Novice needs longwire design suggestion.

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JackJohnsrud

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I have a very large walk up attic in my building in which I'd like to place a longwire or dipole antenna tunable with a transmatch. I'd like it to work for 160-10 meters and also be useful for medium and longwave reception.

The attic is all one room with floor space of 60 x 40 feet. The floor is 50 feet above the ground. The attic roof peaks at about 12 feet. No metal is present.

Given that there is no limitation to the amount of wire I could use, would there be any advantage to various wrap around configurations, for example, creating a web of interconnected wires that follow each beam and rafter with standoff insulators and tuning that with a transmatch?

Would a tunable dipole configuration be preferable to a longwire if directivity of the signal is not a major goal?

The feed line would be about 50 feet. Can coax with a balun be used with a transmatch instead of twin lead line?

Need some antenna experimenters' thoughts.

Thanks.
 

ka3jjz

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Just thinking out loud here - I'm not sure that 160 would be possible without sacrificing some efficiencies with all the twisting that would be needed; even 80 might be iffy at best. Almost certainly going to sacrifice some bandwidth, depending on your license class that might be a concern. Random wires (which is really what you mean here, a longwire is a very different animal, and much too long for your space) can also pose a serious RF hazard, and might not be the best choice for an indoor installation.

However a shortened Carolina Windom or G5RV might be possible. RadioWorks and BuxComm both sell antennas that might fit the bill nicely. 40-10 is a definite possibility with a minimum of twisting and bending, and with the plans as shown, you might even be able to constuct it yourself if you have the parts.

Our HF antennas wiki has these links, and others, too - I'd also spend some time on the AC6V website and look at any plans found there. I've heard of other hams mounting their antennas by bending or stapling them to beams, ect. I would imagine that's not quite as efficient, but it does seem to work.

http://wiki.radioreference.com/index.php/HF_Antennas

Remember to watch the RF exposure limits. Running reduced power will likely help keep the RFI down as well. 73s and GL Mike
 

k9rzz

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Here's what I would do.

Make a 40 meter dipole ... that's approx. 33ft on each side, fed in the center and stretch as much in a straight line as possible, then what's left over on each end, bend it however you need to, trying not to bend it back over on itself. Now, feed it with open wire feeders (like the current "window line" or 300 ohm twin lead would be okay).

With that set up and an antenna tuner in the shack, you can tune the antenna for 40 to 6 meters as is and if you tie the feedline together down at the shack, you'll have a top loaded 50 ft vertical for 80 meters and down.

Another idea would be to make a loop around the perimeter of the attic and feed that with the window line or twin lead fed through an antenna tuner at the shack. That should load up on multiple bands well just as above. Tying the feedline together at the shack, again, would give you a top loaded vertical, which is quite efficient at lower frequencies.

I'm a firm believer on antennas fed with window line or open wire feed line. I've loaded up my 40 meter loop on 80 meters as described above and worked into Europe with 300 watts. I've also loaded up a 20 foot vertical with window line for shortwave listening .. works GREAT.

I don't think using coax and a balun at the antenna will work as well, but maybe someone else has done this.

John K9RZZ
 

kb2vxa

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Hi Jack and readers,

Ah, nice to see the "exclusive" ham community in action. (;->)

Since you have 60' of room to play with methinks a 40M dipole with a balun (ladder line is impractical in this instance) and T match tuner will do it for you. It'll do nicely on 40 through 10 as a dipole and reasonably well on 160 and 80 as a Marconi T with the shield and center conductor tied together and fed opposed to ground. In this configuration the combination of coax and antenna MUST be equal to or greater than a quarter wave long at the lowest operating frequency or your tuner won't like it! You may get some RF in the shack that way but that's a subject for another discussion.

While I much prefer open wire ladder line to all else it requires special considerations that may not be practical to meet indoors. Basically it all depends on how it's routed and you really never know what's in the ceiling and inside the walls, but if you can keep it well away from all conducting materials (and that includes drywall and plaster) you just may have a go. Oh, you think gypsum board is an insulator? When I used it as a base for a high voltage experiment I found out otherwise, electricity is a funny thing and in this case quite painful.

BTW, I know a guy with the dreaded HOA Nazis to contend with so he uses dipoles in the attic and does rather well. He's primarily a CW op and does a little PSK, he just loves QRP. If he can do it so can you, just hope you're not plagued by noise from the house wiring but that too is a subject for another discussion.

Last but not least a mild chuckle;
"Tying the feedline together at the shack, again, would give you a top loaded vertical, which is quite efficient at lower frequencies."

That's the Marconi T, the broadcaster's choice before they got the idea of using steel towers and radials, the groundplane.

"I don't think using coax and a balun at the antenna will work as well, but maybe someone else has done this."

John, unless you've been living on Mars you just MIGHT have heard we've been doing it that way since a pair of bifilar air wound coils constituted a balun, before ferrite cores were invented. (;->) You "don't think" right though, but I'd rather not invite a technical discussion on standing waves and reflecive losses.

The simple explanation for the less technically minded is that open wire paralell line is practically lossless regardless of the standing wave ratio while coax dissapates the reflected energy as heat. It must be that air has less friction than plastic but I could be wrong. (;->)

Since twinlead and "window line" was mentioned I should inform you that a group of hams in New England put it to the test and it fauled miserably. When wet losses skyrocket and when coated with ice it becomes nearly useless. If you look closely at certain manufacturers' specifications (not all of them will reveal this) they'll tell you there is quite a significant difference between wet and dry characteristics. Open wire paralell (ladder) line is immune, it's the plastic that causes all the losses, ceramic spacers are quite another story.

Jack, are you still awake? (;->)
 

prcguy

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I have to agree with this guy, feed a 40m dipole (or the longest thing that will fit) with ladder line or twinlead and a tuner will make it work fine on 40m and up. 60m will be down a bit and 80/160 will suck unless you short the balanced line and load the whole thing up against a lot of counterpoise. The same dipole fed with coax will be very hard to match on most other bands except 15m and line losses will be very high on the out of resonance bands as well. There’s something magic about an antenna fed with balanced line, it just tunes and works well as long as you don’t go too far below where it’s a resonant half wave dipole. I have had very good luck tuning similar antennas with LDG type autotuners and you can solder a PL-259 right to the twinlead and tune away. I also had good success with the narrow range internal tuners supplied in some radios, although some bands required a 4:1 balun to allow the tuner to find a match. The 4:1 balun is not a magic ratio, it just made it different enough so the tuner would work and a 1:1 or just changing the line length would probably have worked the same. You also have to consider the proximity of house wiring, plumbing and chicken wire (under stucco) to the antenna, it can wreak the radiation pattern and efficiency of the antenna.
prcguy


k9rzz said:
Here's what I would do.

Make a 40 meter dipole ... that's approx. 33ft on each side, fed in the center and stretch as much in a straight line as possible, then what's left over on each end, bend it however you need to, trying not to bend it back over on itself. Now, feed it with open wire feeders (like the current "window line" or 300 ohm twin lead would be okay).

With that set up and an antenna tuner in the shack, you can tune the antenna for 40 to 6 meters as is and if you tie the feedline together down at the shack, you'll have a top loaded 50 ft vertical for 80 meters and down.

Another idea would be to make a loop around the perimeter of the attic and feed that with the window line or twin lead fed through an antenna tuner at the shack. That should load up on multiple bands well just as above. Tying the feedline together at the shack, again, would give you a top loaded vertical, which is quite efficient at lower frequencies.

I'm a firm believer on antennas fed with window line or open wire feed line. I've loaded up my 40 meter loop on 80 meters as described above and worked into Europe with 300 watts. I've also loaded up a 20 foot vertical with window line for shortwave listening .. works GREAT.

I don't think using coax and a balun at the antenna will work as well, but maybe someone else has done this.

John K9RZZ
 
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JackJohnsrud

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Sep 14, 2006
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Yes, I'm sort of awake. I'm in the middle of moving right now and am bushed. So hang on to your guywires. I really appreciate the suggestions. It will take me some time to digest it. I haven't thought about antennas since the age of 12 when I was last licensed. Don't think I retained what theory I did understand at the time. Now aged 49, will crawl along as time permits. Thanks one and all.
 
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