Exactly. Dave WX7G
On Mon, Feb 6, 2012 at 1:29 PM, ZR <[email protected]> wrote: > So does simple math when looking for 1/2 wave resonance. This is basically > an OCF dipole which is going to have gobs of shield current thet has to be > surpressed. > > Carl > KM1H > > > > ----- Original Message ----- From: "DAVID CUTHBERT" < > [email protected]> > To: <[email protected]> > Cc: <[email protected]> > Sent: Monday, February 06, 2012 12:06 PM > Subject: Re: Topband: Help With Vertical Antenna Matching > > > 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" <[email protected]> >> 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 >>> >>> >>> >>> ______________________________**_________________ >>> UR RST IS ... ... ..9 QSB QSB - hw? BK >>> >>> ______________________________**_________________ >> UR RST IS ... ... ..9 QSB QSB - hw? BK >> >> >> ----- >> No virus found in this message. >> Checked by AVG - www.avg.com >> Version: 10.0.1424 / Virus Database: 2112/4792 - Release Date: 02/06/12 >> >> > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
