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Author Topic: horizontal loop questions  (Read 312 times)
VA7CPC
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Posts: 2357




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« on: January 27, 2007, 08:59:16 PM »

The metal flashing:

If it's continuous, it will behave like a one-turn, shorted transformer secondary winding.  That will have some nasty effects. Your RF energy will go -- partly -- into heating up the copper.

If it's open-circuited at the antenna feedpoint, that will help.  If it's open-circuited at several places along the antenna run, it shouldn't bother you at all.

It (heaven forbid!) it's grounded -- e.g., into a lightning ground system -- your losses may be worse.

    Charles
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N6AJR
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Posts: 9890




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« Reply #1 on: January 28, 2007, 03:21:04 PM »

seeing how its a new house and all, put up a ships mast with cross arms on each end of the building and mount the antenna there,

tell the nosey neighbors it is a design element, or a lightning protector. or something. you could probably do 30 feet high and 20 feet wiide that way,

or tell them it is a new religion, the church of QSK, and it is religiously signifigant..

this could be too much fun.

let us know how it works out, ( BTW I like your  new house.. )
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W6XE
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Posts: 5




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« Reply #2 on: January 26, 2007, 06:05:25 PM »

Hi all,

My wife and I just moved into our new home a few weeks ago.  It only took about three years from idea to completion Smiley  I've been dreaming the whole time about moving in and putting up an antenna and I'm happy to say, I think the time is finally here!  We're on the top of a ridge, and the shape and topography of our lot and the house are a little unusual.  I think a few pictures are worth several thousand words in this case:

http://tinyurl.com/yrvxsp
http://tinyurl.com/ysr6oq
http://tinyurl.com/yvl6un

My current plan is to put up a rectangular-shaped horizontal loop around the edges of the eaves, a little more than 200 feet in circumference.  I'm primarily interested in working 40 meters up through 10 meters.  I'd feed it with a 4:1 current balun on the roof where the ends of the loop meet, and from there about 30 feet of RG-213 coax through a metal conduit that runs from the roof near the feed point down to my shack in the garage.

A few questions and concerns:

1) There's a narrow metal flashing that runs around the permiter of the eaves.  How concerned should I be about coupling with the flashing?

2) What about the asymmetric length (~80 ft) vs. width (~25 ft) of the rectangle?

3) As you can see in the photos, we're on a pretty steep slope.  The section of the loop along the street side of the house will be 15-20 ft above ground, while the back side will be 35+ ft above ground.  Any way to predict what effect this might have on the radiation pattern?  I poked around with EZNEC a bit, but I couldn't see any way to adjust the model for uneven/sloped ground.

Thanks,

Jim Meehan, W6XE
Oakland, CA
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VA1CQ
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Posts: 70


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« Reply #3 on: January 26, 2007, 07:10:27 PM »

Jim,

I'm no expert, but I have played with a large horizontal loop about 625' in circumference and I am a fan of these antennas. Here are tips from my experience and from what I've been told by others more knowledgable:
1) Feed the loop with open-wire ladder line; don't use coax. Did I mention, feed the loop with open-wire ladder line; don't use coax. The nearby metal will affect things, but at least try open feedline first keeping the feeder as far from metal surroundings as possible. At most, you could use a balun to transition the last 5 feet for connection to your unbalanced radio antenna connector. The reasons:
- You will have a better TX signal due to much lower loss. Although your RG-213 is good quality, when used at high SWR (as a loop will present on some frequencies), the RG-213 will dramatically decrease your actual TX output from the antenna. I experienced this phenomenon first-hand when I tried to use coaxial then switched to 600 ohm ladder line. This is not a theoretical idea. Your signal will truly be better if you don't use coax. Read Nov 2006 QST, page 42.
- The antenna will tune more easily and, on some frequencies, may not need to be tuned.
- With ladder line, the antenna will likely work on all bands/frequencies when using a tuner. With coax, probably not.

2) Don't use a balun. Contradicts Item 1 a bit, doesn't it?

3) Maximize the enclosed area of the loop. This means make it as close to square as possible. The more the loop looks like a folded dipole (long and narrow), the less effective it will be.

4) Elevate it as high above the house as possible. If this means using thinner wire to do so, do that. If the antenna falls down from breaking in the wind occasionally, so what. This is called routine maintenance.

5) Keep the feedline as short as possible.

6) Follow these rules as close as possible, but break them if you must to get the antenna up. Even breaking the rules will still give you a very flexible, effective antenna considering effort and cost.

The horizontal loop when installed as high as possible is a secret that commercial antenna manufacturers don't want you to learn about.

Keep us informed how it goes. I am always planning my next loop antenna.

73, Murray
7J1AQH (pending), VE7HA
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KC6UNM
Member

Posts: 6




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« Reply #4 on: January 26, 2007, 08:28:05 PM »

http://www.cebik.com/

Everything you want to know about loops.

The rectangle will be directional.  Try to make it some multiple of a quarter wave (3/4 wave minimum) for the lowest band of interest and as close to a sqaure or circle as you can.  The takeoff angle for a loop becomes useable for long distance communication at 1.5 to 2 wavelengths of the lowest band you want to use.  At 1 wavelength the thing is basically shooting straight up and straight down.  Think about a quad beam that uses a 1 wavelength elements.  It shoots out of the hole in the square.  As the frequency increases to 1.5 to ideally 2 wavelengths it starts to radiate off the sides.  You mentioned 200 ft.  If you can find some way to add 80 feet you'd have 1 wavelength at 80 meters and although it would be an NVIS antenna for that band suitable for local communications (and really, on 80 thats all I hear) at 40 meters you now have a 2 wavelength loop that will have a useable takeoff angle and on 20 it gets even better.
Thats all I know for now.  I am installing my 440ft (close to 3/4 wave on 160 1.5 on 80) tomorrow but I am feeding mine directly with an autotuner from under my second story eave.  If thats not an option, then open wire line and a tuner will be your best bet.
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WB7AVF
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« Reply #5 on: January 26, 2007, 08:50:24 PM »

All great ideas here OM.

I'm living in a New York (pronounced Neeyu Yoik) apartment. I'd been using a dipole, an end-fed.  I just finished my loop (set up for 40m). It's great! Mind you, I've got some steel fencing on one side, but clear elsewhere and height of about 90'.  

Testing with twin-lead but waiting for the ladder line.  And this feed is short (one and half storeys).

Just a thought from an empirical perspective...
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NE5C
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Posts: 320


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« Reply #6 on: January 27, 2007, 03:34:34 AM »

Hi Jim, I certainly have "no claim to fame" as an "EXPERT" but I believe I have done some pretty strange things with loops that still made me happy. I believe your Idea is "Do-ABLE" especially if you have the support of your wife (smiling here)
I took a look at the 3rd photo also and I wonder (Not to get you into a war with the wife) but - If you couldn't use the "Very Corners of the rooftop" for the supports using some good sturdy insulator supports, and using a lighter gauge, COLORED insulated wire (such as 14 or, even 18 Gauge THHN)COLORED WIRE - less able to be seen or Stand out -which would keep you more square on the over-all shape of the loop from the rooftop corners, (over trying to follow along the eaves Up-Down shape) of the roof line? You said you wanted to use a peice of Conduit to run your feed line (which I would have wished was PVC conduit "over Metal")But I understand shall fit your WANT - over using Open Wire or Twin lead which probably will, react within the metal Conduit! Coax also, may possibly be "IFFY" there with reaction, but you can try an experiment with that by hooking it up and testing the whole thing, and then if you get desired RESULTS - THEN - go with solidifying your installation and feeding through the Conduit (Hopefully not picking up REACTANCE with it)and maybe your conduit is just an Isolated section, alone - used just for this feed, as you desired. All in ALL, I'd say experiment with it and "SEE" because what you end up with may certainly "MAKE YOU, HAPPY!" As far as symetrical shape, I've used some of the most awful creations known to Man-Kind and a total LAUGH to look at -BUT- Gotten Rave Excellent Reports - right along side my friends stations - while TESTING them out. Of course you will need a good Tuner, BUT I would bet that you will come up with something that will bring you some Satisfaction, JIM. And basically you are investing "Your Time" to experiment with it and JUST SEE. Good Luck, Congrats on the new home, and my bet is - that you will get it working for you. God Bless - Jerry.
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