[TowerTalk] 160m 1/4 Wave Vertical Results Thus Far...

[jeremy-ca]

----- Original Message ----- 
From: "W0UN -- John Brosnahan" <shr at swtexas.net>
To: <towertalk at contesting.com>
Sent: Thursday, September 27, 2007 9:52 PM
Subject: Re: [TowerTalk] 160m 1/4 Wave Vertical Results Thus Far...


>
>>
>>3. In sweeping the tower from 1.600 MHz to 2.500 MHz, we found it's
>>natural (no matching other than the 75' of LDF4-50A) resonance at 1.788
>>MHz. The lowest swr observed at this frequency was 2.6/1. At 1.800 it is
>>3.0/1 and slowly rising to 4.0/1 at 2.000 Mhz. Conventional wisdom tells
>>us that the thing is too long (tall).  This still brings up the
>>question, "How can this be the case when the vertical is only 120 ft.-6
>>in. tall to begin with?" What are we missing here guys? The center of
>>the band (1.900 mhz) is what we've been shooting for all along.
>
>
> Steve--
>
> Resonance is not defined as lowest VSWR!  It is the frequency where
> the reactance goes to ZERO and may not be the lowest VSWR.
>
> So the problem you are having to determine the resonant point may be
> a conceptual one.  Clearly 120.5 feet is too short for a "resonance" at
> 1.788 MHz.
>
> I think you need some instrumentation to determine reactance in the
> face of strong signal IMD.
>
> But "resonance" is not really that important.  You clearly have an
> effective antenna with a decent ground plane.  All you need now is
> a matching network to get the VSWR to a value you are happy with
> inside the band.  An L-network should suffice.
>
> BTW  Very few AM boradcast antennas are actually self-resonant
> at their operating frequency.  There is no compelling requirement to
> make the antenna self-resonant.  Just match what you have and get
> on the air.
>
> And there is NO NEED for Pepcid consumption.  You are chasing a goal
> that is irrelevant.
>
> 73  John  W0UN

It appears that you need a higher feed impedence in order to improve the 
bandwidth a bit. Try dropping a wire from the top and 2-3' away. Ground the 
base of the tower to the radials and feed the wire. This will have no effect 
on ground losses, just an impedence step up. Look at it as half of a folded 
dipole. The BC industry calls it a folded unipole.

Carl
KM1H