[amsat-bb] Re: Thoughts on alternative eggbeater build/feeding

[Chad Phillips]

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.

Chad
kg0mw
En-13



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. :) )
>
> Matt
> KC4YLV
>
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