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Author Topic: receiving ant  (Read 486 times)
WB9JOX
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Posts: 103




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« on: December 18, 2002, 12:54:34 AM »

Im looking for a good receiving antenna to construct for 160 meters
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K0BG
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« Reply #1 on: December 18, 2002, 08:35:36 AM »

The Antenna Compendium Number 7 from the ARRL has an excelent article on a 160 m antenna. The book is well worth the $25. You can even order it on line.

Alan, KØBG
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KX7W
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« Reply #2 on: December 18, 2002, 10:31:42 AM »

Do a search on K9AY Loop Antenna on Google and you will find an excellent receiving antenna for 80 and 160.  The antenna books also cover his design.  It is a good antenna and may be homebrewed.  73
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AC5UP
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« Reply #3 on: December 18, 2002, 07:41:32 PM »

This reply is a work in progress as I'll be building a simple BCB receiving loop over the next few days.

Here's an idea of what I have in mind...

http://www.carcanada.net/dx/donloop.html

and a resource page for more ideas:

http://www.mindspring.com/~brucec/amloop.htm

Back in the days when radios glowed in the dark, it wasn't uncommon to find a 'spiderweb' loop in the base of a console radio mounted on a pivot so it could be rotated appropriately. They worked extremely well and would pull in some very healthy DX during the winter months but (like every other antenna) tended to be noise limited during the spring and summer. On the lower bands it's usually not a lack of signal that limits your RX range, it's the noise level, and a tuned loop is generally your best bet for a favorable S/N ratio.

As an aside, about five years ago I picked up an AM/FM stereo receiver that came with a small loop for the AM side that I thought was a joke... Four turns of #22 stranded wire on a 6" square frame. Talk about being wrong... The reception is absolutely amazing. A few weeks ago there was a program on KFAB in Omaha that I wanted to listen to and my favorite 'big wire' was full of co-channel interference that night. The little loop on the stereo beat the pants off the larger antenna despite being many dB down in signal strength as it was able to null out everything I didn't want to hear... I have hopes the larger loop I'm building will be several orders of magnitude mo'better, and if the first attempt works out well I have a spot reserved for an outdoor version with a rotator and remote tuning. I have this bad habit of staying up late when Coast to Coast AM has an interesting guest and some nights I need all the help I can get...

- AC5UP
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N6AJR
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« Reply #4 on: December 19, 2002, 04:02:19 AM »

half wave dipole 133 feet per side (or there abouts) coax fed, with or with out 1 to 1 balun... it works, get it up high in the center and keep the ends up as high as you can while you bend them around the edge of the yard...trim to lowest swr. it works.. and it is cheep... tom N6AJR
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AC5UP
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« Reply #5 on: December 21, 2002, 01:32:47 PM »

Here's a follow-up to my previous post...

Built a resonant loop yesterday and rode the ether with it last night. First impressions are favorable, and here's a copy of what I posted to a mail reflector today:

-------------

I took my own advise and whipped together a receiving loop for the AM broadcast band this week and gave it two thumbs up last night... Despite being indoors and no more than three feet off the ground, it's delivering
good signal levels with much less noise than the random dipole I had been using. Yes, the dipole is about two S-Units 'hotter' than the loop, but the lack of noise more than compensates for the lower signal levels. If I had the loop installed at the same height outdoors, I suspect the difference in signal strength would be negligible while the noise reduction would remain.

As it is I can peak the loop on band noise with all attenuators turned on, and that tells me I'm hearing all the way down to the practical noise floor. Adding a pre-amp won't improve the S/N ratio and while tuning around 160 last night I heard plenty of activity with some very clean copy despite the loop being out of resonance on that band. I still need to dial it in by removing a turn or two...

So... What did I build? Being reluctant to waste materials, I bought a 10' section of 1/2" thick-wall (600 PSI) schedule 40 PVC pipe and one 'X' plus four 'Tee' couplers. Using a tubing cutter, I chopped this in to four 2' sections and eight 3" sections. The four longer sections are glued to the 'X' with the 3" sections used to extend the T's mounted perpendicular on the ends. This gives a 7" spread across the perimeter with approximately 12' of wire in each turn and no wasted PVC. (BTW: I used CPVC glue as it has a slower action and gives a bit more adjustment time before it sets) I also gave the frame a shot of black enamel spray paint so it looks pro-fessional. (Hi!)

The outside T's were notched with a hacksaw blade every 1/2" with a small hole drilled through each T where the center lines cross. One T has a second hole approximately 3/8" below the first hole and away from the outside edge. These holes are used to thread a one-turn coupling loop nested inside the outer turns. I used a junk spool of 20 gauge enameled wire and ended up with 13 turns on the outside loop. A medium-value (70 pf?) padder cap was attached to the ends of the outside loop while the one-turn inside loop was soldered directly to some scrap RG-62. If I didn't have the enameled wire laying around I'd probably have opted for 22 ga telephone hook-up wire or similar. The wire size isn't critical, and there's nothing magic about the 1/2" spacing. Use what you have and choose sturdier materials if it's going outdoors. Mine has less than $10 worth of materials involved and it's moving to the attic once it's dialed in...

I can tune the loop for a clean noise peak up to 1400 kHz or so. Beyond that the padder cap opens up so much that it's out of range. I need to remove a turn or two from the outside loop. The concept at play here is a series L / C circuit being tuned to resonance. The larger the L, the smaller the C and vice-versa (in very broad terms). When the C gets too small it will limit the current in the loop and efficiency drops rapidly. The goal is to adjust the number of turns in the coil until you're using maybe 30-40 pf of capacitance at the high end of the band. On 160 the band is narrow enough that you could probably get away with a fixed silver-mica in the 20 pf range paralleled with another 140 pf of variable capacitance, and this can be in the form of varactor diodes if you want to avoid motorizing a conventional tuning capacitor. A clever guy can figure a way of using a DC block on the coax so the tuning voltage runs up the coax, although this will reduce the signal levels a bit. Since the AM broadcast band covers several octaves (540 to 1700 kHz) I'll need more tuning range in the capacitor, but last night I found the Q is low enough that I'm hearing perfectly readable signals well off the resonance peak despite being S-3 or lower. I have a hunch I'm not going to worry too much about the extreme low end of the band. If I were interested in only the CW or Phone segment of 160, I suspect a fixed value cap would be close enough, although I'd probably weatherproof a trimmer cap just so I could dial it in exactly where I wanted it.

In closing, it takes a little time to get used to seeing the S-Meter ride lower than what you're accustomed to and I'm learning to trust my ears... Around noontime yesterday KRLD in Dallas was an S-5 and barely readable in the noise on my Windom, but 100% copy with moderate noise at an S-2 on the tuned loop. Could have been a quirk in the wave angle, but whenever an indoor antenna near ground level delivers better copy than The Wonder Wire, it tells me I'm on to something... By 6pm KRLD was pushing 10 over and sounding like a local on the loop. Also, I found the loop is very useable for general SWL listening and has no problems with WWV at 10 and 15 MHz. The Wonder Wire will beat the heck out of it on HF, but the loop can be built to fit the back of a closet door and that makes it fully 'rotateable' for GP RX use... All you need is 90 degrees of swing.

- AC5UP
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