Receive only loop vs. small magnetic loop

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KX2P, Nov 10, 2018.

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  1. KX2P

    KX2P Ham Member QRZ Page

    Hi all,

    I've moved to an unfortunate QTH ( a high rise apt on the 17th floor with a tiny balcony), where I can't take the chance of transmitting (fear of being kicked out), and can only enjoy the hobby by listening, for now. What do you guys think would hear better on the HF bands ... a broadbanded small receive-only loop or a small tuned magnetic loop that is designed for both receive and transmit? Which one would be less vulnerable to local noise, and which one would, in general, hear better? Thanks.

    Jeff Frank
  2. VA3VF

    VA3VF Ham Member QRZ Page

    The untuned is less 'hands on'. The tuned one will be more selective.

    The other decision is: active (amplified) or passive. If your QTH is quiet, consider an active loop. For TX, don't forget a TX/RX switch if going with the active.

    I would go with a TX/RX loop. There is always the possibility of operating from a location other than your QTH.
  3. NH7RO

    NH7RO Ham Member QRZ Page

    Neither one would be better than the other, basically. If you were to transmit that's where the ohmic losses really factor in big-time (hence far superior construction practices and materials need be a factor).
    N0TZU and WB5YUZ like this.
  4. KQ9J

    KQ9J Premium Subscriber QRZ Page

    I'd go with the small tuned loop. You might find some success transmitting at low power levels with FT8 or CW. Won't be a blowtorch but you could potentially have some fun and likely remain undetected.
    K4BAD and N0TZU like this.
  5. VA3VF

    VA3VF Ham Member QRZ Page

    For reception only, passive untuned loops are easy to build. Try that first. A quick web search will returns many hits.

    The tuned ones are not that much more difficult to build. The capacitor is key. Again, Google is your friend.

    For TX, the following has good reviews:
    WB5YUZ likes this.
  6. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    I agree.Build your own small receiving loop. Receiving loop construction is simple compared to transmitting loop construction! But read this first; some of the designs out there are not well-thought out and will introduce common-mode noise through the coax. W8JI has some advice on keeping the system isolated from common-mode noise:
    Last edited: Nov 10, 2018
    N0TZU and NH7RO like this.
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    I recently built two receive-only loops, one for 40m and the other for 160m. Pictures at the bottom of my qrz page. Both are remotely tuned using a split-stator capacitor, which is mechanically coupled to a dc reversible gearmotot. This tuning method would let you transmit only at very low power without arcing over. To allow transmitting at ~100W, I would have to replace the capacitors with vacuum variables.

    The advantage of tuning a loop like these is that it produces a signal level that the receiver can deal with, without having to have a preamp. An un-tuned loop must have a preamp to be useable.
    The Q of a tuned loop like this is extremely high, so you have to retune when making even a 10kHz QSY.

    This size of loop, when placed in the clear as shown in the pictures, will receive any signal I can receive on either a 50ft high 40m inverted V or 40ft vertical with no active amplification, albeit at about -10 to -20db in absolute signal strength. The 160m loop receives any signal I can tune on my 558ft horizontal 1 wave-length loop which is ~50ft agl.

    I was hoping that I could use these loops to null out some RFI coming from my neighbor's house, but the noise is radiated from all the wiring in that house, and even though these loops have extremely deep nulls (-50db), the null is so sharp (narrow angle), it is not useful in nulling the noise which seems to come from multiple angles.
    Last edited: Nov 10, 2018
  8. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    And, if the system is designed for a 50 ohm match, tuning for lowest SWR with very low power out would allow you to quickly and easily peak a receiving loop remotely! I use this method for peaking receive only antennas with a common SWR meter and tuner in the shack. Not as impressive as using a computer to tune, but quite effective nonetheless.
    NH7RO likes this.
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    I am using a IC7300 for receiving, so all I have to do is look at the waterfall display to see where the loop is tuned. As you run the tuning motor, the waterfall peak follows. The excitation is just the received noise.

    Both my loops are matched to 50 Ohms. I used my RigExpert to get a SWR50 better than 1.1 at the SWR null. The matching system is a transformer, using a large ferrite core (similar to what would be used in a HF balun). The transformer primary is the loop conductor passing through the core window (single turn). The secondary is 14 turns on the core, which feeds the 50 Ohm coax going to the receiver. The optimum turns radio was experimentally derived by adding/removing secondary turns while sweeping the SWR using the RigExpert.

    btw- my matching system is better than any of ones that W8JI shows on his loop page. My loop has the gap at the top (as it should for best balance), but I found a way to put the motorized tuning capacitor at the bottom of the loop. The loop itself is made from one piece of copper heliax, hence the high Q.
    Last edited: Nov 11, 2018
    NH7RO likes this.
  10. KK5JY

    KK5JY Premium Subscriber QRZ Page


    For SWL use, you will like this design much better.
    NH7RO and W7UUU like this.

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