WTF is a Shunt? Why do I need one? What is this Voodoo? Why would I short out my antenna?
I am going to take a very basic approach to WTF is a Shunt? Why do I need one? What is this Voodoo? Why would I short out my antenna? Shunts provide several benefits including static discharge, antenna impedance matching and SWR tuning, to keep it simple it helps in balancing a system.
There are two things you need to remember when setting up antennas, the SWR, tuning the length or the radiator (Whip or Coil on a Firestick type helical wound antenna) and the feed point impedance. This is where the coax attaches to the antenna, in most cases this SHOULD be a 50 ohm connection but is not.
Most transmitters, radios are designed to work at a 50 ohm impedance in the output to antenna portion of the radio, this is standard for our purpose on 11 meters and for the most part on Ham radio HF.
The coils you see in the images below are some coils that I wound from
This is a trial an error part, most calculators say that 3/4" with 5 winding's is sufficient for 11 meter antenna impedance matching most of the time. There are a lot of variables involved here and a constant is you should never need more than 12 winding's to accomplish a match.
One of the tools to do this correctly is an antenna analyzer, this will show you the feedpoint impedance and makes the guessing game a lot easier.
How do I tune an antenna with a shunt? The way I do it is to bring the antenna to the lowest possible SWR , then add the correct shunt into the circuit. By adding the shunt at this point it should show the same SWR or a bit lower. If it goes up try a different winding or if its adjustable re-adjust the shunt to match the last SWR you had without it. Once you obtain the correct reading you can adjust the antenna to see if you can now tune to a better SWR with a shunt in place. An antenna analyzer really helps with this.
Here is a GREAT article by KB0G regarding how and why to use a shunt. http://www.k0bg.com/match.html
His entire site is a treasure of information and if you want to really get your antenna working I suggest you make the time and read through his site http://www.k0bg.com/ .
If you do it correctly that pesky swr at 1.3, 1.4, 1.6 that you just cannot get lower will be 1.0 before you know it.
For the new folks that are reading this and are scratching their heads wondering WHY IN THE WORLD WOULD I SHORT MY ANTENNA, What is this voodoo? I certainly understand your question and it goes against everything we first learn about antenna's. As you get deeper into the subject you will understand that RF like water will look for the path of least resistance and it gets there by finding a perfectly tuned and matched antenna, you will also learn that there is voltage and current on your antenna and that RF is to a degree its own animal. Yes it gets a bit complicated but as you start understanding these principals your station will benefit.
If you have any questions regarding this please leave them here and we will do the best we can in answering them. There are many awesome antenna and electrical engineering types out there that I am sure can expand on this and I look forward to any constructive input given.
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