Well I just got done constructing my own jpole antenna for the first time I have wanting to build one of these for a while and since my 1/4 wave groundplane got broke from high winds a week ago I decided it is time to build a better antenna.
Just a few things I learned that may be of interest to anyone new to building one of these..
It does not matter which way you connect the coax cable to the feed point of the antenna, the ground braid wire can either be connected to the radiator or the matching section and vice-versa for the internal coax connector. Most pictures I have seen have the internal coax lead connected to the long radiator section, but some other online pictures show it connected to the short matching radiator. This confused me for some time. The good news is it can be done either way.
The bottom part of a jpole where both the radiator and the match connect (usually by a T when using copper tubes) is at ground potential. In other words it doesn't radiate anything and is safe to mount electrically directly to a metal tower. In fact doing so will help lightning protect it.
For lower frequencies the distance between the bottom of the antenna and where the coax clips on will be longer. I find for the middle of the FM broadcast band this is about 4 inches for a good 50 ohm match.
When using PVC pipe and installing the radiators inside you will need to shorten the length of the radiators slightly compared to when outside the pipe because PVC slows down the wave velocity factor.
I built my jpole out of a few short plastic pipes that extended into one peice that was used to hang a sports net. I simply used copper wire and taped it along the side of the pipe rather then putting it inside the pipe to make life a little easier. After a quick tune up I got the SWR down to 2:1. I still need to clip a little bit of the top radiator to get the SWR lower. One thing I will probably try is to cut a few inches off of the wire and then take a long screw and nut, solder the side of the nut on to the wire and then adjust the screw to fine tune the resonance of the antenna. This antenna puts out a lot of field strength. Just with a simple field strength meter it would register a reading on the other side of the house with just a few watts, compared to my 1/4 wave groundplane which didn't do nearly that good. I guess those decibels of gain that jpoles are capable of is no joke!
I will post pictures up soon enough once I get it spray painted grey.
For sure. After going for a ride around town I found that in areas that used to be dead spots with a 1/4 wave gp that we got good signal strength with the jpole. I probably shouldn't call it a jpole as mine doesn't look like a "J" lol. Okay 5/4 wave.
Here's my new Jpole type antenna or 5/4th wave whatever. I took PVC like material (though not exactly) and put the elements on the side and basically taped it using eletrical tape and soldered where the feed point was and then spray painted it grey. Simple but effective. 1.2 SWR as long as the rain doesn't hit it! and... {BTW the wire going to the right of the picture is a long wire antenna used for my AM transmitter along with SWLing.} I know it's never a good idea to show stuff like this but fuck the FCC if they really go out and take time time to identify this shit This is a good antenna. This one is 40 foot lower then my 5/4 wave antenna but yet it puts out a signal that puts my 1/4 wave to shame at the same height. Goes to show how a jpole kicks butt.
I seriously recommend to others here who are into FM broadcasting at <100 watts of power to build or get an antenna like this one. It's easy to build and if you aren't technical you could have someone else do it for you. I could but it's iffy as to how to ship it. I am willing to help you all out though. If anything I plan to write up a report on the jpole and show anyone how to design this antenna with ease. It doesn't require much and even though most of the designes require copper tubes and other BS in reality you can build this antenna out of anything. As long as the SWR is right and you mount it right it will outperform anything else I have ever tried myself!
Best of luck. Thank God these forums are taking off. Glad we have you guys here. Long live the Free Radio Forums.
Edit: BTW I used 6 turns of RG8 Coax (I used it through out the system) at 6 loops for the balun. It's spaced around 6". Not sure if this is useful info but whatever. My tower is grounded but my jpole is not eletrically grounded unlike most online documents say to do. I tested SWR a few feet above earth before testing, I don't see how it makes a huge difference. In reality from my experience it doesn't make a difference. As long as you clip on the coax at the right feed point at ground it seems to be just as good at 20+ feet levels.
This antenna is not connected directly to ground like many examples show. I honestly don't know if that makes a difference but hell why even worry unless it's higher then the trees? (as far as lightning) Just be careful.
Best of luck to all. I really do believe this is the worlds best antenna for pirates on FM so far. If I am wrong then please show me something better.
J-Pole antennas, are very effective and cheap to build. My custom dipole has outperformed everything else I have tried.
Very simple design; both elements radiate; as this is a balanced dipole. As you can see; the SWR is extremely low!
I just did a test using cheap TV rabbit ears, with surprising results! It outperformed both my Maxrad 6600 and 1/4wave dipole; and it was less than 12 feet above ground.
Traffic has been up over 400% on our Outlaw Radio site, and I just fixed a defective link to this site. There is a growing interest in micro broadcasting again.
J-poles rock! They kick some serious butt for FM DX'ing too. I think part of it is because they lower the noise floor,since they are basically a direct DC short at any frequency except (near)resonance. I was hearing all kinds of stuff when I used my J-pole on my tuner.
Well that antenna in the previous photos has been taken down. The thing didn't last long once the weather got to it. Also the rain really messed with the SWR a ton. It would be a tiny bit over 1.5 and then the rain would come down and the SWR would go up to 3! Well I shouldn't have expected much since I built it with an unknown type of plastic poll. Once the heat and then winter cold got to it the damn thing broke.
Oh well I just got done building a new one! This one uses the classic idea of using one long poll, part for the boom to connect to tower and the other portion used for the radiator. Then I simply used an L shaped piece of metal as a bracket to connect to where the tuning stub goes and used an old metal car antenna whip screwed into the bottom of the L as the tuning stub. A few plastic separators (made out of used plastic ink pens) to keep the whip from smacking the main element and connect it up and ready to go. Well that's to put it lightly. Tuning the thing was a mess. I added another whip to the very top with two pipe clamps to attach it. This allowed me to size the poll via the whip for best SWR. Once that was done I adjusted the two connections at the stub point for a good 50 ohm match. Then simply coiled 5 turns of coax 6" diam. for a balun right below the L bracket. Once all was done, voila perfect <1.5 SWR. Best of all the rain should NOT effect this design since the elements are out in the open, rather then inside or along plastic pipe like PVC. Upidy to the tower and now my back aches like hell. Oh well it should be worth it. Like the side chat said, I better never have to climb up to fix this thing again heh. Pics to come soon. This antenna actually looks kind of neat using a whip at the top and for the match stub section.
Here's the plans for the antenna I built drawn in paintbrush hehe..
i built a copper one and it tuned up real easy. my balun and feed line where 9913. i dont know about running high power into it though. we tested mine with a nicom running 125 and the transmitter started slowly folding back. we suspect the balun got pissed off and heated up.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
