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Author Topic: Low profile apartment dipole  (Read 1059 times)
KL3DL
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Posts: 45




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« on: February 27, 2010, 08:56:15 PM »

Good day,

I am a newly licensed general class operator, Feb 2010.

I have just gotten interested in ham radio last December '09. Because of my electrical and technical background I only needed to study the guidelines and radio theory.

So, I have acquired some equipment... a kenwood TS530S, MFJ versa tuner and lightning arrestor.

Now all that is needed is to put up an antenna. Due to the fact that I live in an apartment I did not want to string up a bunch of wires across from the buildings or trees, so I thought of running the antenna along the wooden face of the roof edge and then run the balanced feeder into my 2nd floor bedroom... see image.

http://i138.photobucket.com/albums/q244/Kryoclasm/apartmentdipole.png

This method would render it almost invisible to most folks.

The wires are insulated solid copper and will be held in place with plastic wire/small cable clips. The balanced feeder is 300ohm radio shack outdoor insulated antenna wire as I wanted to forgo coax cable.

I am a bit of a book worm and I try to research everything to the hilt.

Now, I don't have an elmer, I just need some input on how well this method would work. There are no metal sidings, just wood and brick.

Any info would be great, just remember, I am a newbie at this.

Thanks,
Shawn
KF5ETW
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W5WSS
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Posts: 1783




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« Reply #1 on: February 27, 2010, 11:24:10 PM »

It will radiate when one gets it tuned as is always the case, however the horizontal oriented antenna near the ground like 1/4 wavelength just as an example always will radiate at a high angle; just means the bulk of the power into and out of the antenna will manifest at some high angle towards the zenith rather than at the horizon.as it relates to the earth. As an indoor antenna considering most apartments have a standard floor to ceiling heigth of just under 8 ft. one can drop half of a 10 meter dipole leg down vertically from near the ceiling down to the baseboard for example and run the other half leg horizontally along the ceiling. This antenna should be fed at the center with a 1:1 current balun and a short length of 52 ohm coaxial feedline when intended for use on a single band. The antenna will receive and radiate power in a MUCH lower angle than a horizontal antenna as low as yours will be. It is a user choice as to what operating goals you prefer. I like the top fed inverted L dipole myself. Note this is much better than a vertical antenna that is bent over and needs radials. The inverted L top fed is a dipole. and works very well. 73.
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KL3DL
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Posts: 45




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« Reply #2 on: February 28, 2010, 06:03:25 AM »

"As an indoor antenna considering most apartments have a standard floor to ceiling heigth of just under 8 ft. "

Well, this is an outside antenna mounted about 35 feet high at the apex and about 20 feet high at the ends of the 40m band dipole.

I cannot mount it any higher, but I'm worried that because it is attached to the wooden frame of the roof that I cannot space out the other dipoles farther than an inch from each other.

Is that going be a problem?
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KL3DL
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Posts: 45




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« Reply #3 on: February 28, 2010, 06:24:47 AM »

here is the first drawing and a new 3d drawing.

2D face:
http://i138.photobucket.com/albums/q244/Kryoclasm/apartmentdipole.png

3D Corner:
http://s138.photobucket.com/albums/q244/Kryoclasm/?action=view&current=3d.png
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N3OX
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« Reply #4 on: February 28, 2010, 08:45:19 AM »

I think your plan will work fine.  You might not need the extra wires: running the longest one with "tuned feeder," that is, using a 40m dipole fed with 300 ohm line on other bands with a tuner might work as well as the multiple dipoles and is certainly easier to install.

I like multiple dipoles because you don't have to readjust them when you change bands, but the choice of 300 ohm line instead of coax will probably mean you'll need the tuner with the multiband antenna anyway.  The antenna will probably be nearly 50 ohm impedance on the resonant bands, but the 300 ohm line will transform that to some other impedance which the TS-530 might not load into.  So you'll need the tuner to bring that impedance back near something the '530 will deliver full power into. 

If this is to be a stealth installation and you don't want to attract attention, it's may be better to put something up that won't require much tweaking and tuning... The 40m dipole used as a multiband antenna has the advantage that you can just put it up and operate without fiddling with wire lengths. 

If you really do want a multi-band resonant arrangement I would reconsider the use of coax.  It is possible to buy coax that's less noticeable than 300 ohm flat line.  RG-174 is only a tenth of an inch in diameter.  RG-316 is the same size but has Teflon dielectric and handles more power.  I think the adjustment required for this sort of installation is going to be  a liability for a low profile apartment antenna. 

When I lived in an apartment, my best antenna was a hundred foot long multiband doublet.  I used a slingshot to shoot weights trailing approximately 50 feet each of thin magnet wire into two nearby trees and I fed that with a motorized tuner out on my balcony.  I  tried generally to do the slingshotting at night... only once did someone see me doing it and they didn't seem to care too much.

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WX7G
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Posts: 6331




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« Reply #5 on: February 28, 2010, 09:04:20 AM »

It will work fine. Keep the wire away from touching metal as it might arc and start a fire.

Low dipoles can work very well. There is a myth that won't die that says a low dipole equals no DX. Get on the digital modes or CW and you'll work plenty of DX.
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NI3S
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Posts: 67




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« Reply #6 on: February 28, 2010, 10:46:35 AM »

My antenna recently came down due to a huge snow fall.  As a temporary antenna, I used 300 ohm twin lead and built a folded 20m dipole fed with 32' of the same stuff. I attached it to the railing on my deck 15' up.  It tunes up well on both 20m and 40m and it working surprisingly well.
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N5YPJ
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Posts: 642




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« Reply #7 on: February 28, 2010, 11:05:14 AM »

The installation idea will work fine. You may not break a pile up to a DXpedition but you will make contacts.

I am concerned about a couple of things - I would either go with a single 40 mtr element and feed the antenna with 300 ohm wire, this method would probably be the least tweaking needed on the antenna itself. Or get rid of the 300 ohm wire and run RG-8X or RG-58 to the multiple dipoles, if you are getting away with installing the antenna on the facia boards then a small coax shouldn't draw anymore scrutiny than the antenna itself.

Regardless you will need a tuner on either antenna unless you are prepared to tweak each element to get it resonant.

If you have a way of bringing in the flat line without any RFI problems, I'd just go with the flat line and avoid any extra fuss.
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KL3DL
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Posts: 45




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« Reply #8 on: February 28, 2010, 06:27:35 PM »

Great advice all.

I think I will run just the 40m wires with the 300ohm balanced line. I really don't want to hang up a 1:1 balun in plain sight if I used coax.

I have a tuner so what I can gather from your input is it should work fine.

If anyone else has any other info or ideas, I welcome them.

Thank you all.
Shawn
KF5ETW
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W5WSS
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Posts: 1783




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« Reply #9 on: February 28, 2010, 08:46:11 PM »

Oh sorry I thought your height was low and similar to mine of 18 ft max. Had I given more consideration to your question My answer would've read differently. Anyway I actually use a 100 ft doublet that unfortunately is so low that most power manifests in high angles toward the zenith Not my favorite situation but alas The 100 ft doublet and 300 ohm Belden balanced line with a wide range tuner has offered me some enjoyment during this difficult 0 sunspot period over the past six years ,but recently I discovered a better antenna for indoor Hf work the afore mentioned Top fed inverted L doubletas a multi band antenna the 135 ft version when fed with balanced line and a wide range tuner..or...one can simply feed it with coaxial cable and tune the antenna and use as a mono bander. a 16 ft 10 meter version naturally fit the indoor external wall here. I love the little antenna it really works as a symmetrically balanced antenna, has a natural low loss surge impedence lending itself to coaxial feed and presents power manifest into 25 degrees toa or so and works well it so easy to build: 16ft of wire I use the DX Engineering bal-050-h05a 1:1 balun at the feed point then 10 ft of 8x. to the equipment. Not alot of gain but the point here is that none of the horizontal antennas from 18 ft low could nearly the power in the angles needed so whether it was a horizontal loop to double extended zepp they responded simarly to the rule of height above ground mandates earth confluence and toa the top fed inverted L dipole overcomes the difficulty associated with horizontal antennas and height requirements for lowered toa compared to vertically oriented antennas the top fed inverted L dipole is a grat little antenna that is not the same as a bent folded over vertical voltage fed against a radial feild infact the top fed inverted L doublet does not benefit from a radial feild however it does respond smoothly to height increase primarily because of the horizontal leg but smoothly gain increses and toa angle lowers with higher heights as demonstrated in modeling.anyway enjoy our ham bands hope I clarified my thoughts better this time 73
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