Have you ever wanted a "stealth" car antenna that:

- Covers All Bands (510KHz through 450 MHz)
- One Antenna Covers All Radios, Including Amateur and AM/FM
- Cosmetically Pleasing ("Stealth")
- Great Performance (59's)
- Cost $60 to build
- No hole to drill in roof
- Light weight (Antenna assembly weighs less than 2 lbs)

Well, you don't have to wait any more.

Honda Insight with "Stealth" antenna. Where is it?

In the photo above, the "Stealth" antenna is mounted on top of the roof. When fully retracted the only thing visible is the black rain cap.

The "Stealth" antenna deployed to 2 feet,  can fit into any home garage.

"Stealth" antenna deployed to 8 feet for the HF bands (80-10 Meters)

How'd They Do That?
The general design of the antenna is:

- Telescoping  Elements
- Plastic Push Rod
- Plastic Plumbing Pipe

Interior view of upper part of "Stealth" antenna assembly and SGC-230 automatic antenna tuner.

Interior view of lower part of "Stealth" antenna assembly with push rod.

"Stealth" antenna push rod (from a radio control airplane) and locking thumbscrew.

The yellow push rod has color markings to note the push rod length. E.g.:

- Black    = Fully retracted
- Blue      = 450 MHz Band
- Red      = 144 MHz Band
- Green   = HF Band (Tuned with the SGC-230 Automatic Antenna Tuner)

The blue push tube, which houses the push rod, is 8 feet long. It is fixed in place and routed through the front of the vehicle, hidden under the dashboard.

How's it Wired?
The antenna element is made from four interlocking hollow brass tube segments. The telescoping is similar to traditional telescoping antennas (E.g. TV Rabit Ears). Brass provides:

- Solderability
- Compatibility with Off-the-Shelf Plumbing Supplies
- Easy to Cut
- Forgiving and Flexible
- Good Conductor (Brass is an alloy of Copper and Zinc with high corrosion resistance)
- Availability (Any good hobby shop)

The antenna length is continuously adjustable from 24 inches to 8 feet. The length is fixed by tightening a thumb screw inside the Honda.

Electrical connectivity is made by inserting a tapered stainless steel pin into a hole in the plastic pipe. This pin serves two purposes: 1) Makes electrical contact with the antenna and 2) Locks antenna in place.

Electrical and mechanical connection to the antenna is by inserting a stainless steel pin into the antenna.

The "Stealth" antenna needs to be deployed at least 24 inches to lock in place for transmitting. This is because the antenna element needs to be outside the car. This length requires it to be operated as a "harmonic" mode, the best of  which is 3/4 wave on 144 and 450 MHz Band.

The radiation resistance of a harmonic antenna increases with length. This antenna is somewhere in the 75 Ohm range. This offers an SWR of less than 2:1 to the transmitter on VHF and UHF. On the HF bands, the SGC-230 automatic antenna tuner resolves the impedance mismatch and allows the Yaesu FT-100 to dump 100 Watts into the antenna.

This photo shows three clips:

- AM/FM Broadcast radio (Red clip in the foreground)
- Yaesu FT-100 144 and 450 MHz Band (Shown connected to the stainless steel pin in the background)
- SGC-230 HF Band (Black Clip in the middle of the photo)

Performance
Transmit: Good signal reports were obtained on several bands. On HF, two mobile stations reported 5 by 9's on 17 Meter SSB phone). On HF, the SGC-230 is able to lock into a tuning solution and present an SWR of less than 2:1 to the Radio using 100 Watts.

On VHF and UHF all stations reported good signal quality compared to a commercially available 5/8 wave 2 Meter antenna, the SWR is less than 2:1 using 50 Watts of power, primarily because the antenns length is continously adjustable.

Receive: On VHF and UHF, signal strength is the same as a commercially available 5/8 Wave 2 Meter antenna. On HF, signal strength is the same as any 8 Ft antenna element.

Yaesu FT-100 and AM/FM radio, both operating from "Stealth" antenna.
 

Mounting "Opportunity"
Mounting the antenna caused a few challenges and "opportunities":

- Slight obstruction in rear-view mirror.
- Enlarged hole in roof
- Metal doubler on car sheet metal (The Honda Insight already had one)
- Hole in headliner

For easy access to antenna controls (push rod and pin) the antenna has to be mounted close to the driver. The antenna assembly causes a 1 inch obstruction in the rear-view mirror. This obstruction is tolerable and safe. No traffic is blocked by the antenna.

This vehicle already had a hole in the roof for an AM/FM antenna, so no new hole had to be drilled. However, the existing hole had to be enlarged about 1/4 inch. If desired, the "Stealth" antenna could be removed, and the AM/FM antenna re-installed.

At highway speeds with the antenna at 8 ft, there may be too much stress on the vehicle skin near the antenna. Although the car was driven at 61 MPH and no buckling noted, there is the possibility of buckling of the skin area when driving with a relative wind of 80+ MPH. The Honda Insight uses an Aluminum skin and chassis, which is softer than steel, so there may be a need for a larger inside skin doubler around the antenna.

A 1.5 inch hole must be drilled in the headliner to acommodate the "Stealth" antenna. Oh well, no pain, no gain.

Maintenance
A weekly cleaning of the brass element is recommended. CRC contact cleaner and lubricant, or equivalent should keep the antenna mechanically and electrically sound. A one time application of spray silicone to the push rod should be enought to ensure easy antenna movement.

Parts List
8   Concentric Brass Tubes - Starting with 3/8 OD, in decending OD's - Available at Modellbahn Ott Hobbies, Inc.
2   GOLD-N-ROD #506 48 inches - Blue and Gold - For Radio Controlled Planes - Available at Modellbahn Ott Hobbies, Inc.
5ft 1/2 CPVC Plastic Hot Water Pipe - Available at Home Depot
1   CPVC Adapter 1/2 inch to thread NIBCO D-2846
1   CPVC Coupler, 1/2 inch
1   CPVC CAP, 1/2 inch - Drill Slotted Hole
1   PVC CAP, 1 inch - Drill 3/8 Hole, Paint Black, For outside of car
1   O Ring, 1 inch, for outside seal
1   Stainless Steel Pin, Tapered 1/8 inch to 1/16 inch
1   Stainless Steel Clamp, 1.5 inch
3   3/8 Brass Compression Rings
2   Brass Compression Fittings
1   Thumb Screw 1/4
1   Flat Washer, 1 inch ID (used for system ground,  ground braid soldered to one side)
1   1/8 Brass Screw, Soldered on top of telescoping element

OD=Outside Diameter
ID=Inside Diameter
 

Pictorial Diagram - "Stealth" Antenna (Not to scale)

The Future
A 3 pole band switch will be added to ease switching of radios.
The push rod could be driven from a stepper motor/capstan system to fully automate the raising and lowering of the antenna. Currently, manual deployment and retraction with fingers works just fine. Indexing the push rod could be done with an optical encoding method.

About the Author
Tim Drager is a Web Test Engineering Manager for The Vanguard Group. In November, 1997, QST Magazine published Tim's article "Retractable HF Mobile Antenna" in the "Hints and Kinks" section. Tim is an Amateur Extra living in Pottstown, PA.