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Author Topic: Planning my 40m Dipole - Have Questions  (Read 1126 times)
TPELLE
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Posts: 18




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« on: February 13, 2009, 05:40:11 PM »

OK, I'm getting ready to hang my first antenna!  I've been thinking about this, and it turns out that I'm lucky enough to have two trees on opposite sides of my yard that I think are just about far enough apart for 65 feet of wire.  The wire would end up being oriented pretty close to north/south, which will give me my strongest radiation east/west (which is what I think I want, as I'm pretty much right in the middle of the country).  The trees should put the wire about 20 - 30 feet in the air, and the center point should be fairly close to my back deck (about 8' in the air), so I can run my feedline straight down to the deck, then under it and into my basement where the radio will be.

I have a couple of questions, though:

1.  I can probably buy 12 or 14 AWG insulated wire (or maybe get it for free), as long as THHN machine-tool wire will work OK.  Is this OK for the antenna?

2.  I can probably also get some RG6 coax (75 ohm) to use for my feedline for free.  According to a couple of old ARRL handbooks I have, they call for 75 ohm coax for the dipole design in the book.  Is RG6 OK?

3.  Regarding lightning protection, what's a good way to ground the feedline when the radio isn't in use?  I suppose I could drive a ground rod under the location where the feedline attaches to the deck rail, put a connector in the line, then just drop it down and connect it to ground rod.  Any suggestions?

4.  I will need some sort of rope and insulators at each end of the dipole to attach it to the trees.  I'll probably use a pulley at each end to run the ropes over, and just tie one end to one tree, then at the other end run the rope over the pulley to a counterweight to take up the slack as the trees move in the wind.  What kind of rope can I buy at the home improvement center to get the job done?

5.  I recall that, years ago, you could buy insulators at Radio Shack.  If the 'Shack no longer carries them, can I make insulators out of lexan or something?
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W8ZNX
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Posts: 1




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« Reply #1 on: February 14, 2009, 02:49:27 AM »

just put it up
far too many new ops worry too much
about things that do not matter

example
direction / pattern of the antenna

40 meter dipole less that 60 ft high
will have no directivity

you need to put a dipole at least 1/2 wave above
the ground to see any sort of dipole pattern

don't sweat the small stuff
just put up a antenna

use it, if you don't like it take it down try another
if it works great
take it down anyway
and try another

playing with wire antennas is fun

mac
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W8ZNX
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Posts: 1




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« Reply #2 on: February 14, 2009, 02:56:05 AM »

ps nearly any old copper wire will do
and yes ive used #14 THNN house wire

stranded wire stays up better than solid wire

i use insulated THNN stranded wire because its easy to
find not over priced and it works

any old insulators will work
same for the rope to hold it up

coax feed line

one small point make sure the coax you get
does not have aluminum braid/shield
that you can not solder

and yes one more time

don't worry
do it !

mac
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K5LXP
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Posts: 4435


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« Reply #3 on: February 14, 2009, 06:37:08 AM »

I think ZNX is right on target about sweating the details.  When I was a punk-kid novice I made my antennas out of anything I could find, and they worked great.  Sure, a dipole made with nice ceramic insulators, copperweld or flexweave for wire and some fresh off the reel coax will work great and last a long time, but one made out of old telephone wire, scrap plastic for insulators, TV cable fished out of a dumpster and hung up with mason twine will work just as well.  If it comes down, just splice it and put it back up.  Think of it as a "learner's antenna".  Even if the coax has aluminum braid for shield just use some crimp connectors or in a pinch, wire nuts.  Not a permanent solution but one that will get you on the air quickly and cost next to nothing to build.  It'll tide you over until you get time to engineer a more permanent solution, meanwhile you'll be making contacts.


Mark K5LXP
Albuquerque, NM
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AB8ZX
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Posts: 42




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« Reply #4 on: February 14, 2009, 03:45:58 PM »

one thing I would suggest. you said the middle of the antenna will be around 8 feet off of the ground? you might want to try and get that up in the air a little more, if for anything to protect anyone that may come close to the feedpoint while you are transmitting. sometimes I have used long pieces of bamboo as a 'push it up' kind of support. strap it to the deck.

you asked about insulators, you can make insulators out of all sorts of (non conductive)stuff. do you have some scrap plexiglass laying around? cut some pieces of that into squares. using plexiglass also helps an antenna be a little more invisible.

good luck and have fun!
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N2EY
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Posts: 3833




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« Reply #5 on: February 14, 2009, 04:33:52 PM »

"two trees on opposite sides of my yard that I think are just about far enough apart for 65 feet of wire. The wire would end up being oriented pretty close to north/south, which will give me my strongest radiation east/west"

This is all very good.

"The trees should put the wire about 20 - 30 feet in the air"

Higher is better if you can manage it. Much better, actually, but it all depends on the trees. The trick is to get lines into the trees by the Robin Hood (bow and arrow) or Huck Finn (slingshot) methods. Note that in some places those things are considered "weapons" and in others they're not.

"the center point should be fairly close to my back deck (about 8' in the air), so I can run my feedline straight down to the deck, then under it and into my basement where the radio will be."

This is very good too. I presume the deck is 8' up, not the wire.

THHN or other house-wire is fine for an antenna. If you can get scraps of it for free, so much the better. The stuff splices easily (use the "Western Electric splice" and solder with a big iron or torch). 12 is better than 14 because you're using trees.

There are all kinds of specialized antenna wire out there, with various advantages and disadvantages. For your simple coax-fed 40 meter dipole, #12 THHN or similar is just fine.

RG-6 coaxK if you get it free or cheap. If you have to buy coax, RG-8X is about the best choice for this particular job.

The impedance of a half-wave wire dipole *in free space* is about 73 ohms, but proximity to the ground lowers it, so "50 ohm" coax is a tiny bit better. But in practice, on 40 meters with a run of less than 100 feet, it's not a problem. The main issue is to be able to get the SWR to a reasonable value at the shack end of the line.          

"I could drive a ground rod under the location where the feedline attaches to the deck rail, put a connector in the line, then just drop it down and connect it to ground rod."

That's an excellent way to do it. Have a connector with both the center conductor and outer braid grounded connected to the ground rod, so that you just disconnect from the coax going into the shack and connect to ground.
 
Rope, pulleys and insulators are a whole subject in themselves.

In my limited experience, there are two kinds of rope worth using. The first is braided (not twisted) nylon, quarter-inch size. The second is braided Dacron, same size. Both have their advantages and disadvantages.

The nylon stuff is strong, easy to work with and fairly inexpensive but it stretches a little under load. That can be an advantage in situations where the supports move a little. Nylon will eventually need to be replaced from UV exposure, but is good for at least several years in full sun.

Dacron is stronger, more expensive, and it doesn't stretch. It's also more UV resistant. But I've found that knots tied in the common homecenter Dacron rope can be very difficult to untie, so I only use Dacron for special jobs.

I make my insulators from PVC pipe, 1-1/2 or 2 inch size. A piece a couple of inches long with holes at each end does the job. For the center insulator, I use a piece with holes for the wire, then wrap the coax around it and secure the coax to itself with black UV resistant tie-wraps. That way the coax end is pointing down, and water won't get in as easily. I also tie-wrap the coax to the pipe via holes in the pipe in the middle. The coax end should be waterproofed with some sort of goop (I use the rubbery stuff found in the homecenter, or spray with clear Krylon).

I don't know how much your trees move in the wind, but you only need a counterweight and pulley at one end, and that's if the trees move considerably.

Some folks go to a lot of trouble to put eyebolts into their trees, but unless that's done right it can hurt the tree. I prefer to launch lines into a tree, then tie it off to a low branch. The rope needs to be moved every year or so so the tree doesn't grow around it but that's no big deal.

73 es GL de Jim, N2EY
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VA3TSK
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Posts: 14




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« Reply #6 on: February 15, 2009, 10:26:29 PM »

I wrote an article for our club newsletter about making up a quick antenna.  Read "Junk Box G5RV" in the following link to get the idea that you shouldn't worry so much. Throw some wire into the trees, it'll probably work just fine.

http://www.barc.ca/Docs/Newsletters/Nov_Dec_2008.pdf

Cheers,

Greg, VA3TSK
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AA4N
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Posts: 108




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« Reply #7 on: February 16, 2009, 12:57:33 PM »

I like to use the hollow plastic clothesline.  It's almost like tubing.  It's already an insulator, it has some stretch, it's plentiful and cheap as dirt.

I shoot it over the tree and tie it off somewhere on the back side.  That gets the wire up as high as it can go.

My first antenna (a 40m dipole) broke in the wind after a year.  But it was the 14awg wire that broke, not the plastic clothesline.  I leave a little more slack in them now.

73 E E

mike
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VA7CPC
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Posts: 2354




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« Reply #8 on: February 17, 2009, 07:38:38 PM »

>>>   The trees should put the wire about 20 - 30 feet in the air, and the center point should be fairly close to my back deck (about 8' in the air), . . .
<<<

You have the opposite of an "inverted-V" layout -- the ends are higher than the center, for you.

The antenna currents are strongest near the center of the dipole.  So most of your power will be radiating from 8' above ground.  That will give you an NVIS pattern -- strong vertical radiation and good local contacts, but not much DX.

My intuition is that the tighter you string it, and the higher you get the center, the better your results will be.

Try it out, see how it works.  That's the nice thing about wire antennas -- if you don't like it, make another one.

      Charles

PS -- The free version of EZNEC has enough power to let you model the antenna, and see if I'm talking through my hat.  But there's a steep learning curve.
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W4KPA
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Posts: 69




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« Reply #9 on: February 19, 2009, 06:55:11 PM »

Home Depot or Lowes will have the pulleys.  
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KB9CRY
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Posts: 4284


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« Reply #10 on: February 20, 2009, 12:49:06 AM »

1. I can probably buy 12 or 14 AWG insulated wire (or maybe get it for free), as long as THHN machine-tool wire will work OK. Is this OK for the antenna?

Yes

2. I can probably also get some RG6 coax (75 ohm) to use for my feedline for free. According to a couple of old ARRL handbooks I have, they call for 75 ohm coax for the dipole design in the book. Is RG6 OK?

Yes

3. Regarding lightning protection, what's a good way to ground the feedline when the radio isn't in use? I suppose I could drive a ground rod under the location where the feedline attaches to the deck rail, put a connector in the line, then just drop it down and connect it to ground rod. Any suggestions?

No, you route the entire coax to ground and install a proper lighting arrestor there and attach it to the ground rod and then route the coax into the house.

4. I will need some sort of rope and insulators at each end of the dipole to attach it to the trees. I'll probably use a pulley at each end to run the ropes over, and just tie one end to one tree, then at the other end run the rope over the pulley to a counterweight to take up the slack as the trees move in the wind. What kind of rope can I buy at the home improvement center to get the job done?

UV resistant Dacron

5. I recall that, years ago, you could buy insulators at Radio Shack. If the 'Shack no longer carries them, can I make insulators out of lexan or something?

Yes
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AD7WN
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Posts: 113




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« Reply #11 on: February 25, 2009, 07:36:57 PM »

You voiced some concerns on the subject of lightning protection.  There are two aspects of this subject that need to be examined: 1) static discharge, and 2) what happens in the event of a direct lightning strike?

If a metallic conductor is isolated for dc and is elevated above the earth, when a lightning storm approaches, the air around that conductor assumes a net charge.  This makes the surrounding air more conductive and thus invites a direct lightning strike.  If a discharge path is provided from the metallic conductor to ground, the air around the conductor will become more nearly charge-neutral.  When that happens, the lightning streamer will, far more often than not, seek a different path to ground, and the antenna system will be spared.

Half of the antenna system can be rendered charge-neutral by simply grounding the sheath of the coax, preferably directly under the antenna.  The other half of the antenna can be rendered charge-neutral by connecting a high-value resister (something near 100,000 ohms works for me) between center conductor and ground.  I like to put this resistor across the antenna center insulator.  The power rating of this resistor is not important when running 5 watts.

The aforementioned precautions will bleed off the static charge and prevent a lot of direct hits from happening.  But it is not 100% effective, and one needs to consider what happens in the event of a direct lightning strike.

In the event of a direct lightning strike, the antenna and feedline will be largely vaporized.  To prevent this damage from spreading to the equipment (matching network, transceiver, power supply, keyer, operator), the best approach is to disconnect the feedline from the shack for as great a distance as possible whenever a lightning storm is approaching and whenever the equipment is not being used.  The disconnected end of the feedline can be left lying in the dirt.

Direct lightning strikes on amateur antennas are exceedingly rare.  Many antennas have been up for decades without this unhappy event occuring.  But it does happen and the results can be spectacular.  Sure, it's a hassle, but if a strike hits an antenna that's connected to the equipment (and operator), the merits of lightning protection are likely to be only of academic interest to your heirs.

More elaborate lightning protection systems are available commercially.  While somewhat more effective than these simple steps, they are expensive and they are not 100% effective.

The rest of your concerns have been very well covered by others.  Good luck with your installation.

73 de John AD7WN
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KB9CRY
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« Reply #12 on: February 27, 2009, 05:05:11 AM »

the best approach is to disconnect the feedline from the shack for as great a distance as possible whenever a lightning storm is approaching and whenever the equipment is not being used. The disconnected end of the feedline can be left lying in the dirt.

Wrong


Direct lightning strikes on amateur antennas are exceedingly rare.

Wrong


the merits of lightning protection are likely to be only of academic interest to your heirs.


Wrong

they are not 100% effective.

Wrong
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K5END
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Posts: 1309




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« Reply #13 on: February 27, 2009, 11:26:45 AM »

It's hard to understand why the same lightning protection argument and same myths/misunderstandings continue to rear their ugly heads.

If done properly, one can protect himself and QTH "99.99999%" from lightning.

Lightning is an electrical tsunami. Just deal with it properly and you'll be fine.

A Ham with a PROPERLY protected shack is at a MUCH higher risk walking across a parking lot before an approaching storm than he is while operating from within said protected shack during a furious electrical storm.

Some of the advice given above and a large part of the advice given "online" is not correct.  Lightning and tower grounding practices are something I know about, despite being relatively new to amateur radio; but I am not going to get into a debate...again.

I recommend that each OM do his own due diligence and research the topic within recognized sources--and forego the opinions of the "Spit and Whittle Club." Your life (and that of your family, your dog, cat and parakeet) may depend on your own diligence.

For casual reading on the topic, see:

http://www.eham.net/articles/19773

http://www.eham.net/articles/19745




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K5END
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Posts: 1309




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« Reply #14 on: February 27, 2009, 11:27:12 AM »

It's hard to understand why the same lightning protection argument and same myths/misunderstandings continue to rear their ugly heads.

If done properly, one can protect himself and QTH "99.99999%" from lightning.

Lightning is an electrical tsunami. Just deal with it properly and you'll be fine.

A Ham with a PROPERLY protected shack is at a MUCH higher risk walking across a parking lot before an approaching storm than he is while operating from within said protected shack during a furious electrical storm.

Some of the advice given above and a large part of the advice given "online" is not correct.  Lightning and tower grounding practices are something I know about, despite being relatively new to amateur radio; but I am not going to get into a debate...again.

I recommend that each OM do his own due diligence and research the topic within recognized sources--and forego the opinions of the "Spit and Whittle Club." Your life (and that of your family, your dog, cat and parakeet) may depend on your own diligence.

For casual reading on the topic, see:

http://www.eham.net/articles/19773

http://www.eham.net/articles/19745




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