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Author Topic: Window Antenna  (Read 7197 times)
AD0AE
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Posts: 76




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« on: September 26, 2012, 07:21:44 PM »

Hi All-

I have been periodically posting on different possible antenna configurations that 'could' work for my current living situation.  I have just about reached my wits end with doing some sort of wall-mounted indoor antenna.  There is just way too much noise.  So much so that JT65 is having a hard time making it through. 

Just for reference, I live in a 2-bedroom apartment, on the second story (about 8-10 ft off the ground).  I have no deck or balcony.  I am also somewhat hesitant to do anything that might arouse the attention of my management company (who truth be told is pretty lazy, so I doubt they would notice, although they might care.) 

I have toyed with the idea of doing an "invisible type" antenna and I could connect something from the window, over the parking lot, to a fence close by.  The problem I run into is that car antennas might knock the antenna down (or drunk kids).  So I am still a little bit reluctant to do that idea.  I would probably do a random end-fed and see what happens.

The other idea which I am interested in asking about is something that I could pop the window screen off, put an antenna out the window and bring it back in when I am done.  Generally I am operating at night and with fall/winter on us darkness won't be an issue.  So I am thinking something like a MFJ-1622.  Or possibly a buddistick (although I have heard these antennas are very inefficient by an experienced ham in the local club who is a wiz with antennas (Rich-W3ACO))?  Or maybe something similar to either of these?  Speaking of which, how difficult would it be to build something like a MFJ-1622?  The only thing I am not certain about is where to find a good coil calculator online - one that would get a reasonably accurate inductance.  I have an indoor loop antenna, but have been having some issues with it lately (to be posted in the Elmers section soon). 

Anyway, I am just looking for ideas and thoughts about what may or may not be a good idea.  And also something that isn't too stiff on the pocketbook.

Thanks all, I am looking forward to the responses!

73s,
Steve
AD0AE

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AE5JU
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Posts: 223




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« Reply #1 on: September 26, 2012, 08:42:49 PM »

http://www.k7mem.com/Electronic_Notebook/antennas/shortant.html

http://www.k7mem.com/Electronic_Notebook/inductors/coildsgn.html

http://www.k7mem.com/Electronic_Notebook/inductors/coil_ind_calc.html

Paul - AE5JU
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JAHAM2BE
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« Reply #2 on: September 27, 2012, 04:13:29 AM »

There is just way too much noise. [...] I am interested in asking about is something that I could pop the window screen off, put an antenna out the window and bring it back in when I am done.

I didn't see which bands and modes you're interested in and how much power you plan to be running, but: a magnetic loop antenna (more properly known as a small transmitting loop antenna) sounds like it could be a good fit for your situation. The high-Q preselection reduces adjacent channel interference, and the figure-8 pattern allows nulling (60 dB or more attenuation) of nearby localized noise sources. Radiation angles are good even with the antenna mounted at low heights, and best of all, the antenna can be tiny (e.g. 1m diameter) - probably small enough to slip out your window for operating and back in when done.

They require careful construction to achieve good efficiency due to the extremely low radiation resistance, which requires absolute minimization of all ohmic losses. MFJ sells a commercial small loop antenna that reportedly works very well. If you build your own, there are several very knowledgeable folks here on eham that will answer your questions in as much detail as you could possibly require, and then some! Smiley

I have an indoor loop antenna, but have been having some issues with it lately (to be posted in the Elmers section soon).  

A properly constructed small transmitting loop is in my opinion a very good alternative for your semi-stealth situation. What kind of loop do you have now, and what "issues"?
« Last Edit: September 27, 2012, 04:17:19 AM by JAHAM2BE » Logged

AD0AE
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Posts: 76




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« Reply #3 on: September 27, 2012, 07:37:57 AM »

Thanks for the replies back so far.  In response to some of the questions, I would like to operate primarily JT-65, psk31, and possibly voice.  I think JT-65 would be the best given the circumstances.  Bands: 20 m for sure, and anything else I can get away with.  I have always had a soft spot in my heart for 40 m.  I don't know why I but I always feel like I can hear a lot of good action on 40 m.

I appreciate the thought the loop.  I think my next ham radio related purchase will probably be the mfj-935B. As it turns out, I am buddies with the head machinist in our shop at work and he has access to a bunch of 3/4 in copper tubing!  As for the loop antenna issue I have been questioning a few things.
1. How efficient is my loop in reality? (also how can I measure that?  Field strength measurements?)
2.  I have used  a somewhat expected coupling of the loop and a picture does better than attempting to describe it.  (this also leads into a deeper question of how much you can 'trust' SWR measurements through antenna analyzers)
3. Is the butterfly cap I am using a possible problem location - how can I further reduce losses?
Keep an eye out on the elmers forum, these questions are coming very soon along with some pictures.

More than anything I have done some wire loops around the ceiling of my apartment and even the loop is still picking up mostly noise.  I think perhaps I just live in a large Faraday cage!

Back to the original topic, are there any antennas or antenna configurations that might work well as something I could 'put of the window' and then bring back in when I am done?  Something like the MFJ-1622?  Or maybe a buddistick?  Or some sort of compromise antenna that might do the trick?

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WX7G
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« Reply #4 on: September 27, 2012, 08:23:32 AM »

I have an MFJ-933 loop tuner and it might be ideal for your situation. It will tune loop that are 1/6 to 1/4 wavelength in circumference. For example, a square loop that covers the 20, 17, and 15 meter bands is 2.5' on a side. When built with 1/2" copper tubing the radiation efficiency can approach 50%. 
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WB6BYU
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« Reply #5 on: September 27, 2012, 03:28:57 PM »

Here is a very accurate coil calculator:

http://hamwaves.com/antennas/inductance.html


There are various options that you can use sticking out your window.  I used a standard
75m mobile whip from one second story window and it was adequate for contacts over
a few hundred miles, though not particularly efficient.  I used the aluminum window frame
as a ground plane, and fortunately all the windows in the building were bonded together.

In another apartment I stacked my furniture on my balcony and climbed up onto the
roof at 2AM, and strung a thin wire along the peak of the roof tucked under the shingles.
The end dropped down into my window.  Some friends in a 2-story townhouse didn't
have access to the roof, so we tossed a wire over the roof and brought it in through the
downstairs windows on each side, making a large vertical loop with the feedpoint on the
wall right over the rig.

Depending on the bands you want to operate, a 12' to 20' telescoping fiberglass
fishing pole stuck out the window will support a thin wire.  You can use it with
a tuner at the base, or a loading coil (as long as the wire is less than 1/4 wave
long:  beyond that it may need a series capacitor or an "L" network tuner, which
can be homebrewed.)

Here is another approach:

http://www.sm0vpo.com/antennas/balcant1.htm
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N4UM
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Posts: 450




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« Reply #6 on: September 27, 2012, 03:34:52 PM »

You might consider mounting an antenna on your window.  Years ago I briefly played with a loop made from metalic tape (burglar alarm tape as I recall) stuck onto a large glass door in my family room.  I was able to make a few contacts with it.  This was in the pre-digital age.  I don't know how big the windows in your apartment are or whether or not your screens are made of metal - although I would suspect these days they're probably made of fiberglass or plastic.
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WB2WIK
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« Reply #7 on: September 28, 2012, 02:51:28 PM »

I have an MFJ-933 loop tuner and it might be ideal for your situation. It will tune loop that are 1/6 to 1/4 wavelength in circumference. For example, a square loop that covers the 20, 17, and 15 meter bands is 2.5' on a side. When built with 1/2" copper tubing the radiation efficiency can approach 50%.  

^I like that idea, too.  The 1/2" copper tubing idea is neat, but even using #6-8-10 AWG copper wire is quite good, with reasonable radiation efficiency and probably slightly less bandwidth, so you just have to adjust the tuner more often.

Set the loop close to the window.  If your "walls" are just wood/sheet rock that stuff is fairly RF friendly so it may not matter much; but if you have foil-backed insulation, stucco, aluminum siding or anything else that is very RF "unfriendly," being by the window would likely help.

Note you really cannot substitute a "conventional" antenna tuner, the kind you'd use to attach coaxial cable to, for this at all.  The loop tuner is quite special, balanced, and uses specialized components to achieve what it does.
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WX7G
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« Reply #8 on: September 29, 2012, 09:05:57 AM »

Back to the original post; the problem is receive noise, not transmit effectiveness.

Receive noise will not be solved by any antenna inside the apartment and it will not be solved by an unbalanced antenna. The reason the unbalanced antenna will not help is that an unbalanced antenna uses the apartment AC power wiring as the other half of the antenna and the apartment AC wiring is likely hot with RF noise.

That leads us to a dipole outside the apartment. If you want to avoid wires a tubing dipole can be used. Two MFJ telescoping whips attached to the balanced terminals of an antenna tuner will work. Or use the 17' telescoping whips and you can adjust the length to form a resonant dipole from 20 to 10 meters. Angling them like TV rabbit ears will get them away from the apartment. A 1:1 current balun should be used at the antenna to ensure that the coax shield does not become a significant part of the antenna.

Another option is two mobile antennas such as the Buddipole or the MFJ version. Again, place a 1:1 current balun at the antenna.
« Last Edit: September 29, 2012, 09:10:03 AM by WX7G » Logged
WX7G
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« Reply #9 on: September 29, 2012, 12:55:09 PM »

Telescoping rabbit ears dipole:

The 17' telescoping whip part number is MFJ-1979, $59.95. I recommend the MFJ-901B tuner, $99.95. A suitable 1:1 current balun is the MFJ-915, $29.95.

A similar dipole that is off-the-shelf is the Comet H-422, $370. It covers 40, 20, 15, and 10 meters. There is also the Comet CHV-5X covering 40, 20, 15, 10 , and 6 meters, $320.
« Last Edit: September 29, 2012, 01:00:58 PM by WX7G » Logged
N3LCW
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Posts: 133




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« Reply #10 on: October 09, 2012, 05:20:51 PM »

Steve,

I highly recommend reading the book Urban Antennas by RK3ZK Igor Grigorov ( http://www.antennex.com/Sshack/urban/urban.htm )
I picked it up this year and it is full of many unique antenna ideas that should be considered.  I've experimented with a few with very promising results especially on the lower bands 80M to 30M.

You may have to try building a single band resonant antenna.  I have found them more effective over a one-size-fits all solution for multi-band used.

Some variants of magnetic loops made of wire would be low profile and easy to deploy out the window in the evening.  I used one made from a Crappy 10ft telescoping fishing pole.   It was very effective on 40M deployed from hotel room windows on travel and was great on JT65HF.  The loop itself was based on Ben Edington G0CWT's wire magnetic loop design approach (http://g0cwt.co.uk/magloops/new_page_2.htm)

A little ingenuity and perseverance pays off.

Andy
N3LCW
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K2ZS
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« Reply #11 on: October 17, 2012, 03:13:57 PM »

I use an indoor wire loop antenna fed with an SGC-230 at the base and it is fantastic. I have several thousands (well at least 2K) of QSO's logged, and worked and confirmed my first DXCC with it. Yes, the SGC is a bit pricey but after being off the air for 10 years it was worth it for me...

The only bands that are noisy for me are 80 and 160 and I find it is mostly interference from all of the CFL lights the complex installed in the common areas of the building. Keeping that in mind, I try to do most of my operating during daylight hours. I operate almost 100% cw because it is the most reliable mode in my situation but have run RTTY and PSK31 from here. I have to admit however that PSK31, even at the low power recommended, did cause RF issues with my computer.
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