[Elecraft] Unexpected use for a P3

[Jim Sheldon]

Very interesting.  My Butternut HF2V with a homebrew copy of their 160 meter base loading coil is pretty high Q as well and after reading Chuck's post, I just got done checking mine the same way.

The K3's tuner will match this vertical from 1800 up through about 1840 before it goes out of range.  Using Chuck's technique of the P3 and band noise with the internal tuner bypassed, it's pretty easy to see the peak in band noise at the resonant frequency.  The display shows it best when the span is set pretty wide.  You can actually see the SWR curve as a peak in the noise around the resonant point, rolling off fairly sharply either side of it.

To get an accurate look at it, you have to put the ATU in BYPASS mode to make sure you are seeing the true antenna impedance and not the last point the tuner matched it at.

Jim - W0EB

> I've been spending the winter on 160 meters with my Cushcraft MA160V
> top-loaded vertical antenna.  This antenna has been surprisingly
> effective
> for such a short (approx. 36 feet) vertical.  I have a decent,
> though by
> no means
> exceptional, ground system.  The usable bandwidth is on the order
> of 20 KHz,
> and I carefully tuned it for a center frequency of 1820 KHz,
> positioning
> me for
> CW DX fun on Top Band.
>
> Since I obtained the P3 last Fall, I've noticed that I can see the
> rolloff characteristics
> of my antenna simply by looking at the display with band noise.
>
> We had considerable freezing rain and sleet, followed by snow here
> in
> St. Louis
> in the past couple of days.  The top loading elements (coil and top-
> hat
> capacitance)
> have a thick coating of ice on them.  The other night, I turned on
> my K3
> and P3 and
> was prepared for listening for DX at sunset when I noticed the P3
> was
> indicating a
> frequency response rolloff far below what was normal.  In fact my
> antenna had made
> a QSY right out of the band to a new center frequency of roughly
> 1770
> KHz!  Even at
> the 1800 KHz band edge my SWR was about 5.5:1.
>
> Having used this and other top-loaded verticals, I had experienced
> shifts of 15-20 KHz
> due to icy conditions previously.  But never a 50 KHz shift!
>
> I tried using my MFJ-259B Antenna Analyzer to check the resonant
> frequency and
> bandwidth of my antenna, only to discover the lower limit of the
> analyzer is 1800 KHz.
>
> Now, I'm waiting for some of the ice to melt or evaporate from my
> antenna so I can use it
> again.  So I'm watching band noise on my P3, waiting for the
> resonant
> frequency to
> come back up.  Already today, it has moved up about 10 KHz (even
> though
> the temperature
> remains below freezing).
>
> I just thought I'd point out this aspect of the P3 (or other
> panadaptor
> frequency displays) for
> indicating the frequency response characteristics of high-Q
> antennas.
>
> 73,
> Chuck Guenther  NI0C
>
>
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