Change in the P3 noise floor

[Larry K1UO]

A question for you "Tekkies".  Today, for the first time I can recall, I widened the P3 span to the full 200K bandwidth display. While listening on aux receive antennas for 160 meters, I could see a half moon shaped noise floor that started gradually rising from S0 at about 1.775Mhz, peaking around S3 at about 1.830 and slowly declining again until back to S0 level at around 1.875Mhz  (A nice even arc of noise).

Would this be noise from my series fed 115ft top loaded 160/80M xmit tower that I am seeing?  I have a vacuum relay that disconnects the feed /radials from the tower when receiving and I didnt see much if any difference when switching the relay in and out manually.

curious now as to what I am seeing.

Thanks,
Larry  K1UO

[Bob - W6OPO]

Larry,

That is usually due to the antenna's bandwidth especially if an antenna tuner is used.  Really wide spans will show how sharp the antenna is - in other words its bandwidth.

Again if an antenna tuner is employed it will look pretty sharp/narrow compared to no matching network.

73,

Bob - W6OPO

[Nate Bargmann]

* On 2011 17 Dec 08:47 -0600, Larry K1UO wrote:
> curious now as to what I am seeing.

I see about the same thing on 160m.  I have a 3/8 wave center-fed
doublet on 160m fed via 450 ohm window lead and a Palstar AT1500DT
tuner.  As I need to retune on excursions of more than 20 kHz or so, the
peak of the noise on the P3 corresponds to the matched setting of the
tuner.  As the most efficient transfer of energy occurs when impedances
are matched, it makes sense that the P3 would show a peak of noise at
the setting of the conjugate match.  

In other words (for those who believe I'm not clear enough and will want
point that out via private mail--save your bandwidth), the noise floor
peak on MY P3 corresponds to where I have the tuner set for an SWR of
1.0:1 on 160m.  The display is not as pronounced on other bands where
the 2:1 SWR bandwidth is much wider.

73, de Nate N0NB >>

--

"The optimist proclaims that we live in the best of all
possible worlds.  The pessimist fears this is true."

Ham radio, Linux, bikes, and more: http://www.n0nb.us
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