Proper way to install heliax

[VK4WDM]

I was attaching a heliax feed line to a vhf vertical at the Townsville RAAF museum station VK4KG this morning when we were visited by an old RAAF communications tech who watched my endeavors with some interest before saying "you are doing that wrong."

My method is to simply run the feed line down the aluminum mast and secure it with cable ties. His opinion is that some plastic covering should be removed where the heliax first comes in contact with the top of the mast and the outer copper sheath clamped to the mast with a stainless steel clamp. This should be repeated at five foot intervals down the mast and also where the feed line enters the metal-clad building.

Is he right?

73

Wayne VK4WDM

[VK3ALB]

I've never seen it fitted that way. I can understand the requirement to ground the cable before it enters the shack but that's about lightning protection and a whole other story. Did he say why it has to be done in that way and what is significant about 5' clamping intervals?

[VK4GJW]

Our Heliax is grounded, but that is purely for Lightning protection.

[VK6ZFG]

Wayne

What you have been told in my view is bit of an overkill.

One should earth the feeder at the top and bottom to the mast and if a tall mast, say every 20M. The important part however are the top and bottom connections. What these do is prevent the feeder at the base of the mast from being at a voltage different from that of the mast at the base due to the coax connecting up the mast to where a higher voltage will be present at the antenna during a lightning strike. (volts drop occurs during the lightning strike along the mast (or cable if this provides a better path)- the voltage drop is quite large due to the very high lightning currents involved).

Connection to the helical cable should be made in such a way as to exclude the possibility of moisture entering the connection and travelling down the sheath. S/S clamp is fine but I would advise against using the S/S clamp to directly clamp the cable to the mast - unless you can come up with a reliable way of sealing the connection from moisture. Use a suitably sealed cable/cable strap.

By the way the surge diverter is fitted at the base of the mast. The purpose of this is to connect the divert the voltage appearing on the center conductor of the cable (connected to the top of the mast) during a lightning fault to the outer so as to limit the voltage appearing on the centre conductor. Fitting the surge diverter at the top of the mast (as I have seen) is a total waste of time.

[VK4WDM]

Hi Igor

The mast is only 8m high and the feeder swings away from it at the 3.5m point. So from what you are saying is that we should clamp it at the top and where it leaves the mast. Can you advise on the clamp we should be using? The surge diverter is mounted on the shack wall close to where the coax tails enter the shack.

Hi Lou
The reason he gave for the frequent grounding along the mast was to prevent the building up of static charges and reducing noise.

73

Wayne VK4WDM

[VK4REX]

Hi Wayne

The recommended method by Andrews is:

https://www.telcoantennas.com.au/site/m ... xial-cable

Cable runs should be attached to a good electrical ground at the top of the tower near the antenna, at the bottom of the tower, and at the point where the cable enters the transmitter building.

A surge diverter would normally be fitted at the point of entry to the building also

Rex

[VK6ZFG]

Wayne

Installations often need to adopt a compromise.

I would connect the cable where it leaves the mast so at least the outer is only subjected to a bit less than half the volts drop along the mast during a lightning strike.

Connect the surge diverter at the hut entry point to a close by earth stake. I would also put an earth stake connected to the base of the mast. Ideally the bottom of the mast would also be connected to the mast entry point but this is probably not practical.

Ideally the earth stakes should have a low impedance to earth. However achieving this can be difficult (near impossible like over here in Perth with sandy soil). The important thing is for the earth stake to provide a better path to earth than the alternatives available, it will be providing a function. There are other issues that come into play as touch voltage etc but to tackle these one needs to go into a detailed earthing design. Doing the basics is the main thing.

The extent of lightning is a problem in ones area determines to what extent one needs to go with lightning protection. The risk in some areas is greater than others. It is a form of insurance that minimises ones losses as it may not be always possible to totally eliminate them! A lightning strike can contain a massive amount of energy that needs to be dissipated in a very short space of time.

[VK4TI]

The right way

http://www.commscope.com/catalog/doc/pd ... _Cable.pdf

[VK4WDM]

Very helpful info guys. The question now arises, should coax feed lines be installed the same way? My guess is that a lot of hams just lead them from the antennas to rig with perhaps a surge protector in the line.

73

Wayne VK4WDM

[VK4TI]

""Very helpful info guys. The question now arises, should coax feed lines be installed the same way? My guess is that a lot of hams just lead them from the antennas to rig with perhaps a surge protector in the line.

73

Wayne VK4WDM""

I use a similar method on all lines because of the summer storm problems we have here certainly a decent ground on a tower is essentual , because if it tracks into your home you may loose the house The biggest mistake many make is the method used or failure to support a cable correctly

[VK6ZFG]

The support system is of importance. The strength of this determines what fault current it can carry from the mechanical perspective. I can vouch for this, as in one of my earlier lives I was involved in the design of high fault level bus bars systems which where subjected to and passed type approval tests at the then only testing facility in the southern hemisphere in Sydney.

Short term current capacity of the conductors also comes into the equation. The conductors must be of sufficient cross sectional area to be able to carry the fault current for the duration of the fault without melting. I was reminded of this when I put my finger on the bus bars just after the tests and found them more than somewhat hot. I should have known this as I had calculated the temperature rise. My memory must have had a lapse as a result of seeing the tests performed!

[VK3ZAZ]

I was attaching a heliax feed line to a vhf vertical at the Townsville RAAF museum station VK4KG this morning when we were visited by an old RAAF communications tech who watched my endeavors with some interest before saying "you are doing that wrong."

My method is to simply run the feed line down the aluminum mast and secure it with cable ties. His opinion is that some plastic covering should be removed where the heliax first comes in contact with the top of the mast and the outer copper sheath clamped to the mast with a stainless steel clamp. This should be repeated at five foot intervals down the mast and also where the feed line enters the metal-clad building.

Is he right?

73

Wayne VK4WDM

Go check out any BA NTA tower

Heliax is clipped all way down the tower no matter what the size using s/s specially made retaining clips.
AT any turning point the earthing/grounding kit is fitted.
And a central earthing point is made on all towers using copper earthing strap 7cm wide 1cm thick.
At Dundas we have LDf4/50 right up to 6 inch diameter cable worth $1MILL
It all grounded both ends and along the way.
Any horizontal is laid in cable trays or under them with clips.
Any cable ties shock horror are stainless steel variety you will never see insulated ties on large towers.
We don't use surge diverters on transmission lines
I think we do on SAT feeds and RECEIVER Off air RBR feeds
2 cents

I imagine there is an ANDREW app note.


http://www.datasheets.pl/coaxial_cables ... _GUIDE.pdf

[VK4WDM]

copper earthing strap 7cm wide 1cm thick

Obviously there has to be compromise with amateur stations because of the costs and complexity involved. My mast and anything else that needs earthing is done with 12mm diameter multi-strand power cord. At VK4KG we use off cuts of 25mm dia heliax with plastic removed the inner and outer connected together.

73

Wayne VK4WDM

[VK3ZAZ]

copper earthing strap 7cm wide 1cm thick

Obviously there has to be compromise with amateur stations because of the costs and complexity involved. My mast and anything else that needs earthing is done with 12mm diameter multi-strand power cord. At VK4KG we use off cuts of 25mm dia heliax with plastic removed the inner and outer connected together.

73

Wayne VK4WDM

Opinions are everywhere
There is no right way or wrong way and asking amateurs you sure will get misinformation
Luckily at work we don't listen to amateurs even when amateurs who work in professional circles say that industry does.

LOL


Its all on the internet and professionally at that

http://www.solacity.com/docs/polyphaser ... unding.pdf

Diagrams show strike on tower where at 2M agl 28 KV occurs
Right where cables come into most buildings so you ground it fwiw

[VK3ZAZ]

copper earthing strap 7cm wide 1cm thick

Obviously there has to be compromise with amateur stations because of the costs and complexity involved. My mast and anything else that needs earthing is done with 12mm diameter multi-strand power cord. At VK4KG we use off cuts of 25mm dia heliax with plastic removed the inner and outer connected together.

73

Wayne VK4WDM

I suggest you strike up friendship with local SLM/FIM/First responder and go check National sites out to see how minimum contractual lighting protection is done.
All earthing goes to common point, yes we use big green cables like you say but the primary earthing is done by thick copper straps, both on towers and along cable trays.
The earth straps are insulated until point of grounding at central station earth.
We do triennial ground inspections to make sure they are sound and fit for use.

Get it wrong and ???



Cows sheltered near fence in storm that got hit.

Lightning experts from the Electrical & Computer Engineering department are studying ways to reduce the cost of lightning damage by investigating how to repair and protect power lines that are hit by lightning.

Lightning damage to power lines in the U.S. costs almost $1 billion annually, and 30 percent of all power outages are lightning related, according to studies by the Electric Power Research Institute (EPRI). The cost to industry of power failures led EPRI in 1993 to create the predecessor to what a year later became UF’s International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida.

“Industry wants power that stays steady at 60 cycles per second, with no impulses or failures caused by lightning,” says Martin Uman, UF Electrical & Computer Engineering (ECE) professor. “Power should be pure and uninterrupted,” says Uman, who, with ECE Professor Vladimir Rakov, is co-director of the ICLRT.

The ICLRT’s triggered lightning experiments involve underground and overhead power distribution lines built on the site. Presently, there are two different types of overhead lines that are identical to those used by power companies such as Florida Power & Light, which funds the present experiments.

Lightning is triggered by wire-trailing rockets fired toward overhead storm clouds. Lightning vaporizes the wires and follows their traces down in a manner that can be observed and studied. The researchers have investigated what happens when lightning hits power lines, how they fail, how to fix them, how to protect them, and equally important, the physics of that lightning.

A power line collects natural lightning that would otherwise strike in an alley about 100 feet wide on either side of the line. To protect against the effects of lightning strikes, power lines are often equipped with arresters and overhead ground wires. An arrester behaves like an open circuit when the line is in normal operation. When the line is hit by lightning, the arrester acts more like a short circuit. Like throwing a switch, it diverts lightning current to the ground, holds the voltage at safe value, and thereby keeps the line from harm.

The tests at the ICLRT revealed that up to half of all natural lightning strikes will cause the nearest arresters to fail, according to Uman.

The studies revealed that underground power lines are also vulnerable, contrary to previous beliefs, Rakov says. Ground lightning strikes affect underground distribution lines almost as often as they affect overhead lines. Further, the ground rods intended to dissipate the current actually act as interceptors. A percentage of the current does enter the system through the ground rods and can cause damage in the system.

As a further test, the ICLRT conducted an experiment on ground rods of the kind commonly used to protect homes from lightning. The observation that ground rods appeared to transmit the bulk of the current from lightning into the system rather than to “ground” was unexpected.

The researchers built a small structure with a typical lightning rod system at the ICLRT. Lightning rods placed on a home’s roof are supposed to route the current from a strike into the soil via wires connected to buried vertical ground rods. The established international standard for these systems allows no more than 50 percent of a lightning strike’s current to enter a home’s electrical system.

The experiment demonstrated that more than 80 percent of current from a lightning strike flowed into an electrical system when the ground rods were in sandy soil of the kind found in the southeastern states. Sandy soil tends to remain dry beneath the surface and therefore does not conduct electricity well.

Rakov says more research needs to be done to determine how lightning current is distributed through a system. The ICLRT studies have shown that in 50 percent of lightning strikes, the strong initial pulse of the current is followed by a tail of continuing current of variable duration. It is not now known how the continuing currents divide or if they flow through arresters. This is important because arresters are designed for and tested against strong pulses only, Rakov says.

In the meantime, the studies indicate that homeowners should probably use surge protectors at the electric meter and in the home. Wire ring grounding systems are also desirable. These systems involve lightning rods connected to a buried wire loop that circles the house. Because the loops have more surface area than ground rods, they can better dissipate current into the ground.

As for power lines, the solution lies with the power companies, Uman says, given the available results of the experiments. It is theoretically possible to make power lines lightning proof if overhead ground wires are combined with line arresters of the proper power and energy rating. And although there are several types of new lightning elimination or dissipation devices now sold that are advertised as able to protect against lightning by diverting it away from the power lines, they have not been proven to work in that situation, he says.

Writer: Martha Dobson

[VK4WDM]

Hi Steve

A very interesting document. The info on stranded and solid wire compared with copper strap was very enlightning (pun intended ). The challenge is to do an adequate job whilst keeping the cost down to what is acceptable for an "amateur" station that does not have the ability to recover costs.

I will have a poke around the metal recyclers and see if I can pick up some second-hand copper strap. The new stuff is a way out of my price range. I presume that the woven stuff used for automotive grounding is not good enough?

73

Wayne VK4WDM