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Author Topic: Dorm Room VHF/UHF  (Read 7498 times)
KC2YWE
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Posts: 14




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« on: January 13, 2011, 03:52:56 PM »

So I'm living in a dorm room (at a university you've heard of) until May. Because of a hectic last semester, I didn't have time to set up my shack, but this semester should be better.

I have some obvious limitations. Obviously, I can't go out to the roof and mount a nice antenna - if I could, I'd have an awesome range. But I'm about 50 feet up with nothing in front of me for miles, and have a window. However, this obviously limits my line-of-sight to 180 degrees - the other half of the sphere is behind a cumulative 4 feet of walls.

I have a nice mag-mount that I was using at home (stuck to an air conditioner at 30 feet, worked great), but the interference is terrible here. I've already wrecked a clock radio with my 50W transceiver, and I've got much more expensive computers and things at stake here. The few times I've tried going OTA here, I've disrupted my hard drives and crashed my stereo system (no permanent damage). I don't have a very good ground plane here... I'd need to scavenge/salvage/buy a large-ish piece of steel and figure out how to keep it in place.

My eye's been drawn to the Arrow dual-band J-pole: http://www.arrowantennas.com/osj/j-pole.html . I can stick this out my window, and I don't think I'll be given a hard time (though I'll have other uses for it if I am and it doesn't work out). Would this lessen my interference issue? My EMF theory is pretty weak, but for some reason I can't explain, I feel like a J-pole would.

In short, what's the best way for me to run (up to) 50W out my window without screwing anything in or releasing any blue smoke? Can it be done?

Thanks in advance... 73 KC2YWE
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N3OX
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« Reply #1 on: January 13, 2011, 04:39:19 PM »

In short, what's the best way for me to run (up to) 50W out my window without screwing anything in or releasing any blue smoke? Can it be done?

A J-pole would almost certainly work out better than a mag-mount with no ground plane, but it still could cause some extra RFI problems because of currents on the coaxx.  If you think you can get away with sticking something outside the window, I would do a center fed vertical dipole.

A J-pole is an end fed half wavelength vertical fed with a matching stub... the problem is that you can never completely choke off currents from the coax.  A center fed dipole is much easier to choke, keeping all the RF current on antenna elements outside your room.   I don't think anyone builds a cheap-o vertical dipole to stick out the window, but it's a straightforward homebrew project.

Here's the kind of thing I'm thinking of:

http://n3ox.net/files/simple_vertdipole.jpg

That said, you may never be able to run 50W in extremely close proximity to certain electronics stuff, but at least this will give you the best shot at running the most power without having a lot of strong fields right near your gear... or your dorm-mates.
« Last Edit: January 13, 2011, 04:42:25 PM by N3OX » Logged

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

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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« Reply #2 on: January 13, 2011, 05:57:00 PM »

The J-pole might work if you can get it far enough out the window.  But running 50 watts
in a room full of electronics is not a trivial task.

One option might be a small beam on a standoff outside the window, or at least pointing out
through it.  This has two advantages - it directs more of the RF away from electronics inside
the room, and it may a allow you to use lower power.  (A beam with 6dBd gain, for example,
would allow you to run 1/4 of the power to cover the same path.)

Here is an example photo:  http://www.antentop.org/010/ua6hjq_010.htm

There are many different designs, some of which are very easy to build.  A 2-element quad
could lay against the side of your building right below the window outside, with the
reflector flat against the wall.  It will detune the reflector somewhat, depending on the
wall material, but it should give a reasonable signal away from the building with less RF
inside.  You can also use adhesive copper tape to mount a full wave loop directly to
the window glass  - if you make it about twice as wide as it is tall and feed it in one
vertical side you'll have very close to 50 ohms (though metal window blinds, or some
glass treatments, may reduce the effectiveness.) 

Something on the order of a 3-element yagi, 2-element quad, or a Moxon is relatively
small for 2m, and for 440 you can get even more gain in the same boom length.
With a bit of creativity you can come up with a mount that allows you to aim it at the
desired repeater (as long as it is in the available 180 degrees).  I'd suggest the
WA5VJD "Cheap Yagi" designs for very simple construction, but there are many other
options using thin elements and PVC pipe or other simple materials.  With a coat of
paint that matches the outside walls, it might not be too noticeable.

At the moment I'm using a simple UHF ground plane with two radials hanging on the
inside of the window by my desk, held up by a suction cup on the glass.  (Suction
cups with hooks are often sold for attaching Christmas lights.)  A 2m version probably
would be possible on my window, but you'd have to see if yours is large enough.
It all depends on how far away the repeaters are that you are trying to work.

But certainly I'd suggest NOT trying to run the rig at full power in a room full of
consumer electronics - that is asking for trouble.  You might get by if the antenna
was outside, though.
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N3OX
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« Reply #3 on: January 13, 2011, 07:43:08 PM »

One option might be a small beam on a standoff outside the window, or at least pointing out
through it.  This has two advantages - it directs more of the RF away from electronics inside
the room, and it may a allow you to use lower power.  (A beam with 6dBd gain, for example,
would allow you to run 1/4 of the power to cover the same path.)

Here is an example photo:  http://www.antentop.org/010/ua6hjq_010.htm

A Moxon Rectangle would be my choice for a "stick a beam out the window pointed in a fixed direction" antenna.

Not so good if the goal is to run lower power by running high gain... though 3dB will help some.
« Last Edit: January 13, 2011, 07:47:20 PM by N3OX » Logged

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

Monkey/silicon cyborg, beeping at rocks since 1995.
WX7G
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Posts: 5920




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« Reply #4 on: January 14, 2011, 03:17:06 AM »

A 1-wavelength square loop feed on the vertical side will provide a ground independent vertically polarized antenna. The downside is the feedpoint impedance of 120 ohms. Or build a 3-D folded loop for a small, ground independent antenna presenting a 50 ohm load that you can set anywhere in the dorm room.

Link to the 3-D folded loop

http://www.antennex.com/preview/Jun603/intro3dfl.htm

An advantage of a 1-wavelength loop over a dipole is that no balun is required. The loop presents a high common-mode impedance to the coax shield.
« Last Edit: January 14, 2011, 06:43:29 AM by WX7G » Logged
K1CJS
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Posts: 5879




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« Reply #5 on: January 14, 2011, 04:01:50 AM »

Radio Shack has a small ground plane antenna that I've used for VHF/UHF ham bands that works rather well.  It's light and can be put outside a window without too many problems.  Here's the link: 

http://www.radioshack.com/product/index.jsp?productId=2103641

Yeah, it costs $27.00 now, but its a good altenative to blowing out electronics with stray RF.
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KI4SDY
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Posts: 1452




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« Reply #6 on: January 14, 2011, 05:53:54 AM »

I agree with N3OX, your best and least expensive bet for what you described is a homemade center fed dipole. Make each leg of the dipole 19 1/4" long and it should work well. You can move it out from the side of the window a bit on a flat piece of scrap plastic or PVC pole. Be sure to check the SWR of the antenna to prevent damage to your radio. After you get it working good, paint the whole assembly with $.99 flat black spray paint to hide it from complainers or rule enforcers and to cut down on corrosion.  Wink

With an outside antenna, the improvement in signal should be great enough that you will be able to reduce power. The combination of moving the antenna outside and reducing power should eliminate your RF problems.  Grin
« Last Edit: January 14, 2011, 06:41:25 PM by KI4SDY » Logged
AA4PB
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Posts: 12685




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« Reply #7 on: January 14, 2011, 06:12:07 AM »

In my opinion, 50W is way too much power for operating from such a confined area. I'd keep it to 10-15W maximum. A simple dipole is a good way to go. Use some ferrite beads over the coax near the feed point to act as a balun to keep RF currents from flowing back into the room on the shield.
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AB2RC
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« Reply #8 on: January 14, 2011, 09:36:07 AM »

So I'm living in a dorm room (at a university you've heard of) until May. Because of a hectic last semester, I didn't have time to set up my shack, but this semester should be better.

Did you check to seed if your school has a ham radio club, and possibly a club station? If so, join up and use the schools gear and antennas.
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AF6WI
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« Reply #9 on: January 14, 2011, 10:08:06 AM »

I used to live in an apartment with 2 stories. I operated on the 2d floor, and I hung my antenna from the rain gutter. I only operated at 5W, though. 50W in a dorm room -- well, you've heard from others on that. I'm sure you've screwed up not only your stuff but the stuff of others as well who have no idea what happened. Kindly play within the rules of amateur radio.
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N4NYY
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Posts: 4742




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« Reply #10 on: January 14, 2011, 03:25:11 PM »

Quote
One option might be a small beam on a standoff outside the window

I like this idea. It id directional and less likely to cause RF issues inside the dorm. I am not sure about safety issues, but I have a Yaesu 2M HT, that when I keyed up, my paper shredder would start. No joke! Anyway, I found it useful as a remote control paper shredder.

But like I said, at least you won't have to worry about RF or such, as you would with an omni.
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KC8OYE
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Posts: 297




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« Reply #11 on: January 30, 2011, 07:50:43 PM »

I've already wrecked a clock radio with my 50W transceiver,

hahahha you too?

I've got my JetStream JTB-3 antenna sitting here in the room until the weather improves to get it up on the roof (it's snowy and icey right now)
It offers 4.5db on 2m... I keyed up with 25w and immediately took my atomic 'smart clock' out.. it's been steadily gaining time over the last couple of weeks.. its up to about 2-1/2 hours fast now. LOL

everytime I un-key, the touch lamp goes through another cycle Smiley  so it goes low-med-hi-off-low-med-hi-off as I QSO LOL
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K2ID
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« Reply #12 on: March 14, 2011, 07:48:35 PM »

I have and currently still do, use an Arrow Antenna J-Pole. Just lean it against the window and it works real fine. Smiley
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KE4DRN
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Posts: 3714




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« Reply #13 on: March 15, 2011, 08:27:43 PM »

hi,

take a look at this delta loop

http://www.k2zs.com/indoorvhfdelta.php

http://www.eham.net/ehamforum/smf/index.php/topic,3022.0.html



73 james
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W5TTW
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Posts: 42




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« Reply #14 on: March 19, 2011, 09:55:03 PM »

Dude, you're in college and you want to spend your free time talking to us old farts on the radio?  There will be plenty of time for that later.  Take my advice and chase the girls while you can!

Youth is wasted on the young. 
« Last Edit: March 19, 2011, 10:03:09 PM by W5TTW » Logged
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