[TowerTalk] 160 radials

Sat Dec 6 08:38:17 EST 2008

I'm not disagreeing with the fact that for a small number of wires close to or on the ground that they will not have a resonant less than a physical 1/4 wavelength.  What I am saying is that for a large number of radials, whether they are resonant or not is irrelevant since they are now forming a ground screen - not a counterpoise.

Quoting from ON4UN, fourth edition, page 9-14..."As soon as you use a larger number of equally spread radials the resonance effect disappears, and the radials form a disk, which becomes a screen with no resonance characteristics. In this case, we no longer talk about length of radials, but about the diameter of a disk hiding the lossy ground from the antenna...." (emphasis added)

Hence the goal in this case should be to create an efficient screen and not resonance in the radials.

I do agree that for a few radials on the ground or elevated couterpoise radials, achieving resonance is important in order for the 1/4 wavelength radiator to have something to work against.  However, for muliple ground mounted radials, resonance is irrelevant.

Of course, one can discuss which works better - a ground screen or a counterpoise system.  But that's a whole new can of worms, one that has been debated ad nauseum without consensus agreement.

73, Jeff
W3KL

Jeffrey K. Okamitsu, PhD, MBA
+1-609-638-5402

--- On Fri, 12/5/08, Hal Kennedy <halken at comcast.net> wrote:

From: Hal Kennedy <halken at comcast.net>
Subject: RE: [TowerTalk] 160 radials
To: w3kl at w3kl.com
Date: Friday, December 5, 2008, 7:44 PM

Nonsense.  Radials on the ground behave as lossy transmission lines and have a natural resonant frequency.  Their Vf is approx 0.6

Run the experiment.  You will find a nicely defined resonant point on a 10 meter dipole laying on the ground.  X (or j) will =0 at around 20 meters.  

I make no other claim….all the extrapolations of what I said were not said by me.  I make no claim regarding efficiency – I just offer up the idea that if you want an easier impedance to match you can get it with radials shorter than 135 feet.  

I have supplied more intelligent errata to ON4UN and N6BV on their books than most folks…

73
Hal
N4GG (several advanced degrees and founder of 14 electronics companies.  I consider it superfluous at best to list it all in the context of ham radio conversations)

-----Original Message-----
From: w3kl at w3kl.com [mailto:w3kl at w3kl.com] 
Sent: Friday, December 05, 2008 10:29 PM
To: Hal Kennedy; towertalk at contesting.com
Subject: Re: [TowerTalk] 160 radials

For multiple radials laid on the ground or buried just below the ground, there is no "resonance" effect - that is, there is no such thing as a tuned radial for wires laid on the ground or buried just below the ground.

What you are referring to IS true for a counterpoise system - aka "elevated radials".

When you have multiple (greater than 10 say) radials on the ground they simply form a ground screen.  The "need" to extend them 1/4 wavelength out is simply to improve the current collection efficiency of the ground screen - it isn't because you want them to be resonant.

There are volumes written on this.  I recommend ON4UN's book on low band DXing.  W8JI's website may also have something on this as well.

Finally, in order for your assertion that a length of wire in close proximity to the ground has an electrical length greater than it's physical length would be true only if the wire and ground formed a transmission line.  This is how a Beverage antenna works.

However, at HF frequencies (even on 160 m), I think that a wire laid on the ground will not form a transmission line since the current and it's image current are too close in proximity.   However, again what's important is that the radials are forming a ground screen and not the fact that they are or are not resonant.

Said differently....a quarter wavelengtth radiator has the same radiation resistance regardless of the efficiency of the ground.  However, the radiation efficiency of the antenna depends on minimizing ground losses.  Installing an efficient ground screen (radials) helps reduce the ground loss, thereby increasing the radiation efficiency.  This does NOT require the radials to be resonant.

73, Jeff

Jeffrey K. Okamitsu, PhD, MBA
+1-609-638-5402

--- On Fri, 12/5/08, Hal Kennedy <halken at comcast.net> wrote:

From: Hal Kennedy <halken at comcast.net>
Subject: [TowerTalk] 160 radials
To: towertalk at contesting.com
Date: Friday, December 5, 2008, 5:13 PMFor those who believe they are putting 1/4 wave radials on the ground -it might be important to know/remember that the velocity factor of wireon the ground is approx. 0.5.  Quarter wave radials are actually approxa half wave electrically -  which is why it takes so many of them to geta monopole down to 35 ohms - each radial presents a high impedance ifits 135 ft long and on the ground.  BC stations use quarter wavelength(mechanical quarter wave) radials because 120 of them will provide a lowimpedance when placed in parallel and current share well since each area high impedance.  You can easily prove this to yourself.  Lay an 80 meter (or higher inQRG) dipole on the ground and check it with an MFJ.  It will be resonantnear 160 meters.  Do it quick with a 10M dipole - it will be resonantaround 20M.  You can't do that test with a 160 dipole as it will beresonant around 900 KHz and the MFJ won't go that
 low. Want that inverted L impedance down where it should be with very fewradials?  Elevate them to eliminate the above effect, or put them on theground and make them close to an electrical quarter wave - which isaround 80 feet long - not 135 ft. Having an inverted L "too long" is just right.  It moves the maximumcurrent up off the ground and is easily tuned out with series C. 73HalN4GG(Running just great with a low feedpoint impedance from four 80 footradials under the vertical, on the ground)  _______________________________________________   _______________________________________________TowerTalk mailing listTowerTalk at contesting.comhttp://lists.contesting.com/mailman/listinfo/towertalk