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HR receiving loop antenna - Ham Radio Library

HF Receiving loop antenna

  • Prototype 40m Loop - works great!
    Prototype 40m Loop - works great!
  • Final 40m version of the Loop...
    Final 40m version of the Loop...
  • 20m HF Receiving Loop Antenna
    20m HF Receiving Loop Antenna
  • Loop components
    Loop components
  • Completed 20m HF receiving loop
    Completed 20m HF receiving loop
Receiving HF signals at my location is a compromise situation due to the antennas I use and the high noise environment in the city. Although my transceiver has good selectivity and excellent filters, I suffer from high noise conditions that at times make receiving a chore. This is true for both weak-signal DX as well as regional chats with friends in the Southwestern US.
 
I have often wondered whether a loop receiving antenna would be a good solution to my need for better reception but was hesitant to get involved with the typical preamplifiers and monstrous dimensions most of the designs I have seen require. This all changed when I came across a website that described an easy-to-build receiving loop that does not need a preamp. This design does require an external antenna tuner to provide some preselection gain and uses your transceiver's preamp.
 
You simply connect the loop through the feeder to your antenna tuner and feed the tuner into your transceiver's auxillary antenna input connector. You peak the external tuner for maximum signal strength.
 
Remember: this is a RECEIVE only antenna - it is not intended for transmitting! If your rig doesn't have an internal antenna switchover relay you will need to exercise caution doing it manually. I built my first loop for 40/80m in order to improve reception of signals in the Southwest and inadvertently transmitted once on 75m. I was heard weakly, 150 miles away -pumping 300 watts into the loop - it didn't damage anything but you don't want to operate this way for long!
I built this loop directly from the greertech.com website dimensions (see below) and added my own wooden support design made from 1/2X3/4 inch stock from Home Depot. I first built the loops and supported them with a simple 1/2 inch PVC pipe mounting system to evaluate them. I liked this antenna so much that the final versions have a simple wood support built into my operating desk for hand rotation. I can remove the entire antenna system in a manner of minutes when I don't want to use the loop.
 
The photos here show the basic parts and how small the 20m loop really is - each wooden frame piece is 22 inches long. The feeder from the loop to the antenna tuner is 34 inches long, and the loop itself is 5.9 feet in length, per the greertech.com dimensions for RG-58/U coax. If you use a different coax type, make sure you use the appropriate dimensions in the referenced material.
Receiving HF signals at my location is a compromise situation due to the antennas I use and the high noise environment in the city. Although my transceiver has good selectivity and excellent filters, I suffer from high noise conditions that at times make receiving a chore. This is true for both weak-signal DX as well as regional chats with friends in the Southwestern US.
I have often wondered whether a loop receiving antenna would be a good solution to my need for better reception but was hesitant to get involved with the typical preamplifiers and monstrous dimensions most of the designs I have seen require. This all changed when I came across a website that described an easy-to-build receiving loop that does not need a preamp. This design does require an external antenna tuner to provide some preselection gain and uses your transceiver's preamp. You simply connect the loop through the feeder to your antenna tuner and feed the tuner into your transceiver's auxillary antenna input connector. You peak the external tuner for maximum signal strength.
 
Remember: this is a RECEIVE only antenna - it is not intended for transmitting! If your rig doesn't have an internal antenna switchover relay you will need to exercise caution doing it manually.
 
I built my first loop for 40/80m in order to improve reception of signals in the Southwest and inadvertently transmitted once on 75m. I was heard weakly, 150 miles away -pumping 300 watts into the loop - it didn't damage anything but you don't want to operate this way for long!
 
I built this loop directly from the greertech.com website dimensions (see below) and added my own wooden support design made from 1/2X3/4 inch stock from Home Depot. I first built the loops and supported them with a simple 1/2 inch PVC pipe mounting system to evaluate them. I liked this antenna so much that the final versions have a simple wood support built into my operating desk for hand rotation. I can remove the entire antenna system in a manner of minutes when I don't want to use the loop.
The photos here show the basic parts and how small the 20m loop really is - each wooden frame piece is 22 inches long. The feeder from the loop to the antenna tuner is 34 inches long, and the loop itself is 5.9 feet in length, per the greertech.com dimensions for RG-58/U coax. If you use a different coax type, make sure you use the appropriate dimensions in the referenced material.
 
I pegged the pieces of the loop support together using dowels so as not to have any metal inside or close to the loop. I made the shorted end of the loop out of an old "blitz bug" lightning arrestor which was in my junk box that conveniently has female '259 connectors and a screwhole in the barrel. Make sure you short only the braid to the center conductor to the braid and don't short out the center conductors! You could simplify things and skip the connector altogether - just solder the center conductor to the braid to the shield as greertech.com indicates on his website. My first 40m loop was built this way and it took about 15 minutes to construct.
Notice the very important gap in the shield braid on the side of the loop - this is one of the key features that makes this design work.
See the greertech.com website referenced below for the electrical details and principles of operation - this is an excellent source of information for these simple, constant-current magnetic loop antennas.
 
 
RESULTS
This simple and inexpensive loop works remarkably well for something so cheap and simple. It has given me enhanced receive performance in three areas:
  1. Nulling noise sources and interference - Orienting the loop so that the plane of the loop is directly aimed at a noise source (and it works for noise sources either near or far, it doesn't matter) allows you to essentially remove the noise. The nulls are sharp, meaning a slight rotation of the loop will pass through it if you're not careful. Of course, if the noise source is in line with the bearing to the signal you want to receive this is not going to help - but in cases where the bearings are different it is a significant help. I am now able to eliminate some sources of noise that have plagued me for years.
  2. Peaking desired signals - Aim the plane of the loop to maximize the desired signal and other signals are reduced in strength (unless they are on the same bearing). This loop receives a bi-directional figure-8 pattern, where the peak is in the plane of the loop and the null is broadside to the loop. This little antenna has excellent directivity - I have enjoyed orienting the loop for peak signal strength on both DX as well as US stations and for the majority of cases I can determine the Great Circle Bearing within 5 degrees of the actual bearing. If you enjoy RDF (Radio Direction Finding) pursuits on HF, this is a neat way to do it. There are instances where signals don't show much directivity - I have especially noticed this on Near Vertical Incidence Skywave signals.
  3. Lower noise floor - The loop delivers a lower strength signal than a conventional antenna so you need to compensate for this with your transceiver's preamp (or use an external preamp after the antenna coupler and in to the receiver) to bring the signal back up. By doing this in combination with reducing your RF gain and using your AF gain to bring up the signal for comfortable listening you will have reduced the noise floor in your system and will enjoy quieter receive performance. Of course your S-meter won't read like it normally does, but that's a slight penalty to pay when you can hear signals without so much noise!

 

I am pleased with the improvement this loop has provided in my receive capability. I am able to copy some weak-signal DX I previously could not copy at all and I can now listen on the lower bands with the 40/80m loop and enjoy improved noise conditions.

 

Copyright pictures and text to K7ZB