The AM Forum

Since my Inverted L is now scattered to the 4 winds by Thor, I am left contemplating my antenna situation. The stroke hit a tree lower than my antenna peak but roughly even with the sagging part of the L. No telling if the stroke hit the wire or the tree first, but at this point it makes no difference.

My inverted L was 145' total length, with 6-50' radials over very conductive ground, with an extensive lightning protection ground. I don't believe any lightning energy came in via the feedline. Modeling showed very little benefit by going much more than 6 radials over this marshy area.

Looking for ideas. The antenna was tuned and ran as an all bander, and performed well for my porpoises.

However, given I've had two strikes near or on this antenna in the past three years, I wonder if my location may have something to do with it (highest trees in subdivision, etc.).

Thinking of just running enough wire, ie about 66', for a purely vertical antenna, with perhaps a trap in the middle to isolate the bottom half for 40 meters thru 10 meters. No quite sure how to execute this, but researching at the moment.

The bottom line is to have an antenna that has a very small "thumbprint" and hopefully lessen its strike probabilites.

Opinions???? I don't want to reinvent the wheel only to have it destroyed again.

tnx

Philip

Put the antenna back up....You can't transmit without it....If your place is a lightning magnet, the type of antenna isn't gonna make a difference.....Glad your damage wasn't too severe and no one got hurt...

Put yer EL back up. As  S  said, 'it aint gonna make no difference'.

klc

Sounds like spitting in the eye of fate, or daring the thunder gods to strike again. I have to determine if the tree from which it is suspended will live or not. Waiting from a visit from a tree company for a free quote and STILL trying to get a reply from my Ga State Farm guy.

Feeelip....tell THOR to stick it!
The EL you had is one of the better antennas for the low bands. And it sounds like you have the perfect situation there for a Ham station. Light some candles and cover all of the mirrors in your house with blankets during an electrical storm. My wife is from the Philippines and that's what she does. Supposedly a nearby strike can be reflected by the mirror and come inside. I just deal with it!!!!!!
Your pictures looked scary. The discharge from the strike went everywhere!!!
Fred

Phillip,

  Perhaps a coax fed (inside) to OWL fed (outside) antenna like the ZS6BKW would be a good fit?  Look at this link for the idea:

http://www.w5ddl.org/files/Zs6bkw_vs_G5rv_20100221b.pdf

  The coax section could be low loss type like LMR-400 on the bands that show high SWR like 80m. I find it amazing that this antenna will behave on 5 bands without a tuner (so they say).

  An alternative if you can run OWL into the house, is a small vertical loop like what I run here. It is just 16' wide X 8' high, and 8' off the ground. I use it on 40-20-15m. It is fed at a corner with 450 ohm OWL about 30' long to a EFJ Matchbox. It works on 10m if I open the corner opposite the feedpoint. It tunes on 80m as well, but is ~ 10db down from a full sive OCF Inv. V. On 40M the feed at the tuner is very Hi-Z, so any tuner with a balun type output won't work. On 20m I see a significant reduction in noise with this antenna, and some gain...perhaps why I can copy signals on Monday nights 20m group that are pretty weak.

 Jim
WD5JKO

Problem is, I run top band on a small lot.

I would just put it back up if it worked.  How many years have you been there without a lightning hit? 

I've been at my current place for 21 years.  I've had 4 severe lightning hits during that period - severe enough to damage equipment (not necessarily ham equipment), house wiring, etc.  My suspicion here is a power line hit for the wiring and equipment damage.  Twice, I've had to replace the RF amplifier FET in my homebrew solid state receiver.  Those were obvious antenna hits.  I really should disconnect the antenna when I'm not using it, but you know the nature of human beings - I forget to do it!!

Anyway, if you've been there a long time and only now having a problem, probably over the long run you're OK.  If you get hit every storm, you'd probably want to take other measures!!

5 years after erection to first hit. 2nd hit was 3 years later, almost to the day.

Just looked up the specs of the wire I was using for the aerial. It was military phone wire, WD-1/TT, at about 0.022 ohms per foot, or 3.2 ohms DC resistance per my length at 145'.

WD-1/TT =       .022 ohms/ft  or 3.2 ohms for antenna
No. 12 AWG =   .0016 ohms/ft or 0.2 ohms...
No. 13 AWG =   .002 ohms/ft  or 0.29 ohms...
No. 14 AWG =   .0025 ohms/ft  or 0.36 ohms...

I never really bothered to check. However, I wonder how much juice that telephone wire was absorbing in heat? Needless to say, I'm going to use proper antenna wire and not surplus stuff.

It would be worth to copy what you had. Probably #14 is a little better. It's not like the wire is heating from all of the RF amps. D.C. Amps is an issue.
And for erections lasting more than 4 hours; you will need to see a doctor. sick humor sorry
Fred

Fred. Keep your day job.

I use 3 or 4 pair telephone wire for my antennas and it works fine.  I just twist together all the 22ga wires at the ends.  The stuff is stronger that using a single 14ga wire.  The outer jacket holds up very well in the sunlight.  I've had some up for more than 10 years with no problems.

Ask me why I use telephone wire, well I have about 20K feet of the stuff.

Fred

Arborist just left. The pine and water oak both have to come down. My L was hung from the pine.

Phillip,

Staying away from WD-1 is probably good idea.  Unlike conventional telephone wire, field wire is designed primarily for strength and low signaling levels. 

Is not the best in terms of conductivity.  When I was in the business moons ago, some of our wiredogs would have to run multiple lines and parallel them just to get distant analog phones to come up on a digital switchboard.  Was just too much resistance with the analog cards we were using inside digital switch.   

And could still be PIA--despite improved ruggedness still wouldn't survive some of the abuse.   I distinctly remember one very problematic analog run (multiple runs of field wire in parallel) getting a trouble call every single day............until we finally figured out why.  Some of the GI's on site were tooling around in a captured T-62 (Desert Storm) for amusement and those tracks were just wreaking havoc on it.  I had to call their commander--good guy (USAR Combat Engr)--and ask him refrain.   We just couldn't keep splicing--was becoming very marginal.  He'd eventually have to choose between tank rides or calls back home to Mama <hihi>. 

I know some field manuals show WD-1 being used for field expedient antennas but again, that's at tactical power levels and better than nothing which they'd otherwise have. 

73's
Geo W8VG

Going to go back to square one. Not sure what I'm going to put up at the moment. Any form of dipole will have zilch low angle on 80 and 160 at my mounting heights.

How high can you get it up now that a couple of trees are gone?  60 feet or more works VERY well on 75 meters, but you may not have the trees for that....  I used to put Radio Shack TV mast up in 40 foot trees to get 60 feet of height - back in the day.  Worked quite well.  Obviously, the mast was guyed at the top.

Regarding the resistance of the wire

.022 ohms per foot = .072 ohms per meter

From http://chemandy.com/calculators/round-wire-ac-resistance-calculator.htm you can calculate the AC resistance of the wire (ref: skin effect).

At 1 MHz, the AC resistance is .17 ohms per meter (0.052 ohms per foot)
At 3 MHz the AC resistance is  0.28 ohms per meter (0.09 ohms per foot)
At 10 MHz the AC resistance is 0.49 ohms per meter (0.15 ohms per foot)
At 30 MHz the AC resistance is 0.84 ohms per meter(0.26 ohms per foot)

I used EZNEC to calculate the driving point resistance (i.e. at resonance/minimum SWR) of a 3.75MHz dipole (127 feet total length) with 0 resistance wires, and (separately) the same dipole with this type of wire. The two calculated results were: 58.3 ohms and 64 ohms.

Therefore it would appear that your recently departed antenna had about 9% of its total RF input power being lost to heating of the wires on 75 meters

Doing the same calculation with a dipole antenna cut to be resonant on 3.75MHz (127 feet total length), but operated on 19.3 MHz (also a resonant frequency for this antenna), the calculated driving point resistances were 121 ohms and 134 ohms. About 10% of the total RF input power would be lost due to heating of the wires on (approximately) 15 meters. Even though the frequency is about 5x higher in this case, and the resistance of the wire per unit length is about 2.3x higher... the impedance v. position and the current vs position on the 127 ft dipole are such that the loss is only slightly higher on 19.3MHz v. 3.75MHz.

Operating on the top band (160m) is a different matter. Since the radiation resistance of this antenna (the real part of the driving point impedance when the resistance of the wire is set to zero ohms) is only around 2.75 ohms - 4.2 ohms, depending upon the frequency within the 160m band... versus 5.8 ohms - 7.5 ohms with this type of wire...  about half of whatever power you were able to couple into this antenna on 160m (with this type of wire) would have been converted to heat by the resistance of the wire.

Stu

Sometimes industrial and military style "lightning arrestors" can be found for cheap. They may be rated 10KV, some huge amount of amps. The ones I've seen are for aircraft and ships. Not that they won't take damage but might protect your other equipment a little better if placed at the antenna.

Thanks for the calculations. 50% of my power at 160m is unreal.

Still don't know what I am going to do for a permanent solution......

HOWEVER

I do have a spare AT1011/U handy. Its a 32' long military vertical antenna. I can fabricate a base loading coil to make it play well on 160 and 75...it's self resonant on 40m (proof that Shakespeare has hams on its payroll). I can add a couple more 4' sections and get it up to about 40' long. Wouldn't take long to put up.

I disassembled the sad remains of my MIGHTY KA4KOE inverted L feedpoint. The stroke arced CLEAN through the centerpoint to ground and ZORCHED the coaxial connector at that point. However, the coax is pristine from that point to where it goes to the house. I do have/had a decent lightning ground composed of 3- 10' long by 3/4" copper clad steel ground rods interconnected with No. 6 bare CU with UL listed acorn compression clamps. I was planning on installing a shorting stub betwixt the ends of the feedpoint to ground the aerial when not in use. That's a moot point now and not really sure it would've helped me anyway.

What's an opinion of a vertical option? If it plays well I may use it one a permanent basis. I'd have to find something non-metallic to paint the aerial with as the fiberglass radiator tubes will break down after continued use in the UV soaked area of SE JAWJA.

Philip

KA4KOE

AT1011 Calculations, Approximate, 32', ground mounted, used column outlined in red.

Philip

I'm not a fan of the acorn clamps; but they are a listed type.  I'm too cheep to use a good clamp. Right now, I'm using an acorn at the 80m dipole entrance.

I'd suggest a few of the Good Type.
Or, if you want to go hog wild, the exothermic is the way to go. As Tuco said, " I feel a man like you can manage it."


klc

I priced some Erico stuff. Not cheap.

No it ain't.If only one could......

http://www.ab7e.com/exothermic/AB7E_Homebrew_Exothermic_Mold.html

" but in reality you can usually find regular Cadweld weld
metal on eBay for the equivalent of one dollar per shot or less.  I
always look for the largest containers I can (usually 200 grams) because
they usually are the cheapest per gram.  I can typically get three shots
out of one 200 gram cylinder. "

Again, as Tuco said, " I feel a man like you can manage it. "

klc

Blondie!

There are two kinds of people in this world...

.

http://www.hark.com/clips/cmlhdwpjxy-you-dig


klc

MMANA-GAL run, 32' AT1011 extended to 36', capacity hat guys. Ground R doubled to 3.6 ohms vs. 1.8 using same number of radials. Not great, but better nonetheless.