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10-Meter Antenna -- The 'L' You Say

Dick Reid (KK4OBI) on January 20, 2014
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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:

 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

Member Comments:
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10-Meter Antenna -- The 'L' You Say  
by K1WJ on January 20, 2014 Mail this to a friend!
I have had good performance with a 102 inch whip with a single 1/4 wave radial. Base of whip & 1/4w radial are 4ft off the ground. Good for 400khz+ below 2:1 swr.MFJ 6.5ft tripod holds up antenna & small tripod holds up radial on insulated side, 4ft off the ground.
73. K1WJ
RE: 10-Meter Antenna -- The 'L' You Say  
by KK4OBI on January 20, 2014 Mail this to a friend!

Try pointing the arm towards your preferred listening/talking direction. There is 3 or 4 db between front to back.

RE: 10-Meter Antenna -- The 'L' You Say  
by AF5CC on January 20, 2014 Mail this to a friend!
10 meters is NOT the only place you can play DX. You also have privileges on 15, 40 and 80 meters. 15 in particular is a great DX band. 6m also opens up for DX during the summer.

73 John AF5CC
10-Meter Antenna -- The 'L' You Say  
by K1WJ on January 21, 2014 Mail this to a friend!
The 102 in whip is set up as an L, elevated radial is to the east, theory gain is there but make as many contacts to the west. May add west radial at somepoint. 73 David
RE: 10-Meter Antenna -- The 'L' You Say  
by KK4OBI on January 21, 2014 Mail this to a friend!
Modeling your setup shows at 28.4 MHz = 1.3 SWR. Z=41.0.
Resonance is at 28.6 MHz = 1.19 SWR. Z=41.9.
Your front to back is over 7 dBi towards the radial side +/- 100 degrees. Take-off angle is strongest around 60 degrees towards radial side. Radiated efficiency is 33%.

4 feet off ground is a good choice. At 5 feet it becomes a sky-warmer. At lower height, efficiency and projected power fade.

I can see why you are satisfied with your antenna.
RE: 10-Meter Antenna -- The 'L' You Say  
by K1WJ on January 22, 2014 Mail this to a friend!
Dick what would be the numbers if I added another 1/4 wave radial to the west also 4ft off the ground? Would the effiency of 33% increase overall. 33% seems low to me...after all, it is a full 1/4 wave antenna. Is it beacuse the base of antenna is at 4ft?
thanks David
RE: 10-Meter Antenna -- The 'L' You Say  
by KK4OBI on January 22, 2014 Mail this to a friend!
Efficiency is good compared to Hamstick, Budipole, Screwdriver, etc.

A monopole needs something to reflect the radiator image. A single radial is the lowest common denominator. Horizontal radiation is cancelled by radials in opposition to make a more effective vertical antenna. Accordingly, raising efficiency is the age-old exercise of adding radials. Here is some modeling data at 14.1 MHz from "Analysis of HF Monopoles". (Spain)
1 radial, SWR 1.4, Eff. 16%, dBi -1.29, F/B 4.62 dB
4 radials, SWR 1.63, Eff. 27.6%, dBi -0.47
8 radials, SWR 1.75, Eff. 35.2%, dBi 0.56
16 radials, SWR 1.75, Eff. 39.9%, dBi 1.12
32 radials, SWR 1.75, Eff. 45.8%, dBi 1.75
64 radials, SWR 1.75, Eff. 50.7%, dBi 2.19
128 radials, SWR 1.75, Eff. 53.1%, dBi 2.37
RE: 10-Meter Antenna -- The 'L' You Say  
by K1WJ on January 23, 2014 Mail this to a friend!
The figures from spain are for a ground mounted vertical? 4 elevated radials are = to 64 radials ground mounted? That was my general understanding.
My 10m 1/4 wave vertical with 1 elevated must be better than 16%. 73 K1WJ
RE: 10-Meter Antenna -- The 'L' You Say  
by K8QV on January 23, 2014 Mail this to a friend!
This configuration works quite well with the Buddistick and one elevated counterpoise. It seems in practice that a similar horizontal dipole needs to be considerably higher to work as well, but I have no hard data to prove it - I imagine somebody does, though.
10-Meter Antenna -- The 'L' You Say  
by KJ4DAQ on January 23, 2014 Mail this to a friend!
Last month I made a 10 meter dipole antenna. I cut 5 meter 14 gauge copper wire available with me at that time and bought two 10 feet electrical conduit pipes, made the antenna in just one hour and mounted it vertically on the deck. Added a balun and I could make at least ten contact to Europe on 10 meter band. Being a dipole I do not require any radials.
RE: 10-Meter Antenna -- The 'L' You Say  
by KK4OBI on January 23, 2014 Mail this to a friend!
You cannot equate the Spain models for 20 meters to your setup on 10 meters. The value here is that it is a unique study because it included a single radial as he illustrated the effect of radials on efficiency.

On the other hand, I too wondered about the 33% vs 16% between your 10 meter model and his 20 meter model. We both use the 4NEC2 software so it would be interesting to sort out the difference.

RE: 10-Meter Antenna -- The 'L' You Say  
by K1WJ on January 24, 2014 Mail this to a friend!
Dick, Thanks for your time & input, interesting indeed. 73 David K1WJ
10-Meter Antenna -- The 'L' You Say  
by N8TI on January 25, 2014 Mail this to a friend!
I never really thought about an off center dipole using a vertical and then running the ground out along the ground. Not a bad idea when you are in the field.
RE: 10-Meter Antenna -- The 'L' You Say  
by KK4OBI on January 27, 2014 Mail this to a friend!
The modeling from 21 Jan of K1WJ's set-up should be of interest. At 4 feet off ground it performed best. Nearer to ground adsorbed signal.
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