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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
 

Static Discharge from Antenna Can Do Damage to Your Gear.

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Snow This Winter we have got a lot of snow in Southern Finland. A couple of weeks ago we had nearly -20 C cold and it was snowing. When I  turned on my rigs, there was a lot of static noise on the bands making QSOing impossible. Suddenly I saw the linear amp power/swr meter needles to bump erratically. I guessed it was the static coming from the antenna and switched the antenna off by turning the tuner panel antenna selector switch to an empty position having the amplifier on - that was a mistake.

I started hearing a static discharge sparking every second or so within the tuner. I disconnected the coax from the tuner an saw sparking 1-2 times per second at the N-connector between the pin and  the barrel. Then I grounded the antenna and turned off all my gear.

Linear Amplifier SWR Meter ProblemNext day snowing had ceased, I turned the rigs on, everything looked right. I tuned on 80m and saw that the linear amp meters did not show correct values anymore. The power out was barely 200 W and reflected around 30W. However the Antenna tuner meter showed normal 500/0 W (out/reflected). The same for all bands. No changes to any tuner settings from previous were needed to tune to 1:1 SWR with 500-600 W out by the tuner meter. Also the (exciter) Icom power output and SWR were as normal.

I guessed that the static arcing inside the tuner antenna switch had propagated a transient to the linear amplifier and somehow damaged the SWR circuitry.

Furher study with an e-mail and a call to Ameritron support suggested that the Germanium diodes at the amplifier SWR circuitry may be damaged. That was cured by replacing the diodes, but needed some adjustment to get the meters to show correctly again. More about that in a later article.ALS-600X Power/SWR Meter Board Repair I constructed a simple static charge bleeder inside a coax connector, and installed it at the antenna coax end using a t-connector. I also added a grounding switch to the antenna.

Lesson learned: Have a static charge bleeder installed to the antenna to prevent this happening again - and try to avoid working when it is snowing and cold..note that a static bleeder is not a prevention against lightning damage.Antenna Static Bleeder
Last Updated on Friday, 05 February 2010 13:14
 

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QSOmonth - August 2009

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After record number of qsos in July, working returned to normal in August, about 100 qsos per month. First August qso was with 7P8MM (Lesotho) on 30m cw on the 1st August and last on 31st with UN9LBY on 40m cw.

August QSO statistics:

DX 76 (72 on cw, 4 on ssb)
EU 38 (36 on cw, 2 on ssb)
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QSO total 114 (108 on cw, 6 on ssb)

Band
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80m 22 (18 on cw, 2 on ssb)
40m 22 (20 on cw, 2 on ssb)
30m 39 (39 on cw, – on ssb)
20m 27 (25 on cw, 2 on ssb)
17m 5 (5 on cw, – on ssb)
12m 1 (1 on cw, – on ssb)

Band highlights:
80m 3DA0, JA, 4S7, ZS
40m 3DA0, KH2, LU, R1ANB, YB
30m 7P8, 9J2, CX, DU9, V5, VQ9
20m 3DA0, 7P8, 9V1, KH6
17m ZP
12m YO

Last Updated on Monday, 18 January 2010 17:57
 

QSOmonth - July 2009

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After cleaning up a hectic work quarter, July brought me some time to work. Band conditions seem to favor higher frequencies and dx qsos could be made almost everyday. Obvioulsy the Sun is trying to show some activity, though there still were some nice contacting on 80 meters. 20m cw dominated this month. As always all qsos are run with the 80m vertical + tuner. First July qso was with ZS6BQI on 80m cw on the 1st July and last on 31st with ZP6CW on 30m cw.

July QSO statistics:

DX 114 (101 on cw, 13 on ssb)
EU 46 (42 on cw, 4 on ssb)
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QSO total 160 (143 on cw, 17 on ssb)

Band
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80m 22 (21 on cw, 1 on ssb)
40m 22 (17 on cw, 5 on ssb)
30m 11 (11 on cw, – on ssb)
20m 102 (91 on cw, 11 on ssb)
17m 3 (3 on cw, – on ssb)

Band highlights:
80m 3B8, PY0FF, 5N, VK9N
40m 9J2, CX, VK, Z2
30m 3DA0, 7P8, TY5
20m 8Q7, 9M6, 9Q, PV8, Zf1, CP
17m K, PY

Last Updated on Monday, 18 January 2010 17:57
 


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