A conductive mast becomes part of the radiating element. I just proved this with my 70cm two bay J-pole collinear. I got it to tune-up by itself but it changed after lifting it aloft on a metal mast. Yes, I grounded it. I used a Nano VNA with a wide frequency sweep and observed a comb family of SWR notches in a "V" going both higher and lower from the design frequency. These notches moved around while sliding my hand along the mast. To solve this, I installed a pair of back to back pipe fitting transitions, solder to thread, and isolated with a very short threaded PVC section, right under the "T" fitting. Problem solved! Grounded or not still has this problem as there will be standing wave reflections from either a short or open. As reported below, if you are determined to have a DC path, then a quarter wave "hairpin" stub at the isolator might work. A mast is usually cut to an arbitrary length and it's the luck of the draw if it is resonant at the design frequency. This can explain why some people have good results while others not.
So how should you install a j pole. I am getting my technician license soon and am eating to set up a small base station with a j pole... Now I'm confused??? I was going to use a old DTV dish mast but now am thinking not the best way to go.. any help will be awesome thanks..
In stall it on a metal mast if you like, and ground it, because this is nonsense, and grounding the thing is safer for you and your equipment. As far as radio is concerned though, it makes no difference at all.
Sorry, I think this is wrong. DC ground and RF ground are completely different. The j-pole transmission section will be lowest resistance at the frequency it is designed for, and the mast will appear as a high resistance at radio frequency. In any case, the coax of the feed must be grounded before it enters your shack for safety and noise reasons, no? That ground is attached to your antenna and a jpole has a short in it. Same thing.
Thanks for your comment. I have studied this phenomena quite a bit more since the posting of this video. Much of this is documented in patent US10468743B2.
What about static buildup? It doesn't take much charge to fry electronics. If the j-pole is mounted on a plastic mast, can I still ground the antenna cable at the other end?
So long as you provide an adequate choking impedance on the feedline at the antenna end, yes. More information available here... www.hamradio.me/interests/j-pole
A "Mast Decoupling Stub" provides the benefits of choking the mast at the antenna operating frequency, but allowing full conductivity at frequencies below. More info here... www.hamradio.me/?s=mast+decoupling+stub
This is a simulation? And yet, this doesnt factor in that the feed point is not at the base of the J? In fact, the feed point should make the impedance zero at the base of the jpole, so anything below should not be radiating at all.
The current through the base of the J in the J antenna is maximum while voltage is minimum resulting in the lowest impedance, but never zero or no power would flow through this point as it must. There's been much more work on this topic since the release of this video to confirm the tendency for mast currents to flow. See U.S. Patent #10,468,743 for additional details including results from real measurements and a novel solution to thwart mast effects.
I looked on the Arrow site and found GP radial antennas, J antennas, but didn't see J antennas with radials. Regardless, the use of a radial or two 1/4 wave beneath the bottom of the J has merit. I am attempting to patent a similar method using a mast decoupling stub... www.hamradio.me/interests/j-pole/
+wimpog97 Think of this as the need to "RF" isolate. The J-pole as it stands does nothing to prevent flow of current down the mast. A stub can fix this as shown in the article here... www.hamradio.me/antennas/j-pole-antenna-grounding-have-your-j-pole-and-ground-it-too.html ...and a copper cactus approach here... www.hamradio.me/antennas/mast-mountable-j-pole-antenna.html
+wimpog97 If you can live without the robust lightning surge capability of the continuous copper pipe to mounting mast, this is a good plan. K6MHE offers some tips in his writeup... techdoc.kvindesland.no/radio/vhf_antennas/20061030163006789.pdf
+wimpog97 They way to think of it is it's better to "RF" isolate it from the mast. Physical separation via, for example, a piece of PVC certainly performs that function. I build the Mast Mountable J-Pole and use a mast decoupling stub to provide a measure of stopping RF currents from flowing while preserve the ability to flow static charges and lightning induced energies to the antenna mount. Think of the mast decoupling stub as a kind of notch filter that only blocks at the design RF frequency (plus some harmonics).
Ryan Toomey I’ve seen them grounded and un grounded both with excellent results, I myself would think grounding would eliminate static discharge and create a quieter noise floor... what do I know? Lol
Your wrong you should not ground a jpole Ryan Toomey. If you ground it you'll have to re cut your jpole because your adding more length on to it which you shouldn't be .so don't ground it .
A conductive mast becomes part of the radiating element. I just proved this with my 70cm two bay J-pole collinear. I got it to tune-up by itself but it changed after lifting it aloft on a metal mast. Yes, I grounded it. I used a Nano VNA with a wide frequency sweep and observed a comb family of SWR notches in a "V" going both higher and lower from the design frequency. These notches moved around while sliding my hand along the mast. To solve this, I installed a pair of back to back pipe fitting transitions, solder to thread, and isolated with a very short threaded PVC section, right under the "T" fitting. Problem solved! Grounded or not still has this problem as there will be standing wave reflections from either a short or open. As reported below, if you are determined to have a DC path, then a quarter wave "hairpin" stub at the isolator might work. A mast is usually cut to an arbitrary length and it's the luck of the draw if it is resonant at the design frequency. This can explain why some people have good results while others not.
So how should you install a j pole. I am getting my technician license soon and am eating to set up a small base station with a j pole... Now I'm confused??? I was going to use a old DTV dish mast but now am thinking not the best way to go.. any help will be awesome thanks..
i realize I'm kind of off topic but does anyone know a good place to stream new movies online ?
@Danny Koda i would suggest Flixzone. Just search on google for it :)
In stall it on a metal mast if you like, and ground it, because this is nonsense, and grounding the thing is safer for you and your equipment. As far as radio is concerned though, it makes no difference at all.
Sorry, I think this is wrong. DC ground and RF ground are completely different. The j-pole transmission section will be lowest resistance at the frequency it is designed for, and the mast will appear as a high resistance at radio frequency. In any case, the coax of the feed must be grounded before it enters your shack for safety and noise reasons, no? That ground is attached to your antenna and a jpole has a short in it. Same thing.
Thanks for your comment. I have studied this phenomena quite a bit more since the posting of this video. Much of this is documented in patent US10468743B2.
What about static buildup? It doesn't take much charge to fry electronics. If the j-pole is mounted on a plastic mast, can I still ground the antenna cable at the other end?
So long as you provide an adequate choking impedance on the feedline at the antenna end, yes. More information available here... www.hamradio.me/interests/j-pole
You are correct. IMO this guy is wrong.
Interesting.... How was this simulation created?
FDTD was used to create the time step by time step simulation.
what about lightening?
A "Mast Decoupling Stub" provides the benefits of choking the mast at the antenna operating frequency, but allowing full conductivity at frequencies below. More info here... www.hamradio.me/?s=mast+decoupling+stub
This is a simulation? And yet, this doesnt factor in that the feed point is not at the base of the J?
In fact, the feed point should make the impedance zero at the base of the jpole, so anything below should not be radiating at all.
The current through the base of the J in the J antenna is maximum while voltage is minimum resulting in the lowest impedance, but never zero or no power would flow through this point as it must. There's been much more work on this topic since the release of this video to confirm the tendency for mast currents to flow. See U.S. Patent #10,468,743 for additional details including results from real measurements and a novel solution to thwart mast effects.
alright cool but what about ground radials like what arrow antenna sells for their jpole
I looked on the Arrow site and found GP radial antennas, J antennas, but didn't see J antennas with radials. Regardless, the use of a radial or two 1/4 wave beneath the bottom of the J has merit. I am attempting to patent a similar method using a mast decoupling stub...
www.hamradio.me/interests/j-pole/
so it is better to isolate the antenna from the mast?
+wimpog97 Think of this as the need to "RF" isolate. The J-pole as it stands does nothing to prevent flow of current down the mast. A stub can fix this as shown in the article here...
www.hamradio.me/antennas/j-pole-antenna-grounding-have-your-j-pole-and-ground-it-too.html
...and a copper cactus approach here...
www.hamradio.me/antennas/mast-mountable-j-pole-antenna.html
+SkyCharter what if I use a piece of PVC pipe to connect the antenna to a metal mast?
+wimpog97 If you can live without the robust lightning surge capability of the continuous copper pipe to mounting mast, this is a good plan. K6MHE offers some tips in his writeup...
techdoc.kvindesland.no/radio/vhf_antennas/20061030163006789.pdf
+SkyCharter Thank you for these links. I will keep it in the attic, so I guess it is better to isolate it from the mast.
+wimpog97 They way to think of it is it's better to "RF" isolate it from the mast. Physical separation via, for example, a piece of PVC certainly performs that function. I build the Mast Mountable J-Pole and use a mast decoupling stub to provide a measure of stopping RF currents from flowing while preserve the ability to flow static charges and lightning induced energies to the antenna mount. Think of the mast decoupling stub as a kind of notch filter that only blocks at the design RF frequency (plus some harmonics).
if i ground it i lose signal
Not true at all. I have my J-pole mounted directly to a metal mast and my SWR is good.
VSWR critiques feed match only. VSWR of the feedpoint is not a sufficient metric to analyze antenna performance.
Ryan Toomey I’ve seen them grounded and un grounded both with excellent results, I myself would think grounding would eliminate static discharge and create a quieter noise floor... what do I know? Lol
Your wrong you should not ground a jpole Ryan Toomey. If you ground it you'll have to re cut your jpole because your adding more length on to it which you shouldn't be .so don't ground it .
Same here.
this is cactus, not j