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Author Topic: radials underneath vertical dipoles  (Read 1500 times)
K3VAT
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« on: October 03, 2011, 10:29:24 AM »

I have ordered a vertical dipole (actually, asymmetric feed and compressed length tubing radials) from Tom, N6BT, primarily for DX operations on 20M, 30M, and 40M.  The antenna will be on a fiberglass mount with feed point about 15 feet above ‘average’ ground.  Terrain is gradual slope (western MD), mostly downward from the antenna in a wooded area adjacent to and approx. 130’ from the shack.  There are several large trees within ~15’, but none within ~5’ of the antenna.  Coax is a mix of ½” Andrew HL and BuryFlex; rig is K3/100.
Excuse the bandwidth in case this has been covered before.  I’ve done numerous searches (i.e., google) and gleaned over available texts, but haven’t been satisfied by the info.  There have been some suggestions that modeling the above configuration on NEC2 would provide ‘reliable’ results, but this is something that I can’t do.
Question: I’m looking for any findings (ideally, from your experimentation) to either confirm or nullify the proposition that “adding a number of ground radials (let’s say, 12 radials of 35’) beneath the antenna will lower the peak radiation angle and hence increasing the chances of working long haul DX (say, Zone 26 on 40M) with this antenna.”  Some description of your setup would be appreciated.  Of course, anecdotal reports are also welcomed.  Thanks!
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LA1BRA
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« Reply #1 on: October 03, 2011, 11:52:28 AM »

When I posted this, I see only 15 people had read your post..FWIW.
I have a  33 foot tall vertical on my porch with a SGC 230 at the base. the vertical feed point is 1/2 inch above the radial plate. I mention this as when I installed the setup, the feed point was about 8 inches above radial plate, took some testing to see that 1/2 inch above worked better...as in capacitance coupling to earth ( rf ground).
Now I have been reading threads on vertical antennas for years here, just so I could learn what I did not know or at least see the light of others trials and jubilation (pun intended). If you read about elevated radials, you will find much said about sloping radials under verticals. Do some searching here on "elevated radials", I recently read a series of articles on empirical data and testing of your very question.
All that said, I am of the opinion the radials under your vertical should be close to the feed point as I mention in my experience, if elevated, so be it, if stapled to the ground, so be it.
 A friend here has a High Sierra mobile antenna (not the 1800) he had lots of problems with getting the antenna to tune, antenna was mounted on a 8 foot pipe and the ground radials on a plate at the base, it worked...somewhat...so I suggested moving the radials wires up just below feed and it worked better...reading through the HS manual, it does discuss the antenna working like a vertical dipole and how not to make it do that..
Maybe the key thing of interest is simply that, do you want a vertical dipole or a vertical ground plane? Might generate some comments....So, if you mount your antenna and the base is at 15 feet, maybe run the radials from just below the feed point to Terra Firma and then away from antenna mast? or simply elevate them 15 feet above ground.

I just re-read your post, vertical dipole, a dipole is a dipole is a dipole, even a horizontal dipole has it's mirror half, if you will, a vertical mounted ground plane is a much different animal as I have mentioned above..a vertical dipole is the same as horizontal dipole, just mounted vertically

Anyway, my thoughts on the subject, my vertical works pretty good and I have 2 on the front porch (1.5 mtrs above sea level)
73 de LA1BRA aka KB5VWZ tom
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N4JTE
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« Reply #2 on: October 03, 2011, 06:07:00 PM »

Where would you attach the radials ? Any ground screen that decreases ground loss helps any antenna, hence the salt water marsh ideal. I would not consider actual "radials" on a vertical dipole.
Bob
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AJ8MH
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« Reply #3 on: October 03, 2011, 06:20:40 PM »

I'd ask N6BT since this antenna is his design.  It looks like a ground-plane antenna with coils at the feed point, but without more data from him, who knows.

I have been using true vertical dipoles for years and have never used a radial system.  The late L. B. Cebik, W4RNL, said that radials don't do much for a vertical dipole except change the feed impedance.

Here are a couple quotes from "Screening 40 Meter Vertical Arrays" by W4RNL.  "However, unlike the radials of a monopole, the radials beneath a vertical dipole (either full length or shortened) perform no antenna-completing function. Rather, they simply function to raise the conductivity of the soil immediately beneath the antenna."

"Note that none of the radial fields changes the TO angle of the elevation pattern. The TO angle is mostly a function of the far-field reflection zone, with is mostly well outside the radial limits. For the exercises, I did not change the antenna dimensions. Therefore, the major influence of the fields appears in the feedpoint impedance listings. As the size of the radial field increases, the resistive component of the impedance decreases and the reactance becomes more inductive."

"For example, local area treatment may reduce ground losses below the antenna, but that results in a higher TO angle, because the improvement increases local reflection almost straight upward. The lower TO angles that we often associate with the same antenna over very good soil results from improved soil conductivity at considerable distances from the antenna."

"As the table shows, adding 4 radials amounts to wholly wasted effort, since the gain increase is only 0.04 dB. 16 radials provide a 0.3-dB gain increase, perhaps a marginal amount, considering the work involved. If radial installation is easy, we may increase the field to 64 radials and obtain nearly 0.7 dB gain increase."

Remember, we're talking about vertical dipoles, not quarter-wave or 1/2 wave verticals fed at the bottom (monopole).
 
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K0ZN
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« Reply #4 on: October 03, 2011, 06:31:04 PM »

Hi.

In theory, anything that SIGNIFICANTLY reduces the resistance of the ground under a vertical should be an improvement. However, Real world reality is a little different.

A vertical dipole, will in theory benefit from close in radials, but the reality is it is in the Far Field, many wavelengths away that will make the most difference....and
I kinda doubt you plan to make a bunch of radials a couple of thousand feet long! I experimented with a 20 M vertical dipole and compared it to a 20 m
1/4 wave monopole with an excellent ground system (100+ 1/2 wave radials). In no case was the half wave vertical dipole better than the ground mount monopole....but it was always close. I did not have lab equipment, but it appeared that the ground losses probably made the signal differences about what theory would dictate. The radiation angle of the vertical dipole seemed to be just as low as the monopole and it worked very well.  If you have a lot of time and money and access to cheap copper wire, I suppose you could put a bunch of radials under that vertical dipole, but they have to be 1/2 wavelength. i.e. same length as the antenna because the highest current point will be 1/4 wave out away from the antenna. (Personally..... I would not make the effort!). A linear amplifier would probably be a better way to raise your Effective Radiated Power !

Bottomline:  You can dig into the books and look at radiation patterns, etc. to see what happens, but in terms of loss, I would say it is a waste of time to put "short" radials
under a vertical dipole.  "Your results may vary....."

What you DO need to be aware of, and again, the radiation plot patterns will clearly show this; is that you do NOT want to get the lower end of the dipole very high!
This IS a case that (unless you can get the antenna very high) you want the bottom of the vertical dipole no more than 1/8th wave off the ground or else you will
start enlarging a high angle lobe at the expense of your low angle lobe. Again, reality is that you probably only want the bottom of the antenna a foot or so off the ground.  This is by memory, but I believe that if the bottom of your vertical dipole is 1/4 wave off the ground, your main radiation lobe will be at about 45 degrees!  Great for satellite communications, but sure would suck for HF DX.

Lastly, if at all possible, try to bring the transmission line off the antenna at 90 degrees (perpendicular) for 1/4 wave length if possible even if this means you need additional transmission line. This is to reduce antenna currents on the transmission line. If you are feeding it with coax, a balun would be quite beneficial.

73,  K0ZN
« Last Edit: October 03, 2011, 06:34:37 PM by K0ZN » Logged
K3VAT
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« Reply #5 on: October 03, 2011, 06:36:32 PM »

Quote
N4JTE: Where would you attach the radials ?
The radials are not attached to anything; that wouldn't make sense. 
ON4UN calls describes what I'm talking about as a "radial screen" in his book, Low-Band DX'ing, on page 9-29 and for horizontal polarized antennas on page 8-7.  Both N6RK and N6LF have papers that talk about ground screens that are available by search engine.  These papers, and quite a few others, help in the understanding of these the ground screen or the radial screen.  Thanks for the comment. 73, Rich
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W5WSS
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« Reply #6 on: October 03, 2011, 11:11:51 PM »

LB Cebik provided writs on modeling self contained vertical dipoles above various soils with and without radials and various heights above such radial sets. You can read them at the antenna's web. The results noted were mediocre meaning that efforts to lower the toa more as compared to without the radial set, and simultaneously cause the power to manifest within this angle of interest were only small and inconsequential to the effective radiated power or Field strength, amounting to any detectable strength. Again this pertains to elevating a vertical dipole in increments as he searched for improvements. Radials are best utilised in ground mounted versions of the family of antennas. 73 and good reading.
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W8JI
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« Reply #7 on: October 04, 2011, 02:10:20 AM »

This entire topic is complicated. A simple answer is difficult.

Where the feedpoint is has little to do with needing radials.  The current distribution on the antenna and if the antenna connects to the radials is important, but not where the antenna is actually fed.

For example, if I have a 1/4 grounded vertical fed at the base it needs the radials just as bad as the same system fed half way up.  I can move the feedpoint all around and it needs radials the same, because it is still connected to the radials and the current distribution is the same.

A vertical "dipole" still has a strong electric field coupled to the earth, so it might improve a little with radials even when the radials are not connected, but not nearly as much as an antenna connected to the radials with high current in the radials.

As for lowering wave angle, good luck on that. Anything done near the antenna has very little if any effect on wave angle. This has been pretty well known for a long time.

The exception is if the radials are grossly non-symmetrical and carry high current, then they can act like an antenna and radiate. That of course will affect pattern, but not normal radials that are somewhat evenly distributed.
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K3VAT
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« Reply #8 on: October 05, 2011, 05:03:33 PM »

thanks to all who replied about my inquiry about radials under a vertical dipole.  73, Rich, K3VAT
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