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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