Help with Receive Only Low Noise Antenna for 40/80

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WQ6L, Jul 3, 2021.

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  1. WQ6L

    WQ6L XML Subscriber QRZ Page

    Hi Guys,

    I have a 100 ft end-fed long wire that I use on 40/80 meters at 40ft. Great transmit antenna but pretty noisy on these two bands.

    I'd love to get some suggestions/advice re: low-noise receive only antennas. I have a smallish back yard but can get creative to put up a receive-only antenna (magnetic loop or otherwise).

    Thank you!

  2. KS2G

    KS2G Premium Subscriber QRZ Page

    K0UO likes this.
  3. VK3YE

    VK3YE Ham Member QRZ Page

    A loop isn't a bad choice.

    Here's one I built for 1.8 & 3.5 MHz.

    You could have fewer turns to allow it to tune 3.5 & 7 MHz.

    If you're only interested in a narrow section of each band you could use
    pre-set trimmer capacitors and some sort of remote switch (eg with a relay
    powered by DC up the feedline, appropriately isolated from RF with capacitors
    and chokes) to change bands from the shack.

    If you've got some old coax you could use that as a loop. This one works for both
    transmitting and receiving with a tuner box at the feedpoint.

  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Dont forget that a small receiving loop lets you null a noise coming from a single, narrow azimuth. I tried a small receiving loop to null rfi from my neighbor's roof-top solar. I could null the noise from each micro-inverter, but one at a time. When I nulled one, I still received the other 7...

    I tried the MFJ noise-canceller (and more recently a Wimo). Same problem. For any given noise sense antenna position, I could null the noise makers one at time, but was never really successful in practically improving my receiver SNR on desired signals.

    Unless you have only one, dominant, noise source (unlikely), prepare to be disappointed...
    VK3YE and K0UO like this.
  5. N7WR

    N7WR Premium Subscriber QRZ Page

    Look up the EWE receive antenna. Does not take up much room
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    And ewes are not b-a-a-a-a-d.:p

    A small loop on a rotator can be a blessing. But they are directional and unless you have only "one" noise source in one direction, a fixed loop can be disappointing.

    Better, if possible, is to eliminate all the "local" RFI (noise) sources at their origins. That can take some serious work, but it really is possible in many cases.

    I'm in a big city here with hundreds of close neighbors and my noise level using a full-sized 80m doublet is about S2-S3 in a 2.1 kHz BW most of the time, unless there's a lightning storm somewhere that propagates the noise. Noise (RFI) generated by stuff in my own home seems to be nearly zero (after a lot of work), but there's some residual noise from neighbors.

    Since I work CW more than other modes, I can narrow the RX BW down to 150 Hz or so and then the noise level is in the S1 range and I can copy incredibly weak signals using headphones. FT8 is capable of copying signals right at noise level or even below audible noise level, so that might be better, but I stick with CW. I know my RX capability is okay when I call very weak stations and I'm running a kW, and they don't hear me nearly as well as I hear them.

    That's the goal!:)
  7. WA7F

    WA7F Premium Subscriber QRZ Page

    There will never be another ewe! I think that was a song once. :)

    Hey, WA7ARK. Just curious, how close is your neighbor that’s generating the inverter noise? I’ve got a couple of neighbors with new solar installations but, the closest is about a 1/4 mile. I’m getting some occasional noise but, I think it’s from an electric fence.
  8. 4X1SO

    4X1SO Ham Member QRZ Page

    You should check out Don Kirk, WD8DSB's loop. Plans are here:
    The antenna is also available as a kit from DX Engineering. There are videos on Youtube showing how it works. Put it on a small TV rotor.
    Last edited: Jul 4, 2021
    N0TLD likes this.
  9. W1VT

    W1VT Ham Member QRZ Page

    I use an array of eight flag antennas. At one time I used one as a noise or sense antenna, to pick up powerline noise as the 2nd input to a null canceller.
    Then I figured out it was easier to optimize the locations of the loops as needed to reduce the noise from the beginning.
    A lot of times I can just switch between the antennas and choose the best one.

    Just one fixed flag or loop antenna can be extremely useful if it is pointed toward Europe and oriented to minimize noise.

    Zak W1VT
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    About 250ft from my tower, and about 200 ft from my vertical. There are eight individual panels, each with its own micro inverter. The output from the panels is bussed into two 240V pairs, which then backfeed into the main electrical panel. Our houses have underground power from APS, but every foot of of above-ground in-wall house wiring at the neighbor's house used to radiate the RFI, hence the inability to null it with a loop.

    We were able to greatly reduce the interference (S3 down to less than S1) at the source by placing 2ea 240v 20A Corcom Pi-section AC-line filters on the 2ea 240V lines running down from the roof inside EMT between the inverters and the service disconnect switch. I believe that is because the RFI is now not using the house wiring as its "antenna". The filters look like this (but have a higher current rating):

    Those filters reduced the noise to barely detectable (on horisontally polarized antennas) on 160 to 40m. On 20m and higher, I can still see the noise increase by about 1 S-unit as I rotate my HF Yagi in that direction. Not everybody can do what I was able to make happen at the neighbor's house.... I can also use my hand-portable noise finder (TinySA and preamplifed small loop) to "find" that noise by walking up to it... Now, the loop is capable of nulling each individual inverter.

    My 60m to 10m vertical antenna is much more sensitive to picking up the solar system RFI than any horizontally polarized I have tried, including dipoles, my horizontal loop, and my rotatable HF Yagi (SteppIR DB-18). This is why the vertical is my worst antenna HF antenna and is not useful for DX or much of anything else... I did use it as the noise antenna when trying the phase boxes...
    Last edited: Jul 4, 2021

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