2M Ladder Line J-Pole
Update: Since I wrote this, I've seen in several places that
coil chokes are not effective on 2M & higher. My experience
is that they may reduce your SWRs & make a more forgiving
antenna - but this could be bad & good - so do a little research.
Needed: two feet ladder line, 38" stranded-flexible one conductor wire, mini 8X stranded coax (enough to make the coil and hookup pigtail), PL259 with 8X Adaptor, eyelet (or make a loop at top), an SWR meter (many times old CB type SWR meters will cover 2M)
My 450 Ohm stranded Ladder Line 2M J-pole (resonant
< 147.0 MHz) with 25' mini 8X coax that I made for a Fox Hunt. Works well,
low SWR, I pulled it up to a tree limb & I was in business.
On the web everybody has different dimensions - so I made it easy, started long, & started snipping it down to down to where I wanted it resonant.
Overall mine ended up at 19.1" & 55.75" (very bottom to top of eyelet). To make it easy, I didn't cut into the bottom ladder spacer - I attached the coax immediately above the spacer (around 1 7/8" from bottom).
Start with a length of 57", (overall - bottom of stub to top of loop or eyelet) will need some extra length to make a loop at top. Probably cutting the short element at 19" to 19 1/8" (overall - bottom of stub to top) will give adequate results - then you just have to worry about trimming the length of the long element.
Choke coil is 4 turns of coax < 3" diameter.
Center strand of coax goes to the long element.
The coax length is your choice, make it short if you're going to run RG-8 base type coax to it, or maybe 20' to 25' for portable use. You wouldn't want to support a lot of heavy coax with this antenna - it's sturdy, but eventually a solder joint or the eyelet would fail, or the braid may cut through the insulator and short out the coax.
Careful with this step, the heat may melt and cut into the insulation where the coax attaches to ladder line and short the coax if the conductors are not relaxed before soldering.
Once the antenna is finished - seal the coax with silicon to keep water from running down into the jacket. Maybe even some tape over the silicon would keep the silicon from going bad from exposure to the sun and elements.
Don't forget to understand-how-it-goes and slide the PL259 barrel-part on the coax first before you start soldering - tad aggravating if you forget this.
Do your own PL259 soldering method - but I'm not a fan of soldering down inside into those 4 holes, with or without an adaptor. As shown, I flatten out the braid in two or three divisions & hold it down with some steal wire (won't stick to solder), and solder it to the adaptor, this way you can see how it's going and what's been melted.
You may want to put on the PL259 first and ohm it out to make sure it's open - because the antenna itself is a dead short - once it's all connect, you can only hope for the best.
This way, if you mess up soldering, you can undo it all and save your PL259 and adaptor.
Now just screw it into the PL259 and solder the tip. If you don't trust the screwed-on-electrical-connection, you can pre-tin a spot on the adaptor where it touches the PL259 and put a touch of solder - then it's all a electrically dependable connection.
If this is permanent outside antenna using a barrel connector - then be sure to wrap this area with tape (bottom to top) a couple of times to shed and keep water out of the PL259s and coax.
Tuning example for resonance around 146.0 to 146.5:
Let's say you start at 56.5" and you tune-transmit from 144 to 147.5 and the SWR goes from low to high - that means the antenna is now too long because it's happier (more resonant) at low frequencies (a long antenna) and your tuning up to a high frequency of 147.5 which would be a short antenna.
So you cut 1/2" off the tip (cutting makes a shorter antenna = resonant at higher frequencies), now getting close to acceptable - but SWR is still going up when tuning up to 147.5,
So cut another 1/2". Now the SWR goes from high to low when you tune up to 147.5 = means you have cut too much off the antenna, you've made a short antenna - which is happier at higher frequencies.
So now your antenna is too short, so now what - you solder some length back onto it, be happy with it, or start over.
So in the 2M band: 1/4" can equal
about 2 MHz in bandwidth
- so once your just a little on the long side then only trim a 1/16" to 1/8" at a time.
Everything affects the resonant-spot on a homemade antenna, distance and stability between parallel elements, not having a choke coil, size of the coax conductor "Y or T" split and how-where you connect to the elements (once the center conductor is pulled out of the braid - that is where the antenna starts). Always make the driven element longer (slower frequency) - then trim it down to raise the frequency to where you want it.
73 AA4UK ( if you make one, let me know how your antenna turned out )
I will probably redo where the coax attaches and have the center strand of the coax pulling straight down on the long element and have the braid much longer and go the width of the ladder line over to the short element - this would give the antenna more strength, and help detour some of the insulation cutting from the soldering heat.