From: K7GCO@aol.com
Date sent: Sat, 6 Jan 2001 07:36:41 EST
Subject: Re: [TowerTalk] 80 Meter Self Supporting
Vertical.......
Hi Ken,
You made this suggestion before with a dipole, and when I looked
at it on a Smith Chart it wouldn't work at all. With the dipole (and
ladder line) the SWR at the upper band edge was very poor.
With coaxial line, it is better. But it still doesn't quite work like you
think. I think I see what you are missing. You assume the line is
1/2 wl long at the upper and lower band edges, and it isn't. 80
meters is about 14% wide. The band's width makes just as big a
problem in a transmission line as it does an antenna.
> ice loading, try this. Resonant the vertical at 3.6 MHz. Connect a coax
> that is a 1/2 wave at 3.6 MHz (90.2" for .66 VP) or a multiple. In the
> shack install a BC 3 gang variable Xc in a box that is in series with the
> center lead of the coax. Have an insulated shaft coming out of the box.
> Bend some plates over so that the 3 gang variable Xc shorts out at full
> capacity. As you operate higher in frequency the vertical is inductive
> reactive and it reflects the same at the end of the coax. Use the
> variable Xc to tune this out leaving the Resistive part which rises very
> slowing going toward 4 MHz. You will have a low SWR at 4 MHz as the
> reactance is tuned out. 3.6 MHz is selected as a resonant frequency as it
> gives the lowest average SWR over the whole band. It's called "Proper SWR
> Management". One felt it should be resonated at 3.5 MHz for reasons that
> didn't full fill the main goal. I've used this concept in a variety of
On Eznec over a perfect ground, a 12" diameter vertical has a base
impedance of 36 ohms.
The impedance at the low edge is 33-j11.4 ohms, and SWR is
1.65:1. Adding any capacitance would move things in the wrong
direction, so we would "short" the variable and have a 1.65:1 SWR
(assuming a negligible loss line) at the transmitter. No
improvement.
The impedance at 3.8 MHz is 42.6 j21 ohms. The VSWR at the
antenna is 1.61:1. At the other end of the coax the impedance is
49.9+ 24j. It would take 1740 pF of capacitance to cancel that,
and we don't have it. If we did, the SWR would be 1:1.
So we are short on capacitance at 3800. Assuming we just crack
the capacitor open to 1060pF, the reactance will now be -15.4
ohms, and SWR will be 1.35:1
That's low, but not really that much better than we had. If we had a
thousand more pF, then we could start tuning the thing at 3700.
At 4 MHz, the antenna impedance is 50.8 ohms and SWR is
2.27:1. The impedance at the other end of the feedline is 93+37.2j
for a SWR of 2.27:1
At 4 MHz, we'd need 1070 pF. We can almost make that, and the
SWR when tuned with the capacitor meshed would be 1.87:1.
We get a reduction in SWR from 2.27:1 to a low of 1.87:1 when we
use 1060 pF.
This small change in SWR hardly seems worth all the effort!
With a vertical using #12 wire, the SWR with the capacitor you
suggest at band edges is 2:1 and 5:1 respectively when the
capacitor you suggest is tuned for minimum! Hardly a low SWR!!!
This is similar to the results using open wire line and a dipole you
posted earlier.
This is why a matching network require two or more adjustable
components if any impedance range/bandwidth is going to be
covered.
73, Tom W8JI
w8ji@contesting.com
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