Topband: Help With Vertical Antenna Matching
w9ac at arrl.net
Mon Feb 6 09:26:30 PST 2012
What complex Z value do you get after running NEC at 1835 kHz? Can you
attach the .NEC input file?
Does your MFJ analyzer give you complex Z results in a R+jX format, rather
than just Z magnitude? Not sure if the 259B model will do that or not.
Anyway, with complex Z info, it's easy to design either a low-pass or
high-pass L network using TLW software.
----- Original Message -----
From: "DAVID CUTHBERT" <telegrapher9 at gmail.com>
To: <philclements at centurylink.net>
Cc: <topband at contesting.com>
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
> 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
> 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>
>> 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
>> 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
>> 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
>> 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
>> Any help will be greatly appreciated; my old knees have about two more
>> climbs left in them!
>> Thanks in advance.
>> Phil, K5PC
>> UR RST IS ... ... ..9 QSB QSB - hw? BK
> UR RST IS ... ... ..9 QSB QSB - hw? BK
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