I use RG142, captured as MIL surplus, very nice with double silver
plated shields and TFE insulation. It will handle QRO power beyond
30MHz. I can get 9+ turns into a stack of 240-31 cores. The stack then
goes into a 4x4x4 (or larger) PVC Carlon box from Home Depot with
watertight cable grips in and out (McMaster has grips in the sizes
needed), so no PL259/SO239's to try and get watertight. All connections
are soldered and inside the box. The rig end does get a PL259 after the
rotator loop, so that pigtail is otherwise taped to the boom. My goal
was to eliminate as much as possible that could go wrong out at the yagi
feedpoint, i.e. no connectors except the lugs to the driven element. If
the choke can get very close to the feedpoint, then I use #10 size
cable grips for two #10 wires to the feedpoint. Then there is no coax
shield/conductor "split" to try and waterproof.
I also drill 0.1" drain holes in all four corners of the lid which goes
on the down side so even if the gasket leaks, little water gets inside
and the holes are small enough so most bugs stay out. I have 9 of these
chokes on antennas w/o any problems at full QRO, contest mode.
Grant KZ1W
On 4/11/2016 20:58 PM, Roger (K8RI) on TT wrote:
Using BuryFlex(TM) and LOTS of cable pulling lube, I can get one more
turn, "maybe" 2 with luck. The RG400 would allow a lot more turns, so
adding two or 3 would be easy, but somewhere I have to switch to
either LMR400, or BuryFlex and something has to support the larger
coax. Weatherproofing the RG400 ends is no big deal, but connecting to
the larger coax with out adding stress to the RG400 is a bit more
complicated
OTOH adding cores is easy, but far less productive. Adding 2 more
only adds about 33%
Where as using the BuryFlex, the choke just becomes a lump in the
coax, BUT the coax should be supported with a choker "below" the choke
so there is little or no tendency for the center conductor to
migrate. OTOH I've had no problems (yet) with the current installation.
Everything has been open core so far and I intend on keeping it that way.
On 40, I've been able to run the legal limit through 8X but on 75 the
bandwidth prevents that.
Even with the higher loss, 8X with foil and double braid is flexible,
light weight, and low wind resistance. CNT240 was just too fragile
and too brittle. Migration was a problem in small radius bends with
that solid center conductor.
So the next try will likely be 6 cores and 7 turns. Because of the
high imbalance, I think 2 chokes is going to be a fact of life with
this antenna.
73
Roger (K8RI)
On 4/11/2016 Monday 11:14 PM, Jim Brown wrote:
On Mon,4/11/2016 7:06 PM, Roger (K8RI) on TT wrote:
In this case, "I believe" two separate chokes would do much better
than adding more cores, although "I think", shifting the external
portion of the coils into a bundle should lower the frequency of
maximum R.
Yes and no. The problem is that we need to get the choke resonant
near 75M. BTW -- maximum resistance is actually parallel resonance of
the inductance of the coil with its stray capacitance. :) The photo
shows a choke tuned for around 30M. Squeezing the turns together
would probably lower it to 40M. To get the choke down to 75/80M,
you'll need both more L and more C.
Adding 1 or 2 cores?
L increases approximately in proportion to the length of the
ferrite(s), and as the square of the turns. To lower Fr by a factor
of 2, we must increase sqrt of LC by 2, LC must increase by a factor
of 4. So going from 5 cores to 7 won't be enough without also adding
turns. The good news is that doubling the number of turns also
increases Rp, which is the quantity we're looking for. And C
increases approximately in proportion to the number of turns.
So the solution to the problem is to get as many turns as possible
through those cores, adding cores as needed. That means winding the
choke before installing the connector, or using smaller diameter coax.
OR -- look at the bifilar chokes configured as parallel wire
transmission lines. Two 12-turnor 14-turn chokes in series would
probably handle legal limit power assuming CW or SSB and not AM.
The cores are epoxied together with a very thin film so there should
be plenty of dissipation. I'd like to have both chokes "up there",
but I want to keep the load at the feed point as light as possible.
That antenna is currently under somewhere around 200# tension so
even with the choke and roughly 100' of LMR400 there is no
noticeable sag.
In the modeling I've done, and in my experience, a choke at the
feedpoint and another much farther down the line will be as good for
power handling as two up in the air.
73, Jim K9YC
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