The shunt chokes may not be as critical with the slopers, as the voltage
near the top of the tower is probably lower than with a shunt fed tower.
I can provide details of the chokes ala N9NB and W5JAW. They don't rely
on the difference in impedance with frequency, but the fact that they
have high impedance across the driven element but low impedance to
common mode currents from the boom to the element. Their purpose is to
prevent common mode current from flowing through the balun when the
tower is shunt fed. That is explained clearly in the QST article.
While grounding parasitic elements would increase top loading, I don't
think it's necessary, and could detune the elements.
I shunt fed my tower for years without the shunt chokes on the D40 above
the TH7 or the A3 below, and never had a problem. No doubt some power
was warming up the chokes. I think the fact that the TH7 elements are
all connected to the boom probably reduced the current in the baluns.
Other wire antennas and feedlines on the tower can be problematic. When
shunt feeding the tower, everything on it needs to be connected together
for common mode currents, or isolated and separated. For example, a
feedline running up the tower to an inverted V will result in a warm
balun, unless you use a shunt choke. However, that may be more top
loading than you want. I was successful in leaving the wire antenna on a
short side arm, but pulling the feedline away from the tower at about a
45 degree angle. The balun hangs on some insulators.
73,
Scott K9MA
On 12/7/2019 15:13, Dave Hachadorian wrote:
Let's discuss these shunt chokes a bit.
I am feeding my tower on 80 and 160 with a half-sloper fed near the
top of
the tower. The sloper length is changed with a vacuum relay to cover
both
bands. SWR is pretty good.
Up on the tower is an old KLM KT-34XA, with a Cushcraft 40-2 CD 6 feet
above
that. All of the elements on those antennas are insulated, except for
the
reflector of the 40-2CD. So I guess I should be grounding the parasitic
elements of the KT-34XA, and adding shunt chokes to the driven
elements of
both antennas to improve efficiency on 80 and 160?
So for the 40 meter driven element, I guess I need chokes on both
sides of
the split driven element. The choke impedance needs to be low on 80 and
160, but fairly high on 40 so SWR doesn't get affected on 40. The N9NB
article in this May's QST doesn't go into any detail on those shunt
chokes.
What kind of chokes are people using on 40 and with tribanders?
Dave Hachadorian, K6LL
Yuma, AZ
-----Original Message----- From: K9MA
Sent: Saturday, December 7, 2019 12:40 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Shunt Feed Woes
I'm further convinced of the need for shunt chokes on ungrounded driven
elements on shunt fed towers. See the recent QST article by N9NB.
Hopefully, this will be of use to someone else. It may even save someone
having to climb a tower in December.
My 70 foot tower has a D40 dipole at 75 feet, a JK Mid-Tri at 70 feet,
and an A3 at 50 feet fixed SE. Last night in the ARRL 160 contest, I
discovered that after a long run, the SWR of my 160 shunt feed shot up,
as if something were heating. Nothing was warm in the tuner box, so it
had to be something on the tower. (I did try it with a second
amplifier!) Both the upper tribander and the D40 have those shunt
chokes, but the lower A3 tribander, at 50 feet does not. That one is
presently waterlogged or ice-bound, so it's almost 5:1 on 20. However, I
discovered that right after the shunt feed failed, the A3 SWR went DOWN,
then quickly rose again as something cooled off. It must be the balun on
that antenna. I never had that problem before, but that was when all the
TH7 elements were connected to the boom, so the tower has less top
loading above now. In any case, disconnecting the A3 feedline at the
tower base helped: it took longer to overheat the balun, so I can
probably get away with that tonight. I wonder, though, how much of my RF
is going into heating that balun! I guess it's time to build another
shunt choke, and hope for a mild day to climb up there.
73,
Scott K9MA
--
Scott K9MA
k9ma@sdellington.us
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