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Vantastic Amateur Radio Antennas

The RoadTrek design presents some unique challenges to us hams.

Harvey Tetmeyer K5LJM - Silent Key 5 Feb 2005

As everyone knows, antennas should be up as high as possible to work effectively...
......but with all that fiberglass, finding metal, up high, is a problem.

Also, punching holes in a new van is done with a great deal of care and planning.

Here is my first cut at antennas for VHF in the photo to the left and HF to the right.

First, the VHF antenna....

In looking at the available surfaces, the upper rear corner of the driver's door looked promising.

It was the ONLY place that looked promising. So I tried it!

The Results... The 5/8 wave antenna is mounted up and in the clear.

It looks good with the door closed.

Best of all, it works great!

 

The mounting and access holes were drilled in the body and are covered when the door is closed.

It doesn't even look bad with the door open.

Removal of the antenna later will require minimal plugging and can be retouched quite easily.

Note: This picture is with the door open.

The mounting bracket was fabricated from 1/8 inch hardened aluminum. A flat pattern was made from cardboard to define the shape of the aluminum before bending and drilling. The sheet was sheared to the appropriate width and the curved area was cut out with a band saw.

Bending was done "very carefully" with a metal brake (A vice and hammer would have worked just as well... but I had a brake.) Are all those bends necessary? Yes. I wanted to clear the door and conform to the body.

I drilled the holes after the bracket was satisfactorily formed to the desired shape. The mounting holes are for three #10 flat head screws with countersunk holes. The body was drilled and tapped and a rubber gasket was used between the bracket and the body.

The bracket was painted to match the van. The mounting holes and the surface under the antenna mount were masked to give good electrical connections.

The coax must be run to the rear of the mount to clear the door. I tried running it through a hole just below the antenna mount and the door hit the wire. The hole is still in the mounting bracket even though it does not show up in the photos. Adobe Photoshop is wonderful for covering up mistakes... sort of like Sherwin Williams.

The HF antenna was the next project.

Theory says:

- It should be as long and high as possible.
- It should be as far away from flat vertical surfaces
... as possible, otherwise it is a poorly radiating
... transmission line, not an antenna.
- It must be an educated antenna... well grounded.
- It should be tuned and matched, i.e. very low SWR.
- It should not flop around in the wind... be sturdy!
...Movement causes ever-changing dynamic detuning.

Other requirements:

- Don't drill any holes in the new van...
...unless you absolutely have to.
- It should be heard and not seen.
...Like how do you hide a ham antenna? Oh well!

 

 

So here it is... up close. I chose a clamp-on design. The main support is a 2 1/2 inch aluminum channel I found in the scrap. It is clamped to the rear door hinges using four 1/4 inch bolts and backing plates... no holes in the van. Rubber gaskets are used between the channel and the hinges to save the paint. The antenna is supported by another channel bolted to the main channel. The coupler is a common Radio Shack special... 3/8-24 thread on top... coax fitting on the bottom. The support is a Plexiglas insulator mounted to the main channel with a piece of 1/16 steel sheet bent to form an angle bracket. All metal parts were primed and painted with white epoxy appliance paint for color matching and durability. Last, but not least, I used all stainless steel fasteners, i.e. bolts, flat washers and lock nuts. Lock nuts were cheaper than lock washers and regular nuts at my local Ace Hardware store down the street. Besides that, lock nuts have class.

Since any efficient lower frequency HF antenna has a low feed resistance, matching is very important. After forty something years of mobiling, I have finally found the ideal matching network. It has one component plus a connector... or two. It is a little red toroid wound with X number of turns of enameled copper wire, soldered to a male coax plug. The X number of turns varies from about 7 to 12 turns depending on antenna type, mounting location and frequency. Electrically, it is simply an inductor connected between the antenna feed point and ground... super simple. I use my handy dandy Autek antenna analyzer and adjust the number of turns until the match is just about perfect.

To install the inductor, I needed one more component, a coax T connector. That makes it simple to change inductors if I need a different value when switching from band to band. Usually, if the inductance is optimized for the lowest frequency band I will be using, matching on the other bands will be close enough.

Oh yes... I always tune my antennas... often when necessary... as Margie AB5ZX and I travel across the Fruited Plain.

- Harvey K5LJM

Now to the electrical stuff... I forgot to tell you, I masked the bottom of the channel holding the antenna coupler before painting. I wanted it to make good electrical contact. I also ran a short, flexible ground strap from the channel, around the corner of the door and connected it to the body near the bottom hinge The coax follows a similar path under the edge of the door. There is more clearance under the door than around the door on our van. It is also more watertight.

 

 

Since any efficient lower frequency HF antenna has a low feed resistance, matching is very important. After forty something years of mobiling, I have finally found the ideal matching network. It has one component plus a connector... or two. It is a little red toroid wound with X number of turns of enameled copper wire, soldered to a male coax plug. The X number of turns varies from about 7 to 12 turns depending on antenna type, mounting location and frequency. Electrically, it is simply an inductor connected between the antenna feed point and ground... super simple. I use my handy dandy Autek antenna analyzer and adjust the number of turns until the match is just about perfect.

To install the inductor, I needed one more component, a coax T connector. That makes it simple to change inductors if I need a different value when switching from band to band. Usually, if the inductance is optimized for the lowest frequency band I will be using, matching on the other bands will be close enough.

Oh yes... I always tune my antennas... often when necessary... as Margie AB5ZX and I travel across the Fruited Plain.

- Harvey K5LJM

 

Picture lost.  I know Harvey had one here, but somewhere along the line it got separated from this page. Well I kept looking at the parts of screwdriver antenna #27 laying in the garage and just had to complete it and stick it on the van.

I modified the top mounting support to accept the larger diameter base section and added a hose clamp wrapped around a PVC insulator to keep the antenna steady.

I drove down the road with a 5 foot whip on top. When I use it portable, I replace the whip with an 18 foot bamboo fishing pole spiral wrapped with #14 flexible antenna wire. The fishing pole is in 6 foot sections and stores neatly in the shelf above the passenger door.

Revised 7-13-02