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Author Topic: More antenna questions. Grounding and lightning s  (Read 1390 times)
KL0PE
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Posts: 24




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« on: September 23, 2001, 09:15:13 PM »

O.K., I've decided on my antenna: a horizontal loop just short of a full wave on 80 meters (location and available supports limit the size, but the tuner will take care of that).

My plan is to feed the antenna with coax on a far corner about 60 feet from the house with an 8 to 10 foot ground-rod and an in-line surge surpressor connected to the ground rod with copper strap (if I have the budget I may install two or three ground rods).

All right.  First question: Will that be sufficient for lightning safety?  By the way, I plan to unhook the antenna line from my equipment when it's not in use.

Second question is about grounding: My shack is on the second floor of my house.  Is additional grounding needed (for RF) since I'm using a loop antenna, and do I need to ground my equipement for lightning safety?

Third, if question No. 2 is "Yes": How?

I've read pretty much all the lightning safety articles I can get my hands on, and many of them suggest putting a grounded metal plate in the wall of your house and running bulkhead connectors through it for your equipment as well as using it for a common ground.  This is impractical for a second floor installation for a number of reasons, so what are the alternatives?
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WB2WIK
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« Reply #1 on: September 24, 2001, 02:25:53 PM »

If you're making the loop non-resonant, I'm not sure I'd feed it with coax...wherever its impedance happens to fall in the 25 to 100 Ohm range, it will work well, but on frequencies where there is a grosser mismatch, you could be creating problems that are difficult to overcome (with respect to matching, line loss, RFI/RF feedback into the shack, etc).  I'd probably use ladder line and even that may be buried in PVC tubing quite efficiently.

Anyway...

Your safety ground system with the suppressor and ground rod, plus the excellent plan to disconnect the antenna when not in use, sounds adequate to me.  Nothing provides complete protection from a strike, but a disconnection outside the home is pretty close!  A disconnection inside the home offers some modicum of protection, but possibly not much.  I remember seeing one case where a ham disconnected his coaxial feedline and let it (the connector) lay on the floor behind his equipment.  A strike while he was away from home ignited the nearby carpeting and draperies, and the rest, as they say, is a long, sad story.  A disconnection outside the home is obviously better.

As someone posted in the "Strays" section of this site, a quote from Willie Tyler (the ventriloquist):
"The reason lightning doesn't strike twice in the same place is that the same place isn't there the second time."  Great quote.

I live in a non-lightning area now, but when I lived back east (New Jersey), not a day went by in the summer that we didn't have local lightning activity.  Never took a direct strike, but a nearby one blew out virtually all the electronics in the house...no fires, no explosions, just lots of stuff that went "dead" in one second -- probably less.  It took me months to get it all fixed!

I would not recommend grounding "equipment" for lightning safety.  Nobody recommends this -- not NEMA, not the NEC, not the IEC, nobody.  "Lightning grounding" on equipment is considered a no-no, it invites trouble that wouldn't otherwise occur.  The lightning grounds should be OUTSIDE the house.

73 Steve WB2WIK/6

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KL0PE
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Posts: 24




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« Reply #2 on: September 24, 2001, 03:35:51 PM »

As always, thanks for the response.  A quick follow up, though: my guess-timated total length (based on some rough measurements around my property) will be around 248 feet which, if I've done my math right, will make the antenna resonant at 4.05 MHz, just above the top end of the 80 meter band.  Again, if my calculations are correct, that would also make is resonant at around 8 MHz, well outside the 40 meter band.

I guess what I'm getting at, will ladder line really make that much of difference in tuning the antenna as what I've read the length of loops don't need to be critical and ladder line vs. coax doesn't help or hurt the antenna either way.

I guess my other option is to go with  140 foot antenna which is a full wave on 40 meters and just skip 80 meters or try using the loop as a have-wave 80 meter antenna (if things work like that.  Do they?)
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WB2WIK
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« Reply #3 on: September 24, 2001, 04:40:26 PM »

I'd go for the entire 248' if possible, but use ladder line to feed it.  A 248' loop will be far more effective on 75/80m than a shorter one.

The only problems with coax are:

-Coaxial cable was developed specifically to feed matched antenna systems centered on 70 Ohms (RG11) or 50 Ohms (RG8).  It doesn't do well under mismatched conditions.  In fact, coaxial cable has no published "ratings" under mismatched conditions.  Its dielectric withstanding voltage (breakdown); current rating; power rating; attenuation; and everything else are all published under "matched" conditions only.  Mismatched, all bets are off.

-Any transmission line increases in loss (attenuation) when mismatched.  Since a 248' loop will be severely mismatched at some frequencies in the amateur HF bands, and we know this, it would be best to use the lowest-loss transmission line possible, so that when its attenuation is multiplied by the degree of mismatch, the resulting figure is still acceptable.  RG8/U with a 20:1 SWR at 28 MHz has 20 dB/100' loss.  450 Ohm ladder line with a 20:1 SWR at 28 MHz has about 3 dB loss.  Quite a difference.

-450 Ohm ladder line has virtually no chance of "flashing over" (breaking down from excessive voltage) at any amateur power level, even when badly mismatched.  Coaxial cable does.

-Mismatched coaxial cable has RF current flowing on the outer surface of its outer conductor, from the standing wave created by the mismatch itself.  Properly matched coax does not, and is "self shielding," because the outer surface of the braid is at zero RF potential and can be grounded at any location without impacting performance.  But mismatched coax, having RF potential on the outer surface of its braid, cannot be arbitrarily grounded.  Doing so will impact performance, and can even incite cable damage.  Mismatched coaxial cable also tends to radiate (due to the effects noted) and can cause RFI in the shack/home.  The wonderful "self shielding" attribute of coaxial cable is virtually lost when it's terminated in a mismatch.

All in all, I think a good rule for amateurs to follow without resorting to special modeling, calculations and effort is: (In the HF spectrum), when feeding a 50 Ohm antenna (matched load), coax is great and should generally be used.  When feeding a mismatched antenna, ladder line, open wire line or other high-impedance balanced line almost always does a better job.

Ladder line can be installed in PVC pipe and then buried beneath ground, I've done it many times without ill effects.

Of course, the use of ladder line forces the need for an in-shack antenna tuner having BALANCED output terminals and capable of broad-range tuning.  For performance, it's worth it.

73 de Steve WB2WIK/6
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KL0PE
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Posts: 24




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« Reply #4 on: September 24, 2001, 08:50:17 PM »

That's quite an education there.  Thanks.

Frankly, if I could I would use nothing but ladder line.  Unfortunately, my K2 has a strictly unbalanced tuner that is fed with a BNC connector, so somewhere along the way I need to unbalance the line and turn it into coax.  I suppose I could run ladder line all the way back to the house and then swap to coax and feed it the last 20 feet or so to the second story window, but since I have no idea what the impedence mismatch will be I'm at a loss as to what kind of balun to use (4:1?  3:1?  20:1?)

Then again, maybe it will be easier to figure out a way to string up the whole 268 feet for a full wave on 80 and not worry about impedences and mismatched lines.  I should be able to sneak an extra 20 feet from somewhere!

Once again, I thank you for your help.  The advent of the internet has created a new breed of amateur: the Virtual Elmer.  Again, thanks!
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WB2WIK
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Posts: 20666




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« Reply #5 on: September 25, 2001, 11:08:03 AM »

The length of wire required to make the antenna resonate on 80m and match coaxial cable is a bit of an "unknown," unfortunately, because "free space" calculations don't apply well in the real world.  An 80m loop installed about 125 above ground should be about 250 feet long (for 3.75 MHz) and provide a reasonably good match to coax in the 50-70 Ohm range.  However, that match will occur only on 80m, and not other bands.

But, when reality kicks in and we realize our loop won't be 125 feet high, but maybe 30 feet (or whatever), everything changes.  The length required to resonate on 80m will almost undoubtedly be shorter, but how much shorter will depend on soil conductivity and surrounding objects including trees, houses, fences and so forth and is difficult to predict.  The best thing to do, IMHO, is simply install the antenna and measure the results, then trim the antenna for resonance on-site.  This may take a few tries, but it's worth the effort.

Then, to accomplish multi-band operation, assuming that is a goal, I'd still recommend using ladder line and using a manual tuner having balanced output terminals in the shack.  The K2 to tuner interface is, of course, a short length of coaxial cable.  I've gone through this before with the K2...I use the manual tuner with the auto tuner in the K2 switched "off," and achieve a match with the manual tuner.  Then, as I QSY around the band (80m is a BIG band, where there's a huge difference between 3.5 and 4.0 MHz and no antenna is going to cover all that!), I punch in the autotuner in the K2 to "tweak" it for the specific frequency I'm using.  This combination works great.  You use the manual tuner to get the thing tuned on the band of choice, then leave that alone and use the auto tuner to tweak it for the specific frequency used.

The problem with using coax with or without any kind of balun is this: On some frequencies, the loop may become highly capacitive (reactance).  Attaching coaxial cable to it makes this situation considerably worse, since coax has 26.5pF/ft of capacitance, itself.  On a frequency where the loop has R = 500 Ohms and X = -500 Ohms, adding any length of coax to it can make it an unusable antenna with virtually zero efficiency.  On the other hand, in this same situation, adding 450 Ohm ladder line, with its much higher nominal impedance and very low shunt capacitance, will allow the loop to work just fine at the same frequency.

73 de Steve WB2WIK/6
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KL0PE
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Posts: 24




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« Reply #6 on: September 25, 2001, 01:28:42 PM »

Ah, heck.  I knew it couldn't have been that easy.  I'm still waiting for someone to invent an antenna that takes up no space, is resonant on all bands, has a flat 50 ohm impedence, and incredible gain.  But until then, I guess I'll scrabble together a balanced line tuner and use ladder line instead.
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WB2WIK
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Posts: 20666




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« Reply #7 on: September 25, 2001, 07:25:53 PM »

I wrote an article that was published in the April edition of a popular magazine about such an antenna.

As we all know, a good dummy load has a perfect VSWR at all frequencies, the only problem is it lacks any gain.  And a high gain antenna solves the gain problem, but only provides a very narrow-band impedance match.

The solution lies in my once-famous "Octet DL," which is an array of eight stacked dummy loads.  The dummy loads provide the perfect match, and stacking a lot of them provides the gain (of course).

73!

Steve WB2WIK/6
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KL0PE
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Posts: 24




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« Reply #8 on: September 25, 2001, 08:30:09 PM »

Wait a minute.  Would that magazine happen to have been QST?  If it was, I'll admit that I was almost fooled by the article.  I read it through with delight and thought, "This is too good to be true!" and then I caught myself.  I read it through again and realized that what was being described was a dummy load.  Doh!
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