Topband: Confusion in ON4UN's Low Band DXing radial length calculations.
Bill Wichers
billw at waveform.net
Fri Dec 19 13:11:46 EST 2014
What you're missing is that a conductor carrying an *alternating* current, like RF, also has reactance. Reactance is influenced by capacitance and inductance. The earth, being essentially somewhere between a lossy conductor and a lossy dielectric, can influence the electrical properties of the wire. Velocity factor is influenced by properties of the dielectric. Notice how there are feedlines that are the same impedance, and have the same outside diameter but one has a foamed PE dielectric and the other solid (like RG8 and RG213)? The velocity factor is higher in the foamed dielectric. The electrical length of such cables will be different even though the only physical differences are the differing dielectric materials and slightly different gauges of the center conductors.
A "radial" is actually more complex than just a piece of wire on the ground with resistance. It doesn't lose it's length, but the reactive properties of the wire (or any wire for that matter) will change as that wire is either bent, or brought into close proximity with other wires or dielectric materials. Remember that an inductor is also just a piece of wire with resistance, but when that wire is coiled it acts as an inductor more than just a simple piece of wire.
There are also the other reasons for resonant length being less important with radials (many in parallel, etc.) when they're on the ground that have been discussed many times in the past. The DC resistance of the radial itself does not change when brought near the ground, but the AC properties of the wire do.
-Bill KB8WYP
> -----Original Message-----
> From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of
> k8bhz at hughes.net
> Sent: Friday, December 19, 2014 12:21 PM
> To: Tree; topband at contesting.com
> Subject: Re: Topband: Confusion in ON4UN's Low Band DXing radial length
> calculations.
>
> I can’t agree with this “conventional” thinking. Why does a piece of wire
> magically lose it’s length just because you lay it on the ground? The electrical
> length changes because of Vf, and it’s resistance changes because of the
> lossy ground, but it’s still a piece of wire. I’m going to try to attach a posting I
> did back in 2006. If it doesn’t work, I will follow with a separate posting.
>
> Brian K8BHZ
>
> From: Tree
> Sent: Friday, December 19, 2014 10:07 AM
> To: Doug Turnbull
> Cc: k8bhz at hughes.net ; 160
> Subject: Re: Topband: Confusion in ON4UN's Low Band DXing radial length
> calculations.
>
> Radials on the ground do not have a magic length. Worrying about resonance
> for them is not necessary.
>
>
> If you tune a quarter wave wire up in the air - then lay it onto the ground - it
> couples to the ground and is no longer a distinct single piece of wire. Just
> make them an easy length to deal with and put as many of them down as
> you can.
>
>
> Tree N6TR
>
>
> On Fri, Dec 19, 2014 at 2:28 AM, Doug Turnbull <turnbull at net1.ie> wrote:
>
> Brian,
> I understand that the VF varies with soil type. One could just
> compensate by being conservative but who wants to use 30/40% more wire
> than
> needed. Why does the ON4UN book ignore VF when doing the example
> problems?
> Should I shorten to take into account VF?
>
> 73 Doug EI2CN
>
> -----Original Message-----
> From: k8bhz at hughes.net [mailto:k8bhz at hughes.net]
> Sent: 19 December 2014 00:08
> To: Doug Turnbull; Topband at contesting.com
> Subject: Re: Topband: Confusion in ON4UN's Low Band DXing radial
> lengthcalculations.
>
> Hello Doug,
>
> The 50-60% figure depends on your soil conditions, so may vary quite a bit.
> With my poor, sandy soil, the Vf is 67.7% with the radials laying on the
> ground. When I buried them 6", the Vf was 39.8%. Using these shortened
> radials, there wasn't much improvement going beyond 16 radials.
>
> To find out your soil conditions, simply lay a temporary dipole on the
> ground and use an analyzer to find it's resonance. Then trim to length. Now
> you have your first two radials!
>
> Good luck
>
> Brian K8BHZ
>
> -----Original Message-----
> From: Doug Turnbull
> Sent: Thursday, December 18, 2014 4:18 PM
> To: topband at contesting.com
> Subject: Topband: Confusion in ON4UN's Low Band DXing radial
> lengthcalculations.
>
> Dear OMs and Yls,
>
> I am replacing raised radials for 160M inverted L with ground mounted
> radials mostly because I could not readily get the raised radials up high
> enough in my wood and also because of maintenance problems.
>
>
>
> This inverted L goes up 100 feet at its top before levelling out for
> the final 32' or so. It should I believe have a strong vertical element.
>
>
>
> ON4UN's book Low-Band DXing 56th edition is generally excellent but I
> do find the coverage of ground radials both confusing and somewhat
> contradictory. This surprises me for what is pretty much considered the
> bible.
>
>
>
> On page 9-14 the text states that the velocity factor falls for
> ground mounted radials to the "the order of 50-60%, which means that a
> radial that is physically 20 meters long is actually a half-wave long
> electrically!" This example is for 80M not 160M. However in the examples
> found on page 9-15 the velocity factor change is ignored. I understand
> the velocity factor change and have always accepted this. It generally did
> not pay to try and cut radials precisely to a given wavelength. I accept
> the radial length vs. radial number charts but is this an electrical length
> in free space or a length considerably reduced due to velocity factory
> change? Example 3 ignores velocity factor correction and from what I can
> see this correction is ignore in most of the text concerning ground radials.
> What does one do? Who does one believe.
>
>
>
> While I am talking about a 160M inverted L; I did reference the
> SteppIR BigIR vertical manual, page 18. Lengths should be scalable. I
> find no mention of velocity factor and the shortening effect which is
> experienced. The recommendations are not very different from those in
> ON4UNs book. So does this mean one ignores the change in velocity
> factor?
>
>
>
> I appreciate some guidance with this matter. I would like a
> radial field which would take me to within 0.5/1 dB of the maximum
> achievable for reducing near field losses.
>
>
>
> 73 Doug EI2CN
>
>
>
>
>
>
>
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