject: Re: Topband: Voltage rating of matching capacitor(s)
> 
> For the Traditional Cs/Cp Omega Match, Cp the shunt capacitor is 
> between the antenna and ground so it has to withstand substantially 
> the same voltage as the series capacitor, which is many KV.
> 
> In my shunt fed tower setup the shunt capacitor is around 75pF and 
> for that I use a short piece of hardline. This is obviously a fixed value.
> 
> My series capacitor is a gigantic air variable with about 3/8" 
> spacing between plates.
> 
> Paul - KW7Y
> 
> At 05:08 AM 7/29/2009, k8gg at voyager.net wrote:
> >For the Shunt
> >Capacitor:
> >The RMS voltage  V = I x R, so Vrms = 5.5 x 50 = 275
> >volts.
> >The peak voltage (assuming a sine wave) is about 1.42 x Vrms =
> >390.5 volts peak.
> >This has to be adjusted upwards for VSWR, but with
> >a VSWR of 2.5 or less a 1000 volt capacitor should be sufficient for the
> >shunt capacitor between the feed point and ground.
> >
> >For the
> >series capacitor from the feed point to the bottom of the omega match wire
> >or tube,
> >The voltage across the series capacitor depends on the
> >reactance (Xc) times the current.
> >Using 200 pfd as the value of the
> >series capacitor,
> >Xc = -435j ohms.  from the standard formula,
> >Xc = -1/(2 x Pi x F x C)
> >where F is frequency in cycles or Hertz and
> >C = capacitance in farads
> >
> >The voltage across the series
> >capacitor is then Vcs = 5.5 amps x 435 ohms =
> >2391 volts rms and 3395
> >peak volts, at a VSWR of 1.0
> >Multiply the peak value by
> >the VSWR to get a recommended minimum voltage rating
> >for the series
> >capacitor.  In this case I would use a 10 KV vacuum capacitor, since
> >a 2:1 VSWR gives almost 6800 volts across the capacitor.
> >
> >I hope
> >this helps.
> >
> >73 to all
> >
> >George   K8GG
> >
> >
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> >Topband at contesting.com
> >http://lists.contesting.com/mailman/listinfo/topband
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