Active loop antenna for HF

I’ve got an ever increasing noise floor at my QTH so I thought I would try an active HF loop to see if I can null out the worst of the noise.

I found a simple design on-line and used it as the basis of my loop. The original site unfortunately no longer exists but I did manage to save the design here.

Loop-schematic

I followed the design but I placed 2 sockets on the board for the transistors as I wanted to try different combinations. I don’t have many pictures of the build but I used a waterproof box, a section of 15mm PE-X pipe with a thick wire running through it for the loop and filled the completed box with epoxy to hold everything in place.

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The loop is 1M in diameter. The amplifier is powered via the coax using a simple bias T.

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I’ve been testing the loop in the garden on the top of a 2M pole for the past few months. So far I have tried several transistor combinations. The original suggested BC547, BC549, 2N4401 and some 2N2222A. The 2N222A seem to be the best choice. They offer the lowest noise floor with good intermod and strong signal handling.

Over the next few days I’ll be installing the loop in the attic and attaching it to a rotator.

9 Replies to “Active loop antenna for HF”

  1. Hi Matthew, How has the loop performed over the last year? Any tips if it has to be located near your Tx antenna?

  2. Hi Paul.

    It’s been working great on 20M and below all the way down to 160M. I’ve not tried it above 20M yet as I don’t venture up there that often. Recently i’ve been using it for a bit of AM broadcast DX listening and it’s been performing great.

    I don’t TX with the loop in line so I can’t tell you how it performs next to a transmitting HF set but I can tell you how I use it. The loop is located at the bottom of my garden. It’s mounted on a 6′ pole with a small cheap rotator and fed with RG58 coax (i’ll get some pictures when i’m back home in a few weeks time). It’s around 50′ away from my building and about 20′ away from next door. My doublet antenna runs directly overhead at around 25′ and my horizontal loop runs around the building at 30′ high, 50′ away. I run the loop into the RX only antenna input on my TS-590 so the RX input is disabled when transmitting. I’ve not had any problems with it and can use it to putt stations out of the local noise that I can’t even hear on the horazontal loop or the doublet.

    If you want to use it with a seperate receiver whilst transmitting them maybe a relay activated coax switch would be a good idea. That way you could use the HF TX to activate the relay and switch the loop to ground when transmitting. It is a very broadband amplifier so you may have some overloading issues if a 100W TX is sitting next to it! Let me know if you build one and how you get on with it.

  3. Hi Matthew,

    Thanks for the update and great to know it has been a great peformer, especially on 30, 40, 80, which is my main interest (local noise is worst on 40 here).

    I have two plans – one if for my home QTH where I will build 2 and mount them 90 deg heading offset.The pattern seems to be wide with sharp narrow nulls off the side, so I think 2 will work ok like this. At home I am QRO and am concerned about damage to the transistors in the pre-amp. I will do as you say, use a relay in the bias tee to pull the 12v when on Tx. I’ll add a relay in the pre-amps to short the input when there is no DC supply. This should be ok.

    The second one will be a portable station for an upcoming DXpedition. On this we will run three stations concurrently, pushing powers of 400/400/200 watts. In this case second/third transmitter interference will be an issue, so I think all we can do there is run the BPF on the Rx loop coax and see how it works. It will be a good discovery if nothing else.

    I’ll let you know how I get on.
    73,
    PAUL

  4. I like the sound of that Paul and it sounds like it will work OK. I’ll pop my loop through one of my 5B4AGN BPF into a seperate RX and see what happens when I TX on another close band when I get home. Will be interesting to know what happens.

    I’ve got my eye out for a secondhand Wellbrook loop to test it against to see how they perform side by side.

  5. Hi.
    My old wellbrook loop failed a few years ago. I came across this circuit at the time and built it into a waterproof box and used toe old loop.
    Performance is as good if not better than the Wellbrook when it was new

  6. That’s great to know Trevor. It certainly seems to be doing the job here. I’m going to build a batch as part of our club’s buildathon so I may get some custom PCB’s made to make them quicker to knock up.

  7. Hi Matt,
    If you get custom PCBs made, put me down for 6. I’ll pay the share. I have a couple to build. and the PCB would make it easier.
    Cheers.

  8. I’m going to create a PCB via OSHPARK in Portland, OREGON if this circuit seems reasonable – super affordable…

    Looks like a decent circuit — and I am a veteran WELLBROOK user from ago! I have 4 ALA100’s and 2 FLG100’s

  9. Thanks Colin.

    Also, take a look at the “WellGood” from George M1GEO. A great reverse engineer of the well known Wellbrook antenna.

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