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Author Topic: Need antenna advise  (Read 3323 times)

Posts: 2

« on: May 02, 2003, 01:04:59 PM »

OK, friends and neighbors help me out here.
I have about 50 Ft. between two trees on my property.  I want to install a longwire antenna for SWL.  Now, I would also like the longwire to be about 130 Ft in length.  

Here is my question, if I take a 50 Ft run across from one tree to the other and then around an insulator and run the same wire back to the point of beginning and about 6 inches under the first run will that be approximately the same as running it out 100 Ft. in a straight line.  If there is a problem doing it in this manner; how else could I get 100 Ft. with just two trees 50 Ft. apart.

Come on now I know someone can answer this question.
Thank you,

Posts: 21764

« Reply #1 on: May 02, 2003, 06:12:47 PM »

That won't work very well.  If running the wire back and forth to add length was a good solution, then we could start out with two poles separated by 12" and run the wire back and forth 100 times to make a 100' long antenna.  Or maybe two popsicle sticks one inch apart, and wind it with 1200 turns of wire.

There's a difference between a resonant antenna and an efficient one.  You can coil up a bunch of wire and tune it with a capacitor to resonance, and have a perfectly non-reactive circuit, and even adjust it to be whatever impedance you wish, but that doesn't make it an antenna.

In your situation, I'd ask or propose:

-You have two trees separated by fifty feet; how far is either tree from your house?
-If one tree is 50' away from the house, then you have all the space you need for a 100' long antenna, starting at the house.
-If not, you can make the wire into a loop and run it around the perimeter of your property, or around the roof of your house and then to tree #1, and then to tree #2.

All sorts of possibilities.  Not that 100' is a magic length for a wire antenna, especially for shortwave receiving.  100' is 1/4-wavelength at 2.34 MHz, so worked as an end-fed antenna against a good earth ground, a 100' wire antenna will work best at 2.34 MHz.  It will have a very high feedpoint impedance at 4.68 MHz, and might be difficult to match there.  Then, it will work well again at 7.02 MHz, and will be difficult to match at 9.36 MHz.  It's not a magic length.  Actually, a much shorter antenna wire might work far better for higher frequency reception.

Thankfully, a "receiving" antenna is not as critical an application as a transmitting one; this is because the receivers are very forgiving about impedance match, and as long as you plug in an antenna and hear atmospheric noise, you're usually doing okay.

Running the end of the antenna wire into the house (no coax, no feedline, just the wire), and connecting it to a small unbalanced antenna tuner that's also connected to a good earth ground will usually result in fine reception over a broad range of frequencies, and the tuner will help match the antenna to the receiver (by adjusting it every time you change receiving frequencies).

That's likely the best combination, regardless of antenna wire length.



Posts: 293

« Reply #2 on: May 02, 2003, 10:04:48 PM »

For SWL purposes (since modern radios are so sensitive), a 100 ft longwire is usually no better than a 50 ft longwire. Better to place or orient the antenna to make it resistant to noise sources, and simply as high up as possible.

Radio Nederland has always had good advice about SWL antennas:

Posts: 2

« Reply #3 on: May 02, 2003, 11:23:39 PM »

Thanks, Fellows, I will probably just go with the 50 Ft. longwire.  I have the Grundig Satellit 800 Millennium and I just think the outside antenna will be some better.
Thanks again for the info.

« Reply #4 on: May 10, 2003, 12:12:06 PM »

Some other antennas to look up on Google for really great SWL-ing (or amateur radio receive only antennas):

K9AY Loop
Magnetic Loop
Delta Loop
Inverted Vee (a way to "cheat" on space)
terminated folded dipole

Some of these use a terminating resistor that precludes it from being a transmit antenna. Some of these are very broadbanded and omni-directional. Most require some kind of tuned circuit or tuner so they can be resonant on the frequency you are receiving.

With receiving - height also isn't as critical as when transmitting.

Posts: 1

« Reply #5 on: May 16, 2003, 07:22:07 PM »

Go with a horizontal loop that will go the entire distance of the circumference of your property.

Posts: 9930

« Reply #6 on: June 08, 2003, 08:26:52 PM »

You could also use a dipole with the ends drooping down, and use a tuner to work it..

Posts: 352

« Reply #7 on: June 12, 2003, 10:10:12 AM »

I agree with ADVANCED.  Go with a horizontal loop.

What you would be creating is a feedline and it would cancel signals hitting it the way twin lead or ladder line shields itself due to the proximity of conductors.

Go with a loop and make the largest area inside the loop that you can.  Consider a tuner, even though you are not transmitting.  A tuner acts somewhat like a preselector and will help tune the system.

John Pawlicki, K8AG
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