2 meter mobile antenna
Design:
This is a high-efficiency "gain" antenna. It is not mounted permanently: I have a magnet attached to it and "throw" it up on the roof when I want to work a LEO satellite.
The antenna is an atypical vertical: instead of the common 1/4 wl vertical monopole or 14/ wl ground plane with 1/4 wl radials, this design employs a 3/8 wl vertical section and short radials to complete the "ground plane." Effectively, an off-center-fed vertical dipole that does not rely on the earth or, in my case the truck body, to complete the bottom half of the antenna.
This is an important point. The fact the antenna has a fully contained lower half, i.e., the ground plane, makes it very efficient. This is especially significant when compared to my mag-mount monopole antenna where the "ground plane" had to be completed through the coax, through the rig, then to the vehicle body. I suspect the mag-mount was not very efficient at all. The dipole is off-center-fed to get a better match to the feedline and the bottom "half" of the antenna is completed with capacitive reactance from the four shortened radials.
This design has an honest 3 dBi of gain at 6' elevation (2 dBi free-space) with a suitable pattern for LEO communications--favoring the horizon. At 20' high, the same antenna exhibits almost 6 dBi of gain. The feedpoint is a nominal 50 Ohms at 146.850 mHz.
Construction:
The antenna is built using a 3/4" PVC tee, a cap for the top, and a plug (flat on the bottom) for the bottom. The tee is arranged vertically with the top cap drilled for connection of all 5 elements (I used 6-32 stainless steel botls/nuts/washers) and the bottom plug is drilled for connection of a magnet (optional).
The coax is fed thru the open side of the tee and connected directly to the elements via ring lugs. Alternately, a lower-profile version could be constructed substituting a coupling for the tee and drilling a hole in it for the coax to exit. I connected the center of the coax to the mast and the shield to one of the radials using crimp style ring terminals. I then wrapped some small gauge wire around the outside of the cap, connecting all four radials together, and covered the assembly with electrical tape and paint.
The figure at right shows the layout and dimensions (in cm) of the elements. A 25-3/16" (64 cm) vertical section is combined with four 7-3/16" (20 cm) radials. I recommend you make the elements slightly longer and then trim them based on SWR readings. I used 10 gauge insulated wire for all the elements, but 1/8" or 3/16" aluminum rod would be a suitable material--and likely more durable. When tuning for minimum SWR, I eventually pruned the mast to 24-3/4" and the radials to 7" to get a 1.2:1 SWR. That is much better than my mag-mount ever showed so I stopped fine-tuning and put it on the air.
The radials are angled down at about 30 degrees. This angle can be adjusted to get the SWR perfect once the vertical mast is trimmed for best SWR at the desired frequency, but I found the effect minimal. You could also "wind" a coil of a few turns in the center of the mast to lower the profile of the antenna without affecting feedpoint impedance drastically or performance too much. I did that to make it short enough to fit under my garage door header and note no significant difference in SWR or performance.
Performance:
Mission accomplished. I can now get "into" the birds at low elevations. On it's maiden journey out of the garage, I worked four stations (one was marine mobile) on an 8 degree AO-27 pass. If I could hear the bird, I could work it (as long as a "big gun" station did not have the bird already captured). At high elevations I found I could run on low power (about 3 Watts) and capture the bird with little difficulty.
This antenna makes a nice companion to the 70 cm Handi-Tenna or can be used for any fixed, mobile, or portable service. Since the elements are flexible 10 gauge wire, they can be readily folded and unfolded for backpacking.