Wich is the best 160m antenna ?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by CU3AK, Aug 20, 2010.

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  1. K4SAV

    K4SAV Ham Member QRZ Page

    Yeah the ARRL Antenna Book doesn't say anything about it. ON4UN's book did a much better job describing what happens with this feed arrangement, including a method of choking the currents that would otherwise be going into the dirt.

    Jerry, K4SAV
     
  2. NA0AA

    NA0AA Ham Member QRZ Page

    Please reference ARRL Antenna Book, 21st edition, Chapter 6, Page 24, Figure 42.

    This shows an elevated ground plane being fed with the center of the coax against a tower fed negative. I'm willing to concede it might be an error in the book, but I remembered it correctly.

    In the commentary, there is nothing to say how well this actually works or any real specifics.
     
  3. W0BTU

    W0BTU Ham Member QRZ Page

    Awww.. You're not really surprised, are you, Tom? ;-) After all, the "antenna book" brought us the infamous Double Bazooka and J-Pole antennas! ;)
     
  4. N9AAT

    N9AAT Ham Member QRZ Page

    Now THAT's one I'm definately in agreement with. The Double Bazooka is a sham as far as I'm concerned. Why NOT just a diople? I will agree folded dipoles have a slightly wider SWR range, but only slightly if anything.

    Back to the TFD. Look ... at RESONANCE you're not going to be burning power in the resistor. You're resonant so the resistor is living at a place where current is nil. Once you go off resonance it's a different matter. But that's true of ANY antenna. To me, it's all about WHERE you burn off what you can't radiate. Anytime you're high or low in frequency you're going to loose something to impedances. The TFD does NOT make you transmit better at all; it just gives you a place to let your "inefficiency" go. I'd rather it be up at the resistor than down in my rig. At the cut resonant frequency, it's a moot point. Yeah?
     
  5. W8JI

    W8JI Ham Member QRZ Page

    That's an interesting concept Jerry!! A buddy of mine in Alabama installed one, and he was consistently 20-30 dB weaker than his brother who had a normal logical shunt fed tower.

    The tower base is embedded in dirt and thus has to be grounded.

    Since the area of the tower below the attachment point of that messy feed system excites the tower above and below the "elevated radials", I wonder exactly how a person would stop currents in the area below the feedpoint. Those lower section currents oppose radiation from the upper half of the tower, and undo what the elevated radials and upper section attempts to accomplish in way of low-angle radiation.

    I guess one could sectionalize the tower below the feed system, either by adding a tuning system to bring a tower section below the feed system into parallel resonance (hi-Q, narrow bandwidth), or hacksaw the tower legs and insert insulators. Of course feedlines would have to be disconnected also.

    Another method would be to feed the tower 80-150 feet above ground so the section below the elevated feed system has high impedance and carries little current, but this would require having at least 60 feet of tower above the feedpoint with big antennas at the top for top loading.

    I think a mess like that should come with a page of cautions and warnings about how complex or how lucky one must be to make work correctly.

    Of all the feed systems I've seen devised, the elevated radials worked against a grounded tower is about the flakiest backyard concept ever hatched.

    Once a silly idea makes it in print, we are stuck with it for years. This elevated revese fed radial idea is a very silly idea.

    73 Tom
     
  6. K4SAV

    K4SAV Ham Member QRZ Page

    That' pretty much what ON4UN proposed. He proposed adding a tuned trap on the section between the feedpoint and the dirt (same kind of thing you might do for detuning a tower). Logically that makes sense. With normal elevated radials, the radials get the same amount of current as does the vertical tower. If you can make the same thing happen with this inverse feed arrangement, then this should work as well (or within the limits of what the trap and choke can do). The other part of this (which the ARRL Antenna Book also failed to mention) is that a good choke is required on the feedline. The feedline shield is tied to the vertical part of the antenna and now it is being extended below the radials and beyond. That common mode current has to be choked off, for this to work well. Also that feedline can't be run close to the tower section that has the trap.

    I can see where if your friend built his system from the ARRL Antenna Book that he may be a few dBs down.

    So now after choking everything, it doesn't matter whether the coax shield is tied to the vertical part or the radials. The original reason for doing the inverse feed was so the coax could be taped to the bottom part of the tower, but with the addition of the trap that is now not possible.

    Jerry, K4SAV
     
  7. N9AAT

    N9AAT Ham Member QRZ Page

    You guys obviously have some experience with fed towers. Can you suggest some good atricles for a 50' tower? The Handbook?

    Regarding your shield to the vertical tower; interesting in the FAA that we have been opening the insulation and tying the shields of our hardlines to our towers now for several years. Lightning protection. It seems to work pretty well, too, plus lightning rods that offer a 45 degree "cone of protection".
     
  8. K8MCN

    K8MCN XML Subscriber QRZ Page

    Couldn't agree more this book makes great reading, so much so that it has achieved "top of the stack" on the rear of the porcelain throne. Gives so much information and IMHO the ONE book every amateur that builds his own antennas should have handy! So many sources of info combined into it with ample illustrations.
     
    Last edited: Aug 28, 2010
  9. N9AAT

    N9AAT Ham Member QRZ Page

    I just put my order in at Amazon.
     
  10. PA3BHA

    PA3BHA Ham Member QRZ Page

    ha die Cor

    Wat leuk dat ik jou hier tegen kom hihi.leuk verhaal! altijd weer leuk dat jij je keuken bij je hobby betrek;)

    groetjes

    Arno
     
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