Surge Suppressors for Long, Radiating Ground Conductors?

[rfry]

There is a discussion on another Part 15 board to the effect that installing a surge suppressor at the r-f ground terminal of an elevated Part 15 AM transmitter will prevent radiation from a long, conducting path to r-f/electrical ground (something buried in the earth), but still protect the transmitter from electrical transients caused by nearby lightning.

However most surge suppressors provide a continuous, low-impedance electrical path through the device for normal operation, along with a parallel (bypass) path leading to a component that conducts only during the presence of electrical transients. Depending on the port impedances at the time, the bypass path can divert most of the transient voltage to a suitable ground if such is present, and properly connected to it.

So in reality such a surge suppressor on an elevated Part 15 AM transmitter won't do much to reduce r-f radiation from a long, conducting path to ground.

Note that if the long, conducting path to r-f ground (in the earth) and power and program wires all are prevented from radiating, then the performance of an elevated system will be worse than one installed with its base at earth level, using a total radiating length of 3 meters from the top of the whip to where the ground lead enters the earth.

 

[Ermi Roos]

There are actually two other sites involved in this discussion, not only one. I believe that the objective of the surge suppressor is to keep the ground lead disconnected (and not radiating) until a high-voltage surge occurs.

 

[druidhillsradio]

Note that if the long, conducting path to r-f ground (in the earth) and power and program wires all are prevented from radiating, then the performance of an elevated system will be worse than one installed with its base at earth level, using a total radiating length of 3 meters from the top of the whip to where the ground lead enters the earth.

Rich, I think you once posted a graphic with the gestimated FS from such a device that's ground mounted. Is it possible to model it with say 16 then 33 radials to show the effects of FS in relation to reducing ground losses?

 

[rfry]

I believe that the objective of the surge suppressor is to keep the ground lead disconnected (and not radiating) until a high-voltage surge occurs.

Such a belief or objective of an operator/proponent in this regard is, as may be.

But will either of such be realized by the surge suppressor that is proposed or installed? That is the pertinent matter.

In any case, Mr Roos himself pointed out in two subsequent posts in that same thread on the other board that radiation from program and power wires to an elevated "Part 15 AM" transmitter could result in non-compliance with Part 15.

 

[Ermi Roos]

As has been pointed out to me on "the other board," so far NOUOs have been issued only if the main ground lead is long, and the parallel ground leads formed by the power and audio cables have been ignored by the FCC. I don't know if this is a firm FCC policy, or a present enforcement practice that may change in the future.

 

[rfry]

Is it possible to model it with say 16 then 33 radials to show the effects of FS in relation to reducing ground losses?

It is possible using NEC-4, but I don't have NEC-4. My calculations are made using NEC-2D, which cannot model buried conductors.

There are technically legitimate ways of using NEC-2D to emulate the performance of a monopole and its ground system loss with either a perfect or real earth ground plane, and those are the methods I use to generate my graphics.

The r-f loss in a set of buried conductors installed radially around the base of a monopole depends on their length, their number, their physical arrangement, the conductivity of the earth in which they are buried, and the operating frequency. The range of variation possible can extend from about 2 ohms for 120 x 1/4-wave buried wires in a symmetric arrangement to 30 ohms or more for a few short buried wires, or a few ground rods.

 

[druidhillsradio]

As has been pointed out to me on "the other board," so far NOUOs have been issued only if the main ground lead is long, and the parallel ground leads formed by the power and audio cables have been ignored by the FCC. I don't know if this is a firm FCC policy, or a present enforcement practice that may change in the future.

That being said, why not simply discontinue the use of a ground lead? The problem here is that in the case of the Cartwright situation, there was substantial reduction in the signal, indicating little radiation of the power/audio leads from the Rangemaster.

 

[Ermi Roos]

Ken Cartwright did indeed report a loss in range when he disconnected his main ground lead to his 40-foot tower, but the amount of his loss of signal was never quantified. Some quantitative information is available in the NOUO issued to Sean Murta of Liberty 1640 for an overly long ground lead on April 9, 2010. The FCC inspectors found that the field strength with Murta's 20-foot ground lead disconnected was 11.47 dB less than with the ground lead connected. An NEC simulation shows that a 20-foot ground lead increases the radiation resistance compared to a 3-meter vertical monopole at ground level so that there is a power gain in the radiated signal of about 13.62 dB, giving a net power gain of 13.62 - 11.47 = 2.15 dB over an antenna at the level of the earth. This increase in gain of the elevated antenna, with the main ground lead disconnected, over an antenna at ground level increases with increased elevation.

Therefore, if the FCC allows disconnecting the main ground lead, but leaving the the power and audio cable paths to ground in place, the elevated installation has a definite advantage in range over the installation at ground level that increases as elevation increases.

 

[rfry]

Ken Cartwright did indeed report a loss in range when he disconnected his main ground lead to his 40-foot tower, but the amount of his loss of signal was never quantified. Some quantitative information is available in the NOUO issued to Sean Murta of Liberty 1640 for an overly long ground lead on April 9, 2010. The FCC inspectors found that the field strength with Murta's 20-foot ground lead disconnected was 11.47 dB less than with the ground lead connected. An NEC simulation shows that a 20-foot ground lead increases the radiation resistance compared to a 3-meter vertical monopole at ground level so that there is a power gain in the radiated signal of about 13.62 dB, giving a net power gain of 13.62 - 11.47 = 2.15 dB over an antenna at the level of the earth. This increase in gain of the elevated antenna, with the main ground lead disconnected, over an antenna at ground level increases with increased elevation.

Therefore, if the FCC allows disconnecting the main ground lead, but leaving the the power and audio cable paths to ground in place, the elevated installation has a definite advantage in range over the installation at ground level that increases as elevation increases.

The above clip needs to be reconciled by Mr Roos with his post of yesterday, April 8, 2012 on another board, stating (text attributes are mine):

Even with the so-called "ground" wire disconnected from the transmitter, there are still audio and power cables that complete the RF path to ground. Indeed, the transmitter would not work at all without these parallel RF paths, because you can't drive a monopole from only the bottom end without a return path for the displacement current from the antenna to the cold side of the transmitter output.

Transmitters that have been cited for long ground leads have continued to work (although with lower range) with the official ground lead disconnected, at higher field strength than they would have had at the level of the earth. This is because of the higher radiation resistance antennas have in elevated installations because of the parallel paths to ground which were apparently not taken into account by the FCC inspectors.

 

[Ermi Roos]

No reconciliation is necessary because my previous post here is consistent with the post from elsewhere quoted by R. Fry.

 

[PhilB]

However most surge suppressors provide a continuous, low-impedance electrical path through the device for normal operation, along with a parallel (bypass) path leading to a component that conducts only during the presence of electrical transients. Depending on the port impedances at the time, the bypass path can divert most of the transient voltage to a suitable ground if such is present, and properly connected to it.

I just wanted to point out a minor error to help avoid confusion. Surge suppressors, both of the MOV and Gas Discharge variety, present a HIGH-impedance electrical path through the device in normal operation.

Subsequent replies in this thread are consistent with the HIGH-impedance correction, so it doesn't really matter for the context of this thread.

 

[rfry]

Surge suppressors, both of the MOV and Gas Discharge variety, present a HIGH-impedance electrical path through the device in normal operation.

In normal operation, only the bypass path has high impedance. When the transient voltage on the conductor being protected exceeds a threshold value, then the bypass path needs to have a much lower impedance than the normal path through the protector, if the protector is to provide much benefit.

A high impedance during normal operation in the path for the conductor/device being protected (the long "ground" conductor and elevated Part 15 AM transmitter) would produce losses in whatever power would otherwise flow there. For an elevated Part 15 AM transmitter, this loss will be in series with the r-f current flowing into the 3-m whip. This will reduce the fields radiated both by the long ground conductor and the 3-m whip -- assuming that the power and program lines to the transmitter do not radiate.

Of course this is the opposite result that the elevated installation is expected to produce.

The irony of this application is that the surge suppressor is providing a bypass path to earth ground for/from a conductor that is already connected to earth ground.

 

[Ermi Roos]

In the original thread on another website (from which R. Fry is in self-imposed exile), the intent was clearly to use the surge suppressor as a high-impedance to ground until a high-voltage surge switches in a low-impedance path to ground. Whether or not the means suggested will work properly for that purpose would be appropriate to discuss on the original thread.

Posting on an entirely different website to pick nits out of a discussion elsewhere that one cannot log onto, and make a rather trivial point, quite frankly seems creepy to me.

 

[rfry]

In the original thread on another website (from which R. Fry is in self-imposed exile), ...

The reason for that is the "Chief Cook & Bottle Washer" of that website did not like the technical truths I had been posting there, even though he brands the site as "The Reference for Legal, Low Power, License-Free Radio."

When it was implied to me that my posts there might be edited or censored, and that there were a lot of buttons that could be pushed, there was no incentive for me to continue posting on that website.

 

[Rob Martin]

The reason for that is the "Chief Cook & Bottle Washer" of that website did not like the technical truths I had been posting there, even though he brands the site as "The Reference for Legal, Low Power, License-Free Radio."

When it was implied to me that my posts there might be edited or censored, and that there were a lot of buttons that could be pushed, there was no incentive for me to continue posting on that website.

i dont understand how this goes with surge suppresors. this babble makes no sense to me at all. surge supressors either work or not. can you give me an answer in english please??


rob

 

[rfry]

... can you give me an answer in english [sic] please??

Note that all of my technical comments in this thread have been posted in the English language.

 

[rfry]

In the original thread on another website... the intent was clearly to use the surge suppressor as a high-impedance to ground until a high-voltage surge switches in a low-impedance path to ground.

Expecting that such a configuration will provide much protection to an elevated Part 15 AM system is inherently faulty, because it can permit the build-up of static charges on the elevated 3-m whip and transmitter -- which may increase the susceptibility of damage to that hardware by nearby lightning.

Benjamin Franklin discovered this long before Part 15 systems existed, leading to his invention of lightning rods to protect elevated structures. Such lightning rods need to be permanently / continuously connected to a good earth ground by low impedance conductors.

 

[PhilB]

Here is some info on surge suppressors that may clarify things in the context of this thread.

Typical surge suppressors are either of the MOV type or gas discharge type. Both are two-terminal devices that are open circuit (very high resistance) until the voltage across the device exceeds a specified threshold at which they "switch" to a very low resistance.

MOV suppressors aren't the favored type for Part 15 transmitters because they have a fairly high capacitance across the terminals when they are operated at a voltage below threshold, so they can have a major effect on tuning.

Gas discharge suppressors are the favored type. Their below-threshold resistance is on the order of about 10 Gohm (yes, Gohm) and their below-threshold capacitance is less than 1 pF. They have very quick response time and the relatively small PC-mount versions can conduct a surge current up to about 10 kA for a few microseconds.

Transmitters that use a gas-discharge suppressor between the antenna and ground also incorporate a high value parallel resistor (several Megohm) for the purpose of bleeding off slower static potential buildup on the antenna, that if unchecked could eventually reach the gas-discharge flash-over potential.

I haven't been able to find the other forum thread alluded to in this thread, but I assume it discussed using a two-terminal surge suppressor in series with the ground lead at the bottom of an elevated transmitter. This is not the same as an internal transmitter connection as described above. Such an external use, in series with the ground lead would indeed isolate the transmitter from the ground lead until the voltage across the suppressor exceeds the flash-over value, at which the suppressor will go to the low-resistance state. That's good for lightning surge suppression (not a direct strike), but such a connection does not allow for the normal ground connection required for by a vertical antenna. In normal operation, the vertical antenna would be non-functional except for the unpredictable RF to ground path over the power and audio wires.

 

[rfry]

Transmitters that use a gas-discharge suppressor between the antenna and ground also incorporate a high value parallel resistor (several Megohm) for the purpose of bleeding off slower static potential buildup on the antenna, that if unchecked could eventually reach the gas-discharge flash-over potential

Hopefully the text below will not be considered as nit-picky, "anti Part 15" or too "black and white" by those who wish the reality was different.

For effective surge protection, a transmitter with an internal gas-discharge suppressor in parallel with its antenna output terminal and r-f ground terminal still needs a low impedance electrical path to a good earth ground.[note] Otherwise most of the lightning surge can be dissipated within the transmitter.

Installing another gas-discharge suppressor in series with, and at the top of the ground wire+ground lead means that the transmitter will have no active connection to an earth ground during normal operation. It won't have a path to ground until that 2nd suppressor conducts during a surge event. This has three considerations.

1) No r-f current will flow on that path to earth ground during normal operation, which will greatly reduce the coverage area of the transmitter+whip.

2) The function of the several megohm resistor in the transmitter intended to bleed off static charges from the 3-m whip will be defeated, because there will be no path for those charges to reach the earth. Instead they will build up on the circuits of the transmitter to the same level as on the whip.

3) The suppressor in the ground path external to the transmitter can't act until after the suppressor in the transmitter does, so there will be a (very) short period of time when only the transmitter circuits are absorbing the surge.

Note: The top of a long "ground wire" or metal tower, flagpole, TV mast, billboard/building steel frame, water tower etc are not at electrical ground either for the radio frequency of the transmitter, or the transient waveforms generated by a nearby lightning event -- even if such conductors have zero a-c loss where they connect to the earth, at their base.

 

[rfry]

As a followup, the link below shows the effect that using a gas-discharge suppressor at the connection to the ground conductor of a transmitter elevated at 10 meters could have on the groundwave field strength at 0.5 km, versus using a continuous, low-impedance conducting path from the transmitter to an earth ground.

In this analysis the system using the surge suppressor radiates about 10% of the power that it will when not using the suppressor.

http://i62.photobucket.com/albums/h85/rfry-100/Part_15_AM_Surge_Suppressor_Effect.gif