Hi John,
We know that gain is relative as well as absolute – patterns differ between
antennas at the same height so unless you have the identical antennas pointed
in the exact same direction right at the station at the same time, you won’t
really see that 2 db difference – it will be more or less than 2db in actual
cndx than on the antenna test range. And that is assuming the wave angle of
transmission of each antenna is identical which is usually isn’t, especially
when stacks are involved. And then any uneven terrain in the near field can
and will affect the pattern of the antennas differently as well. These are all
reasons why we notice more or less than 2db even when the specs show a 2db
difference.
You bring up a good point about interaction with other antennas – that is yet
another factor affecting the patterns and gain that we see or don’t see.
The final piece is what I like to think of as “takeaway gain” or gain that is
“taken away” or lost from a variety of factors by the match, transmission line
loss, connector loss and all the other things that take place between the
transmitter and the antenna, when they are not optimal. This may differ
between antennas and when added onto the 2db difference already cited, can make
the difference between busting a pileup or not. So it may be 2db in reality
(or not) but if that 2db is at the threshold of being heard or not heard, it is
effectively more like an s-unit.
73
Bob KQ2M
From: john@kk9a.com
Sent: Wednesday, February 27, 2019 8:45 AM
To: towertalk@contesting.com
Cc: KeepWalking188@ac0c.com
Subject: Re: [TowerTalk] JK 402 versus CC XM-240
Nice summary Jeff. Gainwise there is no difference between the Moxon
and the coil loaded 2el. As I stated below, it is hard to get a lot of
gain on 40m with a single antenna. Even if you compare the JK404G
monster 4el full sized element OWA beam against the small loaded JK402
there is only 2dB difference. There are easier ways to obtain 2dB, At my
station I stacked two 2el ObtiBeam OB2-40 coil loaded beams and it works
incredibly well, plus they get along with all of the other antennas on
the tower.
John KK9A
from: Jeff Blaine AC0C
date: Tue Feb 26 20:48:38 EST 2019
Picking the "best" 40m beam is very hard on 40m and requires a
compromise unless you have a big tower, a big rotor, moderate wind/ice
exposure, height limitations and CW/SSB only vs. wideband CW+SSB. Oh,
and a serious budget of course. That's why there are so many different
designs in the market.
A bit of the decision process for both the beam type selection and
suppler may be found here for those interested:
https://ac0c.com/main/page_acc_towersantennas_40m_stack.html
73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com
On 26-Feb-19 10:57 AM, john at kk9a.com wrote:
> Momobeam makes a T element 40m beam also, I used one last year
> operating as
> PJ4R. I have modeled a lot of 40m beams and it is hard to get a lot of
> gain
> with a single antenna. The Moxon is a nice design but it does not defy
> physics.
>
> John KK9A
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