Hello Dick, K9OM:
I have two separate antennas systems on 20, 15 and 10 meters (six systems)
that should be of interest to your question - where I have two stations on
each of these bands. One antenna system is for the RUN station and one
antenna system is for the Search and Pounce (S&P) station (TX interlocked)
at K3LR.
All of the RUN antenna stacks are made from OWA Yagi antennas which have all
elements insulated from the boom with a direct 50 ohm feed from a good coax
current choke (5000 ohms of CMI or better).
The Search and Pounce stacks here (currently) are M2 antennas with grounded
parasitic elements and a DC ground hairpin feed (also 50 ohms). Also using
high quality greater than 5000 ohms CMI common mode feedline chokes.
So the 20, 15 and 10 RUN stacks all use ungrounded elements and the S&P
stacks all use grounded elements currently at K3LR.
At this station - there is no change in the band noise from one antenna
system to the other (nor would I expect there to be) - as listened to on two
radios (same make and model) side by side at the same time. This is physics
and neither the matching system nor the element mount should have any effect
on received noise.
This summer I am changing out the M2 20 meter and 15 meter Yagis' and
replacing them with OWA antennas (ungrounded elements) to increase the VSWR
and pattern performance of the S&P stacked arrays at K3LR.
You tribanders should have close to the same back ground noise. However the
Skyhawk should be more efficient (full size elements). I suspect you have a
local noise problem or as K9YC comments - you have a feedline issue picking
up noise. This can be cured with high performance chokes at the feedpoints
and using correct grounding and bonding techniques (see K9YC's website). It
may also require additional feedline chokes (I do this) at the antenna
switch or at your radio to make the feedlines "cold" to noise mitigation
issues. You could have some loose hardware on the Skyhawk (or close to it).
73
Tim K3LR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Brown
Sent: Sunday, August 19, 2018 2:25 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Do some HF Tri-Banders receive more noise than
others?
On 8/19/2018 11:11 AM, Bob Shohet, KQ2M wrote:
> Dick,
>
> Are your antennas at the same height or different heights? Is the
grounding of each tower the same? Did you use the identical RF chokes on
each antenna?
>
> Any differences of these factors could dramatically affects differences in
noise level experienced between antennas pointed in the same direction;
especially differences in height and RF chokes.
>
> I would also look at proximity of the towers to potential noise sources.
If a potential noise source is 50? from tower 1 and 150? from tower 2, that
alone could make a 1+ s-unit difference in noise level, depending on the
type of noise source and its polarization.
>
> And differences in physical proximity to the noise source could also
affect the beam heading to that noise source for each antenna. So, for
example, in the event of a noise source 50? from tower 1 at beam heading
peaking 270 degrees, at 150? from tower 2, that noise source might have a
beam heading dramatically different. Just another potential factor to
consider.
>
> I do also believe that the physical design comes into play ? from my own
experiences, even some monobanders pick up more noise than others. I have
to believe that that would also hold true for tribanders even if the physics
are different.
All of these comments are well taken. I would like to emphasize the
importance of an effective common mode choke at the feedpoint that
prevents pickup on the feedline from filling in nulls in the antenna's
pattern. In addition, the design of the antennas themselves can strongly
influence how noisy it is by control of its directional pattern in three
dimensions. G0KSC, one of today's leading designers of Yagi antennas,
has emphasized this in the descriptions of his designs. Two excellent
interviews of G0KSC by N0AX were published a few years ago in National
Contest Journal.
73, Jim K9YC
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