Topband: Help With Vertical Antenna Matching

[DAVID CUTHBERT]

Exactly.

   Dave WX7G

On Mon, Feb 6, 2012 at 1:29 PM, ZR <zr at jeremy.mv.com> 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" <
> 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 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
>>>
>>>
>>>
>>> ______________________________**_________________
>>> UR RST IS ... ... ..9 QSB QSB - hw? BK
>>>
>>> ______________________________**_________________
>> UR RST IS ... ... ..9 QSB QSB - hw? BK
>>
>>
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>