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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