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Author Topic: My antenna plan...looking for comments/suggestions  (Read 1102 times)
KL0PE
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Posts: 24




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« on: September 20, 2001, 10:55:18 AM »

I have a very sizable backyard lot but few trees actually on my property.  There are two fairly large trees to one side at around the middle of the yard.  I currently have a random wire strung out the window to one of the trees, but I'm unsatisfied with the performance.

Here's my plan:  I want to install a multiband dipole with the middle attached to the tree as high as possible, one leg attached to the house, and the other attached to the fence.  This will give me an inverted-V.  I want to use either a G5RV or a five band antenna that is described in the June 1995 QST ("Five Bands, No Tuner").  I plan to feed the antenna with ladder-line of roughly 40 feet (to about the bottom of the tree) and will run coax from there to my house, a run of about 60-70 feet or so.  For lightening protection, I plan to ground the shield of the coax to a copper stake driven in at the foot of the tree.  To get the coax from the base of the tree to my shack, well, that's the tricky part.  I am considering burying it (either using direct-bury coax, which I fear will be too expensive for my budget, or channeling it through PVC pipe that will be sealed).

Comments?
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WB6BYU
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Posts: 13574




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« Reply #1 on: September 20, 2001, 11:39:23 AM »

Use the conduit, and make it large enough that you can pull one or
two more lengths of cable through it in the future.  (You never know
what your future upgrade plans may include.)  Also, leave a pull
rope in the conduit to simplify the task of pulling the next cable
through.)

However, before you trench for the conduit or cable, put up the
antenna and run the cable across the ground, and verify that this
antenna/feedline combination will work for you.  Once you have
tried it out and decided that it is, indeed, better than the long wire,
then you can make it permanent.  It can be rather annoying to get
everything installed before you discover that you really should
have used the next size larger coax cable because of losses due
to SWR, but it won't fit through the conduit you installed!

Good luck!  - Dale WB6BYU
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WB2WIK
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Posts: 20666




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« Reply #2 on: September 20, 2001, 12:19:09 PM »

I also wonder how you're going to get the 450 Ohm ladder line-to-coaxial cable interface to work across several bands...?  Unless there's something magic about the dipole, I cannot conceive how any single balun could resolve the wide spread of impedances faced in this situation.  Possibly, as in the G5RV design, the coaxial cable is known to be an attenuator and actually helps smooth the mismatch by its own attenuation; or maybe, 60 feet or so of low-loss cable won't introduce much loss, so the heck with it!

I agree with Dale, I'd use the conduit approach, and leave a pull rope in place for the "next experiment."  Although: RG213/U with a very tough, direct-bury jacket is available inexpensively from Cable X-Perts ($.40/foot).  I've used it, and it's tough; I'm sure it will withstand direct burial, provided you don't put a shovel through it!  Biggest problem I've ever had with such cables outdoors is "critters" chewing on it...some critter (unseen, but probably a squirrel) chewed completely through some RG213/U I had on the ground a couple of years ago...drove me nuts trying to figure out why an antenna that worked fine one day didn't work at all the next!

Good luck with the new antenna!  Although you really should wait for a tornado, blizzard, typhoon or something before doing antenna work: The old rule says only antennas installed under the worst possible conditions really work well.

73 de Steve WB2WIK/6

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KL0PE
Member

Posts: 24




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« Reply #3 on: September 20, 2001, 01:10:16 PM »

Thanks for the comments!  As for my "magic" multi-band dipole, the instructions in the QST article show a 94 foot total length dipole with 41 feet of ladder line and a 1:1 balun to match it to the coax.  According to the article, this gives less than 2.5:1 SWR on 40, 20, 17, 12, and 10 meters and about 7.5:1 on 80 meters.  The first five bands will give my Elecraft K2 internal-ATU no trouble, and I may even have limited success on 80.  As for the G5RV, I've read that it can be a lossy antenna and that coax loss actually contributes to the "flat" SWR (as the old saying goes, even a dummy load has great SWR!)

Any other comments?  I'm also wondering, is grounding the coax shield to a copper rod driven into earth a proper way to protect from lightning?  My wife is very nervous about this and keeps referring to my antennas as "lightning rods!"
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WB2WIK
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Posts: 20666




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« Reply #4 on: September 20, 2001, 01:23:18 PM »

I would agree that 2.5 SWR is not excessive, and the increase in coax loss, based on 60' of RG213/U, is very little.  And I'm glad you're not one of the "if it's not 1:1, there must be something wrong with it!" crowd -- that's getting really old.

As for lightning protection: I believe grounding the outer conductor (shield) of the coax is better than nothing, but won't do the whole job.  A rather HUGE EMP surge could easily be conducted right through the balun and into the coaxial cable, and conducted via the coax's center conductor back into your home and equipment -- presuming the surge doesn't simply blow the balun to dust, which might also happen.  You're surely on the right track, wanting to ground things outside and far away from the home.

I'd probably use a coaxial spark-gap arrestor on the coax side of the balun where the coaxial cable attaches...Alpha-Delta and others make ones that are fine.  You ground the arrestor, and it provides protection against large transients on the center conductor by shorting them out when they exceed a certain breakdown voltage.  You still need an excellent, low-impedance ground at the arrestor, and it's a good idea to keep that away from the house.

73 de Steve WB2WIK/6
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