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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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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