[TowerTalk] Rising Input Z with more radials?

[K4SAV]

One significant point about your inverted L antenna: Your tower is going 
to have a major effect on your antenna.  I have not modeled your tower 
and antenna configuration, but from similar models, I would expect the 
tower to be resonant around 2.5 to 3 MHz. Since your inverted L is only 
5 feet away, the whole structure will be part of the antenna.  If the L 
was farther away, the tower would look like a reflector, but with only 5 
feet separation, there will be heavy coupling between the two and the 
directional pattern will not be effected significantly (since the whole 
system is the radiator).

I don't know how you accurately predict the feed point impedance of this 
system.  You could model the whole thing, but since most simulators 
don't do well at calculating ground losses, you probably won't get an 
accurate answer. I would expect the feedpoint resistance of this system 
to be drastically different from just an L by itself.  My guess, instead 
of 25 ohms for just the inverted L, to something around 5 to 10 ohms for 
the whole system. (For feeding only the L, not feeding the tower.)

You should treat the tower, cables, etc., the same as if it is being 
shut fed, i.e., expect large currents in the tower (half your inverted L 
current is possible). Do the same decoupling that you would do for a 
shunt fed tower.

Tie your radial system into the tower.  Since tower ground currents will 
be significant, you need a low loss path for these currents, not the 
lossy earth.  Since the feedpoint resistance is going to be very low, 
ground losses are going to be very high.

Jerry, K4SAV

hasan schiers wrote:

>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
>
>_______________________________________________
>
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>
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