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Author Topic: G5RV with 450 ohm ladder line  (Read 997 times)
KB3KAQ
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« on: September 30, 2003, 03:02:05 PM »

i recently purchased a 1/2 sized G5RV that is 52' total length and has 17' of 450 ohm ladder line feed that is feed by 50 ohm RG8X.  i wanted to put the antenna in my attic, but didn't know if the ladder line needs to be in the air or if it can be on the floor of the attic.  my attic only has 5' of head room.  if the ladder line can lay on the floor, should i simple fan it out in a random way to no create an inductor or choke?

KB3KAQ
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WB6BYU
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« Reply #1 on: September 30, 2003, 03:55:03 PM »

It is ideal if the feedline can run in a straight line
in the air, but if that doesn't work you can tie it off
to the rafters with string and bend it as needed.  Best
not to bend it too sharply, and laying it against metal
ducts is not a good idea.  Laying it on the floor may
work if it is wood.

You will need to be using a tuner with this antenna
anyway:  changing the shape and orientation of the
feedline - or laying it on a non-metalic floor - will
change the load that the tuner sees somewhat, but
probably not beyond what the tuner can acomodate.  So
just put it up as best you can, adjust the tuner to
account for the result, and use it.
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KE4ZHN
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« Reply #2 on: September 30, 2003, 04:22:28 PM »

While you can do this, I will caution you that your asking for horrible RFI issues running this antenna in your attic.(yes, even running barefoot) The ladderline section of this antenna acts as a sort of impedance transformer, but metal studs in the attic joists and any wiring and or AC ductwork will cause this to radiate in a haphazard fashion. Also, running the matching section parallel to one of the dipoles legs wont help this antennas performance at all. Basically any antenna you use in this fashion is inviting trouble, but if you have no choice, I would try to at least make sure the open wire line is away from any metallic objects. Try to avoid sharp bends, and try to avoid coiling it up. This antenna is designed to have the open wire line hang verticle below its feedpoint for best efficiency, but the way you intend to use it this will not be the case. You may experience hard to tune conditions on various bands and all sorts of rf headaches. Also, due to its shortened size, it wont perform very well on the lower bands. Im sure your probably aware of this already. You may want to try hiding a small wire outside, it will do much better then one severly crippled indoors, but you may not have this option. Youll make contacts with this, but youll be destroying your phone, your TV and anything else that can rectify RF in your home. You may also experience severe rf on your audio on ssb, especially on the higher bands. You may have better luck making a "fan" dipole in the attic and feeding it with a single coax and a 1:1 balun. At least this will eliminate the open wire line being a problem lying loose in the attic. With 52 ft available you can cut one for all bands 20 mtrs. and above. Im not trying to be the bearer of bad news here, but you can expect at least one of the symptoms I described to happen for sure.
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KT8K
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« Reply #3 on: September 30, 2003, 06:08:03 PM »

I agree with ZHN, and only add that, depending on the age and construction of your house, you may be able to dangle the ladder line down inside one of the walls of the house (avoid any ductwork, wiring, etc. as much as possible).  That will let the ladder line hang closer-to-straight and get it away from the antenna itself.

I would consider a fan dipole, too.  It is a resonant antenna on the bands for which it is cut, and that plus a good balun or choke of coax at the feedpoint could very significantly reduce RFI problems.  Being resonant, it would also work well on most of its bands without a tuner.  Keeping in mind that I haven't seen an antenna that covered 75/80M without a tuner, I am toying with some design ideas that might accomplish that, too, though NOT in an attic (too big).  Anyone thinking of a fan dipole for 75 AND 80?

Best of luck and hope to hear you on the air soon.  73 de kt8k - Tim

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WB2WIK
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« Reply #4 on: September 30, 2003, 06:19:13 PM »

Good advice, here.

You didn't say why you want to install the antenna in the attic, but:

-If you don't absolutely have to, don't.  It will work better outdoors, if that's possible to do.  You can camoflage doublet antennas in many ways, including changing the #14 elements to thinner wire; using dark-colored, insulated wire (which is hard to see, usually); and changing the 450 Ohm ladder-line (which is easy to spot) to thinner, lighter 300 Ohm twin lead, which will work just fine but be more "stealthy."

-If you have to install the antenna in the attic, are you in a multi-family building of some sort (apartment, condo, whatever -- something where you share walls with others)?  If so, the RFI problems can be really horrible, so start out with very low power and gradually increase it to see if you get any complaints.  (If your building is owned and occupied solely by you and your family, at least the only complaints you're likely to get are from family members, so hopefully you can deal with those!)

73 & good luck!

WB2WIK/6
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W8JI
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« Reply #5 on: September 30, 2003, 09:11:40 PM »

Any antenna you install in the attic will be an RFI problem.

The same is true for ANY feedline leaving an antenna that is parallel with the antenna.

It can be coax, ladder line, a stub, a matching section, or anything else and it will ALL have the same RF problem when installed that way.

The G5RV won't be any better or any worse than a dipole, a fan dipole, a loop, a slinky, or anything else. It is a distance VS field strength problem. The more effectively the antenna radiates, the more it will couple to everything around the antenna. The only way you reduce that problem is with distance, or less radiated power!!!

If you do install the G5RV (or any other antenna), don't bother with a balun at the feedpoint. Place the balun in the area where the feedline leaves the parallel point to the antenna!!! You are correct in not bunching up the ladder line. Suspend it in the air if you can. Put the balun at the coaxial line end.

73 Tom
   
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KB3KAQ
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« Reply #6 on: September 30, 2003, 09:46:52 PM »

the reason for not putting the antenna outside is a matter of feedline access to the house and the shack in the basement.  i could put the G5RV in the trees, but that presents a challenge of getting it nearly 25 feet off the ground to keep my son away from it.  

then there is the issue of the feed line from the house to the ladder line.  that would have to cut across the middle of the yard at somepoint.

i'm trying to save myselfa ton of hassle, and i thought that putting it in the attic would do that, but it appears it creates much more.  i'll be taking it back to the store and rethinking my antenna dilemma.

time to enlist the help of my local club.
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W8JI
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« Reply #7 on: October 01, 2003, 07:43:05 AM »

Attic antennas are good only if you have no other choice. They will always bring several problems.

Laying a feedline above ground and parallel to the antenna will also always cause some feedline radiation problems. So will end-feeding an antenna, or using a "no radial" vertical.

The most simple basic antenna you could use, and the one that would work the best without any special work, would be a simple dipole of some form. A trap dipole  or G5RV would be the most unobtrusive. Other forms would be more unsightly or create feedline problems.

What I would suggest is you bury a small plastic pipe or conduit to carry a **coaxial** line into the house (or use flooded direct burial cable), and enter the house where the power line and telephone lines enter the house. Then just pick a spot to come up out of the ground where you can hang an antenna outside between the trees. Then you are free to experiment and have fun.

I think your radio-life will be not nearly as enjoyable if you restrict antennas or choose gimmick antennas like "no radial verticals" or minature antennas. You will get tons of well-intentioned but very bad advice about what works and what doesn't if you ask for an indoor antenna, compact antenna, or vertical antenna.

The simplest and most effective hassel-free antenna for a given amount of work is a plain old simple dipole, and second is a trap dipole with GOOD traps if you want multiband operation. Everything else is a compromise from that point on, and will either look much worse or not work nearly as well.

73 Tom
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KB3KAQ
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« Reply #8 on: October 01, 2003, 09:37:27 AM »

i have decided to buy a vertical from a club member for the 10-20 meter and to put up a 40 meter dipole at some point.

i have always planned on putting in a conduit to the antenna "site" to get the coax out of the way and to make it look less obvious.  the 40 meter dipole will "just" fit in my space as an inverted V or sloper.  if i get some traps, i could do 80 meters as well.

once i get an understanding of the limitations of my setup. i'll be able to work to correct them.  so, getting the vertical up will get me on the air, but it will also get me working to better my antennas

-steve
KB3KAQ
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KT8K
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« Reply #9 on: October 01, 2003, 02:26:56 PM »

I bow to W8JI's far superior knowledge of such things (no joke).
You will be SO much happier with a dipole in the trees, and the higher the happier.  
Now, I just need to understand what makes a "better" trap (and who makes better ones).  Would Dr. Sevick's book help me there?
Thanks Tom.  73 de kt8k - Tim
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N6AJR
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« Reply #10 on: October 01, 2003, 06:29:48 PM »

I actually had good luck with a fan dipole on the roof of an apartment building ( one story, )and I used painted magnet wire and 2" square kiddies alphabet blocks to keep it off the roof.  It was held down with thumb tacks and fishingline for insulators..on the back side  of the house and you can't see it from the street. and about $15 for the whole mess.
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W8JI
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« Reply #11 on: October 02, 2003, 10:49:04 AM »

Be sure you use a GOOD radial system on a vertical!!!

If you look at my website on traps:

http://www.w8ji.com/traps.htm

I measured and compared actual traps.

This was part of a talk I gave in the antenna forum at Dayton several years ago.

Manufacturers, through inaccurate and untruthful advertising, have all but made the word "trap" a dirty word. Amateurs now have the idea that any trap antenna is a bad antenna, and actually have accepted much more lossy arrangements as "better" because it has been drummed into them.

Traps aren't all that bad, except coaxial traps. And even they only have about 1dB total loss in a dipole!!!
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K6GC
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« Reply #12 on: October 12, 2003, 12:46:48 PM »

Dear Fellows,

I read W8JI's website with great interest.  It did not mention the Unadilla traps I'm using.

I just purchased a linear amplifier (Ten Tec Titon One Model 425) and boosted my power over the 100 Watt exciter.  When I did so, I had the misfortune to short the capacitor (100 ohms) in one of my 80 meter CW traps.

Even though I purchased the traps a long time ago, when I called Ralph at Unadilla, he sent me a new one for free.

I've been on the air having fun with this trap dipole for a long time and I thought it was "over and above" what could be expected.  When I offered to pay him, Ralph said to me that I should pass my good experience on to others, and that was better than sending him money.

So, here I am telling my story.

By the way, I have no scientific way of judging traps, but I purchased Unadilla Traps because they seem the best available to me.

I don't want to go into it here because I have no scientific measurements, but anybody who is interested can email me and I'll tell you just excactly why I bought these particular traps over all others and why I recommend them to you.

Very 73,

Tracy WB6TMY
Spam Filter at:
http://www.radions.net/spamfltr.htm
You can use my callsign at QSL.net
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