I don't know why I didn't research this more in the past but if I am reading correctly a basic J Pole really has no more or less gain than a Dipole if designed correctly.
Since a J Pole is essentially an end fed dipole with a matching stub then the gain should be equal to that of a simple half wave dipole correct?
I always assumed in the past that a J Pole had slightly more gain over a dipole but how can that be?
The only true benefit I can see after researching J Pole antennas is that you can match it directly to a 50 ohm coax, or even other impedance whereas a dipole is stuck at around 72 ohms.
Has anyone here actually compared these two antennas while properly tuned and in ideal conditions and compared?
I used to use a J Pole all the time in the past, and besides the convenience of an end fed antenna makes for a nice mount Now that I have moved I am back to square one and deciding which of these two antennas to install, and a center fed dipole would work fine in my current situation.
On a side note I also learned that the number of turns of an open feedline balun does make a huge difference. Most sites suggest 6 turns at 6" spaced properly (not bundled) to remove common mode currents traveling along the coax. I found that this actually is a bit overkill and can even cause problems when using too many windings in a open air coaxial balun. I found by taking my field strength meter that while the first 3 or 4 feet of coax going to the antenna was nulled, that at a distant section of the coax I found standing waves radiating along it
I then experimented with different amount of windings and space and found with RG58 that no more than 4 windings about 4" diameter worked the best and reduced all standing waves along any length of coax. I'm willing to bet different types of coaxial cable will affect this greatly. I bring this up because many of you probably have opted for the one size fits all 6 turns 6" diam. balun as shown in many J Pole/Dipole plans, but as I found that doesn't always work to be the truth and experimentation with this can greatly improve your antennas performance.
Basically it says what I already figured, both antennas are damn near identical and what becomes most important is how you want to mount it, center fed or bottom fed. The only real difference comes in with a 1db gain towards the broad side of the matching stub, which assumes so because of the characteristics of the stub matching section.
So this brings me to another thought... J Poles do have directional characteristics! Has anyone ever thought of this here? I had an ah hah moment there. So that's why J Poles perform great for some people and less than perfect for others over a dipole because they fail to realize that the goofy thing is slightly directional, and as we all know with directional antennas, they focus energy in some directions thus naturally giving more gain (like a flash light lens pointing at an object).
So am I way off base here or am I right? J Poles aren't better, they are flawed in such a way to make things work better by accident? Sorry to crash the party but this has been bugging me for some time.
The J-Pole is a DC shunt fed antenna vs the open end dipole. The biggest benefit is your antenna is at ground potential and eliminates static buildup and provides a direct path to ground for any strikes.
A dipole, unless matched with a matching transformer, is wide open and only the ground element would have any direct contact with system ground, leaving the top element unprotected. Most solve this issue by using an inductor at the TX output or after the LPF to provide that DC path for static or strikes.
Safer however to have that discharge taking place out there at the antenna rather than inside the TX.
A J-Pole has a lower angle of radiation than the dipole. However a dipole can be stacked and fed with a phasing harness and provide gain. Folded J-Pole antennas, or Slim Jim antennas can be stacked and fed with phasing harnesses for gain as well. But both the J-Pole and Slim Jim antennas require a bit of beefy mounting schemes because they present a good wind load to the mounting structure. I remember a member here put up a J-Pole and used hose clamps to secure the antenna to the mast. Unfortunately a fair wind came along and tore that thing down like snapping a toothpick.
J-Poles are not really better, they are just another type of antenna and like anything else, has it's benefits and flaws.
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My rule of thumb is: if you want to TX flatside, use a dipole. If you want to TX vertical, use a J-pole.
Can you explain why that is? I must be missing something here. From my studying the dipole = the jpole besides feed point. So why would any of that matter if all was perfect, the sun was shining just right, the moon aligned with my cats ass and so on.
The only difference I can see with the two antennas besides the obvious feed position is that with the J Pole it could accidentally radiate from the matching stub, but in a perfect world that wouldn't happen. So what is the theoretical differnece?
I guess that's what I am asking.
BTW my cats ass has nothing to do with this, though he has been known to adjust faders on the mixer when I am out of the room, and screw with the PC fan, but that's for another thread.
J-pole is an end-fed device. Ideally suited for vertical.
Dipole is a true balanced device that gets messed right up if supported from an end.... unless you go coaxial dipole. Therefore the dipole shines flatside.
Vertically supported dipoles are OK in a colinear array (usually folded dipoles). Not always practical at 3M wavelength compared to a J-pole. You can use them in a Yagi, as long as you watch resonances around the support (mast/guy wires and tower).
ARRL Antenna Anthology goes into more detail on this.
Wow okay I just really confused myself after reading up a lot of stuff on J-Pole designs.
Today I got a basic J up and running but ran into the same old familiar problem with common mode current traveling along the coax even when using a coax air balun.
Two things I read that kind of amazed me a bit..
The nominal "ground" point on a J-pole SHOULD be the center of the bottom shorting bar. If the shield of your coax is grounded at that point, adding some radials could reduce the common mode currents - but that configuration is less prone to them than most designs that hams build. Connecting the shield an inch or two up on one side of the stub means that either the stub isn't balanced, or the shield is going to be hot with RF. Adding radials at the point where the shield connects to the antenna with such a design may reduce the current on the coax (or it might not), but likely will also make the balance WORSE for the matching stub and may increase currents on the support mast instead.
and.. www.w8ji.com/end-fed_vertical.htm On this page^ I see a few graphs that show using the center conductor to the main element and the ground conductor to the stub produces a better pattern. Now I know most websites show it connected this way in the first place, but there are some ham sites showing the connection reversed (ground to long element, center to stub), and to confuse even more some people say it makes no difference, which in theory would be true if the antenna is perfectly balanced? Of course J-Poles never really are perfectly balanced radiators which is why common mode current is seen along the coax when using no balun.
Ugh, so here I am scratching my head again. I will try clamping on the coax with the ground lead to the center of the bottom shorting bar and the center conductor to the long element, then adjust that to get best SWR. Curious to see if this does indeed work better as mentioned in the quote above.
As I have found out the hard way in the past, just because the SWR meter shows a nice low reading like 1:1.5 or less doesn't necessarily mean the J is radiating properly. I find using a field strength meter along with the SWR meter is a must to proof the setup.
I forgot how fussy J-Poles can be. Get the thing working great in one location and it works like garbage when placed somewhere else. I won't dismiss the fact that it works damn good when properly setup though, which after all is my goal ;D
I just about have my new J-Pole built. Comparing it to a simple cut dipole that I taped to two yard sticks taped together and A/Bing them I found almost no difference between the two for relative field strength.
I am having some issues with the balun on the J-Pole, but heck I had similar issues with the temporary test dipole.
Funny thing I noticed is that almost all the sites that have the calculators give me numbers which seem to be a bit short with the calculated frequency. No problem though as I made the stub fish hook portion of the antenna clamped and able to be moved up or down the main pole to adjust frequency.
I plan on eventually making this a "Super J-Pole" by adding another 1/2 wave section on top with a 1/2 wave matching section. This should double my gain if all is tuned properly but will make for a 12 foot antenna by itself for ~96MHz!
I made this whole antenna adjustable from the matching section clamps, to the whole J section being adjustable, and the stub being able to be pushed closer or farther away from the main element.
So I have lots of ways to tweak this antenna to get every last bit of resonating signal out of it.
Curious if anyone else here has made a Super J-Pole, or has used stacked Dipoles and what your experience was with it, or in comparison.
When I get done building this thing I will post some pics and info on how it works out for me. Here's a super J calculator for anyone interested in making their own... www.hamuniverse.com/superjpolecal.html
Like I said I found the length of the main poll should be a few inches longer than the calculator says it should be. Experiment, experiment, experiment.
I find the calculations on that page are more correct than the online j-pole calculators for ham radio work. Not sure why it's different, but I found from my own tests that my antennas resonating dimensions were more accurate to the calculations on that page than on the Ham radio calculator page sites.
With FM broadcast for some reason the dimensions seem to have to be bigger than even calculated from the online calculators dedicated to ham 2 meter plans.
I found this out the hard way after using a field strength meter near the antenna and hand adjusting the elements. After finding the proper resonating point (highest current field strength) the measured lengths were identical to that page.
I tried the super J-Pole and compared it to the regular J-Pole and didn't notice any difference in signal strength. Then after thinking about the gain factors I can see why it makes little difference. The theoretical gain would be a bit less than 3db which is negligible over the regular J-Pole.
After some research the gain of the super would probably be much less than 3db anyways, more like 1.5 to 2db in reality because of the lack of space between the top half wave element and the bottom. So I figured it's probably not worth the effort. Besides it makes tuning up the J-Pole that much harder when you tack on another half wave section plus the phasing stub.
The gain may be useful if you were to run a great length of coax to make up for the loss in coax, but the gain factor is still so little in comparison that I don't see the real advantage.
I may change my mind in the future, but for now with driving around and taking S meter measurements I don't see much of any advantage.
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.
Also studying a bit on coax length vs decibel loss I realized how little difference 10s of feet of less/more coax makes if using something like RG8/U or better. So adding a bit more coax to get that antenna up 10 or 20 feet higher makes a huge difference in signal strength compared to the loss added with the extra coax. This also applies to using phased arrays like stacked dipoles, super J-Poles and so on. Buying expensive LMR-400 or 600 coax won't help a heck of a lot either. The loss at 100 feet of coax is around .6 or .7db. My old cheaper RG8/U has 1.7db loss at 100 feet. Take the difference 1.7-0.6 is only around 1db loss. That's so little that going for the expensive coax shouldn't make any noticeable difference!
As the old rule applies... height, height and height makes all the difference on FM broadcast.
Hey All: In this post it states that "The fully constructed antenna is then clamped to a grounded metal pole" How important is this? my j-pole is attached to the roof and is not grounded, should it be? Thanks. Sixer
Hey All: In this post it states that "The fully constructed antenna is then clamped to a grounded metal pole" How important is this? my j-pole is attached to the roof and is not grounded, should it be? Thanks. Sixer
In my opinion it really doesn't matter and probably is easier to tune up the J-Pole when it isn't mounted to a metal pole or tower. The rule of thumb is to attach it to your support mast and then do any final tuneups. With a metal pole that can make things slightly more complicated. The only real trade off I can find is that being on a metal mast allows it to be grounded directly to protect against lightning.
The bottom of the J where the two poles connect is technically electrical ground, so anything below that shouldn't matter metal or not. However in real life a metal mast does seem to make things act screwy if not properly tuned up and can accidentally act as part of the radiator if things aren't damn near perfect in the final setup. Heck even placing the coax against the metal pole seemed to effect my SWR a bit making the balun turns critical.
However I am in preference in using the grounded mast for the added lightning protection and support strength so for me it was worth the extra work.
As far as performance, metal grounded mast or not, both setups should perform near identical if properly setup. YMMV.
Last update (I think) before I take some pictures and upload..
Today after playing around I seen that my J-Pole was slightly higher in frequency than intended. It had lower SWR at 97.3 than it did 96.9. So took it down and did a tuneup.
Odd thing I noticed is that the length between the bottom of the J joint and where the feed point is was 5 1/2" instead of 5". I guess it all depends on frequency and other odd factors. I got 5 1/2" AFTER tuning up the stub and main element. It was quite the job, because tuning one element screwed with the other, and the end goal was to find lowest SWR while also finding the proper resonating point.
It's interesting how the online ham radio calculators for J-Poles all miscalculate that distance. 2.5" may be around the right feed point for 145MHz but for 96MHz it's around 5 1/2" for me. The link I shared earlier above confirms that for FM broadcast frequencies that it's around 5 or so inches from bottom to feed point clamps.
It seems funny to me that when I think of this antenna and how it's tuned to resonate at the frequency I want it reminds me of a trombone musical instrument. In a way they do have similarities!
When I had the antenna down I used a hot glue gun to glue around and cover over the back end of the SO-259 (socket) connector. This is a good tip I learned you can all do. Hot glue has no relative resistance so you can use it to glue a big bulge on the back of the connector where the soldered wire comes out to go to the clamp on the elements. This will protect water from screwing with SWR, or ice and snow causing a resistive path from the outer part of the connector to the inner on the back of the 259 socket.
With this antenna as is I get an SWR of 1:1.3 I do believe this is slightly erroneous though because even with a proper dummy load I get 1:1.2. So it's possible the SWR is even lower than suspected. It would probably be 1:1 SWR if it weren't for those old PL-259 connectors, or the old coax, or the position of the moon and my luck that day.
Sorry for making this thread a long ramble but I think it will be helpful to those like me who appreciated posts like this when I just started out.
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