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Strange Lightning Protector Question

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Ok, I bought one of the MFJ Guardian Angel "lightning protectors" to help bleed static off, as I always do for my other antennas.

I planned to use it on my new 800mhz wilson yagi, and I got everything together last night.

The final check showed a short of shield to center conductor, which is strange since all components had already tested good (cables / connectors, "lightning protector", etc.).

After much frustration I determined that with the "lightning protector" removed, there was no short.

Yet the protector itself, when tested alone, was not shorted, only when inline connected to the antenna and ground.

Now, I do have an odd ground setup. For this spot I got lazy, and for a ground used a ground wire attached to the ground wire of the rooftop air conditioner nearby. This is of course the same wire that runs in a sheath with #6 240 volt ac wires that power the air conditioner.

Now I was wondering if somehow this was the cause, due to induction?

The strange thing is another antenna (1/4 ground plane), on the same mast, uses the same ground wire, and it does not have it's center conductor shorted to ground with the EXACT SAME GUARDIAN ANGEL LIGHTING PROTECTOR INSTALLED IN IT'S LINE.

I know there is also neutral to ground voltage in a home electrical system (though in small amounts), but admit I am not well versed in this topic.

The thing is, the "proper" way to ground it would be to have the ground wire from the discharge unit go to the middle of an array of ground rods below, but these WOULD BE TIED TO THE SAME HOUSEHOLD ELECTRICAL GROUND I AM USING AT THE MOMENT. So what is the difference if any?

I guess the unit could be bad, but the little tube looks perfect, and I have never had a bad one before. Can these things be defective and do things like this?

This antenna is for one of my cheaper scanners, used to scan ONE AGENCY. I don't really care to go plop down a hundred bucks or more for a top notch discharge unit if I don't have to, but if this thing is defective I am done with this brand.

Thoughts?
 

prcguy

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Some antennas will show the center conductor shorted to ground, did you check this on the 800 yagi separately? If the 800 yagi is internally grounded you probably don't need the lighting arrestor if the coax is properly grounded. I can;t imagine an 800 yagi having a static buildup problem in general, the driven element has very little surface area exposed to blowing wind, snow, etc.
 

Raccon

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PHOENIX_SCANNER said:
Now, I do have an odd ground setup. For this spot I got lazy, and for a ground used a ground wire attached to the ground wire of the rooftop air conditioner nearby. This is of course the same wire that runs in a sheath with #6 240 volt ac wires that power the air conditioner.
In this case it's not called a ground but protective earth (PE). Technically it is ground but it has a specific function and must be separated from the lightning protection system as the PE may often be wired through different distribution boxes or sockets/switches even and is not designed to carry large currents.

The PE protects human life when the circuit is equipped with an RCD or ELCB. Depending on the environment and device such current may be very small (in the mA - range).

The thing is, the "proper" way to ground it would be to have the ground wire from the discharge unit go to the middle of an array of ground rods below, but these WOULD BE TIED TO THE SAME HOUSEHOLD ELECTRICAL GROUND I AM USING AT THE MOMENT. So what is the difference if any?
A ground wire for lightning protection normally has a larger diameter since the current rating is higher, and the wire should go as direct as possible to ground, outside of the building where the lightning protector should be located, too. The connection to the houshold electrical ground is done at the main grounding bar or rod, see the links in this post.

Using the PE as ground for lightning protection is dangerous and probably a violation of the NEC standard.
 

jim202

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New Orleans region
The MFJ-272 lightning protector uses a gas tube for the protection. Under normal
conditions, this should not show up as a short from center conductor to the shield.
If by some chance you are seeing a short with both the surge protector and the
antenna connected, then you have a problem.

You have sort of done your homework by removing the different connections. I
would suggest that you go back and try again. It may prove to be that you have
a grounded antenna your trying to connect to. Many antennas use a grounded feed
point and as such will show up as a short between the center conductor of the coax
cable and the shield.

If when you get through looking for your problem, the short is still there, but not
when the surge protector is removed, better look some more. Did you make up
the coax connectors? Is there a chance that when you made the connectors up,
that you had a strand of the shield and it is moving when you flex the cable at
the connector causing the short to show up?

Having made up coax connectors for over 40 years now, I am very careful to
make sure that I don't have a shield strand of wire long enough to make a connection
to the center conductor. With some of the connectors, you don't get a second chance
in making them up. You do it correct the first time or cut it off and start over.

Jim



PHOENIX_SCANNER said:
Ok, I bought one of the MFJ Guardian Angel "lightning protectors" to help bleed static off, as I always do for my other antennas.

I planned to use it on my new 800mhz wilson yagi, and I got everything together last night.

The final check showed a short of shield to center conductor, which is strange since all components had already tested good (cables / connectors, "lightning protector", etc.).

After much frustration I determined that with the "lightning protector" removed, there was no short.

Yet the protector itself, when tested alone, was not shorted, only when inline connected to the antenna and ground.


Thoughts?
 

OceanaRadio

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Joined
Dec 19, 2002
Messages
150
Location
Virginia Beach, VA
If you're using a DC-ground antenna as most are, you cannot "check for shorts in the system" with a DC-meter. Use an SWR meter during transmit tests and your Mk-1 Mod-0 Ear for listening tests in a receive-only station.

Good advice given earlier about not ever connecting your latest lightning rod to the HVAC ground wire. Strange as it may sound, all rooftop equipment must be bonded to any lighting protection system (franklin rods). The air terminals must have the primary downconductors to earth ground rods, and the bonding to rooftop equipment only serves to help maintain equipotential throught the system, and prevent flash-overs. So you can and should bond all rooftop systems but the main downconductors must be heavy copper. The prices of that are shocking, o,k. pun intended.

For the some of the reasons Racoon also already listed, I will add that this is why MOV-type power-strip surge protectors are such a mistake. They take potentially high surge voltages from the Line voltage and short it throught those grossly undersized "ground" wiring that are only deisigned to carry milliamps to trip a circuit breaker before your heart trips too. Your home's internal ground wiring was never designed to handle lightning currents or even normal surge voltages from motor-starting events. The Neutral (common) wiring however IS sized to handle anything the hot can give it including surge voltages. Line-to-Neutral surge protection is the only kind of protection that should be employed indoors in my opinion. It is slightly more expensive and is not available at hardware or home improvement centers. It should also use only silicone avalanching diodes which have a much longer life than MOV's and also don't fail shorted which can cause a fire during the last-gasp of that $10 power-strip's cheap MOV's.

Jack
 
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Messages
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Location
Phoenix Arizona
Solved

Thanks for all the great answers, I learned quite a bit from the posts, and the experience.

To begin with I took the advice and ran a true ground wire to four rods in a y pattern spaced out properly, and connected in the middle. Household ground rod connects to these ground rods now.

But that wasn't what was causing the short, that was so obvious it was funny. I was just too tired and it was getting dark the last day, so I didn't apply logic fully.

Turns out, the WHOLE antenna, not just the element, acts as the antenna on this, and I am guessing most yagis. The element, the "non-driven" elements, and the square tube boom they run through, and the clamp assembly, of course. All of this acts as "the antenna", and if you mount the metal clamp to the metal main mast, and then ground the mast, your center conductor is now grounded, and the whole antenna mast and ground wire are what you have for an antenna, not good of course.

To be fair to wilson, the instructions say you do not need to ground the antenna. It would have been more accurate to say YOU MUST NOT GROUND THE ANTENNA. I wrapped my mast with electrical tape in one spot, and just installed the surge protector inline, and if any static comes down the line it will short to ground via the gas tube. Thats it, the antenna mount does not go to ground, which makes sense when I think more about it.

Normally with my ground plane antennas I am used to the antenna base being separate from the center element, and the base be attached to the grounded mast.

There are just different rules at work here, that's all.

As a side note, not only does the yagi work, it solved a perplexing problem. In the P25 simulcast system I am trying to monitor (4 sites within range of my location), somehow they all interfered with eachother, and by aiming this antenna to maily pick up the closest site, 95% of all cut-out and break-ups are now gone. I don't fully understand why one "simulcast" site would interfere with another site transmitting the same thing at the same time (at least in theory), but results don't lie. Credit to someone else on RR forums for the tip, I think he was back east though, but same type of system, same issue, same solution.

This antenna has been a true Godsend, allowing me to enjoy monitoring the PRWN system, which includes Phoenix Police and Fire, the best listening in central Arizona as it so happens.

All of this forced me to learn more about lightning protection and grounding, so I am glad it happened this way.
 
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