multiple J-poles, one coax - nuts or wizardry?

[Opcom]

This would be one for any VHF enthusiasts who are also antenna gurus.

Having seen a mobile HF rig using an antenna made of a piece of steel mast and three separate loading coils/heads/whips on it for 40/20/15M, all at the same time, and having had the OM claim they did not interfere, great SWR, fine results blah blah blah.., and knowing Hustler sells this setup, here is a question about wierd things like this:

If I have three J-poles, for example 6M, 2M, 70cM, and place them at somewhat diverse angles like in the picture so they cannot interfere with each other too much RF-radiation-wise, and mount them all to a common base plate, will this scheme apply?

Or, if I have the three, or more, and mount them some distance (2-3FT) from each other, and feed with one cable and TEEs?

(there is a rumor about a so called "cactus 5-band dipole that can vbe fed with one coax)

A similar scenario would be with the bands closer, such as a setup with one for the civil air band (118-136), and one for 2M?

I am told that the off-resonance antenna is a moot point when there is a resonant one available at the connection.
Some OM told me that will work because the signal goes into the resonant antenna, and is reflected/rejected by the rest of them.

Furthermore, what about more antennas? Even going as far as 4 or 5? (If tilting them -Within reason since the angle of radiation is important.

I don't know how to model this (draw the picture and magically see the swr and radiation plot), nor know where the good free software to do this is.. but maybe someone has tried this scheme and already knows if it's nuts or not.

The question comes up because a certain marsupiality thinks (1 feedline) = (1 antenna). I might need to do this; there is life above 50MHz.

[KX5JT]

The thinking behind the hustler setup is similar to the fan dipole (or parallel dipole).  The signal will see the path of least resistance and go for the antenna (or "resonator") that is resonant.  In reality there is a bit of interaction and some adjustments are made on fan dipoles.  As far as working for the j-pole situation.... one sure way to find out!

[KA1ZGC]

Without running the numbers, it could electrically work, I suppose; but intuition tells me the results will be less than earth-shattering. Several other factors rear their ugly heads in that situation.

A few things come to mind, here:

Only one radiator will actually be vertical. That's an issue at VHF/UHF. The non-vertical radiators will then only perform effectively along the bearing perpendicular to the slant.

In other words, let's say you have three j-poles: one in the center, one leaning to the left, one to the right, like in the pic you provided. Your best pattern will be forward and backward. To the left and right, your pattern will exhibit serious uptilt or downtilt, reducing your gain. That's not even figuring in parasitic effects of the other radiators.

Even your forward and backward gain will be reduced, because of polarization losses (you're not actually vertical in any direction). You could easily lose a few dB that way.

It could work, but you'll only have j-pole performance at best, and even that will only happen with the truly vertical radiator.

There are other multiband antennas out there. Getting one radiator to show a 50 ohm resistive load on multiple bands is not easy, but not impossible. It is very much a necessity, since variable tuned output circuits in solid-state VHF/UHF gear is unheard of.

If you just want one antenna to cover all your vertically-polarized VHF/UHF needs and are willing to settle for just moderate gain, I would look at a discone first. They are extremely versatile antennas, and give a good match across hundreds of megahertz of spectrum.

There are other, more band-specific multiband ants out there that exhibit better gain, but the discone really is a smooth operator.

So yes, it can work, but you may pass the point of diminishing returns rather quickly, I think.

Again, I did no math to come up with any of this, but that's my $0.02.

[KD6VXI]

Patrick,

Having been researching this myself, I have run across more than a couple.

Do a google search, dual band j pole, and see what ya get.

I'll be building one, but too bad I was too shortsighted to get the MFJ269, and opted for the 259, so a plot is out of the question on UHF, but I would like to borrow one if I can

I'm also looking at a discone for 100 and up.

--Shane

[KF1Z]

If I recall correctly ( IF!)

The multiband J Is made of a j-pole at your lowest frequency, Out of large diameter pipe.

Then a "J" for each band on the main pole (2 meter for example)...
That's why it looks "cactus".

But, you have to run a seperate coax for each band up inside the pipe.

Then use a diplexer at the base, so you can use one feedline to the shack.

http://www.csee.wvu.edu/~w8cul/index_files/CACTUS.doc

[KA2QFX]

I've built a few 300 ohm ribbon J-poles for easily hung travel antennas, but aside from that convenience I think the J-pole isn't a steller performer.  For what you intend to do I would build a discone antenna. At it's lowest frequency it has virtually no gain but it presents a nice impedance from it's fundamental up and covers many octaves of frequency. 6M to 450MHz would not be a problem and it covers everything in between as well! 
The J-pole on the other hand has two rather frequency specific elements. The quarter wave driven section and the modified radiator, which is cut to tune out the reactance of the shunt feed. 
But if I WERE to build a three band J-pole I would build them all on one pipe. UHF on top, VHF below that and 6 meter at the base. They would need to be tuned up in that order too, but at least you'd have a relatively even pattern. 
As far as feeding them with one coax I can't say.   I've used fan dipoles for 30 years and I find VERY little little interaction between elements (at their resonant frequency), whether the elements are 5" apart or completely 90 degrees apart. Yes the power divides up nicely given that each element closley approximates the feedline impedance at resonance. IOW, power distribution is proportional to impedance ratios relative to feedline Zo.   So using a fan dipole (approximately 70 ohms resonant) does NOT work well if fed with 450 ladder line, no matter what you use for a tuner!
I would suspect a properly tuned J-pole would be as selective in power distribution. And you can get them to exactly 50 ohms resistive by adjusting the shunt feed tap point and readjusting the radiator length to compensate for the shunt feed reactance.
Keep in mind that a J-pole is some number of end fed half wave radiators (+/- some reactance tuning). So given bandwidth tradoffs, you could add 1/2 wavelengths to the radiator for additional gain.
Some experimenting sounds in order and it would be quite the plumbing project.  All the radiator sections would need to be adjustable above the 1/4 wave section as well as the strap on the shunt feed point and even the 1/4 wave section length might need to have a little adjustment too.  These are a bitch to tune frankly. 
In my experience; try to establish the resonance of the 1/4 section EXACTLY before adding feedline and radiator. I found the other two variables easier to deal with, but if the 1/4 section is off you'll never get the proper current to voltage conversion to drive the radiator.
If you decide to give it a try publish your results.  I'd love to see the results.  There's several good references around the web.


Regards,
Mark

[Opcom]

Yow! ainfo.com wants ~ $1400 for the 40-500MHz 300W CW/600W PEP discone! And this is from China! Must be very well made. I know there are cheaper ones, but all I have seen are PW rated or require a bottom loaded vertical radiator on top for some reason. 6M rig is 250W FM, can be modulated. Any leads on reasonable priced 6M discones? Just exploring options - no decision yet.

[KA1ZGC]

That's ridiculous. A $70 discone from radio shack covers more spectrum than that.

Not much to limit power out of a discone except any reactance that might be added to any whip element.

Keep looking, or get yeself to a hardware store and build one. No antenna is worth that amount of money unless it sleeps with you first.

[WD8BIL]

A 5/8 wave 2 meter J pole works well on 432mhz also. Seems to me some 6 meter traps and extensions necessary to make it play on 6 is all you'd need.

[KD6VXI]

Yow! ainfo.com wants ~ $1400 for the 40-500MHz 300W CW/600W PEP discone! And this is from China! Must be very well made. I know there are cheaper ones, but all I have seen are PW rated or require a bottom loaded vertical radiator on top for some reason. 6M rig is 250W FM, can be modulated. Any leads on reasonable priced 6M discones? Just exploring options - no decision yet.

The discone only covers a decade of freq coverage.

IE, it will cover from about 120 to 1200, 140 to 1400, etc.

The radiator on the top, with the load, is the device that allows the antenna to get do 6 meters!  And the reason the power level is SO low on them.

You can do the same thing with a hamstick on a homemade discone...

OR, if you don't need the bandwith on the upper end, you can build your own discone that will cover from 6 meters to 70 cm, playing with it a little, I'm sure..  It'd be a stretch, but one that was totally doable.

8 spokes in the top and the bottom is ideal for the skeleton, with 1/3 wavelength giving the best profile for both the disc and cone.

Hope it helps!  Roll your own, no need to give them idiots 1400 dollars for a 60 dollar discone and a 6 mtr hamstick!

--Shane

[KA2QFX]

The reason that you get acceptable response from the 2 meter j-pole on 440 MHz is that it's operating on it's third harmonic.  Since the 1/4 wave stub's open end provides the voltage feed for the 1/2 wave radiator above on 2 it acts as a 3/4 wave stub (also a voltage node on it's open end) on 440 feeding a 3/2 wave radiator above.
Unfortunatley, if you were to add a 2m trap and extension to the radiator for 6m operation the stub, being less than 1/10 wave, wouldn't be able to drive the radiator at that frequency; and you'd lose 440 operation as well.

BTW, the J-pole is commonly referred to as 5/8 wave. It's actually a shortend 3/4 wave. The shortened 1/2 wave radiator compensates for the inductance introduced by tapping the up on the 1/4 wave feed stub.  It's perhaps a moot point, but I think it's easier to understand the antenna from that point of view.

Thanks for taking care of the Tron Buddly!
Regards,
Mark

[KD6VXI]

BTW, the J-pole is commonly referred to as 5/8 wave. It's actually a shortend 3/4 wave. The shortened 1/2 wave radiator compensates for the inductance introduced by tapping the up on the 1/4 wave feed stub.  It's perhaps a moot point, but I think it's easier to understand the antenna from that point of view.

Thanks for taking care of the Tron Buddly!
Regards,
Mark

That's not entirely true, either..

The J Pole gets its name more from physical consideration.

The J Pole is ANY length of radiator (although years ago they started calling the 5/8 a Super J Pole for some reason, bigger = better).  The lower section is a quarter wave matching network.

Try it, that same network, connected to nearly ANYTHING that will radiate, and you will be able to tap a decent match out of it to nearly any feedline (all being within reason, of course).

They have yagi's being fed by a J now, 5/8 verticals, etc.

The thing to remember about a J Pole is this:  To get the lower wave angles, a la the NEC type programs, you HAVE to use either balanced line or decouple the coax, otherwise, your coax will really destroy the pattern, the swr can change when the wind blows, etc.

I've done LOTSA studying up on J Poles..  Originally the zep antennas, and someone decided to plumbers delight it, and stand it on end, rather than drag the thing around!

--Shane

[kb3ouk]

I remember talking to a guy a while ago who had a low band Motorola radio that he wanted to use on 10 and 6 meter FM. Anyway he said that when they would install these rigs(in police cars, if I remember right), they would put a whip tuned for the low end of the VHF lowband on one side, and another whip tuned for the high end on the other side. Then he said that they would put something like a T connector on the coax and run two coaxs from that to the pair of antennas. Then no matter what frequency the radio was tuned to, the RF would radiate out the antenna that was the closest match to the frequency the radio was operating on.
Shelby KB3OUK