What I really wanted was a small hand-held antenna I could fit under/behind the seat of my truck and just point out the window during a LEO pass. I recently conducted some experiments on a 4-element beam antenna for mode J downlink where I employed a folded-dipole feed to transform the feed impedance up from 25 Ohms to 100 Ohms--making circular polarization matching easy. I still had these parts laying around the garage (a.k.a. the R&D; lab) and ...
First, the design criteria:
This first criterion rules out any sort of circular polarity scheme. The second, allowing for a handle, limits the actual antenna length to about 12". This last goal, of course, is up to the builder, but easily accomplished.
I designed this antenna as a very high-gain 3-element Yagi-Uda parasitic array. It has almost 10 dBi of free-space gain (9.7 dBi free-space and 15 dBi at 5' elevation). This high gain is at the sacrifice of two key variables: front-to-back ratio and feedpoint impedance. The E-Plane azimuth plot at the right depicts the gain pattern. The minimal front-to-back ratio, about 8:1, should not pose a problem unless you are in an area of extremely high intermod interference. The nominal feedpoint impedance is a very low 12.5 Ohms for a conventional dipole feed. Instead, a folded-dipole feed element is used to transform the feedpoint impedance up to 50 Ohms (4:1 ratio). Of course, this antenna is linearly polarized.
The basic antenna is built around a "boom" of 1/2" PVC pipe cut to 18" length. The antenna itself uses 12" of the boom length and I left 5" at the rear for the "handle" and 1" at the front to attach a PVC cap--gives it that finished look.A PVC coupling is used at the feedpoint to connect a couple of # 6-32 x 1/2" stainless steel bolts/nuts/washers to the coaxial cable (with crimpt style ring terminals). All three elements are formed from 10 gauge wire (insulated). The table below lists the dimensions and the sketch depicts the layout. These dimensions are fairly critical and you should strive for +/- 1/16" accuracy. All copper wire is used and the coax (with ring lugs) connects directly to the driven element to minimize Ohmic losses.
|Ref||13.00 (33 cm)||0.00|
|DE||See Sketch||4.50 (11.5 cm)|
|D1||12.00 (30.5 cm)||12.00 (30.5 cm)|
Install the driven element (make sightly larger and trim for SWR) and the director, but just try taping the reflector to the boom first. After adjusting the driven element for best SWR at 436.8 mHz, you can move the reflector slightly to get the SWR perfect. I used glue to hold the reflector and director in place and then spray painted the whole assembly a traditional aluminum color for a nice finished look. After all, if you are going to wave this thing around in the air and point it up in the sky, you want it to look profesional for the dumbfounded onlookers :-)
I used RG-8X and not the more common RG-58. For an equivalent 10' (2.5 m) of coax, RG-58 has 1.2 dB loss while RG-8X has only 0.8 dB loss. With LEO satellites, every fraction of a dB counts.
|Testing an early prototype. Note the 3/4" PVC boom.||My lovely assistant, Lauren, out in the antenna "test range."|
As the bird (AO-27) comes over the horizon, I hear the tell-tale signs of FM quieting. By 5 degrees above the horizon I can make out about 50 percent of the in-and-out audio. By 10 degrees above the horizon, the signal is strong and by 15 degrees it is full quieting. In terms of comparison, I get full-quieting at about 5 degrees with my Arrow Antenna.
I can also confirm (again) the polarization of choice for AO-27 is vertical. At times, a slight tilt of the antenna improves reception, but placing it horizontal completely obscures the signal.
This antenna makes a nice companion to the 2 meter vertical and can be used mobile, like I use it, or as a fairly light and compact backpacking setup.
(C) 2000, Gerald R. Brown, K5OE