The online calculator I used for my j-pole had the feed point at 2.88 inches for 96.3mhz , I trusted these numbers so I soldered my clamps to the pipe for better contact. Now after reading this thread I know I need to get up on the roof and and re tune it I think that 5.5 inches would be a good starting point.
Kage can you tell me the other lengths you got for 96mhz. My swr meter died recently so I am waiting for a new one. As I said in another post I ordered a 5/8 wave from AAREFF it just cleared customs this morning and is on its way from Montreal I should have it in a week or so. Sixer
Kage can you tell me the other lengths you got for 96mhz. My swr meter died recently so I am waiting for a new one.
I got right around 90" for the 3/4 wave section and 30" for the 1/4 wave stub with a spacing of exactly 2 7/8" between the poles. As said I got around 5 1/2" between the bottom and where the clamps are. I think the clamps themselves may play into part of that so it may actually be more close to 5".
I wouldn't even touch the tuning of a J-Pole without the SWR meter. It's just next to impossible to get correct by just guessing. At the bare minimal I would at least use a field strength meter and run the transmitter on low power and adjust the elements and feed point for highest field strength near the center of the 3/4 wave section.
The best way to tune up is using the SWR and the field strength meter if possible. If no field strength meter is available then just tune it up for lowest SWR and test your transmitter above and below the frequency you plan to use to find where the center of the resonating point is. Once you find that point it should have the lowest SWR on your channel, but will rise in SWR when tuning the transmitter up or down from your frequency.
Once working properly it should give you the same field strength that a regular dipole would, so if you have a spare dipole antenna laying around (or make a quick one) you can compare the two to make sure the J-Pole is working up to snuff.
Thanks for the numbers, I know guessing is not the way to do it but my swr meter failed last week and I just built this antenna to get me by until my 5/8 wave shows up. sixer
So after measuring and tuning to your specs. my reception and range are GREAT ( I know get a swr meter) I had to lengthen my main by 4 inches and move my feed point about 3 inches but everything sounds great now. I got my new antenna today but can't install it until wednesday, with it properly tuned ( from the factory) and an advertised gain of 4.8dbi it should be an improvement over my homemade untuned j pole. Thanks again for the numbers. Sixer
So after measuring and tuning to your specs. my reception and range are GREAT ( I know get a swr meter) I had to lengthen my main by 4 inches and move my feed point about 3 inches but everything sounds great now. I got my new antenna today but can't install it until wednesday, with it properly tuned ( from the factory) and an advertised gain of 4.8dbi it should be an improvement over my homemade untuned j pole. Thanks again for the numbers. Sixer
Let us know how the two antennas compare. If the J-Pole is working correctly which it sounds like it is for you then it should have around 3dB or more of gain over a 1/4 wave ground plane antenna. I assume your 5/8 wave 4.8dB would also be that of compared to over a ground plane.
So that would theoretically be a difference of 1.8dB of gain between your J-Pole and your new 5/8 wave antenna. That's actually not a large enough difference that you will probably notice any difference in reception results as long as your antenna is at least 25 feet off the ground.
Of course theory != reality all the time. So definitely let us know how all works out!
I don't have a lot of options for mounting antennas, the new one is an aluminum design and weighs about half of the copper j-pole so I should be able to get it higher. I will let you know tomorrow... Sixer
I can see where dual phased dipoles would be no different. They would also have 3db gain over a single dipole. It takes 6db gain to notice much of a difference on the receiving side, which would mean you'd need 4 phased stacked dipoles to double the "power" over a single dipole if I have my math correct.
Phased dipoles give much more usable signal where you want it! It's all to do with the radiation angle. A lot of the energy from a single dipole goes (almost) straight up - this is wasted energy! When you stack dipoles, you get a radiation shape that looks like a doughnut - flat to the ground.
VHF Band II signals always go to the horizon - make the horizon distance as large as possible by putting your antenna as high as possible. Transmitter power just determines how strong the signal is when it gets to the horizon. Stacking dipoles also helps at distance because the field strength is doubled where you want it - little of the signal is wasted upwards or downwards.
I got my new 5/8th antenna up today so now for the comparison,
I definitely am getting more range than the guesstimated j-pole, maybe a mile or two, but I notice the signal is way clearer all the way out,less static for sure.
South and west I'm at about 5.5 miles, north and east about +-3.5 miles. South and west is clear all the way out but north and east there are houses trees mountains hills etc.
Is this normal for <7 watts of power? I am covering a little more area than before so I am hoping to get some new listeners on board. Sixer
Phased dipoles give much more usable signal where you want it! It's all to do with the radiation angle. A lot of the energy from a single dipole goes (almost) straight up - this is wasted energy! When you stack dipoles, you get a radiation shape that looks like a doughnut - flat to the ground.
Yep. And changing the phase of the upper bay/s to the lower bay/s allows for sharper "beam tilt" to really get that signal down where it's really needed, able to get inside buildings and congested areas. Stacked C-POL's give that and help eliminate the multipath.
THIS may be of interest as it focuses on exactly this issue, and the "ZORCH" effect.
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South and west I'm at about 5.5 miles, north and east about +-3.5 miles. South and west is clear all the way out but north and east there are houses trees mountains hills etc.
Is this normal for <7 watts of power? I am covering a little more area than before so I am hoping to get some new listeners on board. Sixer
Sounds about right to me. Of course so many things can change that though like terrain, how open the area is, time of year, and how large/small your city is.
For me I get about 2 to 3 miles into my city with 5 watts and a J-Pole. I live on the edge of town though. Now if I take the back roads outside of town I can get a clean signal out there at least 5 miles. In fact it seems the city is where the troubles begin. When in the middle of the city my signal is alright in the car but I get dropouts when driving because of tall metal buildings and such. When outside of town that signal just keeps going and going, even with the city between the radio and my station. It seems that as soon as we get on the outside of town that is when the signal comes in flawlessly for miles.
It should be obvious that height is the issue. Once you get above the level of most of the buildings then all will be much better. Of course even then though there is the issue of the receiving end itself being between metal buildings or in a radio "dead zone". It's inevitable, even with powerful radio stations that have all the money in the world to get huge towers and powerful transmitters.
I found this page and the chart shown to be very close to accurate for the average setup.
Keep in mind for FM broadcasting which uses a larger bandwidth than ham radio work in VHF that you can expect slightly less range than even that chart shows. Especially with stereo which introduces its own noise with distant signals. But it does give a general idea.
Random thought... I was reading where J-Pole antennas have a lower angle of radiation if the antenna is decoupled from the tower itself electrically. Well maybe this can be an advantage to people who can not setup a tall pole/tower. If the electrical coupling characteristics of a metal mast throws off the j-pole so that it shoots at a higher angle then why not utilize that for low mast antenna installs?
Maybe this is a bit of a goof but I actually did find that my j-pole works best on my new install while directly electrically coupled to ground to my tower, however I am only at 30' here before the antenna. Maybe at higher elevations of mast over ground the opposite would be true and to use the antenna decoupled electrically from the metal mast?
From all the papers I have read it sounds like the difference between the two (decoupled/coupled to mast electrically) makes little difference besides take off angle and lightnight protection. Supposedly decoupled is the way to go since the mast will have less effect with common mode currents along the coax if not using a balun which most of us use anyways.
Just found this tidbit of idea interesting. Maybe sometimes it is better to use an antenna with a higher angle of radiation when we do not have the height of tower to support it, like take for example a 1/4 wave ground plane antenna near... the dirt. Makes little sense to push a low angle radiating antenna towards the horizon through houses and buildings if low to the ground!
Maybe this is why some people swear by 1/4 wave groundplanes compared to horizon shooting 5/8s or J-Poles? Maybe it is nothing more than the height of the aerial subjective to its surroundings as to how it performs in reality? This leads me to seriously reconsider what I know about antennas. Maybe the worst antenna for someone may be the best answer for someone else all depending on topography?