[amsat-bb] Re: Thoughts on alternative eggbeater build/feeding
KG0MW at greatplainsphotography.com
Sun Jan 27 13:10:07 PST 2008
Thanks for your thoughts Jonny. I am curious to see how that will work.
I am in the process of totally rebuilding my station after 4 years qrt
and am open to all the options. Egg Beater antennas are certainly on my
list of things to build. I LOVE my long boom yagis, but the EB's would
be fun to play with.
jonny 290 wrote:
> Ye olde bird nerds,
> I was sketching out some plans to rebuild my practically-failed
> Eggbeater antenna project for
> 70cm, as I am tired of cranking the rotator knob every pass and want to
> try out a good omni satellite antenna, which I
> can hang from the top limb of a tree 30 feet above my shack window (best
> option on my city lot).
> My plan is to build on the K5OE Eggbeater II antenna. One thing that's
> always bugged me about these antennas
> is that most are implemented using a 93 ohm coaxial section to phase the
> loops. This is hard stuff to find, and though
> I know some hams say, "anything below 1.5 SWR is OK", I'm a
> perfectionist and want to try to polish it up.
> The current antenna has a phasing section made out of two runs of RG-8X
> coax with the shields shorted and insulated, centers fed as a balanced
> 100 ohm (theoretical) matching section. It was difficult to build and I
> doubt its integrity because it's coiled up so tightly;
> I don't think that the foam core coax retains its impedance well when
> folded up in 1" PVC.
> I was thinking about the feeding issues, then I recalled that one
> popular CP yagi antenna design is to space the horizontal and vertical
> elements 1/4 wavelength along the axis, and then feed them in phase. It
> then came to me that it may be possible to build an eggbeater with the
> two elements separated vertically by 1/4 free space wavelength, fed with
> simple half-wave coax sections to replicate the two loops' 100 ohm
> impedance into a simple coax tee. Polarity switching should be possible
> with a coax relay and a 1/2 wavelength delay line. Radials would be
> placed 1/4 wave apart as well, same spacing as the loops.
> The advantages of this to me seem numerous. First of all, the free space
> quarter wavelength spacing is easier to accurately implement than a 1/4
> wave phasing section using coaxial cable at 70cm, and the
> power/currents/etc split to the two antennas are more balanced as long
> as you build the 1/2 wave sections to equal lengths, leading to improved
> circularity skyward. You don't need to find rare coax for the phasing
> section, and instead of cramming several folded up or coiled runs of
> coax inside a narrow (for aesthetics) PVC tube, you only need to run two
> half-wavelength (or any multiple of half-wavelength) runs further down
> the mast, where the tee or relay could be ziptied.
> So, that's my plan. If it makes sense, I'm going to give it a shot with
> 1/4" copper fridge tubing for the loops and some sort of sturdy wire
> (coathangers? heh) for the radials, using LMR240 coax for the two
> halfwave sections. If anybody has any input or comments, I'd love to
> hear them.
> (P.S. If somebody's done this and I'm reinventing the wheel, please let
> me know. :) )
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