Simple 10-Meter Antenna... the "L" you say?

Ten meters has been open recently. As a new Technician Class licensee this is the only place I can play DX. It brings back many fond memories of times like this 50 - 60 years ago. So naturally I want to jump in with the only antenna I had, a problematic quad-band whip on my motorhome.

The "10-meter" loading coil on the whip was evidently for the Asian market because it resonated around 27 MHz. By removing the wrap and re-tuning with copper or aluminum sleeves the antenna could be brought to 28.4 MHz at 1:1.0 VSWR and 50 ohms... for while. Then, in the sun and weather, it would drift.

During those halcyon times when the antenna held tune I worked the Caribbean, South America, the Azores, West Africa, etc. However problems of an intermittent connection in the antenna, rising SWR and corrosion in the mount were aggravating.

To heck with the quad-band antenna, I dug out a magnetic mount, short CB whip and cut off a couple inches so it would tune to 10-meters.

Mounted on the hood the SWR was high. On the motorhome roof I tried grounding the whip to the luggage rack which was electrically part of the coach frame. Still high SWR. What to do.

I had been modeling and prototyping meander antennas. In part of the work related to tuning I had used extended radiators... sort of "pining a tail on the donkey". I thought that also might be a very simple approach for tuning a whip antenna.

First, I created a ground connection for the tail at the base of the whip.
Then, by attaching a length of wire with a telescoping antenna, I ran that along the motorhome's fiberglass roof. By simply adjusting the tail to a bit more than twice the length of the whip, it tuned beautifully.

1.0:1 VSWR at 47 ohms 50 KHz wide.
28.4 MHz +/- 150 KHz at 1.5:1 VSWR.

Almost too simple.
I was back to working DX in the Technician band.

Why does this set-up work so well?

A search turned up an article by L. B. Cebek, W4RNL:
http://www.antennex.com/preview/Folder01/lant/lant.htm

My odd antenna fed 1/3 off-center I know is related to a Windom off-center-fed antenna. The shape is what Cebek calls an "L-antenna", a dipole in an L-shape.

Well with this success I got to thinking: a proper center-fed, quarter-wave radiator placed higher should be even better... and more conventional. Looking for something metallic around 8 feet long in my stuff was a dead-end. Then I thought of the 1-inch diameter mast for my car top sailboat. It measured 8' 3". Perfect! I lashed it to a wood pole about 12 feet up and connected the center coax conductor to the mast bottom. To the coax shield I connected the "L-wire" (the tail on the donkey) and tuned its length with a small telescoping antenna. To isolate the feed line at the base of the sailboat mast I used ferrite clip-ons and a choke balun. Five turns of RG59/U at 4" diameter dips nicely in the 10-meter band.

I know that the lower Q of a thick radiator gives broader resonance, but up close and personal the results were an eye-opener:

- 1.0:1 VSWR at 41 ohms 130 KHz wide.
- 28.4 MHz +/- 200 KHz at 1.2:1 VSWR.
- Bandwidth from 28.0 to 30.0 MHz (7.3%)!

The band scope came alive. Ear-wise I heard perceptibly better signals... 1 to 2 S units better.

Basically, the L-antenna is a slightly off-center fed dipole. The vertical element only has to be close to frequency. Adjusting the length of the horizontal leg does the fine-tuning. Ah, the unnecessary effort folks spend on symmetry.

However, because the antenna does not have opposing radials to cancel horizontal radiation, it has a mixed pattern that has slightly asymmetric gain towards the "L-arm". W4RNL says that with DX signal-bounce this mixed vertical-horizontal pattern is negated.

Impedance is largely controlled by L-angle and secondarily by elevation, element diameter, off-center-ness and nearby things like the roof that screw up the radiation pattern. At 90 degrees an "L-antenna" models around 40-50 ohms impedance... down from around the 75 ohms of the straight dipole. Some adjustment of impedance is possible by adjusting angle. I found the impedance went from 34 to 44 ohms if the L-arm lay down-roof or up-roof. If sag in the wire was eliminated to not touch the roof, I got 47 ohms for a 90-degree L-arm in free air.

Impedance is lowest as long as you keep the load point gap near the center of the dipole. However, if impedance is less than 50 ohms you can also adjust that upwards by making the vertical radiator longer and the L-arm shorter to make a more off-center dipole. Exploring this I was able to raise the impedance by lengthening the vertical-arm about 12 inches and tuning the 90 degree L-arm in free air to a perfect 50 ohm, 1.0:1 match broadly centered on 28.4 MHz.

This is one "L" of a simple antenna.

Dick Reid, KK4OBI