Topics

Roof versus ground


Dave
 

Has anyone figured out just how far above the dirt can the antenna’s feed point be before you stop using random radials and start using a tuned counterpoise? I’m wondering about mounting on a fence post.

~73~
Dave
N1EYO@...
Merritt Island Florida


Steven AC2XM
 

I don't have any real experience with this, but the Severns paper at https://rudys.typepad.com/files/qex-may-jun-2012-1.pdf may be helpful.

Figure 17 seems to suggest that once the radials are a few feet above ground they are no longer affected much by the ground. But, it also seems that the correct length of resonant radials does depend on the proximity to ground.

I get the impression that the correct length of slightly elevated radials is tricky to calculate, and that you'll have to carefully tune them for your situation. The detailed recommendations are in the "Final Comments" section on page 35 of the Severns paper.

Steve
AC2XM

On 4/15/20 8:44 PM, David N1EYO wrote:
Has anyone figured out just how far above the dirt can the antenna’s feed point be before you stop using random radials and start using a tuned counterpoise? I’m wondering about mounting on a fence post.
~73~
Dave
N1EYO@...
Merritt Island Florida


Federico IK3UMT
 

I have experienced tuned radials to be completely useless and time-wasting unless the feedpoint is high in wavelength terms from real ground.

I moved from 4 tuned radials per band to 30 x 33ft radials @ 10ft from ground without noting almost any effect in tuning.

http://ik3umt.ir3ip.net/nuovogp.htm 

(I'm trying to restore the english page...)


Federico
ik3umt


 

This would be a good question for QST "the dr. is in" column if it hasn't already appeared.
Al
WB9UVJ


Tim N8NEU
 

I have about 16 radials on the ground. The grass has pulled them in. They measure in length: 2 each 4 feet, 2 each 6 feet, 2 each 8 feet, 2 each 12 feet, 2 each 16 feet, 2 each at 18 and 2 each 20 feet. They are spaced in a circle with the matching lengths 180 degree opposite each other. All my SWRs are below 2.0. 80 meters is a little tight but doable.

My Butternut is is located in my upper yard. The ground is a rich dirt with shale rock scattered everywhere in the subsurface. Living in Pennsylvania, my home is over an empty coal mine.

With all the talk on the length and number of radials required by the Butternut,  I am confused why mine works as good as it does. I have no plans on disrupting my present setup.


Tim - N8NEU
DMR ID 3158619
FN00ah

On Apr 17, 2020, at 06:03, Federico IK3UMT <ik3umt@...> wrote:
I have experienced tuned radials to be completely useless and time-wasting unless the feedpoint is high in wavelength terms from real ground.

I moved from 4 tuned radials per band to 30 x 33ft radials @ 10ft from ground without noting almost any effect in tuning.

http://ik3umt.ir3ip.net/nuovogp.htm 

(I'm trying to restore the english page...)


Federico
ik3umt


Federico IK3UMT
 

Tim, basically radials lying on the ground has nothing to do with tuned radials. 
Ground plane and elevated ground plane are two completely different things (where elevated means the ground, due to its "electric" distance has little or no effect on radials tuning).
In your setup, like mine was despite 10ft from ground, you shouldn't bother with lenghts (unless they are too much short like few feet are)
You have room for 20ft radius, go for 16 radials of that length , that is quite the minimum lenght that accomplish a good compromise for lower bands.
If more room is available, I'll go for 16 more 30-35ft radials added to already improved 16x20ft
It doesn't matter if they are "too long" for higher bands, you need a rich ground screen not a resonant one.

Federico
ik3umt


Tim N8NEU
 

Federico, Thank you for the information.  I will keep it in mind when I relocate after this terrible pandemic is over with. But for now my theory is, if it aint broke dont mess with it.


Tim - N8NEU
DMR ID 3158619
FN00ah

On Apr 20, 2020, at 02:52, Federico IK3UMT <ik3umt@...> wrote:
Tim, basically radials lying on the ground has nothing to do with tuned radials. 
Ground plane and elevated ground plane are two completely different things (where elevated means the ground, due to its "electric" distance has little or no effect on radials tuning).
In your setup, like mine was despite 10ft from ground, you shouldn't bother with lenghts (unless they are too much short like few feet are)
You have room for 20ft radius, go for 16 radials of that length , that is quite the minimum lenght that accomplish a good compromise for lower bands.
If more room is available, I'll go for 16 more 30-35ft radials added to already improved 16x20ft
It doesn't matter if they are "too long" for higher bands, you need a rich ground screen not a resonant one.

Federico
ik3umt


VE9AA - Mike
 

I'm surprised our sysop/admin Engineer AC8DE Scott hasn't weighed in. He must be busy.

I have my HF9V around the 7' mark, off the back of a shed.  I have many (RAISED) tuned radials and partially disagree (somewhat) with Federico)

If you have your Hf9V @ (let's say) 4' on a fence post and have your radials all on the ground, I think you may be disappointed in its performance and SWR curves.

Think of 2 halves of a dipole.  Now make your dipole vertical.  Now, put the upper half of the dipole vertical at 4' above the ground.  Now...separate the bottom part of the dipole 4' away from the vertical part of the dipole.  Now lay those radials on the ground.

I am not at all saying it won't work.  It will work. HOw well is the unknown.

In fact, I did just that on a recent trip to Cape Breton NS (I operated as VE1TTT , one of my other calls).  Instead of an HF9V, I used a motorized screwdriver-type antenna at a cabin. 

Before I left, I setup everything just as I had envisioned it would be on my trip.  In my driveway at home all was good and my radials were all (almost) on the ground in a 360* pattern., however when I setup at the cabin, because of space and walking path contraints, the antenna had to be up on a small deck raised off the ground, then the radials went down some stairs, then eventually (barely) spread out in a slim/tight figure-8 pattern on the ground.  All the tuning I had done at home was completely out the window and I found the antenna a poor performer, with high SWR's and I wasn't even able to achieve resonance on 80m (but was at home)

Did it work? Yes.

Did it work as well as in my driveway under optimum radial placements? No.

YMMV

GL

Mike VE9AA

On Wed, Apr 15, 2020 at 09:44 PM, David N1EYO wrote:
Has anyone figured out just how far above the dirt can the antenna’s feed point be before you stop using random radials and start using a tuned counterpoise? I’m wondering about mounting on a fence post.

~73~
Dave
N1EYO@...
Merritt Island Flori

 
--
Mike VE9AA


Scott AC8DE
 

Yes Mike VE9AA, I have been very busy.  I will now chime in.

 

I think there is some misunderstanding on some of the answer.  I believe the real question here is a twofold question; two separate questions if you will.

 

  1. How far can one separate the feed point from the radial field?
  2. How high above the ground must tuned radials be to work somewhat effectively.

 

ANSWER to #1 - Regardless of which solution you decide to run with (ground radials or elevated), the feed point and where they converge in relation to the feed point must always be close in order for the antenna to function as designed.  I strongly suggest trying to keep this within 3-4 inches with 6 inches as a maximum.  It can be "stretched" to 12 inches, but that may cause problems on 10M.  Once you get beyond that, the tuning gets "soft" and the SWR rises and you'll have problems on the upper bands.  You'll also increases your potential for RFI tremendously.  As someone else already pointed out, think of it as the same as the two halves of a dipole.  They MUST stay close to each other to be able to function.

 

I suggest studying these two charts.  Note I these charts are focusing on SWR strictly and NOT system efficiency, which are two different subject.

 

Radial Count Effect on SWR

 

26" VS 4" high Feed Point

 

So as you can see, there is a correlation with radial count and feed point height from the radial field and how this affects SWR.  If you go with low radial count AND highly separated feed point, your results will be... well, not good.  And I can assure you the system efficiency takes a real hit as well when these are separated and the radial count is low.

 

So then we get to question #2;  How high does a radial field have to be above the ground to effectively "insulate" the radiator from the capacitive effects of the ground.  Read Rudy Severns stuff if you have the time.  Here are some rules of thumb and things to consider.

 

  • At least 1/4 wave above the ground is ideal.  Obviously, not practical when you get below 10 meters.  in other words, it's not practical at all.
  • 4 tuned radials has the same performance as 64 ground mounted.  Makes elevated sound appealing, no?  I can assure you it is not easy to accomplish well.  You have to physically tune them with an analyzer and if they are close to anything, it throws the tune.  No, they cannot be laid on a rooftop.  It is not a calculate and cut to length thing to do it right.
  • To properly insulate the radiator from the ground effects, it will take more than 4 tuned radials per band if you are below 1/4 wave from the ground.  How many depends on how high you are.  I've not seen any empirical research done on this.  Would be a real time eater to do.
  • Once you get to a certain number of radials above the ground, they no longer have to be tuned nor do you have to be 1/4 wave above the ground.  It's a lot of radials though to get there and the mechanics of this arrangement is complicated and looks like a mess. (Not wife friendly.)  You get to a point where the metal in the elevated radials starts approximating a metal roof.  A high count long length elevated radial system is killer, but not easy to do.

 

OK, now that I've spewed all that; What will actually work?  I've helped set-up 2 opposing cage dipole multiband radials mounted along a wooden fence line with the feed point about 6 feet off the ground.  Theoretically, that is FAR from ideal.  But when we tested, it seemed to hear and transmit pretty well.  Probably not as efficient as it could be, but sometimes, you just have to live with what you can accomplish.

 

73,

Scott AC8DE

 

-----Original Message-----
From: Butternut@groups.io On Behalf Of David N1EYO
Sent: Wednesday, April 15, 2020 8:44 PM
To: butternut@groups.io
Subject: [butternut] Roof versus ground

 

Has anyone figured out just how far above the dirt can the antenna’s feed point be before you stop using random radials and start using a tuned counterpoise?   I’m wondering about mounting on a fence post.

 

~73~

Dave

N1EYO@...

Merritt Island  Florida

 

 

 

 


Bruce VK3WL
 

Hi Scott.  Useful post and tnx.

Are you able to point me to a post, article or give me advice about mounting mounting a Butternut multiband vertical on the metal roof of my house pse?

I've successfully done this before with trapped verticals but have heard that a Butternut is tricky under such a set up.  Is this true?

Cheers and tnx in advance. 


--
Bruce R Kendall
VK3WL
9V1WL


VE9AA - Mike
 

Bruce

This is not the EXACT answer you were looking for however K9YC has a great article on this general subject.
If I were lucky enough to own a metal roof, I would just mount it up there instead of spending time here (smile)

Just do it ! ;-_)

and the link to Jim's article:

http://audiosystemsgroup.com/VerticalHeight.pdf

--
Mike VE9AA


Scott AC8DE
 

Bruce,

 

I would not say it is tricky exactly, you just need to be creative.  But on a metal roof, you do NOT want to use the roof mount kit (RMK-II), as this raises the feed point entirely too high.  The roof mount kit is for non-conductive roofs, where you are running elevated radials.  For a metal roof, you want to fabricate a "Cup" of sorts that is attached to the roof to stabilize the bottom of the antenna, which will be the insulator.  You should NOT use the A tube at all, but rather the insulator sits in your shallow "cup".  You want your feed point to as close as possible to the metal roof.  Then just figure out guy points to hold the antenna vertical.

 

You'll want to make sure your metal roof's panels are all electrically bonded together.

 

If you do those 2 things, your Butternut will BOOM.  The bonded metal roof mount is the king of all vertical antenna mounts.

 

73,

Scott AC8DE

 

From: Butternut@groups.io On Behalf Of Bruce Kendall
Sent: Tuesday, June 16, 2020 9:26 PM
To: Butternut@groups.io
Subject: Re: [butternut] Roof versus ground

 

Hi Scott.  Useful post and tnx.

Are you able to point me to a post, article or give me advice about mounting mounting a Butternut multiband vertical on the metal roof of my house pse?

I've successfully done this before with trapped verticals but have heard that a Butternut is tricky under such a set up.  Is this true?

Cheers and tnx in advance. 


--
Bruce R Kendall
VK3WL
9V1WL