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The 160 Meter Loop |
Greg Ordy
I have another web page which describes an 80 meter receiving loop that I have used over the last few years. That loop was actually built several years after my first 160 meter loop. The 80 meter loop was built because I felt that the 160 meter loop was useful, and worth the effort. The 160 meter loop, due to the length of the loop perimeter, is a physically larger project. After almost 10 years out in the yard, the loop needed to be rebuilt. The weak links in this implementation are the wooden dowels that act as spreader arms to support the loop. Although the dowels are made of hardwood, and have been stained to add protection, they simply rot and break after some number of years. The length of service will probably depend upon the local weather. Since I was rebuilding the loop I thought I would take a few pictures of how it's put together. I suspect that if the wood was replaced with a material such as fiberglass, the loop would last a very long time.
This loop is in the shape of a hexagon. The signal output of a loop increases as its area increases. For a given length of wire, the greatest area is contained when the wire is formed into a circle. Since perfect circles tend to be hard to construct, they are approximated with other geometric shapes. The more sides on a polygon, the greater the area for a fixed perimeter. The area of a circle is the perimeter squared divided by 12.56 (4 times PI). The area of a hexagon is the perimeter squared divided by 13.86. The area of a square is the perimeter squared divided by 16. Finally, the area of an equilateral triangle is the perimeter squared divided by 18. As the number of sides decreases, the area also drops.
A few words on loop performance, in my opinion. When I first experimented with using a small receiving loop, I was very impressed with its performance. My transmitting antennas at the time, both on 160 and 80 meters, were verticals. Verticals have a reputation for being noisy on reception, and the loop seemed to pair up well with the vertical. As I have used a few more alternative receiving antennas, my perspective on the loop has changed, and it has lost some of its value.
The loop is not very effective as a DX antenna. This is because the loop response pattern is straight up. DX signals usually arrive at much lower take-off angles. The loop has very low signal output. A preamp can be used, but I tend to follow the conventional wisdom that says if I can hear background noise, then a preamp will simply increase the noise and the signal by the same amount, so why bother?
A loop does have two sharp broadside nulls. These nulls are at a very low angle. The nulls can by used to attenuate specific local noise sources, or, for direction finding. I used this mode once with an 80 meter loop, when I was trying to locate a noise source.
For me, the loop has become what I call my summer ragchewing antenna. The high take-off angle is perfect for local NVIS communication, and the improvement in RDF (receiving directivity factor) helps reduce ear fatigue, especially during the high static summer months. This is when paired with a vertical for transmitting. If the antenna alternative is something like a dipole or inverted vee, then the loop offers less value, in my opinion. I noticed this value reduction when I put up my own 80 meter inverted vee. I had also done some other experimenting with using a long dipole a few feet off of the ground as a receiving antenna, and that worked well for local signals on several bands.
If you are interested in a receiving antenna which is also a DX antenna, then I would suggest trying a K9AY or Flag/Pennant before a loop. Those antennas will also operate on several bands, whereas the loop sharply tunes to a single frequency on one band. The RDF numbers presented on another page tend to follow my own subjective evaluation of receiving antennas. Like so many things in life, build one of each and find the best solution for your situation.
The perimeter length of the loop is 24 feet, and it is in a hexagon shape. Each loop support arm is 4 feet long. Four of the arms are made from 1/2" wooden dowels. The remaining two arms are part of the vertical aluminum tube which supports the entire antenna.
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The 160 Meter Loop |
The antenna schematic is identical to the 80 meter loop schematic shown on another page. The loop itself is 24 feet of 0.405 inch diameter coaxial cable. This is a shielded loop. The cable braid is opened at the top of the loop. The loop is resonated at 1.850 MHz with approximately 350 pf of capacitance. A trimmer capacitor adjusts the resonant frequency. A ferrite-core transformer is used to step up the low loop impedance so that it is a match to the 50 Ohm transmission line. The
Connect the antenna to an antenna analyzer. Set the analyzer to the desired frequency on the 160 meter band. If your analyzer provides antenna reactance (X) data, adjust the trimmer for resonance, that is, reactance equals zero. If not, adjust the trimmer for the lowest SWR. If you are using the matching transformer, you should be able to achieve a 1:1 SWR. I add or subtract turns on the secondary until I get a 1:1 SWR (once I am at resonance). If you are not using the matching transformer, then the point of lowest SWR is probably not the point of resonance, but they should be close.
You can also adjust the loop without test equipment by peaking the noise level at the desired frequency.
Assuming the addition of a transformer to match the loop to the coax and provide a 1:1 SWR at resonance, the 2:1 SWR bandwidth was measured to be 36 KHz. Once you get approximately 50 KHz away from resonance, the loop signal (and noise) output will drop by several dB. This is not really a problem, as most all receivers have more than enough gain to compensate. Still, if you want maximum output from the loop, you should adjust the resonance point to your desired frequency.
It is almost a necessity to use a radio that has the ability to accept a separate receive antenna. Fortunately, this feature is becoming standard on most all recent vintage radios.
There are a number of ways to mount the antenna. The 80 meter loop was small enough that it could be used inside as well as outside. This antenna is so large that I can only imagine it being mounted outside. I have strapped the loop to deck railings, and hung it from a tree. It is usually mounted inside a pipe which I buried in the ground. This pipe mount allows the loop to be rotated, and I have several small antennas which can be dropped into the same pipe when I want to change the antenna lineup. That mounting location is in the middle of a small ground radial system consisting of 8 radials each 10 feet long. I connect the radials to the loop ground when it is mounted in the pipe. Having the ground radial system certainly does not hurt the loop performance, and from time to time I get the sense that it helps increase the signal output. It's not a big factor with a loop.
These loop antennas make good reference antennas since they are small and inexpensive. If you have a specific local noise source, you can point the null at the noise, and really knock down the noise. They are also about as a small as you are going to make an antenna for 160 meters.
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