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Author Topic: Best low cost antenna for a school rooftop?  (Read 1074 times)
WP4NXA
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Posts: 48




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« on: January 22, 2009, 03:01:03 PM »

Hi. I'm looking for a HF multiband antenna that would be practical for a school rooftop (flat) that'll cost less than $100 with minimal installation.

Note that the rooftop to the station is a very short distance... probably less than 30ft of feedline.

I've been reading about the G5RV but I get the impression that the invert v configuration (space saving) isn't as promising.

Thanks.
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N4NYY
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« Reply #1 on: January 22, 2009, 03:10:29 PM »

Don't run it in a inverted V. Run it straight.
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K1BXI
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« Reply #2 on: January 22, 2009, 03:49:01 PM »

You didn't say what kind of support and how high above the roof you can get the center. Since this is a school and a public building what ever you do will have to meet the approval of the powers that be. And be aware of any liability you may encounter.  

The G5RV is no different than a coax fed dipole as far as out straight vs inverted V. No less than a 90 degree angle at the apex though with the feed line at right angles to the wire and the G5RV will work on 80, 40, 20 and 12 meters well. It's not exactly an ALL band antenna. Mine serves me well in the limited space that I have.


Most any wire dipole you could make yourself at less expense than a commercial one and probably work as well. Good luck.

John

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KB9CRY
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« Reply #3 on: January 22, 2009, 03:57:06 PM »

Homemade vertical
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N3JBH
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« Reply #4 on: January 22, 2009, 04:16:01 PM »

Wow !!! you aint asking alot. needs to be multiband, cheap, easy to install, and use a small space. Maybe a dummy load fit the needs Ok kidding. Maybe you can find a used Hustler 6 BVT that be best i could think of.
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N6AJR
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« Reply #5 on: January 22, 2009, 05:01:10 PM »

fan dipole, make your own, bend the ends if needed
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N4JTE
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« Reply #6 on: January 22, 2009, 05:22:02 PM »

If you are trying to demonstrate radio during school hours, daylight, run the longest flat top you can and feed with 450 ohm ladderline, not sure if you have a tuner for open line but you will get more bang for the buck with lousy conditions.
Good luck, keep us posted.
Bob
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KI4VEO
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« Reply #7 on: January 22, 2009, 05:32:11 PM »

A full wave loop would be my suggestion.  Square or delta (3 sided) configuration.  About 260 feet circumference, with a decent tuner and some open wire feedline will give you full coverage from 80 through 10 meters.

Low receive noise figure and pretty easy to tune.  I've used my horizontal delta loop for QSO's thoughout Europe, South America, the Caribbean and most of the USA...all with 150 watts, or less.  Even had a QSO a few times into Australia on 40 meters.

I used egg insulators, AWG 14 stranded wire from Home Depot, and string trimmer line for support.  Up 50 feet, supported by 3 trees, and some homemade 600 ohm open wire feedline.  It's suffered through all sorts of weather here in Virginia without a single problem.
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N3OX
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« Reply #8 on: January 22, 2009, 08:01:26 PM »

I would recommend a pair of dipoles, maybe with a common feedpoint, as high as you can get them, cut for the 20m and 17m bands.

You don't need 15m and up right now... they're dead.

You don't need 40m and down ... you're going to be operating in daylight.

30m is cool but it's all CW

So that leaves 20m and 17m, so you should do those and do them right.  They're cheap, simple, and high performance for the cost.  You don't have to worry about using a tuner... at most you have a switch.

You can add antennas for other bands as the sunspot cycle improves.

A G5RV or the longest flat top you can put up or a full wave loop is just more wire than you need up there, in my opinion.  Maybe your goals differ from what I'm imagining, and I don't know KP4 propagation, but I'd think you only really need a couple of routinely open bands.

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

Monkey/silicon cyborg, beeping at rocks since 1995.
N3JBH
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« Reply #9 on: January 23, 2009, 04:28:52 AM »

"30m is cool but it's all CW"

I would not say that. It has digital modes such as PSK and RTTY as well.   Jeff
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WA3SKN
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« Reply #10 on: January 23, 2009, 05:26:53 AM »

OK, flat school roof, $100 budget, and Puerto Rico wx, and daytime operations.
The answer will be something home made.
I prefer a loop with ladder line and a tuner.  Lightning protection could be an issue.
Fan dipoles are always a possible solution.
And a homemade vertical certainly has merit and a small footprint.  And Dan is right about 20 and 17 meters being the bands to design for!
How well is the roof grounding? And how large is the roof? For 20 meters you need about 33 ft for 1/2 wavelength-16.5 ft for 1/4 wavelength.  For 17 you will need 26 feet for 1/2 or 13 ft for 1/4 wavelength.  Is there anything on the roof to mount a vertical on?

-Mike.
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KB8NDM
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« Reply #11 on: January 23, 2009, 05:27:45 AM »

Sure, but that just looks like error-prone IM. Smiley
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KB8NDM
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« Reply #12 on: January 23, 2009, 05:28:19 AM »

of course I meant PSK31 and RTTY.
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N4OGW
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« Reply #13 on: January 23, 2009, 08:19:03 AM »

I have been involved in a several university ham clubs with flat-roof antenna locations...I would avoid dipoles, loops, and other wire antennas if you want to operate on lower freq bands (40m,80m). Flat roofs almost always have plenty of metal in them, and you will have a hard time getting a horizontal antenna to work properly unless you can get it very high above the roof (50+ feet).

So a multi-band vertical would probably work better. It is of course more expensive than a simple dipole, but I bet you can find a local ham who is willing to donate one.

For the higher HF bands (20,15,10) this is less of a problem, as it is easier to get the antenna a reasonable distance above the roof in terms of wavelengths.

Tor
N4OGW
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N5LRZ
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« Reply #14 on: January 23, 2009, 11:39:51 AM »

If you are looking for a commercial antenna, how about looking into the trap dipole for 10\15\20\40 meters sold by DX Engineering manufactured by Hustler.  It costs just over 100 bucks for the 4 band model.  

Personal experience speaking, you may be tempted to buy the 5 band with 80 meters but overcome the temptation.  The 80 meter is nothing more than a big loading coil with a little stinger--in other words not very broad in useable frequencies.
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