Outside Antennas - DC grounded or not?

Forum for CB, PMR, Ham, Business users, GMRS, Repeaters and all other TX Chat

Outside Antennas - DC grounded or not?

Postby G4RMT » Fri Jul 15, 2016 11:18 am

Anyone got any views on lightning issues.

Lots of the commercial antenna systems make big bones in their advertising about the antenna being DC grounded - having electrical continuity - effectively a DC short across the terminals. This means that some perfectly decent types of antenna can't be used as they are floating - simple half wave dipoles for example. Good matches to the feeder cable and simple construction. Commercial designs seem to prefer folded dipoles with their higher impedance and then a matching network to bring it down to the feeder, as in a balun. Verticals are good examples of where some designs float, and others are grounded.

My view is that my research suggests that a metal object in the sky makes a good lightning attractor, and a few modern piece of research suggest that a grounded metal object in the sky can initiate lightning. On a commercial tower it's clearly something built into the design - with proper heavy duty lightning protection. This makes it sensible that all the antennas are bonded to the structure. When I was at college we had an electrostatic voltmeter, and connecting this to a radio antenna on the roof showed the amazing voltages building up in storms. Some people are very safety conscious and remove the connections to their aerial systems, others don't bother. I've got a few aerials up here - some show a DC short, others don't.

How important is it in reality? I just have a thing that says inviting lightning strikes by grounding the aerials might increase the chances of a hit. what do you think?
G4RMT
 
Posts: 1115
Joined: Sat Feb 07, 2015 5:45 pm

Re: Outside Antennas - DC grounded or not?

Postby m0lsx » Fri Jul 15, 2016 7:16 pm

When I lived over looking the Wash back in the early 80's. I heard a clicking sound in the corner. When I investigated I found sparks jumping the gap between the earth wire & my long wire on my SWL radio :shocker: :shocker: . What made it a bigger learning point was the fact that there was no lightning around that night. That was a lesson learned, a bleed point can be useful.
Have you looked at kite lifted antennas? They all have a static bleed point. Or they do if the person knows what they are doing. Wind & rain induced static can be a massive issue & yes even in the UK.
A static bleed point is no real good when it comes to lightning, but a close call can be suffered safely with no loss of ATU or radio.
All a static bleed point does is it simply gives electrical build up in the antenna an easy path to earth. One of the old methods is a spark plug, the modern method is a bleed resistor to earth & that could be simply be an appropriate resistor between earth wire & long wire on the back of an ATU. But preferably something more efficient.
Here I earth what I can & hang the ends of my various coax outside of the shack when lightning threatens. But having had my hand on the coax when a massive bolt of lightning lit the room up a few months ago, I am not now so sure disconnecting coax is such a bright idea.
My Diamond CP-6, does not like any form of earthing & SWR goes through the roof if I use even a lightning resistor, so I have just earthed the pole the antenna is mounted to. Hoping that gives a easier route to earth. But most of my antennas have lightning resistors fitted, not that I have any real faith in them & on my longwire I simply use a 9:1 complete with earth rod at it's feed point. Again in the hope that lightning would take the shortest & easiest route there too. Also the earth rod helps to reduce static at the receiver a little.
My Windom is in the loft & it's there so I can have a lightning season safer antenna.
I cannot remember who told me, but it was once told that following WW2 a lot of churches in France suffered a lot of damaging lightning hits. The reason they discovered was that the French resistance had cut the earth straps on Churches using the top as an antenna & the bottom as their earth. Once fixed the rate of strikes reduced. So I take the view that earthing probably does not increase the risk of a hit, it simply reduces the chance of a damaging strike.
Buy a database from Kimmy JS19 via http://ukscanningdirectory.co.uk/
Or do Google search of this forum via https://www.google.com/cse/home?cx=partner-pub-6291336405621919:2662881632
73 De Alan (M0LSX.)
http://www.qrz.com/db/M0LSX"
User avatar
m0lsx
 
Posts: 5380
Joined: Wed Nov 21, 2012 9:14 pm
Location: Norwich. TG21.

Re: Outside Antennas - DC grounded or not?

Postby G4RMT » Sat Jul 16, 2016 9:13 am

I'd heard that story but it was missing the part about why they were cut. Interesting!

I suppose you have to accept that in the case of a hit - destruction is inevitable and maybe having an easy path saves the house so equipment is kind of sacrificial. Always thought the kite fliers must be mad to do it on any day when storms are possible, but they're probably mad anyway!
G4RMT
 
Posts: 1115
Joined: Sat Feb 07, 2015 5:45 pm

Re: Outside Antennas - DC grounded or not?

Postby m0lsx » Sat Jul 16, 2016 9:57 am

Image

Image

My 5 to 6 foot wingspan Cody Kite, lifting a antennas.
Cody was Samuel Franklin Cody also known as Colonel Cody. He was American & did come to Britain as part of a wild west show, but he was not Buffalo Bill Cody. Cody kites were designed to lift men & he routinely lifted men to over 2000 feet. Part of the design of the Cody was borrowed from the Wright Brothers & that tied him up in a legal wrangle with them.
Although kites did have some success & although they had many successful military trials, Aircraft were developing at the same time & Samual moved over to aircraft becoming the first person to officially fly a aircraft in Britain, flying a distance of just under 1500 feet in British Army Aeroplane Number One... He later set a British endurance record of 1 mile & won the Michelin Cup for the longest British Flight, which was a flight of well over 4 hours long.
Samual died in a airplane crash in 1913 & was buried with full military honours.
Samual Cody's British wife Lela is the Great Grandmother of the BBC's John Simpson.
As much of the RF from a antenna emanates from the lower part of the radiator, long ground mounted antennas defeat the object of lifting an antenna, so something like a Zepp makes an ideal kite lifted antenna as like that you get the radiator as high as possible, although the lower image of my kite, shows a centre fed 80 meter quarter wave being lifted as an inverted V. It was a bodged together antenna using borrowed bits. Everyone kept saying where's your antenna, so someone said I'm up for it if you are. We worked all over Europe using 1 watt SSB.
Buy a database from Kimmy JS19 via http://ukscanningdirectory.co.uk/
Or do Google search of this forum via https://www.google.com/cse/home?cx=partner-pub-6291336405621919:2662881632
73 De Alan (M0LSX.)
http://www.qrz.com/db/M0LSX"
User avatar
m0lsx
 
Posts: 5380
Joined: Wed Nov 21, 2012 9:14 pm
Location: Norwich. TG21.

Re: Outside Antennas - DC grounded or not?

Postby ODuffer » Wed Dec 07, 2016 11:47 am

Your not supposed to ground external antennas, due to potential(pun intended) issues with Protective Multiple Earthing.

See http://rsgb.org/main/files/2012/11/EMC07-final.pdf

The sparks observed could be obtained if there is a PME fault condition :shocker:
ODuffer
 
Posts: 13
Joined: Wed Apr 08, 2015 11:17 am
Location: IO83 (QRM Alley)

Re: Outside Antennas - DC grounded or not?

Postby G4RMT » Thu Dec 08, 2016 2:20 pm

That's a very ambiguous and confusing document in that it states that the ground should be bonded - but in the case of doing this to an outside aerial, even 10mm2 cable is going to be a problem with something like a distant outside aerial. Plus, if the aerial is say, a tower, then this also extending the equi-potential zone outside the dwelling - another can of worms dealt with by a different section of the regs. It's possible you could also have to consider this a temporary electrical connection, meaning more problems.

I'm not actually sure myself, if grounding of a tower or aerial system comes under the electrical regulations as there is no mains power involvement, outside the transmitter end?
G4RMT
 
Posts: 1115
Joined: Sat Feb 07, 2015 5:45 pm

Re: Outside Antennas - DC grounded or not?

Postby m0lsx » Thu Dec 08, 2016 3:20 pm

ODuffer wrote: The sparks observed could be obtained if there is a PME fault condition :shocker:


I don't think so.

m0lsx wrote:When I lived over looking the Wash back in the early 80's. I heard a clicking sound in the corner. When I investigated I found sparks jumping the gap between the earth wire & my long wire on my SWL radio :shocker: :shocker: .


PME was not around in the 80's.
Buy a database from Kimmy JS19 via http://ukscanningdirectory.co.uk/
Or do Google search of this forum via https://www.google.com/cse/home?cx=partner-pub-6291336405621919:2662881632
73 De Alan (M0LSX.)
http://www.qrz.com/db/M0LSX"
User avatar
m0lsx
 
Posts: 5380
Joined: Wed Nov 21, 2012 9:14 pm
Location: Norwich. TG21.

Re: Outside Antennas - DC grounded or not?

Postby G4RMT » Thu Dec 08, 2016 6:43 pm

This seems to have nothing to do with the actual topic - how did we make the leap to mains wiring systems - which, from my own frequent experience cause quite different problems - hum being the prime issue where the ground potential creeps away from the potential of another location. Just a few volts difference induces all sorts of nasty noises in the audio systems.
G4RMT
 
Posts: 1115
Joined: Sat Feb 07, 2015 5:45 pm

Re: Outside Antennas - DC grounded or not?

Postby m0lsx » Thu Dec 08, 2016 7:34 pm

The two are linked, as I understand it. As I seem to remember reading that if any of the earth paths are of different value to another. That is where lightning strikes cause the real damage.
We can have several different earthing systems on our stations & it's the lack of congruence between them, that can sometimes be the issue.
Buy a database from Kimmy JS19 via http://ukscanningdirectory.co.uk/
Or do Google search of this forum via https://www.google.com/cse/home?cx=partner-pub-6291336405621919:2662881632
73 De Alan (M0LSX.)
http://www.qrz.com/db/M0LSX"
User avatar
m0lsx
 
Posts: 5380
Joined: Wed Nov 21, 2012 9:14 pm
Location: Norwich. TG21.


Return to The TX'ers Forum

Who is online

Users browsing this forum: No registered users and 1 guest