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Author Topic: more on lightning arrestor & other antenna issues !  (Read 10995 times)
9H1FQ
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Posts: 194




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« on: September 02, 2011, 10:32:26 PM »

Hi all.


I have these issues with my antenna:

My antenna is on top of a three storey building. I have a 1000 liters water storage tank, connected to the house water supply, by plastic pipes and   is always full. Can I use it as an RF ground ?

I will be installing a remote auto  atu. I am afraid that it will not survive long, as my Qth is prone to thunderstorms and lightning in winter. A knife switch is fine to ground the antenna, but it is not practical for me to go up and down frequently to the rooftop. A remote knife switch does not exists, and I am afraid to fit an RF relay.

A gas filled arrestor will be the ultimate solution, but they are only available for 50 ohms coax ! Cannot be used on open feeders. A spark gap will only work at a relatively high voltage.

Any comments please ?

Thnaks

Paul
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An unknown gem of the Mediterranean sea is the Island of Malta. blessed with warm sun , all year round.
In just one day, you can see places which will make you travel in history from the Prehistoric temples throu the middle ages, the Knights of Malta, all the way to world war two. All the major civ
W8JI
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« Reply #1 on: September 03, 2011, 03:27:03 AM »

A remote relay will work fine, and be much better than any gas tube or spark gap. There are suitable relays.

Is the water tank metal?

What particular type of tuner do you have?

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9H1FQ
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Posts: 194




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« Reply #2 on: September 03, 2011, 03:58:56 AM »

The tank is plastic and I intend to fit an SG239 remote auto atu.
Can give some examples of suitable relays please ?
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An unknown gem of the Mediterranean sea is the Island of Malta. blessed with warm sun , all year round.
In just one day, you can see places which will make you travel in history from the Prehistoric temples throu the middle ages, the Knights of Malta, all the way to world war two. All the major civ
AA4PB
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Posts: 15019




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« Reply #3 on: September 03, 2011, 07:57:59 AM »

If the tank is plastic then you can't use it for an RF or a lightning protection ground. The fresh water isn't going to provide any sort of effective ground. For lightning protection you can run a #6 gauge wire down a tank leg to several 8-foot ground rods (there may already be one on the tank or the building). For an RF ground for the tuner you are going to have to string out a number of radials or at least get connected to building steel.

Grounding of the antenna (when not in use) to protect the tuner input can be accomplished with a high-voltage open-frame relay mounted inside a water-proof PVC electrical box. Again, you need a good ground at the tuner in order for this to be effective.

What type of antenna are you considering on the tank?
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Bob  AA4PB
Garrisonville, VA
9H1FQ
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Posts: 194




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« Reply #4 on: September 03, 2011, 08:36:29 AM »

At present I have a G5RV, but the intension is to experiment with various others, verticals and wire
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An unknown gem of the Mediterranean sea is the Island of Malta. blessed with warm sun , all year round.
In just one day, you can see places which will make you travel in history from the Prehistoric temples throu the middle ages, the Knights of Malta, all the way to world war two. All the major civ
W8JI
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Posts: 9748


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« Reply #5 on: September 03, 2011, 02:58:20 PM »

The tank is plastic and I intend to fit an SG239 remote auto atu.
Can give some examples of suitable relays please ?

You need a high voltage high current relay with good insulation, and a proper way to wire things.

Like this:

http://www.dxengineering.com/Parts.asp?ID=3571&PLID=85&SecID=33&DeptID={5B8FC0A3-1D03-4603-B76D-F4F0DA53D8C1}&PartNo=DXE%2DRLY%2DSD
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AA4PB
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Posts: 15019




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« Reply #6 on: September 03, 2011, 05:50:06 PM »

The SG2327 is limited to 200W PEP so I did the following calculation based on that maximum power:

25 Ohms = 71V at 2.8A
50 Ohms = 100V at 2A
2000 Ohms = 633V at 0.1A

The relay from DXE is rated at 6KV and 15A maximum. That means that with 200W output the impedance could be as low as 1 Ohm or as high as 180K Ohms and still be within relay specs.

I just ordered a couple for the same application - to short the antenna (an end-fed inverted L) to ground when the tuner is not powered.

« Last Edit: September 03, 2011, 05:52:29 PM by AA4PB » Logged

Bob  AA4PB
Garrisonville, VA
9H1FQ
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Posts: 194




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« Reply #7 on: September 03, 2011, 11:04:33 PM »

Thanks.
The shorting relay is the best for lightning. The ideal would be a Ross shorting bar relays, but they are made for very high power and very expensive.

 http://www.surplussales.com/Relays/RossRelays.html

 But I will still have the problem with statics. Once I had an R4 vertical and I was lucky that my TS850 has a GDT gas tube at the input, that saved it all. There were no thunderstorms at the time, just clear blue skies. The problem is that GDT are always quoted as 50 ohms ! That would have solved the problem of statics for the atu. Incidently, GDT do not have to be replaced, once discharged, do they ? I understand that the gas will return to its normal state.

Charles, I have sent you an email  about this subject on    at contesting dot com on the 2nd September.
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An unknown gem of the Mediterranean sea is the Island of Malta. blessed with warm sun , all year round.
In just one day, you can see places which will make you travel in history from the Prehistoric temples throu the middle ages, the Knights of Malta, all the way to world war two. All the major civ
KF7CG
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Posts: 1213




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« Reply #8 on: September 06, 2011, 10:37:45 AM »

When working with insulated and fairly long wires, an electrical storm is not required to have electro-static discharges and the resultant damage. Unless a moderate (<50,000 ohms or so) dc resistance is provided to ground insulated wires can develop quite high voltages in moderate wind storms.

Have you ever noticed that a hot stick is used to grab the cable from a helicopter if the cable is wire. This is due to the high voltages that can be generated by the static electrical effects of the rotor downwash. The same effect applies in a windstorm with a well insulated antena. A good choke to ground is a worthwhile preventative while operating.

KF7CG
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9H1FQ
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Posts: 194




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« Reply #9 on: September 06, 2011, 02:03:13 PM »

I remember when we use to build an 807 pa stage with a pi output network, we used to include a choke at the output, just in case the dc blocking capacitor fails.
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An unknown gem of the Mediterranean sea is the Island of Malta. blessed with warm sun , all year round.
In just one day, you can see places which will make you travel in history from the Prehistoric temples throu the middle ages, the Knights of Malta, all the way to world war two. All the major civ
AA4PB
Member

Posts: 15019




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« Reply #10 on: September 06, 2011, 03:43:46 PM »

The SGC tuners always have a DC path between the input and output via the inductors. Even when it has no power applied the path exists through all of the inductors in series. That means that you could handle the static discharge bleed-off issue with a resistor across the radio side of the tuner where the impedance should always be close to 50 Ohms.

The problem with doing anything on the antenna side of these tuners is that the impedance and voltage can be very high at the antenna connection, depending on the frequency and the length of the antenna.
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Bob  AA4PB
Garrisonville, VA
AA4HA
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Posts: 2630




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« Reply #11 on: September 07, 2011, 06:02:43 PM »

A gas tube arrestor does not have a 50 ohm impedance. Many of the holders of gas tubes may be arranged for 50 ohm coaxial connectors (PL-259, Type-N, etc...).

For ladder line you would need three gas tubes, line1-line2, line1-ground, line2-ground. The challenge would be to get gas tubes that are rated for the maximum voltage during transmit and the ability to carry sufficient current to ground. This is very similar to the protective devices that are used on telephone equipment where the surge protector is a three lead device to provide a path to ground for "tip" and "ring".

A gas tube has several ratings that you would need to take into consideration. Take a look at the specifications for these devices off of eBay;  360135457873

Below the trigger voltage gas tubes will have a very high resistance (tens of megaohms), there is a point where the gas tube begins to "glow" (below trigger voltage) and a small amount of current begins to flow (a few mA), then there is the trigger voltage (for the tubes listed on the ebay posting it is between 300-500 VDC). There is a maximum striking voltage (800 V for these). Glow currents (1.1 A), max AC current (20 A) and peak current (20 kA). A few years ago I purchased a case of these things (960 to be exact) and use them at the knife switch disconnect enclosures for the ladder lines. A direct lightning strike will vaporize the leads off of an individual protector so I have 5-6 of these tubes across each ladder line.

For my ladder lines I bring them down into a stainless steel NEMA 4X enclosure through ceramic high voltage insulating bushings on top of the enclosure. There is a copper bus bar inside of the enclosure and the center leg of each three lead protector is screw lugged into the bus bar and hard soldered into the ladder line leads. There are a pair of open air chokes (#16 AWG, 6 wraps around a 1/2" piece of PVC plastic pipe) and then the connection runs down to a 3PDT knife switch (600 volt, 200 amp rated, ceramic base and copper bus bar).

One position on the knife switch shorts the antenna ladder line leads to the copper backplane. The other position lines up the ladder line to the antenna balun.

The antenna balun has another gas tube protector that is bulkhead mounted at the bottom of the stainless steel enclosure on the Type-N connector. From there the coax runs up the side of the house about 16 feet to where there is a 6" wide copper bus bar in the lower part of a partially open window with another bulkhead protector. Everything (window entrance bulkhead panel, stainless steel enclosure for the gas tubes and knife switches and coaxial "ground kits" are all tied into a 4' x 6' copper mesh grid that is buried a foot down. 2" copper strap ties it all together.

-----------------------
For broadcast facilities they will use a really expensive gas tube protector (that costs a few thousand dollars) or air terminals (balls or horns). The idea is to allow for the lighting to arc a few inches but for the arc to not be sustained while the transmitter continues to operate. If the gap/ voltage/ dielectric is set too low the arc will continue after the lightning transient and burn up the transmitter (as the transmitter is now supplying the power for the arc)).

Since you are putting an antenna on top of a plastic tank and the water is for all intents and purposes an insulator, you need to provide a nice, low impedance path to earth. Much lower in impedance than your antenna system. "Think" like lightning, it is in such a hurry to balance the charge it is going to rush through anything go dissipate. It "likes" fat, low inductance paths with lots of surface area (strapping is better than a round conductor), no bends that even look like an inductor (smooth transitions between conductive surfaces) and connections that are more like permanent attachments and have no resistive component (brazed connections are good, exothermic connections are great, making everything into one piece is even better).

My little air chokes between the cabinet bushings (6 turns of #16 on 1/2" PVC) is really just to provide a bit of series inductance to allow the gas tubes to do their job in diverting the current to ground. As far as being contributors to the antenna system they are irrelevant but just give enough impedance to stretch out and delay the current component of the lightning transient.

Normally I leave the knife switch in the "antenna shorted to ground" position and there is a "tell tale" on the cabinet so I can look out the window and see the switch position. I get really antsy when the thunder and lightning has started and I realize that I left things connected and need to go darting outside to throw those switches and run back inside. If I could set up a pneumatic actuator to operate the handle I would be really happy... let lightning try to follow a piece of plastic pressurized tubing back up the switch handle. (air motors and pneumatic systems are used for other high voltage applications because they are inherently insulated).
« Last Edit: September 07, 2011, 06:07:40 PM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
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