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Static bleeder resistor?

Posted: Fri Mar 08, 2024 7:38 pm
by King Croccy
A little while back, I had a rig that come in for a service. On inspection, it had one of those Ebay "170w" amplifiers inside. I noticed the original builder appeared to make some kind of protection against static by fitting a 14k7 resistor across the so239 terminals (center core and ground). I am just wondering if this is a good way to bleed static from an "open" style dipole antenna with that particular amplifier?

I know that these type of antennas aren't quite as good as a dc shorted equivalent but just wondered if it would be effective to protect the mosfet?

Re: Static bleeder resistor?

Posted: Sat Mar 09, 2024 12:44 am
by Albert H
No. If you get static build-up on an aerial, you can get gigantic voltages down the coax. I've seen the results of induced lightning damage into a rig - it wasn't a direct strike, but every single component in the rig was completely fried, and there was a nasty burn mark inside the lid!

DC-shorted aerials tend to suffer much less from this kind of issue, but I recall a very nasty accident that a friend of mine had when installing up an East London block. He went to plug in the link receive aerial lead, and there was a colossal flash and he was thrown across the tank room! The link receive aerial was a TV Yagi (so DC-shorted), and it came down from the roof to the rig using the usual cheap brown-sheathed TX Coax. I can only assume that static had built up on the outer of the RX downlead, and bringing the plug close to the earthed rig case caused it to flask over!

Re: Static bleeder resistor?

Posted: Sun Mar 10, 2024 5:46 am
by radium98
what i knew , they put a coil of 1 uH about 20E

Re: Static bleeder resistor?

Posted: Sun Mar 10, 2024 5:33 pm
by radionortheast
I have a short loop about 48 cms on my dipole connected between the ground and the main element, thought it sits in the loft not doing much, I did get static build up, maybe due to electrical cables or pylon. I once blew an expensive pcs transmitter, had a none dc short aerial pushed it up throught the roof where I used to live, think it was more the wind action. It was durring a thunder storm it blew the mosfet, I connected up the spare transmitter and the samething happened again, without a transmitter for weeks on end. I was able to hear a signal still on just 10mw, a mile away, was not without a signal…
I did look up bleeder resistors, you could never find information about them, about what values are needed for fm, you would think it would be easy way to protect a transmitter, I don’t think they could really be trusted. The transmitter had thermal protection which saved it quite alot when early aerials partly blew down, :D I think static protection at the transmitter level is hard to implement and what you’ve got with the resistor is abit of a bodge.

Re: Static bleeder resistor?

Posted: Sun Mar 10, 2024 5:44 pm
by Krakatoa
I don't have empirical experience, but logic tells me that a simple 1W 1k to 10k resistor between live and ground at the aerial's socket should stop static buildup between the elements of a typical open dipole or ground plane antenna.
A less electrically invasive approach should be a gas-filled surge arrestor: these have negligible parasitic capacitance (good for RF) but the firing voltage is quite high (in the hundreds of volts) but quite fast acting, btw.

Re: Static bleeder resistor?

Posted: Mon Mar 11, 2024 10:58 pm
by Albert H
The only time I've found bleeder resistors worthwhile is in receivers with really sensitive dual-gate FET input transistors. The Yaesu handheld radios used to be notorious for the static destruction of their input FETs (it was a common repair). It was possible to kill the FET gate with static just connecting the rubber helical aerial! The fix was a 10k resistor across the aerial terminal to the case, and a more robust input FET - the original was a 3SK88, and the improvement was a BF981 and a couple of resistor changes to tweak the bias.

Most of the gas-fitted surge arrestors are too slow to protect many semiconductors. The only ones that really worked were the Tritium-filled ones. Unfortunately, these are no longer available because the "green" numpties were concerned that they were radioactive - they were a lot less radioactive than a luminous watch, but the idiots howled "they're deadly!" and they were banned.

In my professional life, I did a lot of work finding ways of protecting telecommunications gear (like modems and so on) from lightning-induced spikes on phone lines. To that end, I spent some time in a high voltage lab making simulated lightning strikes with a Van Der Graff Generator! The only protection that really worked were the Tritium-filled GDTs, and you needed two of them (each leg of the line to ground) and a really solid mains earth wire! We tried all sorts of other technologies (including "high-speed" VDRs), and nothing worked nearly as well - nothing (apart from the GDTs) was fast enough to protect semiconductors!

Re: Static bleeder resistor?

Posted: Fri Mar 15, 2024 12:03 am
by thewisepranker
Albert H wrote: Sat Mar 09, 2024 12:44 am No. If you get static build-up on an aerial, you can get gigantic voltages down the coax. I've seen the results of induced lightning damage into a rig - it wasn't a direct strike, but every single component in the rig was completely fried, and there was a nasty burn mark inside the lid!
Technically speaking, static in the context of an aerial would be induced due to the interaction of water molecules in windy, foggy conditions due to friction, much like the way that the Van de Graaff generator works. A bleeder resistor wouldn't be required here unless the transmitter were expected to be connected to the antenna whilst unpowered for quite a while. I'd expect the output FET to be able to handle the static during normal operation, dissipated via the D-S resistance, both to ground and up to supply depending on the charge polarity. The parasitic body diodes would handle half of this on their own, even when off.

Lightning on the other hand - it's always at least 100x more violent than you think it is, whether induced or direct! Trying to protect against it is expensive and complicated, for pirate radio I just wouldn't bother. You might be tempted to just put a GDT across the RF output but be prepared to suffer the seemingly random impedance matching consequences.

You can buy off-the-shelf devices but they're not cheap. Be dubious of physically small devices that claim to protect against lightning. Direct strikes can explode trees that weigh several tons into lots of pieces and indirect strikes can induce 100s if not 1000s of Amps into nearby wiring.

I've done this in the real world for an RF device that was going to fly on a military aircraft and it gets really complicated, really quickly. For indirect lightning you've got to transformer couple the estimated current and voltage waveform into the total cross-sectional area of the coax shield, then look at the mutual inductance and the cross-sectional area of the centre conductor (the transformer secondary) as a ratio of what is effectively now the primary (the shield) to see what the differential voltage is going to be, all whilst trying to figure out what the source and load impedances are. Once you've done all of that, you've got to integrate the damped sinusoidal waveform that you're given into something you can work with as an equivalent constant current and a duration in order to figure out what protection device you need and whether it'll survive or not. You'll then go and consult datasheets, only to find out that you've done all of it to the wrong time constant, current limit, voltage limit, and/or severity because your spec. is a MIL standard and the device was tested to an RTCA standard.