Loop Upgrade

vagrant

ker-muhj-uhn
Joined
Nov 19, 2005
Messages
1,587
Location
California
Well, after reading a few websites today and using an online loop calculator, I upgraded my Chameleon F-Loop by doubling the efficiency. I did this based on the numbers for increased loop diameter, as well as the diameter of the conductor (the coax) itself. I was using 94" of LMR-400 that was supplied with the loop. I had 120" of LDF4-50A (1/2" Heliax) on a shelf. This upgrade I realized I had sitting on a shelf, others have known for decades. Still, I'm new here in this RF world. Anyways, for loops I learned the coax needs to be shorted out. I also needed to use N to PL259 adapters to connect the items I had. Rather than butcher a perfectly good N connector already in place, I shorted out two N female to PL259 adapters by cutting a bit of copper tape and pushing several of them down into each well of the PL259. I first notched the center bit of tape so the pin would easily, yet snugly fit through. I checked the continuity on a meter and it was good to go. (Later, each adapter got a bit of paint sprayed on it to make them easy to identify)

For testing, I initially swept various frequencies using the 94" of LMR-400 and then progressively with the 1/2" Heliax. Hell, it looked good before shorting it out, but it definitely improved after shorting. I use an inexpensive vector impedance analyzer to view the plots. Best results were from 15-20 meters. On 40 meters the efficiency is very poor, but it works. I would have to rig up something to hold 16+ feet of 1/2" Heliax to get some decent numbers on 40m and at that point things are less portable.

Anyways, $200+ for a solid aluminum loop sold by Chameleon was not a solution for me. I enjoy portable use so I needed that kind of solution, versus strapping a 36" diameter loop to the top of the vehicle. Also, 120" of 1/2" Heliax is very stiff, so it retains the loop shape when pulled open and does not require a vertical support. On 12 to 10 meters the shorter 94" LMR-400 is probably better.

I am unsure if the copper shield of the Heliax will be an improvement over aluminum, but as the aluminum is 1" in diameter I think it may win the numbers. Still...not as portable and $200 more.
 
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