Glenn,
you had mentioned somewhere that the choke should be at least 12 feet from the antenna. Should the RG 400 be longer in order to provide for this? Or use coax jumper and barrel connector to get it away from the antenna?
The latter, Scott. The reason for the 12 feet or so of coax in the clear is to provide an effective radiator as the other "half" of the antenna.
I believe when you're saying "other half" of the antenna you're referring to the Alpha 10-160m HF JPole Sr, correct? For everyone's benefit (and I'm trying to keep this straight myself), you made this comment a few days ago:
The Alpha Antenna 10-160m JPole Sr is an end fed antenna. Like all end feds, the obvious wire is not the complete antenna. The coax cable outer shield forms the other part of the antenna. No amount of grounding or adding of counterpoises will completely eliminate that effect.
You can experiment with a purposeful counterpoise for your antenna that may provide some relief. But you should move the antenna as far away from the house as possible and then use two proper common mode chokes on the coax. One choke should be 12 feet or more from the feedpoint of the antenna. The second choke should be located at a point before the coax enters the house.
I did experiment with different counterpoises and they had no effect on the background noise. Without some major re-design and location of the shack, I'm probably going to try keeping the antenna location for now, which is supported by an 8 ft non-conducting mast clamped to the main sewer vent stack on the roof. The other end is tied off to a tree at the edge of the property.
As you saw in
that photo of the kludged CMC, the feedpoint is just inches (< 1ft) from the top of the mast, so my run is ~ 9 ft down the mast to the roof, then ~ 12 ft across the roof before it runs ~ 5ft down (6-7 ft including drip loop) to enter through a plywood insert in the window of the garage shack.
Here's another photo taken standing outside the window of the garage shack where the feedline enters, looking up at the feedpoint of the Alpha (the 2m/70cm JPole is seen to the right). So you see, I don't have a lot of space out there. Your comment referred to coax "
in the clear" ...
The latter, Scott. The reason for the 12 feet or so of coax in the clear is to provide [. . .]
...but I don't have 12 ft in the clear. 12 ft from the feed point and I'm laying on the garage roof, and the entry to the shack is just several more feet away. I've got 8-9 feet in the clear, though.
What's special about this 12 ft? Would I maybe want to put the CMC at the
base of the roof mast? Or what if I put it at the top, right at the feed point, where I installed my kludgey CMC? Or is all of that ~12ft of coax laying on my roof and the ~5-7 ft hanging off the roof before going in the window an effective radiating part of the antenna, and I would lose something by choking elsewhere?
In any case, I think I would
NOT build any additional lengths
into the CMC, but keep those as short as possible because that makes them the most versatile. Then one can always adjust lengths and distances from feed points, entry points, etc. by means of jumpers, leaving the CMC equally usable at any location.
If, on the other hand, the antenna is a center fed dipole then the first choke can go right at the feedpoint because you don't want any part of the coax to contribute in a meaningful way to the radiation or reception of RF.
And when you refer to two chokes, I assume that'd mean two
sets of chokes? For example, in my case of wanting to cover all bands I would have three differently wound chokes in series; you're suggesting a second set of three? I may just start with one set of three and see how that goes, location To Be Determined.
Dave