[TowerTalk] Shunt Feed Woes

[K9MA]

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 at 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 at sdellington.us