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Antenna Static Charge Bleeder

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{jcomments on}Static Bleeder Attached with a T-Connector to Antenna CoaxStatic discharge transient damaged my linear amplifier swr circuitry when it was snowing in a dry and cold Winter day (Static Discharge from Antenna Can Do Damage to Your Gear). So I needed a way to discharge the static charge which can build up in the antenna.

Antenna acts a a capacitor against ground. The 80m 1/4-wave vertical antenna capacitance is in the order of 100-200pF. Snowflakes and rain can and will carry a charge and charge the antenna (capacitor). This resulted in arcing at antenna coax connector in the hamschack end. In my case there were 1-2 sparks per second. Dry air breakdown voltage is about 33kV/cm with smooth surfaces. At the N-connector pin tip the charge density is highest, causing the potential being highest, too. If the tip to barrell gap is about 2 mm, the potential difference was in the order of 6 kV. This built up 1-2 times per second.

Using a bleeder resistor accross the antenna and ground prevents the charge building up to any significant level, if the RC time constant is significantly shorter than the charge build-up time. The bleeder resistance should be non-inductive and has to be large enough not to add any reactance and affect the antenna characteristics. And it should be able to withstand high voltages in case of mismatch.

Three 10 Megaohm 3.5kV Resistors in Parallel I  found 10 Megaohm  3,5 kV resistors at a local supplier. This suggests the resistors are about 1 Watt max. Using three of those parallel for 3.3 Mohm and assuming a conservative high value of 1000pF for the capacitance gives the RC time constant as 3.3 ms (milliseconds). This means that max 10 ms is needed to discharge the voltage to ca 5% of initial. A magnitude or more shorter time  than the charge build-up time.

The bleeder with tree resistors can handle 3 W (continuous) dissipation and 3.5kV max. If the applied power is max 1.5kW and max voltage is 3.5 kV the impedance across the bleeder insertion point can be max 8000 ohm. For practical lossy antennas this is high value and the coax cables and connectors will arc before. I decided not to use the resistors in series, just to make the construction simple.

Bleeder Resistors Soldered Inside an UHF ConnectorThe bleeder was constructed with the three 10 Mohm 1W /3.5 kV high voltage resistors in parallel. Added a layer of electrical insulation tape over the resistors and soldered the bunch inside an UHF -connector, one end to the pin and the other ends to the end of the connector body.  Note: screw the connector outer barrel into the connector body before soldering the resistor ends to the connector body - or you cannot screw the outer barrel to the body - I found it the hard way.

Do not Solder the Resistor Ends to the Connector Body Before you Screw the Outer Barrel ;o) The Bleeder is then connected with a t-connector to the antenna coax.

There has been no bleeder warming nor any noticeable changes in tuning of the antenna. I typically run 500W and  the antenna mismatch can be severe on some bands.

Note: This is NOT a protection against lightning!









Last Updated on Tuesday, 30 March 2010 08:47  

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