receive antennas only

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

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  1. N9AAT

    N9AAT Ham Member QRZ Page

    You MIGHT want to try an AMRAD antenna. I built one and it was fun. In fact, I built two; one for the house that works with 24 volts and another with a mag mount for my car on 12 volts (you have to use a different transistor).

    The AMRAD is an old design, basically a short whip used as an E-Field probe. Both of my whips are auto antennas bought at AutoZone.

    No tuning and no SWR to worry about. In fact, you can enlarge the built-in toroidal divider to become a multicoupler feeding as many receivers as you want.

    It is an active antenna, which means you have to build the board and house it in a piece of PVC with a heat sink, and you have to build a low-noise 24-volt power supply to handle about 100 ma. I use the classic CP-666 transistor, which is still available from Crystalonics. It's the single most expensive part. I think I spent a total of about $90 after I added a few "impulse shopping" things. :D

    Far Circuits in Dundee, IL makes, or used to make, the circuit boards for the active antenna and the power supply for a few bucks. Well worth it. E-mail them to see if they still have some.

    Basically, the AMRAD has a single transistor that serves not as a preamp but an impedance matcher, the same way car AM radios work, which is why they're so good. The CP-666 however has a much broader frequency range than AM radios but the same high dynamic range. I've never seen mine overloaded and the sensitivity is great from 150 kHz to about 25 MHz.

    Yes, E-field antennas pick up more noise than H-Field loops, but they're still just as sensitive and easy to hear signals in my personal experience. I'd compare my AMRAD's to any of my dipoles or my multiband vertical.

    Good to have the big-dog, the 24-volt version, on the roof with something like chicken wire under it for a counterpoise to keep local noise away. My car version works well with a mag mount, a strong one, and the lower power transistor does not need a heat sink. The home version needs about 6" of 5/8 copper tubing to radiate heat from the CP-666, and yes it does get hot. And yes, mine stays on 24/7.

    One of the LF groups recently used an AMRAD to re-create the first transatlantic contact. :eek:

    Some have said the AMRAD is great with the new software-defined receivers; for changing bands rapidly, good sensitivity, not having to worry about antena bandwidth, and just general operating convenience.

    I've never heard of one that works too well above 25 MHz, where preamps start to get important. The AMRAD is NOT a preamp ... it's an impedance matcher.

    At 1600 kHz a standard car radio with a standard antenna would see about 100 dB of signal loss if it did not perform impedance matching in the first stage.

    The AMRAD is great! The only problem is, you can't transmit out through it.

    I wish you could. :D
     
  2. W0BTU

    W0BTU Ham Member QRZ Page

    Well, you said you once tried his design, and I kind of assumed since they worked that well for you, that's what you did; and the wire under the antenna reduced the losses and made it work that well.

    What cores did you use that would take 100 watts? Those little binocular balun cores are only good for about 30 watts, according to Tom. And that's another reason I assumed you used Misek's design, because IIRC, you're using those cores.

    Anyway, when I first built my Beverages, I tried calling some strong stations with less than 30 watts, but they never heard me. It's pretty amazing to me that they worked as you say.
     
  3. KM1H

    KM1H Ham Member QRZ Page

    None of his designs worked worth a damn, especially the 2 wire.

    It wasnt until the Topband Reflector and I got ON4UN's 1st Revision that performance improved and it took 4 more revisions, Internet searching, and a lot of actual building and testing on my part to get them to where they are today. Ive only been using the binocular cores a short time....maybe I should take the TS-830 (Its lighter) out to one of them and see how much RF they can handle:)

    Those TX contacts were about 20 to 25 years ago starting with the Belrose comments in one of the mags, maybe QST. Cores were probably FT-140 but I dont remember the mix but likely 43 or 77. The rig was a TS-940 in both cases but again dont remember details such as using the built in ATU.

    Carl
     
    Last edited: Mar 13, 2011
  4. W0BTU

    W0BTU Ham Member QRZ Page

    I keep thinking about this. (Respectfully, I'm not 100.000% sure that you weren't pulling our leg.) Regardless, just I might try this again (with 3 stacked binocular cores on SSB, the wires tied together, pointing NE), just for fun.

    Please tell us about that Beverage. So you used six 135' radials. Did you have the far end terminated, etc.?
     
  5. KM1H

    KM1H Ham Member QRZ Page

    Nope, its not a joke Mike.

    I removed the 2W 470 Ohm resistor and used a 600 Ohm 100W glass non inductive one for the test; it was something Id been hanging on to since the 60's.

    The 135' radials were the "in thing" back then and obviously not resonant when laying on the ground. Id always felt they offered some directivity altho later articles say different. However it took a lot or reading plus bench and outside testing last year to get the new 2 wire Beverages to work well as most of what was published just didnt work well at all. There is still way too much ambiguity and often deliberately missing info mixed in.

    Carl
     
  6. W0BTU

    W0BTU Ham Member QRZ Page

    I guess I'll have to wind a transformer that'll take 100W and try that again sometime. Probably won't terminate the end when I do.

    I would really like to discuss that with you sometime. I've also noticed that there is no one single place, either in a book or on the Internet, where you can go to learn everything that you need to build a good working bi-directional Beverage antenna. It was because of that that I decided to create that page about Beverages on my Web site (which is by no means finished). I think some input from you, perhaps going into some detail about what you mean, would help make that page a better one.

    Thanks, Carl.
     
  7. W0BTU

    W0BTU Ham Member QRZ Page

    I think what you had was a more of a terminated long wire than a Beverage. Depending on the height, the difference between the two could get blurred. How high was it, how long was it, and what bands did you do this on?
     
  8. KM1H

    KM1H Ham Member QRZ Page

    Why would you make that assumption without knowing anything about the antenna?

    It was 750' long, about 7' high, and was used for years as a highly directional Beverage. This includes both QTH's.
    The slightly different TX termination value wouldnt make much difference other than possibly a bit less F/B; the matching transformer was 9:1 and fed with 75 Ohm hardline. It was only used that way for a few days and on receive it appeared to work the same as before.

    Carl
     
  9. W0BTU

    W0BTU Ham Member QRZ Page

    Thanks. What bands did you do this on, Carl?
     
    Last edited: Mar 18, 2011
  10. KM1H

    KM1H Ham Member QRZ Page

    Only 160 to TX for long distance and a few times on regional area 75 SSB where signal reports confirmed the high directivity. And as high as 10M for RX when snow static wiped out the regular antennas, a preamp was necessary on 20 and up and it was still marginal on 15 and up....strictly a temporary compromise.

    Carl
     
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