Hello Martin,
One sure way to break any common connection to the other elements is
to use a line isolator/ 1:1 in line with each elements feed line. There is
NO common ground connection at that point. You have a balanced antenna
element to start with and an unbalanced feed line so putting a balun in each
line would make sense and with the feed impedance being about 100 ohms a
matching transformer would be prudent and would save the matching problem
unless you used a 1/4 wave 75 Ohm stub on each feed element. You would still
have an unbalanced stub to a balanced load issue. Interesting conundrum.
Michael Baker K7DD
k7dd@cox.net
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Martin Sole
Sent: Sunday, May 23, 2010 10:05 PM
To: TowerTalk@contesting.com
Subject: [TowerTalk] Quad questions
I am about to start work on re-installing my Lightning Bolt 2 element quad.
Since we were granted access to 10, 18 and 24MHz I have decided to add these
bands back on to the spreaders but I am mindful of the issues in having 5
bands on 1 quad. From what I have read I believe the first important
requirement is to feed each band separately. To do this I intend to use a
boom mounted remote coaxial switch but have a few questions.
1. I plan to have coaxial baluns on each feed point but am unclear if
this will provide sufficient isolation for the common connected shield at
the switch box? Do I need to isolate the entire feed to each element or is
switching the centre of the coax sufficient?
2. Cebik states that it is beneficial, especially for the 20m and 10m
loops, to ensure the 20m loop is open when operating on 10m. With the length
of open coax from the 20m feed point to the switch box acting as a
transmission line transformer the length would seem critical to ensure an
open circuit feedpoint? Presumably it needs to be a multiple of half
wavelengths at 10m?
3. What about the other bands? It seems prudent to ensure feeding the
12m and 10m driven elements with lengths from the switch box that make them
appear open when on the other band. Helping to minimize interaction?
4. I want to add a 30m folded loop to the driven element and a 40m
folded dipole to the reflector element using short spreader extensions along
the lines of EI7BA, see http://www.iol.ie/~bravo/Cubical%20Quad.htm.
Wondering about the impact of the 30m loop on 10m and the 40m folded dipole
on 15m?
Grateful for any input.
Martin, HS0ZED
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