K5OE, Eggbeater Antenna

Eggbeater Antenna

These are probably the easiest and cheapest satellite antennas you can build. They make a great "starter" system to hear and work the birds without spending a fortune. The eggbeater is the one on the far left (sorry so small). The 40/80/160 m trapped vertical with tophat(s) is gone now. The 2 meter version is hidden behind the chimney.

Theory:
The "eggbeater" antenna is an omni-directional antenna employing circular polarization to maximize signal capture from low Earth orbit (LEO) satellites. It is, basically, just two full wave loops fed in quadrature. This version employs a 90-degree "phasing" line to provide a fixed right-hand circular polarization (RHCP). It also uses a pair of parasitic reflector elements to focus more of the radiation pattern overhead. This effect makes it a "gain" antenna, but that gain is at the expense of low-elevaton reception. To the horizon it is linear-horizontally polarized. As the pattern moves up in elevation, it becomes more and more RHCP. The sketch at the left depicts the nominal impedances present in the design. Note how well this design matches to 50 Ohm cable without any complicated mechanisms.

Construction:
The antenna is formed around a 1" PVC coupling with 4 holes drilled in it, 90 degrees apart. The two "loops" are connected on the outside of the coupling and the coax connections are made on the inside of the coupling. Approximately 3/8 of a wavelelngth below the loops is a pair of reflectors in the same plane as the loops. Make the loops and the reflectors per the table below, adding 2" to the loop length and phasing cable length to allow for connections.

Table I: Nominal Dimensions (in.)
70 cm 2 meter

Loops 25-3/4" 6'-4-1/2"

Phasing Line 5-1/2" 1'- 4-1/2"

Reflectors 13-1/4" 3'-2-1/4"

Dist. To Ref. 8-1/2" 2'-5"

Table I: Nominal Dimensions (in.)
	70 cm	2 meter
Loops	25-3/4"	6'-4-1/2"
Phasing Line	5-1/2"	1'- 4-1/2"
Reflectors	13-1/4"	3'-2-1/4"
Dist. To Ref.	8-1/2"	2'-5"

I used 6-32 stainless steel hardware, but 4-40 or 8-32 would be good also. The 50 Ohm coax (9913F or equal) is connected to one loop and the 93 Ohm phasing line (RG-62) is "jumpered" from one loop to another. See the sketch to the right and the detail below. I have found crimp-on ring-lugs to work very well for connecting the coax to the hardware.

Calibration:
1) With the reflectors installed, attach the 50 Ohm cable to two loops without the phasing line (connect center to two adjacent ends and the shield to the opposite two ends) and adjust the length of the loops for minimum SWR.

2) Install the phasing line per the sketch below (view looking up from bottom of antenna) and re-test for SWR. Adjust the length of the phasing line slightly if SWR improves off-frequency.

3) Adjust reflector spacing (not length), moving up and down slightly to achieve minimum SWR (should be less than 1.5:1).

Performance:
This antenna performs quite well on satellite passes above 30 degrees, but I found the addition of the reflectors noticably degraded the low-angle performance. I also built the 2 meter version and was impressed at what a good antenna it became for mode A uplink to RS-12/13. I was not able to confirm the commercial version's claim of 6 dB gain; measuring about 4.5 dB of gain at the lobe maxima (at 60 degrees elevation per my testing).

In terms of real-world performance, the 70 cm antenna will be great for FO-29, fair for FO-20, and only so-so for AO-27. The 2 meter version, with 25 Watts, will easily work FO-29 and AO-27, as well as RS-12/13, but 100 Watts is needed for effective uplink to FO-20 and RS-15. The biggest drawback to this antenna is it's fixed circular polarity, making it subject to the "nulls" often experienced from satellites due to the random downlink signal polarization.