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Old 3rd Jul 2020, 11:43 am   #1
crackle
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Default Static on antennas

My CRT SS6900N was damaged again yesterday during a short thunderstorm. I was caught out having a nap and was awoken by the thunder.
I went into my shack to find the radio was down on power, it turns out one of the OP MOSFET's had blown again.

This has now happened 5 times, (lucky the IRF520's are relatively cheep from Farnell) and heavy rain/thunder storms seem to be common on most occasions.

The 6900N is normally connected to a vertical Imax2000 antenna for use on 11m & 10m. Which is a grounded radiator type and the braid is connected to a new earth bus bar wired direct back to the main house earth at the incoming mains. I have also taken steps to fit a band pass filter across the antenna (adding another DC path to earth) and a low pass filter before the antenna connects to the change over switch. The coax from one of the tuner outputs also connects to the other half of the switch. This change over switch will enable my Yaesu 857D to connect into the Imax vertical.
I feel it is unlikely that static is building up on the Imax antenna.

I think the problem is my G5RV junior which is feeding into a length of coax going into the shack. I have a "Dirty Balun" on the coax near where it feeds onto the G5RV to help stop RF feeding back up the coax braid.

I suspect static is coming in on the balanced antenna, and down the coax into the antenna tuner. Here it builds up to a high voltage until it arcs across the tuning capacitor to ground. I believe it is this that is what we hear in the TV amp and speakers.
I am not quite certain how it gets into the 6900N to damage the OP MOSFET's but the coax from the 6900n does connect into the same earth bus bar as the G5RV.

Would an UNUN help here on the G5RV, between the balanced feeder and the coax to give a DC path to ground for both halves of the dipole, and hence help prevent static build up.

Thanks
Mike
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Old 3rd Jul 2020, 12:36 pm   #2
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Default Re: Static on antennas

Big RF choke across the capacitor?
 
Old 3rd Jul 2020, 7:37 pm   #3
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Default Re: Static on antennas

Please can you be more specific, what would constitute a "Big RF choke".
The ATU is a MFJ 949E
Would it not interfere with the tuning.

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thanks
Mike
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Old 3rd Jul 2020, 8:02 pm   #4
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Default Re: Static on antennas

Hi , At the end of the balenced feeder put two spark plugs to ground. No good for a direct strike but will tap of induced voltage and may help. Thread is M14 x1.25 on the plugs I have. Some plug types have an integral resistor which may not be as reliable in this service.
I bought all the bits at an autojumble but have yet to build it so no operational experience I am afraid.

PS. I can lend you the thread tap if needed.
Pete

Last edited by G4_Pete; 3rd Jul 2020 at 8:05 pm. Reason: added P.S
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Old 3rd Jul 2020, 9:26 pm   #5
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Default Re: Static on antennas

Surely the Voltage for a spark to jump a plug at atmospheric pressure is at least 20 or so Kilo Volt ?
A 2W 1-5 Meg Ohm resistor wired across a PL259 into a T piece connector would allow induced Voltage to bleed down to ground without adversely impacting signal strength. ISTR that this is often employed inside commercial Antennas,also an old trick is to use a wire ended Neon lamp without series resistor as these Ionise at around 85V or so and were commonly fitted in WW2 Aircraft receivers.
I would certainly agree about the use of a spark gap to protect a disconnected or isolated antenna from all but a direct strike.

Mike
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Old 4th Jul 2020, 12:15 am   #6
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Default Re: Static on antennas

A sparkplug will arc over at a lot less than 20kV at atmospheric pressure. It does need a good fraction of that at 10bar+ on wide open throttle in an engine though.


The usual gap is about 30 thou or about 0.75mm but the contacts are anything but smooth and the air in a storm is likely to be very humid all of which brings down the nominal 3kv/mm breakdown figure considerably.
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Old 4th Jul 2020, 12:44 am   #7
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Default Re: Static on antennas

100k resistor from each leg of the balanced feeder to ground is an old timers way of bleeding off static.
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Old 4th Jul 2020, 8:51 am   #8
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Default Re: Static on antennas

Mike,
Just out of interest do you have a circuit of the pa stage and the conections to the radio RF socket. I have read all your previous posts including the shack layout thread and just wondered how the transients are getting into the pa stage and across the mosfets. I am guessing these radios were also designed for mobile whip use where there is going to be plenty of static build up and transients so I cant help wondering if there is onboard protection that has failed previously and has left your new mosfets exposed.
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Old 4th Jul 2020, 11:50 am   #9
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Default Re: Static on antennas

Mike,
Make sure you have a good short RF earth connection to the tuner. Someone will probably say don't do that as it is against electricity regulations but it has never bothered me.

You have explained you have your G5RV connected to a coax via a choke balun, therefore one side of your G5RV is already grounded. There is no static leak in your model of tuner so the other side of the G5RV is floating. Connect a 100K 1W from the centre to ground at the two aerial connector coax sockets.

If you brought the twin feeder all the way to the tuner's balanced connector both sides would be grounded for static via its internal balun.

73, Andrew
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Old 4th Jul 2020, 2:44 pm   #10
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Default Re: Static on antennas

Quote:
what would constitute a "Big RF choke".
Something physically big enough not to get upset by a few hundred volts at the transmitting frequency. I used "big" to discriminate from common or garden types. As said in previous posts a bleed resistor will do as well, as low as a few k ohms won't absorb much power (maybe a percent tops).
 
Old 4th Jul 2020, 4:49 pm   #11
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Default Re: Static on antennas

Quote:
Originally Posted by GW3OQK Andrew View Post
Mike,
Make sure you have a good short RF earth connection to the tuner. Someone will probably say don't do that as it is against electricity regulations but it has never bothered me.

You have explained you have your G5RV connected to a coax via a choke balun, therefore one side of your G5RV is already grounded. There is no static leak in your model of tuner so the other side of the G5RV is floating. Connect a 100K 1W from the centre to ground at the two aerial connector coax sockets.

73, Andrew
I assume this would be the same as fitting a 100k resistor across the balanced feeder up to the dipole where it joins onto the coax using a connector block, as in diagram?
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Old 4th Jul 2020, 5:27 pm   #12
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Default Re: Static on antennas

Wonder if it's worth considering a lightning conductor, perhaps not the cheapest option but if you can source the copper, then not a problem. Of course the lightning conductor is not to receive a direct hit, just to disipate the static electricity in the air around it's vicinity. It might well prevent repeated repair jobs to the radios.
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Old 4th Jul 2020, 6:16 pm   #13
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Default Re: Static on antennas

The B6122 and BD272 Marconi HF transmitters at Skelton (and elsewhere) had a balanced RF output of 328 Ohm Z into open wire feeders. Each feeder leg at the RF output stage had a big RF static leak choke to earth. Can't remember the value, but like shunting each leg with +j10K, or something like that.
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Old 4th Jul 2020, 7:47 pm   #14
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Default Re: Static on antennas

You have to view the entire G5RV and the downlead as one capacitor plate. If you disconnect your feeder, connect a wire ended neon across it and ground you will find it may light when a charged rain cloud is overhead,it doesn't have to be a lightning storm.
If you can use or extend the balanced feeder you can make use of the un-bal transformer already inside the tuner, connecting a jumper across "wire" and one balun terminal.
This transformer is centre tapped to ground and may offer a useful discharge path.
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Old 4th Jul 2020, 8:47 pm   #15
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Default Re: Static on antennas

In times when I've experienced static-buildup-on-antennas I've been happy to fit a low-wattage high-value *wirewound* resistor or two from the static-afflicted parts to the RF ground.

Something like 10KOhms 5 Watts.

The wirewound-ness provides a HF/VHF-choke effect, so it doesn't cause significant waste of RF power or 'RF unbalance' - and a resistor of 10K to the local RF-ground won't cause any significant risk-currents if your DNO's neutral/grounding arrangement fails so the neutral/ground 'floats' towards the supply-phase voltage.
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Old 4th Jul 2020, 10:48 pm   #16
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Default Re: Static on antennas

Quote:
Originally Posted by Restoration73 View Post
If you can use or extend the balanced feeder you can make use of the un-bal transformer already inside the tuner, connecting a jumper across "wire" and one balun terminal.
This transformer is centre tapped to ground and may offer a useful discharge path.
I believed the length of the balanced down feeder from the dipole is a critical length and is designed like that in order to make the aerial work.

I could always revert back to a long random wire and use the balun.

So another idea then is to use a choke or 10k resistor from the floating half of the dipole to an RF ground.

thanks
Mike
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Old 5th Jul 2020, 1:04 am   #17
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Default Re: Static on antennas

Whatever you do on one side of a dipole, you need to do on the other, regardless of whether you have coax and a balun/unun fitted or not.

You need to think in terms of RF, not DC as to what it does to the characteristics of the antenna, even though what you are actually trying to achieve is an earthed path to bleed the static off from both legs of the dipole.

Not bleeding the static before the coax connection has been known to let static punch through the dielectric of some types of coax, leaving it very lossy and prone to arcing.

More modern coax types may be less susceptible to this type of punch through.
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Old 5th Jul 2020, 7:22 am   #18
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Default Re: Static on antennas

Quote:
Originally Posted by Terry_VK5TM View Post
Whatever you do on one side of a dipole, you need to do on the other, regardless of whether you have coax and a balun/unun fitted or not.
Looking at my drawing I did in post #11, I cant see how I can do the same to one side and the same to the other side when I have shown the bleed resistor across the dipole feeder.
Remember one side of the dipole is already connected to the earth through the coax screen. Also I would plan to sink an earth rod in under where the dipole feeder joins the coax.

If anyone thinks a common mode choke would be better than the "dirty" choke I can fit one of those, if you can give me the details.

Mike
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Old 5th Jul 2020, 7:48 am   #19
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Default Re: Static on antennas

The resistor at the top might be better placed at the feeder/coax joint.

I was thinking more along the lines of where the feeder and coax meet, adding a resistor to each side of the feeder to ground rather than across the dipole to keep the static out of the coax.
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Old 5th Jul 2020, 8:45 am   #20
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Default Re: Static on antennas

Hi Terry
The coax is already at DC ground potential, connected to ground in the shack, I have shown the resistor across the bottom of the balanced feeder where it joins the coax.
My plan will be to add another 8 foot earth rod at this point when the ground is softer.
Mike
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