Topband: Help With Vertical Antenna Matching

[DAVID CUTHBERT]

Phil, NEC shows that shortening the radials to 110' will resonate things at
1.8 MHz. The ends are 10' from ground. The radial length can be used to
adjust the antenna to resonance.

The input impedance is 100 ohms.

A balun having a lossy 400 ohm impedance is needed to reduce the worse case
feed line shield current to 20 dB below the total radial current.

Dave WX7G
On Feb 5, 2012 3:49 PM, "Phil Clements" <philclements at centurylink.net>
wrote:

> I need the help from one of the Guru's here on the group....
> I need to know how to match the 160 meter vertical ground plane here. It
> worked fine for years until I removed 80 meter 4-square from the tower. I
> have no modeling capabilities here.
>
> The set-up is this:
> A 200 foot Rohn 25 tower, with insulators at the 60 foot level.
> Three elevated radials at the 60 foot level; sloping down to about 10 feet
> off the ground at the far ends. They are c. 110 electrical degrees long.
> The vertical radiator is c. 140 feet long.
> There is a large 1:1 balun at the feed point.
> The feed line is 450 feet of 7/8 helix to the ham shack.
>
>
> The reading on the 259B at the feed point is c. 3:1 VSWR, with a slight dip
> to 2.7:1 at 2.45 mhz.
>
> I am at my wits end here; several trips up the tower produced no good
> results. There are many systems on Google and YouTube, but the matching
> systems are for antennas shorter that 1/4 wave length.
> The old matching system when the 80 m 4-square  was co-located was a 600 pf
> series capacitor from the center  of the coax to the base of the vertical
> element. The three radials are joined together at the feed point, and
> connected to the coax shield.
> The VSWR back then was 1.25:1 @ 1835 khz. , and was a "killer" antenna on
> 160!
>
> Any help will be greatly appreciated; my old knees have about two more
> climbs left in them!
>
> Thanks in advance.
>
> (((73)))
> Phil, K5PC
>
>
>
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