Wednesday, February 10, 2010

Building the AA2TX Parasitic (Or 'Passive') Lindenblad for 70cm

The AA2TX parasitic lindenblad, which was published in Feb 2010 QST (and has been available online for some time), is proving to be an enjoyable project. I have modified some components because I could not find them here in Atlantic Canada. The #8 aluminum wire for the passive elements is not in our big box stores (different wiring regulations?), and the PVC ferrules require a special order, at least at this point in the year, when no sane person would be installing eavestroughing. So I made do with what I had, and the results are encouraging.

I used 1/2" wood dowling for the horizontal members and #12 copper wire for the parasitic elements. I used exactly the same measurements as in the published project. I'm sealing the wood to ensure that it survives the weather, and I fixed the elements in their holes in the wood using a glue gun.

I'm getting about a 1.5 SWR at 435-436 MHz, which I consider acceptable. Moreover, the SWR drops as I tune higher. I believe this is a good situation, since ice and rain tends to drop the resonant fequency of an antenna.

One tip for anyone who is testing this as they build. Before you take any SWR measurements, make asbolutely sure that the feedline is tightly affixed to the vertical element and has its ferrites properly placed. With the feedline hanging loosely and about 1/2" away from the vertical element, I got very different results. Moreover, in my experience, the vertical element without the 'hub' will produce an extremely low SWR over a large part of 70cm. If you aren't getting that, don't fiddle with the hub; look into if your cable is correctly attached to the elements and if the coax is properly choked at the bottom of the lower element.

I will finish this version by using the Marine Goop recommended by Tony to seal and glue. I expect the doweling will survive nicely, even when affixed to the PVC hub with hot glue or epoxy: there just isn't much force on them.

8 comments:

  1. Hi Bruce, A friend and I also built this antenna and observed the same thing you did: the distance between the coax feed and the lower dipole element dramatically affects the SWR. In our case we substituted for the LMR-240 called for in the design. When we swept the antenna we found a nice SWR about 420 MHz, but not at 436 (this with the coax tight against the dipole as instructed). We were just getting ready to trim the dipole when my build partner (George VE2NGH) noticed that after releasing the cable ties holding the coax against the dipole we were getting the SWR we wanted at the desired frequency! So for us with different coax the answer was no trimming and space between the dipole and the coax.

    The moral seems to be use LMR-240 tight against the dipole if at all possible. If you must substitute the coax, you may have to adjust the spacing between the dipole and coax to get the resonance you want.

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  2. Malcom et al. --

    A bit of an update. I have upgraded to LMR240 for the feedline, and am feeding that directly to a weather-proofed filter and preamp combo. The SWR is even better, and I'm finding this an excellent omnidirectional antenna.

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  3. Did anyone consider modifying the central centre-fed dipole to an end-fed.. like a J-pole? Then you can get rid of the lossy ferrite, have no more de-tuning effects from the co-ax and have a handy pole to drill and mount the circular converter?
    Hugh G6AIG

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  4. Hi,

    Wood is used to survive the weather and copper is best for the wire of LMR 240. We can use aluminium wire for it. Thanks a lot..

    LMR 400

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  5. Aluminium Cable Manufacturers ? A renowned aluminium cable manufacturers in India and one of the leading aluminium cable manufacturers and suppliers in Delhi India.

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  6. Nice blog dear I like your blog it’s looking so cool according to blog theme. I would like to join your blog again and again. aluminium cable, copper wire.

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  7. Hi Bruce,
    pls tell me what are the current trends of Parasitic Lindenblad antenna?

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  8. You can feed the dipole THRU the lower leg. Is a very common system called "Coaxial Antenna". The ferrite bead will be in the same place.
    Regards.
    Edgardo Maffia LU1AR
    Buenos Aires - Argentina

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