10 meter J-Pole antenna
I'm beginning construction of a 10 meter antenna for installation onto the top of my palm tree. It will end up being about 35 feet above ground level to the tip of the antenna.
I have enough wire to make a 1/2 wave upper section but I am a little stumped on the bottom section. I plan to use 300 ohm twin lead for it. I need to know if simply making a 1/4 wave section using the 300 ohm twin lead, connecting it to the 1/2 wave section above it, and then simply tuning it by finding a resonant point near the bottom of the "J" section is all I need to do.
Is it that simple? I have made a 2 meter J-pole and it was that easy using all 1/2" copper pipe. I guess I just need to know if the 300 ohm twin lead will function as the "J" section.
Thanks and 73's de N7USR Greg
The matching system below the the half wave radiator is not exactly a quarter wave long but close to it plus a shorter shorted stub across the coax. The high impedance at the end of the radiator is transformed down to a lower impedance (50 Ohms) about an electrical quarter wave down the 300 ohm ribbon plus some capacitive reactance that you tune out with the short shorted stub. So two things need to be juggled- the tap point down about a quarter wave and then the length of the shorted stub. Basically you adjust for 50 Ohms with some remaining reactance (Z = 50 - jx )that gets tuned-out by the shorted stub (inductive reactance) at the bottom. I think the shorted stub is about 0.015 wave lengths long ( a shorted stub less than 1/4 wavelength is an inductive reactance so X is minus jX. The result is 50-jX+jX resulting in 50 Ohms for a very nice match). It takes a bit of tinkering but the results will be satisfying This is a good learning project to help understand the Smith Chart. When you understand the Smith Chart, you can visualize the whole matching process in your head. 73,
Try something like K6AER's, but scaled down for 10M. Here's the link:
73, Steve, NL7W
Not in but around Palmer, Alaska
Avatar: my Iditarod sleddog mutt - Yukon
"Saruman believes it is only great power that can hold evil in check, but that is not what I have found. I found it is the small everyday deeds of ordinary folk that keep the darkness at bay: small acts of kindness and love. Why Bilbo Baggins? Perhaps because I am afraid, and he gives me courage." - Gandalf the Grey, The Hobbit: An Unexpected Journey
Thanks a million !
Hey thanks ! I appreciate the replies.
I'll have a look at them and see if this helps me. I have seen the Smith chart in books and other places but never had anyone help me understand it. I guess now is the time.
I used this link to build one
Sense only the top 1/2 wave does the radiating you can make the matching network out of coax instead of 300ohm, I worked great for me:
1/2 wave aluminum tubing is alot easier to deal with than a full length J pole
Have fun N4aeq
Can you are useful such scheme of the co-ordination?
This is realized on resounding coaxial "glass".
The whole this system tunes in in 2 stages:
1 - At first, tunes in the glass - is simply changed its long. First, it is necessary to take the cable, little longer of design, but afterwards checking on SWR-meter it is necessary to find the minimum. Abbreviating glass - adjust the minimum on necessary frequency.
Then, similarly, possible select the vibrator long .
The not bad antenna is got.
No truths in extremitys... she is somewhere between...
This is how Mike, K6AER built his J-pole. You can easily scale it down for 10 meters.
I am not personally fond of a J-pole. Results are not necessarily repeatable as they are with other type of antennas. I do discuss some of the J-pole myths here:
Terry Graves, K7FE
Chief Editor, QRZ.COM
"Some people call CW a MODE but in
reality it is an autonomous LANGUAGE."
That's a sizable vibrator.
Originally Posted by RU9CA