Help With an Inverted L

[chrissim]

Hello:

I'm confused on a couple of issues regarding an inverted 'L" antenna for multiband use. It has been suggested elsewhere that I use a DXEngineering BAL200H10AT (a 4:1) balun. Here is the first issue.

If the antenna is unbalanced as is the coax, why then would I use a balun? All I can imagine is that the impedances change as one cycles through the bands.

I've also always understood that one simply does not use coax for multiband use (other than proper harmonics of a resonant frequency). How are losses mitigated using coax and multibanding using an inverted L?

I also understand that an inverted L shows near 50 Ohm when the feedpoint is near the ground. What happens if the feedpoint is raised to 20 feet above the ground?

I've been experimenting with an L for 40 meters. The feedpoint is roughly 20 in the air with with a 50/50 vertical/horizontal orientation. I am using a choke at the feedpoint and it is cut for 40, but tunes up well and easily for 30 meters (and works well).

I'd like to bring the wire down and no longer depend on tree limbs for vertical support. Instead, I'd like to use aluminum pipe for the vertical, and a limb to support the horizontal wire. I'd like to raise the vertical further from ground level and run an elevated counterpoise system.

I have referenced the Antenna Book, Cebik's "Straightening Out the Inverted L," and Low Band Dxing but am still uncertain which path to take.

Thanks for the insight and suggestions.

 

[dsalomon]

To start, I am ASSUMING you are going to use this for transmit, so my answer will be tailored to that.

Re: balun, one side of the balun goes to ground and the other to the antenna. The impedance of a properly built and mounted inverted L is such that to get to 50ohms, you need the balun. This is not so critical for receiving as it is for transmit (but still matters). Also, an inverted L REQUIRES tuned radials, usually 3. Can you use it without radials...yes. However, your impedance will be off and it will not tune as designed.

Also, an inverted L should be grounded at the antenna.

The best way to feed an inverted L is with ladder line, up to the shack entrance. Put your balun at the shack entrance, close to where it will connect to the rig. Then use a short run of coax from the balun to the rig. Ladder line is MUCH lower loss than coax and more of your signal will get to the antenna than by using straight coax alone. You can try bringing the ladder line all the way into the shack and stick the balun right at the radio. However, you may or may not have RF issues this way. The only way to know is to try it.

A 20 foot feedpoint is a too high. They're generally mounted at about 10 feet, with the elevated, tuned radials immediately below (i.e. also at 10 feet).

Good luck - David, AG4F

 

[prcguy]

The best way to feed an inverted L for multiband use is with an auto antenna tuner at the base with the base being on the ground with lots of ground radials or a ground screen. The inverted L types I most often see are between 75 and 100ft long with about 25ft of vertical height and the rest horizontal.

On the lower bands like 160 through 40m they perform well as an NVIS antenna with a good amount of signal going upward. On the higher bands above 40m you get some usable low angle takeoff. When I'm on the east coast listening to 160m I hear a lot of people using an inverted L on 160.
prcguy