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1/2 Heliax questions


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#1 jsouth

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Posted 17 March 2020 - 09:17 PM

Just inherited some 1/2 Heliax and looking for some learning. I will be running about 75 feet. The bend radius will be difficult on both ends but I think will be easiest at the antenna for the drip loop. At the radio, I plan to use a 3 foot piece of LMR400 or the like to give the flexibility needed.
If I did the same at the antenna with a short jumper, will having two jumpers create too much loss?
Any advice will be great.

#2 Lscott

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Posted 17 March 2020 - 10:52 PM

If you’re going to splice in jumpers be sure to use a constant impedance connector like type “ N”. Every connector has a small insertion loss. Usually a fraction of a db for good quality ones. The small power loss shouldn’t be very noticeable. Also be VERY sure to weather proof the splice points if outside. It’s also a good idea to do the same at the antenna base where the cable connects.

#3 Jones

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Posted 18 March 2020 - 09:17 AM

I think you may be confusing a drip loop for a common-mode choke.

 

If you have a proper antenna, with a true unbalanced feed point, you do not need a choke at the antenna.  Those are only required if you have one of those cheap ham-type J-poles, or other balanced feed antenna.

 

If you do need a choke on your antenna, do NOT make it by coiling 1/2" line.  Leave the 1/2" straight up to the base of the antenna, and use a 3-foot jumper of RG-8x, RG-58u, or similar small coax to make your coil, which should be 4 or 5 turns about 6 inches in diameter.  You can also make a common mode choke by taking a foot-long jumper of RG-213 or LMR-400 and put ferrite clamp-on chokes all the way from one end to the other.  Again, on most decent commercial antennas, this is not needed.

 

A drip-loop is simply a low-point in the coax right before it comes into your building, so that rain running down the coax will drip off onto the ground, rather than get funneled into your house. This is not even needed in all installations, as sometimes the coax is not running down a tower, but rather across a roof, under a soffit, or into a conduit.

 

Short jumpers at each end aren't going to hurt you much. As Lscott posted, use type "N" connectors where-ever possible for lowest loss.


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#4 jsouth

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Posted 19 March 2020 - 01:47 PM

Great advice. Thank you.




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