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"... Comparisons Between Ground Surface and Elevated Radials"




 
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Author Topic: "... Comparisons Between Ground Surface and Elevated Radials"  (Read 18552 times)
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« on: June 11, 2011, 08:52:44 PM »

I came across this information today while searching Google.  Here is the link:   http://rudys.typepad.com/files/qex-ground-systems-part-3.pdf
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« Reply #1 on: June 12, 2011, 12:02:50 AM »

I concur with Rudy's findings after working in broadcast for years and around many vertical antenna systems.

One also finds that the fewer radials, the wider the operating frequency range is as the base impedance remains almost constant over a whole ham band (for a 1/4 or 5/8 wave vertical).  It makes a matching unit much simpler, at least on 160 & 75.

Elevating radials a bit has always been operationally preferred by me, but most locations dictate they be buried for longevity, security and/or liability, especially in broadcast applications.

73DG
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« Reply #2 on: June 12, 2011, 02:07:24 AM »

One also finds that the fewer radials, the wider the operating frequency range is as the base impedance remains almost constant over a whole ham band (for a 1/4 or 5/8 wave vertical).  I makes a matching unit much simpler, at least on 160 & 75.

So does adding series and/or parallel resistors between transmitter and antenna.  Or you can transmit into a dummy load and get a perfect match all the way across the band, and the impedance remains almost constant over several bands.

But a simple matching unit and a "good match" don't guarantee a good signal.

http://www.eham.net/articles/14905

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« Reply #3 on: June 12, 2011, 07:23:25 AM »

I came across this information today while searching Google.  Here is the link:   http://rudys.typepad.com/files/qex-ground-systems-part-3.pdf
73,
Chuck

That's the guy who has gotten a lot of mileage from coming up with stuff about radials from modeling.  He might be the one who had the article on the best radial ground system design if you have a finite amount of wire [and you'll never be able to add any more ever again].   Roll Eyes  As some already know, I'm not exactly wild about what I call armchair antenna experts--I prefer test range results but I realize for hams we don't get much access to antenna test ranges.   There are two great reports published by K7LXC on some commercial yagi and vertical antenna tests conducted on a range of their own making that are really interesting but you have to pay a nominal fee to get copies--they are not free on-line.

But back to the elevated radials--I've never tried them but I have heard enough about them to convince me they work very well if they are set up correctly.  For hams on the low bands, that last part is the catch.  They have to be elevated a certain fraction of a wavelength on the operating frequency.  I think around 0.15 lambda as I recall.  That's around 50 feet on 160 and few hams are able to elevate the feed point and four radials that high.  The need is to isolate the radials from earth.   In the past I've had various hams claim to me that they get good results with their radials elevated some ridiculously low height:  1.  What does "good" mean here, 2.  local results vary with ground conductivity, 3.  height with antennas obviously is a sliding scale performance wise--it is not like putting the radials at 40 feet results in the antenna shutting down and you seeing a short circuit at the feedpoint.  So, IOW, they work really well at the height needed for isolation, but not nearly as well at say 15 feet (all comments pertain to 160 m.).   But the only way to really prove this is to A/B the two heights with other things being equal, and I do not know of any hams (or broadcasters) able to do that.   So what's left is the whole thing with hams and antennas where you have guys with "elevated" radials 8 feet off the ground on 160, telling themselves and the rest of us this is just as good as 100 on the ground, because the motivation for this belief has more to do with avoiding the labor and cost of 100 radials on or below grade. 

I've stuck to low bands here because the elevated radial thing comes up with 75 and 160 verticals most often--it is less of an issue on the higher bands probably because it is simply easier there to properly pull it off, so hams just do it and don't give it much thought or comment and actually on 20 m. and higher, most hams put up something other than a vertical anyway.
 


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« Reply #4 on: June 12, 2011, 11:02:36 AM »

Good one Don Grin

Rudy's results are fine for Rudy's back yard and others with a similar ground conductivity and for 40M ....only

Since verticals are so ground dependent there is no one size fits all answer but some think so and fill up a lot of magazine and web pages.

Carl
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« Reply #5 on: June 12, 2011, 01:09:20 PM »

I came across this information today while searching Google.  Here is the link:   http://rudys.typepad.com/files/qex-ground-systems-part-3.pdf
73,
Chuck

That's the guy who has gotten a lot of mileage from coming up with stuff about radials from modeling

...

Severns ran many, many careful experiments not just models.  All caveats apply and he included them in his series of articles -- forty meters, his ground conductivity, etc.  Even with the caveats, I find his results more interesting than endless EZ-NEC hypotheticals.
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« Reply #6 on: June 12, 2011, 01:38:51 PM »

Another difference between ground-mounted radials and elevated are that elevated radials become resonant to the operating frequency, while ground-mounted become non-resonant accumulators of ground current, effectively lowering ground resistance.  Therefore, a vertical with elevated radials will have narrower bandwidth than one with ground mounted. But a small number of radials still allows a  lot of ground resistance from the lossy earth figure into the equation.  120 ground-mounted radials, each 0.4λ, is said to be a near-perfect ground plane, but most AM BC stations don't even use that much. Ground-mounted radials are a diminishing-return phenomenon; the more wire you have already put into the earth, the less improvement is realised by putting in more.  120 radials is probably overkill; the consensus seems to be that you don't see much improvement past about 60.

Naturally, with fewer radials which add more ground resistance, you get wider bandwidth, but more rf is wasted heating up lossy earth.  The ground plane can be thought of as a shield to isolate the lossy earth from the radiating part of the antenna.  The more complete that shield, the better the isolation, the  less the loss, but the narrower the bandwidth until a point of negligible ground resistance occurs.

Another way to think of it is to compare a typical broadcast installation with 120 quarter-wave ground mounted  radials, to a highly elevated quarter-wave ground plane, such as a CB or 2m ground plane with 3 or 4 radials, mounted a half wavelength or more above ground. Each antenna shows good efficiency and low ground loss, but it makes logical sense that between these two extremes, the closer the radial system is to the surface of the earth, the more radials are needed to maintain complete isolation.

The ground plane may or may not serve as a missing portion of the self-resonant antenna.  A ground mounted half-wave vertical will resonate perfectly with no ground system at all, but a  lot of the rf is still wasted as heat due to the proximity of the lossy earth. With a 1/4λ vertical, the ground plane becomes an integral part of the resonant length of the antenna.  These two functions of the radials are distinct from each other. One elevated radial, usually called a "counterpoise", was  long used for Hammy Hambone 160m inverted-L antennas, dating back to the 20's and earlier. Its function was primarily to achieve resonance in the L-shaped wire, but it did little to shield the radiating antenna from the lossy earth. But raising the counterpoise several feet above ground improves overall efficiency, since the vertical part of the antenna is better isolated from ground by distance, and the counterpoise itself does some radiating as a low-to-the-ground horizontal antenna, along with the horizontal part of the inverted-L.

In addition, a 1:1 match many not necessarily indicate the best efficiency in the antenna.  Take, for example, the quarter-wave vertical.  Its base impedance is about 36Ω. With negligible ground loss, connecting a 50Ω coax transmission line directly between the base of the antenna and the ground system will result in a SWR of 1.39:1. Removing enough radials to increase the the ground  resistance to 14Ω, brings the total base resistance to 50Ω, a perfect 1:1 SWR. But now, 28% of the transmitter power is dissipated in ground loss. The additional loss in the coax due to the 1.39:1 SWR will be almost negligible. So you are much better off  running the SWR at 1.39:1, than disconnecting radials to bring the SWR to 1:1.  You could still bring the SWR to 1:1 by inserting a matching network between the feed line and the base of the vertical, but if the transmitter can be loaded up to full power at 1.39:1 SWR, you probably have less loss in the coax than you would in the matching network that brings it to 1:1.

This problem becomes even more acute with a shortened vertical, which inevitably has lower radiation resistance, for example an 1/8λ vertical, whose resistance is about 12Ω. Now, disconnecting radials until a perfect match is achieved, means that 38Ω is effectively inserted in series with the antenna, yielding an efficiency of 24%. Even with the 4:1 SWR, the antenna will still be more efficient with negligible ground loss.  In this case, a matching network should substantially improve efficiency regardless of the number of radials used.

I downloaded  the whole series of QEX articles.  The writer claims that his modelling figures closely matched experimental data with actual test antennas and radial systems he erected for the purpose.
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« Reply #7 on: June 12, 2011, 02:55:49 PM »

http://www.antennasbyn6lf.com/design_of_radial_ground_systems

Here is Rudy's actual website, which is easier to read and has interactive posts from readers.
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« Reply #8 on: June 12, 2011, 04:02:29 PM »

Yep, tuning/optimizing  a vertical's SWR by adding or reducing radials is a very common mistake, as heard on the air.  As the radials are added and the vertical's input swings from >50 ohms down into the ~30 ohm area, the swr will get perfect and then move away again - until it is later matched at the feedpoint using some type of technique.

The following applies to coax-fed antennas, though openwire is basically the same if the tuner is set in the ballpark:

When it comes to general antenna tuning, I have an old rule.  First design the antenna using a Nec modeling program. Build the antenna to the model's specs. Then adjust it for maximum gain and/or maximum front-back.    The LAST thing to do is match the feedpoint to the coax. This may mean adjusting the length of the driven element and juggling the matching network to get a 1:1  50 ohm J0 match.  ie, SWR matching comes LAST because everything else effects it. SWR has very little effect on front-back and gain tuning, as long as it's in the rough ballpark when tuned for these other parameters.

Some guys have been known to adjust reflectors/directors, height above ground or lower the dipole legs to an inv vee, etc., to adjust their swr.  NG.

Once an antenna is positioned and adjusted for performance optimally, (if required)  THEN the swr fine-tuning matching gets done - last. The other parameters are not touched after the match is optimized except for possibly tiny fine-tuning adjustments for maximum f-b, if desired.

T
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« Reply #9 on: June 12, 2011, 10:25:02 PM »

I came across this information today while searching Google.  Here is the link:   http://rudys.typepad.com/files/qex-ground-systems-part-3.pdf
73,
Chuck

That's the guy who has gotten a lot of mileage from coming up with stuff about radials from modeling

...

Severns ran many, many careful experiments not just models.  All caveats apply and he included them in his series of articles -- forty meters, his ground conductivity, etc.  Even with the caveats, I find his results more interesting than endless EZ-NEC hypotheticals.

I agree with all of this and  my comments may have been on the harsh side and thanks to Chuck for posting the link to the article by the way, and I saw the caveats etc.   I guess I don't understand his point because unless I misunderstood him, the radials are all only a few inches off the ground, less than two feet and his plots tell me things don't start getting good until you have a lot of them, not just four or six, but ~ 30.  Well, to me, he's modeling lots of radials close to ground and probably from a RF standpoint, the same as insulated wire radials lying on the ground, which is what Doty tested and wrote about in his paper I have a link to in the Printed Material section.   Doty did his work on a range; no modeling, because it was done around 30 years ago, before powerful computing was commonly available.  (Not saying anything bad about modeling!)  From a practical point, who's going to put up radials a foot or two high.  I must have gotten something wrong.
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« Reply #10 on: June 12, 2011, 10:48:36 PM »

I should have sat down and read the paper thoroughly before commenting.  There were some key points I missed in haste earlier.  As he says at the end, it would be very interesting to see his ideas explored with very lossy earth such as sand or rock, especially on the low bands. 
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« Reply #11 on: June 13, 2011, 04:16:21 AM »

"very lossy earth such as sand or rock"--Exactly what we have at both our QTH AZ & NM.

When the antennas are done, I'll make measurements with my Potomac FIM-41 and post the results.

That is my story and I'm sticking to it... Angry

73DG
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« Reply #12 on: June 13, 2011, 10:40:23 AM »

........... it would be very interesting to see his ideas explored with very lossy earth such as sand or rock, especially on the low bands. 

    That is our situation also in Anza-Borrego, DM13tf, California desert. The antenna at our alt-QTH is a 175ft inverted L with 4 elevated radials at 8-10 ft. The end of the antenna and the radials meet at the edge of my porch eaves to a tuner then a short coax to the radio. Its temporary setup but has made contacts on 160,75/80,20 and 17 meters with 100W or less on SSB and 20-25 on 75AM.
     I haven't made any measurements, soil resistance, antenna characteristics etc but will when its time to make the antenna more than temporary. At first the idea was just have some wire in the air that fit the area to get on the air and elevated radials were best in that situation. Its worked so well its worth trying a more solid installation.


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« Reply #13 on: June 13, 2011, 11:36:20 AM »

Some places in NM can have quite high soil conductivity when it rains.


"very lossy earth such as sand or rock"--Exactly what we have at both our QTH AZ & NM.

When the antennas are done, I'll make measurements with my Potomac FIM-41 and post the results.

That is my story and I'm sticking to it... Angry

73DG
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« Reply #14 on: June 13, 2011, 12:18:15 PM »

Some places in NM can have quite high soil conductivity when it rains.


I wish it were so.  Our cabin is at 8,000' elevation on very deep sandstone. 

Even ground rods are tough to put in and practically useless.  The requirement by the utility is wishful thinking.

The good part is drilling for a tower base will yield an immovable object.

73DG
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« Reply #15 on: June 13, 2011, 03:03:26 PM »

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Some places in NM can have quite high soil conductivity when it rains.

I dont even get that benefit on this granite rockpile after a 3 day deluge!  Therefore its elevated radials all the way. It takes around 20-25 on 160 at 10-12' high to get the point of no improvement; I tried 32 but nada. They are 135' long but since they run thru the woods over tree branches or loosely stapled to pine trunks they are likely no longer resonant.

Bandwidth is the typical 70 or so KHz of a dipole/inverted vee. Its cut for 1835 (Im a CW DXer first) but the amp or the Viking I will load it all the way to 2000.

Carl
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« Reply #16 on: June 13, 2011, 03:23:01 PM »

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Some places in NM can have quite high soil conductivity when it rains.

I dont even get that benefit on this granite rockpile after a 3 day deluge!  Therefore its elevated radials all the way.......... They are 135' long but since they run thru the woods over tree branches or loosely stapled to pine trunks they are likely no longer resonant.


The inv L is 70ft vertical to a palm tree with a 100 foot tail going east with a slight downward slope. Two of the radials are about 130ft +/- what was on the roll with the other two at 70-75 feet all with nail up fence wire insulators. The house is adobe with 6X6 H beam frames supporting the high end of both roofs. The steel frames and all the metal roofing is tied together then to where I connected the radials, forgot to mention that, 30X 70of metal roofing.

EdZ KG6UTS
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« Reply #17 on: June 13, 2011, 08:46:27 PM »

If you look at buried radials as a part of the circuit between a Marconi vertical and its mirror at the ground, in many areas a foot of earth acts as a lossy resistance in the buried radial circuit, giving elevated radials an advantage.

Using theory alone to determine if elevated radials will be better than buried ones in any given location is like debating how many angels can dance on the head of a pin.

You have to make measurements for ANY given QTH.. Use an RF ammeter and a calibrated wattmeter like a Bird. ADD some elevated radials.Or a buried cast iron bathtub. Or ground rods. Read the antenna's RF current for a given wattage, like 100 watts. If the current goes up, the ground losses are going down and the elevated radials are helping. Sometimes, the opposite happens. So forgetabout using elevated radials in that case.

This procedure is no more different than our ham ancestors in the 1930s used. The more antenna RF current for a given DC input, the better the antenna is working because its losses are lower. They didn't have Bird wattmeters back then. Bigger, thicker wires, maybe a top hat..The more amps into a vertical for a given DC input or power, the better. It's that simple. Measure the vertical's base Z in that manner and use it for a guide. Repeat after me:The more RF amps the better.

In general, verticals in the low-conductivity parts of the USA will get their butts kicked by a high horizontal radiator because "real" ground is so far below the surface. Verticals here in the Great American Desert or on piles of granite in the Eastern CONUS tend to suck on HF.
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« Reply #18 on: June 13, 2011, 10:00:06 PM »

<<<When the antennas are done, I'll make measurements with my Potomac FIM-41 and post the results.>>>

DG I'll be real interested in what you find out.   Not arguing; this is where I'm coming from:  When I decided the best antenna for 160 transmitting I could get on my lot would be an inverted L, I started gathering information about ground systems.  I had to decide what kind of wire to use, bury or lay on the ground, solid or stranded, copper or ??, how many, how long, how to lay them out blah blah blah...one thing that sunk in was that the higher the radiation resistance of the vertical the better.  Actually the real factor is the difference in radiation and ground resistance (I don't really know why off hand).  With an average inverted L going vertical 1/8 w. on 160 or less, before going horizontal, you typically see around 14 ohms resistance, +- 4 ohms or so.   If you can't go vertical any higher, that's it, so what's left to do to improve things (from what I read) is reduce the ground resistance to increase the difference between the two, and that's about it.  After that you can make small but cumulative gains from feedline improvements, maybe using a cage driven element, tuning at the feedpoint and tx gain by Eimac.    Except for the Eimac part these last changes are more satisfying for someone who is obsessive like me than anything else but probably make no noticeable signal strength improvements.  After digesting about 20 papers on radials written here and there over the years until I was sick of the subject, it all kept coming down to put down as many as you can on the ground and fit them into whatever space you have and operate.

If I can be convinced that there's a way to achieve this with a few elevated radials I'll accept it; but since my antenna is already pretty much built with 101 radials on the ground I have to admit it would take an iron clad smoking gun proof of around 6 dB gain or better with the elevated deal for me to think about changing things here  Grin  But anyway, I'm always interested in honest to goodness test results especially with decent gear like a Potomac FIM.

73

Rob 
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« Reply #19 on: June 13, 2011, 11:25:43 PM »

I can't say from experience, but I read somewhere that one thing you DON'T want to do is to mix buried radials with elevated ones. Performance turned out to be worse than with either the buried or the elevated radials used alone.

One disadvantage of elevated radials is that the effective height of a given vertical radiator is shortened by the distance of the common point of the radials above ground. But if you happened to have a tower that was too tall for the band you want to use, the radials could be attached to the tower 1/4λ down from the top, and the top part shunt fed from the common point of the radials. The radials could even be made to serve as a set of guy wires, since a ground plane may use sloping radials.
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« Reply #20 on: June 14, 2011, 12:56:55 AM »

Quote
Some places in NM can have quite high soil conductivity when it rains.

I dont even get that benefit on this granite rockpile after a 3 day deluge!  Therefore its elevated radials all the way. It takes around 20-25 on 160 at 10-12' high to get the point of no improvement; I tried 32 but nada. They are 135' long but since they run thru the woods over tree branches or loosely stapled to pine trunks they are likely no longer resonant.

Bandwidth is the typical 70 or so KHz of a dipole/inverted vee. Its cut for 1835 (Im a CW DXer first) but the amp or the Viking I will load it all the way to 2000.

Carl
Carl,  Won't some of those elevated radials running through trees have some very high voltage points?  Are you going to set the place on fire? 
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« Reply #21 on: June 14, 2011, 02:03:30 AM »

The plan here in AZ is to put up two Rohn 45 series-fed radiators at 5/8u for 75 @ 1/4u apart.  Happens to be a bit long for 1/4u height on 160, but OK. 

Since we own the dirt, and nothing else will be happening on it, I plan to use elevated (aprx 18" up) radials, starting with 8. 

This is modeled somewhat after the vertical at K7ER (SK) in Paradise Valley AZ, which ran WAS easily with a modified 20V3 on 160 and was proof enough for me.  (Elliott 7ER did a field measurement series with his FIM years back and got the mV/M he wanted with just 8 radials)

Our dirt here is somewhat lower than a '3' conductivity, and buried radial have always proven to move the base impedance so crazily when wet from a storm some BC stations went off the air due to the mismatch.

At first I'll test with the meter at a first bounce distance, and then double up the radials and test again, also keeping a sweep of the base impedance across both band freqs with my OIB in every case.

All the necessary hardware is here on the ground, but the tower field area is currently occupied with building materials for the ongoing QTH.  Time will have to pass a bit before starting that project.

The New Mexico QTH will get something else for a radiator.  I have a 60' heavy self-supporter here, and I'd like to haul it up there and hang a giant log off it.  Maybe.  Whatever, down the road.

Passions, passions.  Like opinions and sphincters, we all got 'em.

73DG
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« Reply #22 on: June 14, 2011, 03:37:34 AM »

I would try the elevated radials, and keep adding more and more until there is no more change in impedance or field strength with additional radials. Elevated radials should help the wet/dry problem, but you will have to pay more attention to the exact length of the radials.  They should all be the same length (preferably a quarter wave) and laid out as symmetrically as possible.

According to that QEX article, if you bury the radials, resonance effects would cause the signal to actually drop with lengths exceeding 1/8λ, with only 8 radials. At only 18" up, it would be interesting to see if you experience that effect, whether the radials act more like ground mounted or more like an elevated ground plane. In the worst case, the solution would be to add more radials, since they found that with 16 or more, performance improved with length. So just keep on adding radials until you reach the point of diminishing returns... and hope it doesn't ever rain enough that grass and weeds start to grow, and that a buffalo herd doesn't pass through.
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« Reply #23 on: June 14, 2011, 12:29:30 PM »

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Carl,  Won't some of those elevated radials running through trees have some very high voltage points?  Are you going to set the place on fire?


Nothing has burnt down in 22 years at the 1200W CW/SSB level and the few times Ive gone a bit beyond 1500 PEP on AM  Shocked. Im using a mix of #16 and 18 insulated and enameled wire.

Maybe modeling will tell about the voltages but I doubt there is anything significant.

Carl
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« Reply #24 on: June 16, 2011, 11:43:17 AM »

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Carl,  Won't some of those elevated radials running through trees have some very high voltage points?  Are you going to set the place on fire?


Nothing has burnt down in 22 years at the 1200W CW/SSB level and the few times Ive gone a bit beyond 1500 PEP on AM  Shocked. Im using a mix of #16 and 18 insulated and enameled wire.

Maybe modeling will tell about the voltages but I doubt there is anything significant.

Carl

And from my play days with higher power, I do not think there are fire issues unless you are running  5KW of carrier or the strange problem of a very low-to-ground dipole on 160M or 80M.
As for radials, I agree with Carl.
Fred
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Fred KC4MOP
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