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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