Reducing precipitation static

The following discussion about how to reduce precipitation static took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.

On 12-Feb-2005 K2TXB wrote:
Maybe someone can help me with a problem I have had here for many years and with a number of different antennas. Precipitation static. I know, everyone has it. But mine is so strong that it regularly blows out the gaasfet on my tower mounted 2 meter preamp. When the noise gets to 20 over S9 or a little above, the preamp will almost always blow. This has happened mostly with rain static, but in the January VHF contest it happened with snow static too.

It does not have to actually be raining or snowing for me to receive the static. If there is precipitation somewhere nearby and I point the antenna in that direction I often get the static. So I have had the experience of having the preamp blow because I turned the antenna into a storm, unknowingly.

I have received a lot of suggestions and none have helped so far.

1. Ground the tower - done and no difference. 
2. Use separate power leads up the tower to power the preamp - done. 
3. Put an AC supply on the tower and don't run DC up the tower. 
4. Use separate shielded cables for the preamp power leads vs the relay control leads - they are already separate and shielded, but run in the same bundle. 
5. Use more diodes/capacitors/chokes on the outside and inside of the preamp - I have doides across every relay coil and the preamp has a .001 disc ceramic and a 1 or 2 MFd tantalum mounted directly across the DC input to ground, as well as the normal filtering inside the preamp.

None of these ideas have any merit (to solve this problem), because 
a) the voltage regulator in the preamp never blows out, and 
b) it never happens if the antenna relays are left in the transmit position. 

With power disconnected from the relays, the antenna is connected to a separate feedline that goes down to the transmitter only, and the preamp input is terminated with a 50 ohm load. All suggestions that this is some kind of switching transient or induced pulse on the control lines are incorrect because when the preamp blows the equipment is just sitting there in receive. Also with the antenna relays in the transmit position (but everything powered up) I can sit through a lightning storm and the preamp will survive (and I have done exactly that countless times).

It is not only precip static that will easily blow the preamp. During periods when there may be a storm elsewhere, if I point the antenna towards where the storm is, I the spikes are strong enough to blow the preamp even though I cannot hear thunder and see no lightning.

I went into all that detail to forestall the numerous suggestions along those lines that I would surely get if I didn't explain.

It is clear that the only mechanism that is blowing the preamp is energy being fed into the input connector of the preamp during normal receive periods. I had this problem on my array of 4 KLM 16LBX and also on a single 16LBX, and now on my M2 8WL - 24 elements, 52.5 foot boom, at 100 feet. I do not hear any precip static on my low EME antenna that is only 20 feet high.

I have the same exact problem with 2 different preamps, admittedly of a very similar design. But here is a strange thing. I have NEVER lost a preamp due to precip static if the preamp is in the shack instead of on the tower. The only difference is 140 feet of 7/8 hardline between the antenna and the preamp, yet the preamp seems to survive and the rain static never gets as strong as 20/9 when the preamp is in the shack! (But I also hear better with the preamp on the tower.)

Some have suggested using antennas with grounded elements. Of course that is not easy as I would have to discard an expensive antenna and there is no equivalent antenna made with grounded elements. Some experts have said that element grounding is not important as long as the driven element is grounded. The M2 antenna uses a T-match and a balun loop. Of course the shield of the coax is grounded to the boom, but I don't believe that the DE is actually grounded. Is there any way to ground it without causing problems? Will it help?

Are there any other ideas? For many years I have been trying to come up with answers to this problem, I have talked with many experienced VHF'ers and have never found anyone who has had this problem or who has even heard of anyone else having this problem. Suggestions???

On 12-Feb-2005 W5UC wrote:
One other possibility. Switch the pre-amp to a 50 ohm load during those times. Admittedly, it will take you off of the air, but that may be better than climbing up there to change the pre-amp on a regular basis.

On 12-Feb-2005 K2TXB wrote:
Hi Mike. I already do that. And it does save the preamp. But it is not always easy to do. I have been sitting here working, deeply involved in the programming I was doing and had a rain storm start up. Even though my desk faces a bay window, I am so deeply involved that I do not recognize the fact that a storm is starting and my system is in danger until it is too late. In the January contest it was my own fault. I decided to try to operate even with the high noise level, knowing that I was taking a chance. After 3 contacts it blew. It was snowing hard but there was little if any wind. I operated for the next 11 hours using the EME antenna at 20 feet for receive. Talk about your "alligator station", hi!

On 12-Feb-2005 G8WRB wrote:
How about a 100k of so resistor across the input connector? That should have negligable effect on the NF.

How is your pre-amp connected - is it DC coupled? If so, then my idea won't work, and I can't understand how static will cause you much hassle in this case.

On 12-Feb-2005 K2TXB wrote:
Hello David. Yes, that has been suggested. But the input to the preamp is into an air wound coil. One end is grounded and the gate is tapped onto the coil. So the gate is grounded anyway.

And, YES, I don't understand either, hi.

On 12-Feb-2005 AL7EB wrote:
Russ,

A couple quick impressions and I will let Leif and the experts advise you: I wonder if the tall towers are building up static voltage? You have them grounded for lightning...yep, I see you do. The RF input of the preamps is capacitively coupled, I suppose (normal)? How about a dc-ground that is ac above ground on the inputs?

I'm going by the fact that when your preamps are terminated by 50-ohms they are OK. I wonder if a shorted stub at a frequency removed from the operating frequency could solve the static build-up.

BTW my four M2 xpol-20's are at 50-feet and I have never heard rain static. Perhaps living in a maritime climate is why? We see lightning only once every 3-4 years (very rare). Static build-up during deep-cold wx with winds (also rare) is more likely to generate high static potentials...not a time from assembling gasfet circuits :-) Hmm maybe your tall towers are seeing dry-wind derived static build-up?

On 12-Feb-2005 G8WRB wrote:
What makes you so sure it's static on the input, rather than transients on the supply fed to it? Currents through the wires to the relays will induce a magnetic field. If these run parallel to the power supply leads, there will be maximum pickup.

If you can take the top off the dead GaAs FET and photograph it under a microscope, you might be able be able to twist the arm of whoever makes them to make some comment about what destroyed it. I've known this to happen on larger ICs, but I guess its not impossible on transistors. They might be able to see if the gate is punctured, or whether it was the drain that got damaged. If you do that make sure you take the most rigid of static precautions while doing this - there is no point in inducing further damage to confuse anyone looking at the photograph.

Feed though capacitors are next to useless unless they have some impedance to work in. The loss through them is always quoted assuming 50 ohms, but low impedance power supplies are not 50 ohm sources. Neither are voltage regulators.

Why not publish your circuit, including switching relays on the web and let others comment? We might see what is wrong with the design.

On 12-Feb-2005 PY1RO wrote:
Russ,

The 140' of 7/8 inch cable in front of your preamp (when it is inside the shack) act like a capacitor across the input to the preamp. It probably is large enough to short a good portion of the spikes to ground.

Couldn?t one use two crosspolarized diodes across the preamps input? In the old days we used that as noise-"limiters" on the audio output on receivers to saveguard the ears from the QRN crashes on 160m.....

The diodes work like low voltage zeners, letting pass signals below the zener threshold, shorting out what is above. Of course I am only talking theory, I have no idea what these diodes would do to the wanted signal on V/UHF.

Just a thought...

On 12-Feb-2005 K2TXB wrote:
Rolf, thanks for the suggestion. I might have to try that, but diodes across the preamp input do more than act as spike suppressors. They also make great mixers. I live in a high RF environment, as does anyone on the east coast between Washington DC and Boston, and I am sure that those diodes would introduce all kinds of mixing problems from the strong local transmitters. I live less than a mile from several cell sites and VHF transmitters of various types. There is a strong HAM FM repeater just 1 MHz above the 2 meter calling frequency and a number of strong Packet stations on 144.390 - It's a wonder I can hear anything at all on 2 meters as it is! So I don't want to introduce a source of mixing if I can help it.

But I agree that it might solve the rain static problem.

PS: I like your explanation about the 140 feet of coax acting like a capacitor. Of course that capacitor is then connected directly across the input tuned circuit to the preamp. That must change the tuning - but these things are "broad as a barn door" so it probably doesn't cause much of a problem. But I wonder if an equivalent capacitance inside the preamp would help. Thinking further about it, probably the inductance of the coax has as much to do with it as the capacity...

On 12-Feb-2005 W7EME wrote:
I am not sure if my arrangement is any better than others, but here it is:

First of all, at a lack of enough conductors to feed my additional relays and preamps, I went to a 14 watt solar panel and a gel cell, pure battery DC. This DC circuit is a very small physical package and likely does not open it self to very much outside influence. This circuit feeds my preamps. Isolated from all else, with all the hardware on the tower, out of the antenna near field. I run a second relay (and a little more loss) after my T/R relay which is normally closed to TX. This second relay is to a small 50 Ohm load in the normally closed (TX) position, and connects my rx line and preamp to antennas when energized to the RX position, through external contacts on the T/R relay.

A have huge amounts of wire up out here for VLF work and have regular arcing at a discharge point outside here (corona gap ball type as on a broadcast tower) in even mild winds. I however never lost a preamp at VHF and Up except to my own errors.

On 12-Feb-2005 K2TXB wrote:
> What makes you so sure it's static on the input, rather than transients 
> on the supply fed to it? Currents through the wires to the relays will 
> induce a magnetic field. If these run parallel to the power supply 
> leads, there will be maximum pickup.

I agree, but the thing is, the preamp never blows when the input is terminated. The power wires are still connected, the preamp is still powered up - the only difference is that the antenna is not connected. So I cannot see how magnetic fields and effects with power wiring can be the problem. Perhaps I am missing something?

> If you can take the top off the dead GaAs FET and photograph it under a 
> microscope, you might be able be able to twist the arm of whoever makes 
> them to make some comment about what destroyed it. I've known this to 
> happen on larger ICs, but I guess its not impossible on transistors. 
> They might be able to see if the gate is punctured, or whether it was 
> the drain that got damaged. If you do that make sure you take the most 
> rigid of static precautions while doing this - there is no point in 
> inducing further damage to confuse anyone looking at the photograph.

That's an interesting idea. I don't know anyone who has an "in" with a manufacturer, but I wonder if someone with a bonding facility would know enough to analyze a blown transistor.

> Feed though capacitors are next to useless unless they have some 
> impedance to work in. The loss through them is always quoted assuming 50 
> ohms, but low impedance power supplies are not 50 ohm sources. Neither 
> are voltage regulators. > 
> Why not publish your circuit, including switching relays on the web and 
> let others comment? We might see what is wrong with the design.

Ok, I will try to do that in a day or two. It is going to take me some time to put it together.

On 12-Feb-2005 G8WRB wrote:
>I agree, but the thing is, the preamp never blows when the input is 
>terminated. The power wires are still connected, the preamp is still 
>powered up - the only difference is that the antenna is not connected. So I 
>cannot see how magnetic fields and effects with power wiring can be the 
>problem. Perhaps I am missing something? 

Well it depends on how much time you spend with it terminated, and how much on the antenna. If you only terminate it for test periods of a few hours, but leave it on the antennas for days at a time, then statistically you are more likely to get damage when the device is on the anteanna, even if its nothing to do with what's on its input.

I know it is said that static damage does not always appear immediately to devices, but that too much static can damage them, causing a premature, but not immediate death. It's not something I know much about.

>That's an interesting idea. I don't know anyone who has an "in" with a 
>manufacturer, but I wonder if someone with a bonding facility would know 
>enough to analyze a blown transistor. >

I suspect if you could get a decent photograph of a few blown ones, and phone the manufactuer up, sooner or later you will get some engineer who will say send over the jpegs and I'll have a look at them. Or ask their QA department. If you make a device and someone says they keep blowing up, and provides some evidence, you would be a bit stupid not to at least look at it.

Or stick them on a web site (with a username/password) and let the manufacturer know about it. Then it won't appear you are knocking them. Give them enough time to respond, and if that does not work make the photos public. (If its any help, I could put them on a web site for you that requires a username and password to access the pages. Drop me an email off the list.)

They might say send us one back and we will analyse it. This is what happened to somene I know who blew up an expensive IC. The manufacturer provided a photograph, showing the damage and basically saying it was his fault. I'm sure the cost of the time for them to open it, photograph it, deduce the problem, send the photograph back ... must have much greater than the cost of just sending him another IC. But I guess they wanted to know why it blew, and keep a good reputation.

I don't know how practical this is with microwave transistors, but it might be possible. Size might be the problem. Whereas a Pentium chip is pretty big so damage is more likely to remain localised. This might not be so on a FET.

I guess static, by its very nature generates wide band noise - that is why you can hear it on a receive at any frequency. Hence stubs, inducators etc are never going to work, since they must always be open at 144MHz, or whatever your receive frequency is.

Thinking about it more, even a 560 or 1k Ohm resistor across the input would have negligable effect on the NF, but must do a pretty good job at discharging any static.

On 12-Feb-2005 SM5BSZ wrote:
> Maybe you or someone can help me with a problem I have had here 
> for many years and with a number of different antennas. 
> Precipitation static. I know, everyone has it.

??? Everyone has what? Certainly I have static rain or snow now and then but it is not much of a problem since the QRN is removed quite efficiently with the wideband noise blanker.

> But mine is so strong that it regularly blows out the gaasfet 
> on my tower mounted 2 meter preamp.

This is why I wonder "what ?" above. There is no reason at all for the preamp to blow unless your tower is directly hit by a lightening stroke.

> When the noise gets to 20 over S9 or a little above, the preamp 
> will almost always blow. This has happened mostly with rain 
> static, but in the January VHF contest it happened with snow 
> static too. 

Maybe your center conductor is not properly grounded. You may check the DC resistance from the center conductor of your cable to the screen at the preamp input. Probably you find an open circuit. That is what you would have with a folded balun if it is isolated from the boom tube. It is also what you have with a straight dipole if it is fed with a sleeve balun.

The DC resistance of the antenna cable at the preamp input MUST be zero ohms. If it is open circuit, the center conductor will get charged by the static rain. If your preamp has a capacitive input the center conductor will charge until the voltage is big enough for a small arc across the input capacitor. The wideband pulse will kill the transistor.

With a tapped coil input your preamp would be safe as long as you do not operate the relay. The charged raindrops represent a true current source. They feed charge (current) into the elements they hit and the current has to go somewhere.....

A shorted 0.25 wl stub at the antenna side of your relay is one way of solving the problem (I think)

> It does not have to actually be raining or snowing for me to 
> receive the static. ??

> If there is precipitation somewhere nearby and I point the antenna 
> in that direction I often get the static. So I have had the 
> experience of having the preamp blow because I turned the antenna 
> into a storm, unknowingly. 

?????????????????????? This does not make any sense to me.

> I have received a lot of suggestions and none have helped so far.
> 1. Ground the tower - done and no difference.
> 2. Use separate power leads up the tower to power the preamp - done.
> 3. Put an AC supply on the tower and don't run DC up the tower.
> 4. Use separate shielded cables for the preamp power leads vs the
> relay control leads - they are already separate and shielded,
> but run in the same bundle.
> 5 . Use more diodes/capacitors/chokes on the outside and inside of
> the preamp - I have doides across every relay coil and the preamp
> has a .001 disc ceramic and a 1 or 2 MFd tantalum mounted directly
> across the DC input to ground, as well as the normal filtering
> inside the preamp.
> None of these ideas have any merit (to solve this problem),

OK. That is what I would have guessed....

> because a) the voltage regulator in the preamp never blows out,
> and b) it never happens if the antenna relays are left in the
> transmit position.

Another indication that DC grounding of the input side is missing.

> With power disconnected from the relays, the antenna is connected
> to a separate feedline that goes down to the transmitter only,
> and the preamp input is terminated with a 50 ohm load. All
> suggestions that this is some kind of switching transient or
> induced pulse on the control lines are incorrect because when the
> preamp blows the equipment is just sitting there in receive.

OK. Just as expected.

> Also with the antenna relays in the transmit position (but everything
> powered up) I can sit through a lightning storm and the preamp will
> survive (and I have done exactly that countless times).

OK. Just as expected.

> It is not only precip static that will easily blow the preamp.
> During periods when there may be a storm elsewhere, if I point
> the antenna towards where the storm is, I the spikes are strong
> enough to blow the preamp even though I cannot hear thunder and
> see no lightning.

?? This makes no sense to me. Are you sure there was no rain or snow and that it really made any difference where your antenna was pointed?

The antenna is narrowband. I do not think it has the directivity over a large enough bandwidth to pick up the electromagnetic pulse of distant lightening discharges.

> I went into all that detail to forestall the numerous suggestions
> along those lines that I would surely get if I didn't explain.
> It is clear that the only mechanism that is blowing the preamp
> is energy being fed into the input connector of the preamp
> during normal receive periods.

OK.

> I had this problem on my array of 4 KLM 16LBX and also on a single
> 16LBX, and now on my M2 8WL - 24 elements, 52.5 foot boom, at 100
> feet. I do not hear any precip static on my low EME antenna that
> is only 20 feet high.
> I have the same exact problem with 2 different preamps, admittedly
> of a very similar design. But here is a strange thing. I have
> NEVER lost a preamp due to precip static if the preamp is in the
> shack instead of on the tower. The only difference is 140 feet
> of 7/8 hardline between the antenna and the preamp, yet the preamp
> seems to survive and the rain static never gets as strong as 20/9
> when the preamp is in the shack!

Hmm, I would expect the rain static to be 20 dB (or more) above S9 also with the preamp in the schack. The 140 feet of hardline will protect your preamp because the dielectric has some resistivity. The current that the rain/snow forces into the center conductor will go to ground (the screen) through the dielectric of the cable. If the voltage drop is smaller than the voltage that can cause arcs across the pre-amp input capacitor all will be fine.

> (But I also hear better with the preamp on the tower.):-)
> Some have suggested using antennas with grounded elements.

The elements make no difference except for the radiator.

> Of course that is not easy as I would have to discard an
> expensive antenna and there is no equivalent antenna made
> with grounded elements.

With a folded dipole, thin wires between the center and the braid of your coaxial cable should solve the problem.

> Some experts have said that element grounding is not
> important as long as the driven element is grounded.

Yes, I agree on this.

> The M2 antenna uses a T-match and a balun loop. Of course
> the shield of the coax is grounded to the boom, but I don't
> believe that the DE is actually grounded. Is there any way
> to ground it without causing problems? Will it help?

Yes. The element is maybe 6 mm in diameter. If you route two wires of say 2 mm diameter from a point 10 mm outside the boom to the brais of the feed cable you have introduced a fully symmetrical perturbation. The inductance of these wires will be much higher than the inductance through the element itself so you would not change the properties of the element at all for 144 MHz signals. For common mode signals including the DC current injected by rain, the wires would be an excellent short to ground.

As an alternative you may add a 0.25 wl shorted stub at the antenna side of your relay.

On 13-Feb-2005 GM4ISM wrote:
A device for reducing precipitation static was used in the UK many years in the days of Band I TV. The static problem was reduced by putting a lightning spike at the top of the tower with (4?) resonant (1/4wave) radials below the spike. (They were folded in a narrow V to make a 'thick' element to increase effective bandwidth This was widely used on the BBC Band I system in Scotland (where I must admit it rains a bit) I never saw these in use, but I dismantled a few HI. Must be about 20 years ago that the last one was removed from the receive tower at Granton On Spey in the highlands. They were reputed to work, and somewhere I had a BBC research dept. paper on them, Will try to find it Maybe worth a try??

On 13-Feb-2005 W7QX wrote:
That sounds like the old lightning rod theory (which I believe) except they are using a magnetic ground plane (resonant) instead of a ground wire. If the tower is only holding a single band antenna, I am sure that it is worth a try.

I used to use 2N4416s (JFETs) a while back and every time we would have a thunderstorm, I would have to change the JFET. Fortunately, it was an easy task to run up the tower and change the preamp, at that time. This was on 2 meters back in Illinois.

On 1-Apr-2014 VE3XQQ wrote:
During my days as a Canadian Air-force Avionics tech when we would have receive problems with poor S/N it was usually when the static wicks would fail. Several of these carbon fiber wicks were along the trailing end of any flight surface.

Static comes is two basic forms:

1. weather derived, ie winds and storms
2. atmospheric, the electrostatic pontenial between the ground and the upper parts of breathable atmosphere.

 The two are not mutually exclusive and tend to combine.

 Some amateurs who have a stack of yagis on a tower have found the upper one to be nearly useless on receive because of P-static, and this is with dc grounded mono-banders using a beta match. Some success has been had with putting some kind of discharge units on ends of each element. Corona balls of an inch or so seem to help as do 2-4 inch capacity hats mounted on the ends.

 The solutions are not a 100% solution but seem help greatly.


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1 user comments


#1 - W3ATM - 10/Feb/2010 - 18:18
My HF antennas were having static buildup during windy snow storms. I put a 100K ohm resistor from the ladder line inputs (both sides) to ground. This bled off the static buildup but did not effect my transmission or receiving. Good luck in solving your problem!
73 Bill

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