Hi Low Band people,
Nice presentation Steve. I would like to point out that Beverage users or
even other phased antenna users should not use paralleling two 75 ohm coaxes
and then matching them back to 75 for the receiver. While on the surface that
all sounds good there is a little problem for the individual antennas. If you
look from the wire through the Beverage transformer to the parallel junction
the antenna transformer actually sees the other antenna at 75 ohms in parallel
with the rig matching transformers 75 ohms. This causes the real load impedance
on the Beverage transformer to be 37.5 ohms and not 75. This may cause the
Beverage to not have the anticipated impedance flat curve or possible
performance problems. This could also make the end load resistor not be an
anticipated value. If you use a delay line in one antenna for stagger, the
following should be noted. It has been my experience that although
theoretically you only need to match at least one end of a coax for the phase
delay in circuit to match the proper terminated value, I have found that
terminating both ends reduces problems from component tolerances not
terminating the proper coax impedance causing unwanted phase shifts. Using a
Magic Tee will help terminating the coaxes with their proper impedance keeping
the phase delay where you expect it. Early designers of passive 4 or more
element receiving arrays missed this point when making combiners for arrays
using paralleled transmission lines.
Bottom line is to yes yes, use a Magic Tee or some version of a hybrid
combiner for combining which will isolate between antennas.. Then the antennas
as well as the receiver will see the 75 ohm load as likely designed.
Here is a nice technical tutorial on the Magic Tee.
https://michaelgellis.tripod.com/magict.html
Lee K7TJR OR
-----Original Message-----
From: Topband <topband-bounces+k7tjr=msn.com@contesting.com> On Behalf Of
donovanf@erols.com
Sent: Tuesday, February 9, 2021 3:28 PM
To: topband <topband@contesting.com>
Subject: Re: Topband: The Magic-T
This is the corrected URL for Steve's Magic-T video
www.youtube.com/watch?v=_Bbt1IcC4bk
73
Frank
W3LPL
----- Original Message -----
From: "VE6WZ_Steve" <ve6wz@shaw.ca>
To: "topband" <topband@contesting.com>
Sent: Tuesday, February 9, 2021 10:29:54 PM
Subject: Topband: The Magic-T
Today I uploaded a video to my RX series which explains the Magic-T combiner.
I have received a few emails asking me “how do I make the magic-T”? so I
thought a video would be helpful.
I show the method for winding and building the Magic-T combiner and I explain
how it is used in phased RX antennas.
The associated 2:1 impedance transformer is discussed, and I show the
difference between an isolated and auto-transformer architecture.
I also discuss an error that can be made when building phased RX antennas if
care is not taken to avoid an unintentional 180 phase shift at one of the
elements, especially if using home-brew matching boxes.
YouTube video: https://youtu.be/_Bbt1IcC4b <https://youtu.be/_Bbt1IcC4b>-
73, de steve ve6wz
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