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[TowerTalk] Determining "True" Resonance of Verticals

To: <towertalk@contesting.com>
Subject: [TowerTalk] Determining "True" Resonance of Verticals
From: n0tt1@juno.com (n0tt1@juno.com)
Date: Sun, 5 Aug 2001 00:49:05 +0100
Hi Rick,

Your method sounds doable and perhaps more accurate than
mine....I would have to improvise...no Minicircuits in my
junque box hi hi...lots of toroids though.  So I'll see what
I come up with.  I might want to try your method after I
have done the initial, course tuneup.

Right now I'm getting fairly good resolution using the
259-B and tuning VERY carefully for a minimum reading
on the LCD reactance ("X") scale.  I can usually "rock" the
tuning knob just enough to find the "center" and read the frequency.
I do this several times and throw out the "worst" reading
and average the rest.  It's a slow process under a blazing hot
sun with a heat index of over 100! hi hi

My vertical will be a five band trap antenna...20m on down
thru 160m, and will be a portable unit.  I have limited the radials
to a count of 38 each one being about 80 feet long.  

Thanks again for your input!

73,
Charlie, N0TT

On Sat, 4 Aug 2001 12:03:01 -0700 "Richard Karlquist"
<richard@karlquist.com> writes:
> I've done a lot of measurments of verticals, and I wanted to
> get the best possible answer for resonant frequency and
> feed resistance.  My 259-B just wasn't accurate enough for
> this task, (nor does MFJ claim that it is).  Then there is
> the additional headache of BC station interference.
> 
> I built a Mini-circuits transformer and a 100 ohm miniature
> cermet pot into a little box, and wired it up like a noise
> bridge, except the "noise" was provided through an SMB
> connector from a signal generator.  I pre-tested the bridge
> on the bench with known loads and it was extremely accurate.
> Didn't take much work to build it either.
> 
> You could probably butcher an RX noise bridge to make an
> equivalent bridge by shorting out the capacitors in the
> bridge and bringing out the bridge drive to a connector
> instead of the noise generator.  Seemed to me it was
> less work to just build it from scratch.
> 
> The test procedure is to listen to the signal geneator with
> a receiver, and adjust the pot for minimum S-meter reading.
> Then move the frequency up or down until the best null is found,
> of course renulling the pot if necessary as you proceed.
> The frequency resolution is 10 kHz or better.  A carrier
> works better than a noise generator if you want a lot of
> accuracy.
> 
> Now here's a little trick that seems obvious, but I haven't
> seen it in the literature.  After you get the pot nulled
> at resonance, you measure the DC resistance of the pot with
> an ohmmeter.  This number is the RF resistance of the vertical.
> This is more accurate than trying to put marks on the box with
> a pointer knob, like you see on an RX noise bridge.  You do
> have to verify that the pot you use doesn't do something
> funny at RF to make its resistance vary vs DC.  Most small
> pots are OK, except obviously wirewound.
> 
> The little box I used was 1x1x3 inches, so I could wire it
> into the antenna feed terminals with short wires.
> 
> I also built a balanced version of this bridge to verify that
> the Zo of my open wire line was really 450 ohms.
> 
> 
> Rick N6RK
> 
> 
> richard@karlquist.com
> www.karlquist.com
> 
> -----Original Message-----
> From: owner-towertalk@contesting.com
> [mailto:owner-towertalk@contesting.com]On Behalf Of n0tt1@juno.com
> Sent: Saturday, August 04, 2001 8:46 AM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Determining "True" Resonance of Verticals
> 
> 
> 
> Hi Gentlemen,
> 
> This isn't exactly tower talk...maybe close enough...
> 
> I have tried several methods to measure *resonance* of a
> vertical 20m antenna (which will become a multiband
> trap vertical).
> 
> In tuning a antenna, I consider resonance and impedance
> two separate subjects.  Lowest SWR to me doesn't mean
> "resonance".  At this point in testing I don't really care
> about the "input" impedance at the coax connector...the
> antenna will be matched to the line later.
> 
> I have made many measurements using a GDO and a
> MFJ 259-B analyzer and I get different readings of
> resonance depending on what method I use to couple
> RF to the antenna.  When I used the GDO, the frequency
> was monitored with a frequency counter of known accuracy.
> 
> I'm thinking that the lowest "X" (reactance) indication
> on the 259-B would be THE resonant frequency, and
> that I should connect the instrument with the shortest
> possible leads (like a double-male coax connector)
> to the "input" of the antenna.  Is this correct?  Or what
> IS the best way?
> 
> BTW, the "X" reading has been 6 to 7 ohms with the
> 259-B and "X" will read zero when the 259-B is
> terminated with a 50 ohm load.
> 
> 73,
> Charlie, N0TT
> 
> 
> ________________________________________________________________
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________________________________________________________________
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Juno offers FREE or PREMIUM Internet access for less!
Join Juno today!  For your FREE software, visit:
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List Sponsored by AN Wireless:  AN Wireless handles Rohn tower systems,
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supporting towers up to 96 feet for under $1500!!  http://www.anwireless.com

-----
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