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Impedance bump from coax sheld grounding?
3

Impedance bump from coax sheld grounding?

Impedance bump from coax sheld grounding?

(OP)
After installation of multi-mode coax lightning arrestors, I have referenced several techincal papers that call for shield grounding at building entrance (done) and also at the bottom of the vertical drop and top of tower. My question relates to HF transmitting to a horizontal wire dipole antenna w/1:1 current balun. I have been warned by a colleague that microwave towers (where most grounding references come from) and HF do not behave alike WRT to standing waves on the coax feedline. Is there experience that would indicate that HF feedlines require more than one shield grounding to ensure the proper operation of TVSS/lightning arrestors? Would a second coax shield-ground at the bottom of the feedline's vertical drop be likely to cause an impedance "bump"?

TIA, Jack Painter
Virginia Beach, Va

RE: Impedance bump from coax sheld grounding?

2
1) Title: 'Impedance bump from coax shield grounding?'

Ideally, no.  The RF is supposed to be ~inside~ the coaxial cable. The RF should 'have no idea' about what might be going on outside the shield.

In practice, RF is often coupled onto the outside of the coaxial cable and causes 'unbalanced currents' where the total shield current (inside + outside) is different than that on the center conductor.

Your 1:1 current balun is supposed to help in forcing the currents to be balanced.  If the installation is not perfectly symmetrical, then the coax can have currents induced (onto the outside).  A coil of coax can be used to choke-off such currents - you'll often see such coaxial baluns.

If there are currents on the outside of the coax (likely), then grounding the coax will cause those current paths to change. If the installation is ideal (unlikely), then it shouldn't cause any noticeable change.

Taking your post title even more literally (probably not your intent), the term 'impedance bump' strictly means that the characteristic impedance of the cable is changed at the point where the grounding block or lightning arrestor is inserted.

Such impedance bumps are usually not significant if they are physically small in terms of wavelength.  Therefore, at HF frequencies (where the wavelengths are long), impedance bumps are usually considered to be a non-issue and are rarely mentioned.  They are an issue on the higher bands.

All the above is just to respond to your title...

2) Towers responding differently to microwave and HF

An important concept to keep in mind is that a grounding point becomes a high impedance point only a quarter wavelength away.  You can use a metal pipe as an insulator if it well grounded at one end and one quarter lamdba in length.

If your tower is significantly affecting your SWR, then find the new resonance point, calculate and adjust your dipole length to retune (if the SWR is an issue).

3) Lightning

Lightning and lightning protection is a topic that inspires much debate.  There are many experts and even more that consider themselves 'experts'.  I don't think that I'm an expert on the subject, but I've seen much obvious 'BS' from those that present themselves as experts.  Be warned...

There is at least one point that is pretty much universally agreed: grounding is good, and the more the better.

But it is worth noting that commercial aircraft have excellent lightning protection, are typically hit about once per year (most often with no damage to their electronics), and are obviously not 'grounded' in the same manner as often recommended by the 'experts' for ground installations.

Obviously, there is more to lightning protection than just traditional earth grounding.

Putting your system into a metal box (airplane, commercial building) is an excellent first step.  Then the concept of entrance protection can be made to work.  The metal box can be flying 10,000 feet in the air, not grounded and the lightning protection system would still work.

Wooden building are totally different animals.  Many experts do not acknoledge this important point when they recommend traditional entrance protection but fail to protect long signal wires inside the RF-transparent building.

Relative costs may also be an issue...

RE: Impedance bump from coax sheld grounding?

(OP)
Thanks for the reminder about impedance-bump, re: not materially affecting MF/HF work for short lambda insertions.  I am of the opinion that additional shield grounding will not affect the balanced operations at resonant frequencies of the dipole antennas. But at higher frequencies where antenna tuners are utilized there would be some inevitable RF on the shield as you commented. Insignificant perhaps, and worth the added protection provided by two shield grounds (each). The station is required to be operable during thunderstorms, and an extensive grounding and bonding system are in place, along with TVSS at the AC service entrance and the AC load center for the station.

Many Thanks,

Jack Painter
USCG Oceana Radio
USCG Auxiliary Communications Facility
Virginia Beach, Va

RE: Impedance bump from coax sheld grounding?

Very interesting.  I especially like the bit about operating during lightning storms - yikes!


Off topic: Since you're USCG, do you happen to have any leads on Digital Selective Calling (DSC) VHF-FM (Marine band) radios....-> FOR AIRCRAFT <- (*)~ ?

(*This implies that white plastic radios need not apply. Also, 19" rack radios are out as well.  Need to be either +28Vdc or 115/400Hz aircraft power; as well as all the other aircraft & mil-spec requirements.)

If not a radio, then a DSC controller would work as well provided it is intended for airborne use.

It seems that the entire marine radio system is slowly shifting to DSC and Ch 70 and all that, but that no one (so far as we've been able to find) has done anything to address the requirements of aircraft that have Marine SAR responsibilities.

Thanks for any leads that you might have.

RE: Impedance bump from coax sheld grounding?

I agree with VE1BLL that short discontinuities are not too important in HF and a balun reduces the current on the outer braid of the coax.  You can also have large induced currents from radiated RF but fitting a number of lossy toroids along the cable skin will absorb them if they worry you.  

Lightning protection needs some thought to be effective.  The ground conductors must be very low inductance especially from in-line gas discharge devices in the coax line.  Low inductance calls for large surface area conductors, short and wide, in fact Length < 5x Width.

Another effect to watch out for is the huge magnetic field that surrounds a lightning ground cable when it's hit.  I remember an airfield control tower which had several heavy copper bus bars to ground from spikes on the roof.  Got hit by lightning and all the LEDs in the equipment room exploded like detonators from the induced voltages due to the magnetic field around the ground conductors.

The fix for this was to replace the bus bars with very heavy coax ground leads.  The inner and outer conductors were shorted at the ground, but open at the top and this gave a good degree of shieldong to the magnetic field.

RE: Impedance bump from coax sheld grounding?

(OP)
VE1BLL, I spoke with the USCG Auxiliary Air Commander for the District this afternoon, and they are using off the shelf marine vhf radios mounted aft of the cockpit. They will discuss your question about DSC capability at a meeting later this week at USCG AirSta Elizabeth City. The C-130's and H-60's here obviously use faceplate remote units that connect to rather large tranceivers under the flight decks, that I have observed.

Since Rescue-21 is still a ways off from implementation, neither of us saw FM-DSC as a near-future requirement. Any radio made after 1995 already has DSC capability, and it will automatically go to Ch-70 if it detects a DSC distress call. You should then shift manually to Ch-16 156.800 Mhz and listen for pertinent Mayday information. Here in the Hampton Roads Virginia area there are 1-2 accidental DSC Ch-70 distress calls per month. HF-DSC-GMDSS traffic however averages up to 1,000 safety test messages a day received at my station. I only guard two HF channels at a time for DSC, as 90% of the traffic is on 8, 12, or 16 mhz depending on time of day.

Brian, thanks for your reply. I use special non-degrading silicon avalanching diode surge protection at AC mains entrance and station load center entrance. This form of TVSS returns all surge voltage (not 330v+ as others do) to neutral line where it belongs. Surge voltage from EMF should never be sent to AC ground, always to neutral line. For coaxial suppressors I use the multi-attack mode arrestors by Idustrial Communication Engineers. Gas tube is only a last resort for those, with circuitry bleeding static off the center conductor at all times, and up to 20kva inductor control before the gas tube would kick in.

Your comments on my lightning protection web pages are welcome: http://members.cox.net/pc-usa/station/grounding.htm

Best regards,

Jack
Virginia Beach

RE: Impedance bump from coax sheld grounding?

Jack, you have some good material on your lightning page and I found it very interesting, although there are a couple of points I'd like to make.  

I agree that the silicon diodes you advised clamp at an accurately known figure and work very fast, however MOVs are in fact also very fast absorbers functioning in sub/low nano-sec times.  Like any other device their lead and wiring inductance will slow this.  

Their life is not 10 shocks, but an area under the curve issue and if carrying direct lightning surges they wont last long, however a typical lifetime, @ 100 Amps, 8x20 uS pulse shape is 1000 surges.  As they rely on the series surge impedance of the power line to drop the voltage spike (around 30-50 ohms in a typical industrial installation), a 100A spike drops 3-5KV between the fuse box and the MOV with the voltage across the MOV rising to around 1.5-2x line voltage.  

If you have a clamping surge suppressor in front of a line filter, you will lengthen the pulse width on the output of the filter.  Put suppressors on filter outputs where you can.

In regard to lightning jumping to other things on aircraft we found that with the radar/radome, the positioning of diverter strips needed to ensure that when a lightning leader attached to the radome, the shortest distance to a conductor neded to be to a diverter or it was likely to punch through the radome to the radar structure behind if that was closer.

RE: Impedance bump from coax sheld grounding?

(OP)
Brian, thanks. My website reference to MOV-life of 10 surges does accurate apply to all UL approved power-strip surge suppressors, of the garden variety found in most homes in America. I "accidentally" applied that fact to large MOV's that are very properly installed at installations from electrical generating stations to industrial buildings, always outside the entrance to AC power. But I have never heard that they could ever survive 1,000 surges, since by design they lose a little ability with any and every significant voltage application. Anyway, I will remove that comment about the outside line MOV's!

I'm sure you're aware that professional MOV applications use software to monitor the status of the MOV's and preventative maintenance allows their replacement when degradation becomes significant. You can pay to have your power company install simple MOV's before your meter, or you can pay a lot more to have a contractor install software monitored ones there too. I estimated that even if more than 6,000v ever hit my meter (exploding it) there would never be more than 6,000v make it past my whole house TVSS or station load center TVSS, both of equal capability (the Transtector I2*10 Fortress's) After them I still use more silicon TVSS (this time in the form of Transtector normal-mode protection power strips) to apply power to equipment.

Good comment on TVSS after line conditioning. I have had many discussions with the APC company about their back up power systems, which is my only line conditoning. They have their head up inside somewhere about not using anything except their brand name TVSS (which is all common-mode reference to ground crap) when connecting to their battery back ups. I protect my APC/UPS system with silicon. In the (common) event of loss of AC power,I must have at least one receiver with UPS capability until the generator comes on line to restore transmitter power. The battery switchover of my APC product is line conditioned and I have been happy with it's all too common usage. (Virginia Power is an "overhead company" as they always tells us. What they mean is, tree limbs and ocean spume will cause your power to drop often).

In re: aircraft lightning, I had a great tour of the NOAA P3 hurricane hunter last Spring, and they pointed out many dozens of hole repairs in the fueselage where lightning chose _not_ to leave the aircraft via the provided discharge points. Fighter a/c in this operating area, as well as all of our CG a/c take it on the nose quite often.

Cheers,

Jack
Virginia Beach, Va

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