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Receiving loops - part I. (by Merv, K9FD)

30. prosince 2006 v 19:07 | Petr, OK1RP |  Antennas
Hi,
one of the best chance how to improve the receiving performance in suburban areas where is not possible to install Beverages or even Delta etc. is making the receiving loop.
Which design, which size, how to make it, is the pre-amp needed and other FAQ I will try to reply in an articles about the receiving loops as follows...
Be sure that I tested all of them step by step and performed very well. I will try to put some comments for each nor let You know how it worked in my QTH.
The K9FD Receiving Loopby Merv Schweigert, K9FD

Editor's note: Merv Schweigert, K9FD, has been working jusl about everything on the 40, 80 and 160 meter band" this season. In this article he shares some of his experiences with receiving antennas. lf you live on a typica! suburban lot you might want to read this article carefully as you won't need forty acres to use this antenna. A few months ago I lost access to my faim neighbor's corn field that I used to utilize for Beverage antennas in the winter months. In the past I would erect a few terminated Beverage antennas towards the east and south to enhance my receiving capability on the low frequency bands. After losing this superior receiving capability, I set out to find some kind of replacement system that would again allow me to be viable on the low bands.
Early Receiving Antenna Trials
My first attempt at improved reception was a low dipole for 160 meters. This antenna was installed at the 8' level. After trying it for a few months, I abandoned it as copy was typically better on my transmit antenna, an inverted L at 75'. The search for Q5 copy continued.

My next receiving antenna project was a 2 wire Beverage that is described in the Beverage Antenna Handbook, written by Victor Misek, W1WCR. This antenna is 238' long with the wires about 6' off the ground and utilizes a tuning network that changes the phase angle to null the noise off the rear. This antenna worked fairly well, to the south, but because of some power line noise to the north that always seemed to surface as I was trying to make a DX contact this system was also setaside.

The third attempt at improving my receiving capability was to try a receiving loop. My information said that the loop has a terrific null off of the side and if shielded can be quite effective. The first receiving loop I tried was made with RG-59 coax, as recommended and it did work rather well. You could rotate the loop and null the noise down to zero and still hear signals. The problem that I encountered with loop nr.l was that the signal strength was low and a preamp would be required to correct this condition. When you run a kilowatt in close proximity to a loop you can imagine what will happen to the preamp unless switching considerations are taken. This switching concept is certainly possible, but a more elegant solution with less complexity was tried.

The next loop I constructed was similar to the RG-59 loop except that I used 3/4" 75 ohm TV hardline in the construction. This cable has a much larger center conductor. I am very much impressed with the results. The signal gain is much better than the RG-59 loop and no preamp is required. This loop works well on nulling powerline noise and can also help you null out the noise produced by approaching thunder storms.
Update: Some readers complained about the loop wiring. The wiring is as just the same, as in other loop antennas. The coax braids are connected via the small metal box. I suggest to mount PL-259 connectors on each end of the hardline, if a 1/2" hardline is used, it is quite easy. Mount 3 pieces of SO-239 receptacles into metal box and finished!
And what new is on this antenna? Such loops are mostly made of a length of RG-58 on a wooden cross arm holder. This loop is made of TV hardline and is nearly circular. The result is much stronger output, therefore no preamp needed!
Constructing the Loop
This receiving loop is constructed from a 20' long length if 3/4" diameter 75 obm cable TV hardline. Form the length into a circle and using a pipe cutter, cut through the shield at the top center of the loop in two places about 1" apart. Then, using a hacksaw, carefully split the 1" section from the hardline and peel it off. To strengthen this gap, place a 6" long piece of (split) 1" PVC pipe over the gap and secure it to the cable using a couple of hose clamps.

You can house the trimmer capacitor in either a metal or plastic enclosure. Use a grid dip meter to tune the loop for resonance on the band of your choice. I used a 400 pf. fixed mica capacitor with a 400 pf trimmer in parallel for the system at K9FD. Conclusion - In using this loop I sometimes see some directive properties on DX signals. Some publications say that the loop is omnidirectional to sky wave and only has directional properties to local noise. I have no answer to this as it is not apparent on every DX signal. What I do know is that the use of the 3/4" diameter 75 ohm cable TV hardline for constructing the loop receiving antenna makes a huge difference.

You will also find that the hardline loop will require less of a support structure. When placing your loop in your yard try to locate it as far away from your transmit antenna and other structures as much as possible.

This antenna is not a substitute for an 80 acre Beverage farm, but if you are on a small lot and are plagued by power line or other man made noise this receiving loop will improve your receiving capability on the low frequency bands. You won't work all of the DX, but you will get a much bigger share than before.
73 Merv, K9FD
(orig.published in The Low Band Monitor in Nov 1994)
...will be continued
 

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