Thanks,
I need to digest this.
mike va3mw
On Thu, Oct 26, 2017 at 8:31 AM, k8bhz@alphacomm.net <k8bhz@alphacomm.net>
wrote:
> This issue does indeed keep coming up, so I am posting my response (of 3
> years ago) again for K7EG, N7WS, VA3MVW, W0MU, and others. The shortened
> radials are still in use here, with 235 countries worked on 160. The Vp
> shortening depends entirely on soil type, so math will only get you so
> close. Better to actually measure them in place, in your ground. Only two
> radials are necessary to do that.
>
> Brian K8BHZ
>
> ----- Forwarded Message -----
>
> From: k8bhz@hughes.net
> To: topband@contesting.com
> Sent: Friday, December 19, 2014 12:23:52 PM
> Subject: Topband: Fw: Shortened Radial Experiments
>
> From: Brian Mattson
> Sent: Saturday, October 14, 2006 7:56 PM
> To: topband@contesting.com
> Subject: Shortened Radial Experiments
>
> In response to Eddy's (VE3CUI) question about anyone using "shortened
> radials", I have been using these for two years now, with very good results.
>
> Coming to TopBand after decades on VHF, I was confused by the myriad of
> opinions on radials. Comments like "resonance disappears after a few buried
> radials" and "longer is better" were often seen. As a degreed electrical
> engineer, I was puzzled by the abandonment of the laws of physics once a
> radial was buried, or laid along the ground. Sure, the velocity factor &
> loss factors change significantly once a radial gets near, or below,
> ground, but basic electrical laws must still apply.
>
> As I first got on TopBand in the dead of winter, I used the single
> elevated radial as discussed in "Low Band DXing". Pointed towards Europe,
> and about 5 feet off the ground, it worked surprisingly well. However, when
> it came time to upgrade the ground system, confusion set in with all the
> conflicting opinions I read. Fortunately, I ran across Rudy Severns' (N6LF)
> article on "Verticals, Ground Systems and Some History" in QST (July 2000).
> ( As an electrical engineer in the switching power supply industry, I have
> learned to listen when Rudy speaks!). One comment that really caught my
> attention was on page 41: "For the 0.1 wavelength high (vertical) antenna,
> if we have a good ground screen out to a distance of 0.1 wavelength, we'll
> eliminate over 90% of the ground loss!". The lightbulb came on right then.
> I could instantly visualize an Electrostatics Fields class representation
> of a ground referenced hemispheric field intensity bubble with a radius of
> the vertical height. I use
> a phased pair of inverted L's for my transmit antenna, and each has
> around 50 feet of vertical rise, so a system of enough 50 foot radials
> should suffice. But the nagging thought of resonant length still bothered
> me. Time to experiment (play).
>
> The inherent beauty of a quarter wavelength radial is in it's impedance
> transformation properties. Basically, the higher the impedance on one end,
> the lower the impedance on the other end. As the far end of the radial is
> open circuited, the antenna end is as low as possible, and it is
> non-reactive. Two opposing radial elements look suspiciously like a dipole,
> so that's where I began. All my measuring was done at 1.83 MHz, so a
> free-space dipole would be about 269 feet & have an impedance around 73
> ohms. All my experimenting was done with #14 solid insulated THHN copper
> wire.
>
> My first experiment was to construct a full size dipole and lay it on the
> ground. The resulting dipole was well below the lower operating frequency
> of the MFJ analyzer, so pruning was in order. I finally achieved resonance
> with a length of 182 feet! Rs was 130 ohms. So the velocity factor was
> thus: 182/269 = 0.677. So Eddy, don't take the 0.5 number from "Low Band
> DXing" as gospel, as it depends a lot on the type of soil you have. My soil
> is sandy (almost like beach sand). Note too that the ground proximity has
> increased Rs substantially. Next, I buried the dipole in a slit trench
> approximately 6" deep. Again, the dipole was way too long. To prune the
> buried dipole, I found it easiest to have the ends bent up so that they
> protrude just above ground & place a bright colored "wire nut" on the end
> (so I could find it again!). The resonant length of the dipole was now 107
> feet! Rs was 148 ohms. The buried velocity factor was: 107/269 = 0.398.
> Note that burying the dipole has add
> ed even more losses to Rs.
>
> The result of experimenting thus far resulted in a resonant radial length
> (in my soil) of 53.5 feet (half of the dipole). With my 50 foot vertical
> inverted L's, I was ecstatic. But how many radials would I need?
>
> I constructed another buried dipole of 107 feet length, at right angles to
> the first, and so their centers were coincident. This gave me four radials.
> I tested the second dipole as a separate entity, and it's numbers were very
> close to the first. Next, I connected the two dipoles together (two
> adjacent wires as one node/ the other two adjacent ones as the other). I
> was astounded when the resonant frequency plummeted!! I almost gave up at
> this point. As a VHFer, I knew that whether a ground plane has two or four
> radials shouldn't make much difference. Indeed, some conicals feature solid
> sheet ground screens. In any event, the quarter wavelength dimension
> shouldn't change much. After stewing on this for a few days, I realized
> that I had constructed a "Fan Dipole" which greatly increased the
> capacitance to ground, thus lowering the resonant frequency. I then came up
> with what I consider to be my only "original" contribution to this
> experimenting. By connecting up opposing pairs
> of radials as one node, and the other two opposing pair as the other
> node. sanity was restored. I was pleased to see that Rs dropped almost
> exactly in half (75 ohms), as two parallel impedances should. The basic
> laws of physics were still intact! For want of a better name, I refer this
> connection scheme as cross-connected dipoles. Realizing that with many
> additional radials being added, the "cross-connection" scheme could easily
> get lost. The solution was to have TWO connecting rings at the radial
> junctions. The radials are then alternated from one ring to the other, so
> that each ring has half the radials, but with NO adjacent ones. For
> operating, the two rings are both connected to the coax shield, but for
> testing, the two rings are separated to connect to the analyzer. One
> curious effect was noted when the resonant frequency dropped slightly
> (about 36 kHz). Pruning the radials by 6" restored 1.830MHz numbers.
> (Radials now 53 feet each). This slight (second order) effect is
> probably due to increased capacity, even with the cross connected
> configuration.
>
> I then doubled the number of dipoles to four ( 8 radials), the
> cross-connected dipoles again dropped to half Rs (now 38 ohms). Again I had
> to prune the radial length (now 51').
>
> Then the number of dipoles was doubled to eight (16 radials). Rs was now
> 15 ohms. Elements again trimmed (now 49').
>
> Finally, the number of dipoles was doubled to sixteen (32 radials). Rs was
> now 7 ohms. Elements now trimmed to 48'.
>
> Please note that all the Rs readings were cross-connected dipoles in the
> ground and NOT the antenna impedance.
>
> I then added my resonant vertical (50 feet vertical, rest in the top-hat).
>
> The antenna measurements were: 56 ohms with two radials. 43 ohms with
> four radials. 30 ohms with eight radials. 26 ohms with 16 radials. And, 24
> ohms with 32 radials.
>
> One great feature of short radials that everyone seems to agree on is that
> FEWER of them are required. From the antenna measurements, you can see that
> doubling the amount of copper (& labor!) resulted in only 2 ohms
> improvement from 16 to 32 radials. My second antenna only has 16 radials.
>
> My 48' or 49' radials are an efficient match for my 50 foot verticals, but
> if I were to have a full-size (135 foot) vertical, I would still go to
> resonant tuning. In this case, in my soil, the 3/4 wavelength radials would
> probably end up around 3X48' = 144'. (possibly slightly shorter due to the
> second order effect).
>
> Thanks to Rudy for his inspiration!
>
> Sorry for the long message, but I think it's sound.
>
> Best Regards,
>
> Brian Mattson K8BHZ
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