receive antennas only

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB9MZ, Mar 8, 2011.

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  1. WV6U

    WV6U Premium Subscriber QRZ Page

    I have a noob question about receive antennas. Is it a common practise to use them for regular operation? Almost on every other QSO, the operator is complaining about noise on the band so I am curious as to why HF receive antennas are not as widely offered by antenna manufacturers or mentioned in operation books. For example, in the ARRL handbook, I don't recall seeing anything like install a RX antenna for lower noise and use another antenna for TX. While this approach seems more logical to me, since I live in an apartment surrounded by all sorts of noise. And, is a small loop a good RX antenna to start with for apartment usage?
     
  2. W0BTU

    W0BTU Ham Member QRZ Page

    That should be a worthwhile project, and should work for you from 80 through 20.
     
  3. W0BTU

    W0BTU Ham Member QRZ Page

    That is a very good question. It should be, but special receiving antennas are used by a very small percentage of the total ham population. However, ~98% the people who score well in contests on 160, 80, and 40 are ALL using them. The remaining 2% are using a switchable-direction phased array or rotatable beams; and I'll wager than even then, most of that group has them.


    That's a very good observation. I don't know quite how to answer that, but I guess it's just not presented like it should be in the books.

    In Low Band Dxing by ON4UN, it IS presented. But I don't think the ARRL makes a strong enough case for it.

    Frankly, one of the reasons I created http://www.w0btu.com/Beverage_antennas.html was to help spread the word about receiving antennas.

    Perhaps. Sometimes. I once used a small loop, but I built it to null out my ham neighbor who lived across the street, when both of us operated on the same band. It does not work anywhere near as good as other types of receive-only antennas, such as Beverage, the K9AY loop, flag, or arrays of small verticals.

    In fact, a loop inside your house will probably only pick up more noise from various electronic equipment, as compared to an outdoor dipole or vertical. But it seems to work for N3OX; check out http://www.n3ox.net/projects/rxloop/. Dan has another, better one outside (http://www.n3ox.net/projects/flag/) and I'll bet he'll tell you that works a heck of a lot better than the indoor loop.

    There are companies that sell Beverage antennas and arrays of phased short verticals.
     
    Last edited: May 6, 2011
  4. W8JI

    W8JI Ham Member QRZ Page

  5. K3ROJ

    K3ROJ Ham Member QRZ Page

    The only way I was able to decrease the static crashes on 160 meters was to use a closed horizontal loop with loading coils since the loop is not resonant by itself. I run dual diversity reception with a Flex 5000A and it's second built-in receiver. This setup can also be used for full breakin by using clamping diodes on the Flex 2nd receiver antenna port. Contest stations who typically run 35 wpm or more are easy to work with QSK (full breakin)
     
  6. N3OX

    N3OX Ham Member QRZ Page

    It's a little apples to oranges. I made the mistake of writing all my website articles in the present tense... they don't end up reflecting the history of my station. I don't live in an apartment anymore. I'm renting a house in the same town and that's when I put up the flag.

    I've edited that loop page now to tell more of the history and should go back and fix some other old ones. But the long story short is this: I eventually figured out that the main noise that was killing me on 80m and 160m in the apartment was my desktop computer in the other room. When I was living in the apartment I just turned it off after I discovered this. I fixed that computer recently because my wife wanted to use it for some things while I was operating 160m. I think they just left all the line cord EMI suppression components out of the power supply because a Corcom line filter in an external box took it from S9+10dB noise on 160 down to the point where I couldn't even hear whether it was on or off.

    I would say the two take home things I learned from apartment living and what I've learned since were these:

    1) The worst noise, the one I really needed the loop for, was my own fault. Always best to find and fix problems you have control over first. The loop actually helped me find the noise as I realized my "null pointing" directions as I moved the loop around the living room were triangulating something really, really close by. But once I shut down that computer, my doublet and my loop were about the same for reception. Still a lot of noise from others' stuff but it wasn't totally overwhelming, and it wasn't too different on the two antennas. (it shouldn't be if neither is picking up lots of near-field noise)

    2) It pays to be nearly paranoid about common mode currents when you're trying to receive in a harsh noise environment. Pretty much anything metallic that was hooked into the building seemed noisy. When I moved from a wire fed against my balcony rail to a doublet, that greatly improved the noise. The more I choked the feedline on my doublet to make sure it floated from the building, the better the noise got. I should have replaced or augmented the balun in my tuner. It was a 1:1 current balun but not quite up to the task I was asking it to do.

    I thought maybe I'd ground the tuner (http://n3ox.net/projects/servo/) case to the railing, figuring the balun would be enough, but when I connected a ground wire from the case to the railing, the noise would go up a pretty noticeable amount (probably 6dB+). So I left the tuner case floating and added extra chokes on the coax and control lines.

    I think a magnetic loop antenna even if just for reception could be a pretty good idea for lots of apartment dwellers, but I think the REASON why it works well is summed up in this quote from Tom's page on noise:

    I think a lot of people who build magloops, flags, and other well-decoupled antennas for the low bands probably see this a lot. There are a lot of people who seem to really get blown away by the performance of their flag, but if you look at Tom's site on RDF you'll see that there's not a huge advantage over a vertical. This is borne out here under the right conditions. My TX antenna is a straight-up vertical and if I attenuate it down to the level of the flag and listen in stereo on both, the difference can be very, very subtle on a quiet night. On a stormy spring night when storms are to the SW and I'm listening to Europe, the F/B of the flag can make me feel like I'm in a little bubble of quiet. But on a normal night with noise coming from all directions, my flag is probably only a few dB better signal to noise than my vertical, consistent with the prediction. On a quiet night I NEVER get any kind of light switch effect listening on my flag vs. my vertical provided I set up the test right (enough attenuation on my vertical so I don't blow my cheap front end away).

    But some people report that they can't hear anything on their vertical but they can on their magloop or flag. This has to be some kind of nearfield coupling problem, common mode problem, or receiver overload problem.

    Of course for near-field coupling and common-mode noise it's generally better to get the antennas outside and far away from the dwelling. And I think it's worth thinking about receive-only antennas for apartments, but I think for anyone who wants to try it, it pays to sort out the reasons why they work and what they can do vs. what they mythically do. Unless you only have one loud source, a magloop's nulls won't help you but its common mode decoupling will. But maybe it's a simpler apartment setup on the higher HF bands to just switch to a balanced TX/RX antenna with a killer balun. That worked pretty well for me and I didn't even get to the point where I had a super balun.
     
  7. W8JI

    W8JI Ham Member QRZ Page

    That's correct. Anytime we change something we also change many things we don't even realize we changed. We then attribute 100% of the results to something that isn't actually the thing that made the actual difference.

    1.) Small loops and other lossy antennas act like an attenuator, reducing noise because they reduce sensitivity.

    2.) An antenna change always changes many things particular to our specific system that we don't even think of.

    Sometimes it is all emotional. For example I introduced the directivity concept (RDF) because a popular 160 operator was swearing two beverages fed broadside, even with very close spacing, improved S/N ratio because he saw the gain go up 3 dB in models. This happens because unless you have a stable reference antenna to compare with, no one would ever notice a 3 dB change from day to day. Once we do the work, we expect the results. Our brains make sure we have the results. :)

    There have even been articles in QST claiming extra antenna system gain improves receiving ability by the same amount it does transmitting, and we all know that is false.
     
  8. W0BTU

    W0BTU Ham Member QRZ Page

    That was my experience, also. No computer back in those days, but very seldom was the S/N ratio any better on my 3' dia. remote loop than my inverted-V.
     
  9. WV6U

    WV6U Premium Subscriber QRZ Page

    Yes, I did read W8JI's site about receiving loops and also saw N30X's page. Both are great resources and I don't think I am going to argue about the physics of loops :)

    That said, as N30X points out on his page about loop antenna, is that it might be a good tool to find sources of noise and kill them. Also, my dipole on the deck is fixed. I cannot raise or move it back and forth to experiment. Not so with the loop. I can move it in, out and around the apartment to see what works best. Directivity helps too. For example, my VLF loop antenna picks up a lot of local noise but when pointed in the right direction, the interesting signals clearly come up above noise.

    If I am lucky, I might be able to get away with strapping a small loop on the deck's railing on a pole and push it away from the building :)
     
  10. WV6U

    WV6U Premium Subscriber QRZ Page

    Tom, a questions about T1 from loop at end of the page - it provides isolation and impedance transformation. Right? For a multi-band receive antenna, how would impedance matching work? I could possibly use a regular tuner for matching but that would :
    1. make operation very cumbersome - set xcvr to desired freq, switch to RX antenna, key at 5W, tune the antenna via tuner, switch to TX antenna, tune again and then transmit.
    2. And, would require more substantial capacitors on the loop to allow tuner operation (typically, a tuner requires a minimum of 5W and on say, 80m, a small loop can easily get up to 5kV with 5W).

    Thanks.
     
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