I got away with not bypassing the balun on my TH7 for years. However,
with all the other elements grounded, there may not have been much
current flowing through the balun in shunt feed mode.
I would suggest modeling your tower, but including only the boom and
driven elements of the Mid-Tri, plus whatever other loading there may
be. Assume the driven elements are connected to the boom. (I'm presuming
the 10-20 m ones are through the shunt choke.) Look at the current to
the 40 m driven element, estimate the impedance of the balun on 160, and
calculate the power dissipated. This should be a reasonable worst-case
estimate.
73,
Scott K9MA
On 1/27/2020 10:35, Ignacy Misztal wrote:
I am getting JK Mid Tri 40 installed tomorrow on a 100ft tower. I have
questions about shunt feeding on 160m.
Scott K9MA has sent me lots of information on his shunt feeding of Jk Mid
Tri (not 40), also referring to an article by N9NB/W5JAW in QST. They
advocate adding an extra toroid, like current balun with wires at one end
grounded. This is to short circuit the insulated driven element.
In my case, I would also have to do it for the 40m element. The feeding is
some 6 ft away from the boom, and if something goes wrong, it will be hard
to fix.
A few questions.
Is shunting less critical with 100ft than 70ft tower?
Did anybody experienced heating of the balun when it is not bypassed? I
know that there would not much with the 61 mix, but could be bad with 31
(that I avoid like hell for higher power operations).
Shunting would be equivalent to connecting both coax connections of the
balun to the mast. Shortcuts would be ground just the coax(es) at the top
to the mast (or tower), or even shorting and grounding the coax(es) at the
bottom of the tower. Did anyone do anything similar?
The mast and the tower are separated by the rotor. Does the rotor provide
enough short or does the mast need to be shorted to the tower separately?
If there are tall trees around, would it be better just to use the tower as
support for 1 to 4 inv L?
Ignacy, NO9E
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Scott K9MA
k9ma@sdellington.us
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