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Old 10-23-2014, 6:49 PM
hertzian hertzian is offline
Join Date: May 2009
Posts: 1,982

I second the simplicity of the inverted-L, despite what appears below!

When using one, I recommend putting an inline 1:1 RF choke (Like an MFJ 915) AWAY from the feedpoint, so that you can take advantage of the both modes of the coax (common and transmission) serving double duty as both a transmission line, and a "counterpoise" of only one wire. This assumes that your coax is on the ground and not in the air. A 12-20 foot jumper (not critical) or so will do - as the ground is going to detune the common mode anyway - and of course another coax run from the other side of the choke to the shack.

Note: In my case, I use a short one designed mainly for 25 meters and higher in freq, with an LDG 4:1 UNUN at the feedpoint near the ground. The counterpoise is a 12-foot (not critical) lmr-400 jumper before it sees the 1:1 choke. From there, about a 20 foot run to the shack which keeps additional swr losses somewhat low. The system is purposely non-resonant, and the losses are low enough that a shack tuner is not really necessary.

It is only 18 feet total - 8 feet vertical, and 10 feet horizontal making it stealthy. At night, there is so much signal on the low bands anyway, that this still works ok on rx down to about 5 mhz. If I wanted better low-end, a total of 31 or 43 feet works well. Just saying that under certain circumstances, having a very long inverted L is actually not good on the high bands due to the reception pattern, which is already a tad high anyway.

If the coax is also the *one and only* counterpoise, then for the best general directional pattern, run it underneath the horizontal wire. Otherwise, your pattern may have a really funky skew to it. More counterpoise wires spaced around the feedpoint (also not critical in length when on the ground - more short is better than a few long) alleviate this issue.

Any version of bending will do, like half horizontal and half vertical, but any combination not exceeding about a 1/3 ratio either way kind of classifies what an inverted L is. My preference is to run as much vertical as I can before exceeding 2/3 of the length, otherwise I'm making a true vertical and losing some stealth.

There is more to it like additional swr loss, and reception lobes vs length of wire, but I'll leave it there for now.

The 1:1 choke away from the feedpoint accomplishes 3 things:
1) Coax still acts like a normal transmission line
2) Purposely using part of the coax common-mode as a *limited* choked counterpoise.
3) Helps prevent shack noise from traveling back UP the coax to your feedpoint and back to the shack again.

Ideally run more counterpoise wires. Still, without anything but the coax, the antenna is going to use the shield as the counterpoise anyway, so you might as well put a choke on it to accomplish the above - but to do so you choke it away from the feedpoint, not near it. If you choke it in the shack at the rig, you kind of defeat the purpose. You could choke at the feedpoint if you have multiple wires, but why not make use of the nature of the beast?

Last edited by hertzian; 10-23-2014 at 8:07 PM..
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