Hi Tom. Your radial/ground loss measurements were repeatedly referenced in
John Devoldere's book, so I thought I would point a question your way (and
the way of any other TowerTalkians who might wish to make observations)
Starting point for an 80m inverted L with sloping top loading wire: (spaced
5' and parallel to my HDBX-48 tower with 18' mast and a small T-6 (13-30
mhz) Log Periodic @ 52' level. (Obviously the tower is going to have an
effect, but that is a limitation of my physical supports.)
The antenna: Inverted L, sloping top wire.
45' vertical wire, 25.8' sloping wire at the top, 29 feet high at the far
end. #12
THHN Insulated copper wire, stranded. Feedpoint is mounted 7" above the
earth, directly to a 3' ground rod (strictly for mechanical support, as I
had a prefab mount with 3/8x24 on one end and SO-29 on the other)
Predicted Radiation Resistance (Devoldere): 25.8 ohms, very good ground
(rich pastoral, midwest).
OK, here is my summary table for up to 8 radials, 66 feet long, insulated
wire, laying on the ground: (all measurements at the shack end of 55' of
LMR-400 coax, buried) (MFJ-269 antenna analyzer values)
Radials 2:1 Fo 2:1 BW Z VSWR @ Fo
0 3340 3522 3920 580 36,0 1.3
2 3354 3524 3774 420 45,0 1.0
4 3419 3533 3741 322 60,1 1.2
8 3445 3550 3742 297 65,4 1.3 (reactance would not go lower than 4)
The formatting is bad, but in order, left to right:
Number of Radials
Lower 2:1 vswr point
Resonant Freq point (Fo)
Upper 2:1 point
Bandwidth in kilohertz
Impedance (R,+/- j) at resonance as shown on the MFJ 269 .
I'm a bit confused by the rising feedpoint impedance with increased radial
numbers, yet the narrowing down of the 2:1 swr bandwidth does indicate
decreased losses. It also indicates (since it continues to decrease), that I
should add another 8 radials to see if I begin approaching the asymptote
(sp?). The
rate of change is slowing quite a bit, so I may be reaching the point of
diminishing returns faster than I think.
Initially I thought I could just subtract the predicted radiation resistance
(25.8 ohms) from the measured feedpoint impedance at resonance, and have an
indication of ground loss in ohms. This would then let me calculate
efficiency. However, the values shown in the table above don't make sense
to that way of thinking. Conceptually, what am I doing wrong, in attempting
to analyze the table of values above?
Any explanations for the data are most welcome.
73,
p.s. the antenna seems to be doing a very good job at > 1500 km with my
limited number of observed signals and reports. (compared to a Carolina
Windom 80 at 45')
...hasan, N0AN
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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