I use an end-fed wire [1/2 wave on 80] from MyAntennas.com run along the
top of a wood fence about 1.8 meters high. It has a transformer in a
sealed box. The outer surface of the coax seems to serve as the
counterpoise. It enters the house through a steel pipe near the ground
into a closet and thence to the K3/100.
It works as good as one could expect on 80, 60, 40, 30, and 20. 40 seems
to be it's sweet spot. It works on 160 ... not great ... but I make
Q's. It's OK on 17 and 15, the SFI has prevented much testing on 12 and
10 although it has SWR minimums [1.3:1 or so] there. It has 1.2:1 SWR
on 6 on which I have made exactly zero contacts when the band was open
this last summer and could hear lots of stations.
Apparently low SWR does not guarantee low loss, however any antenna is
better than no antenna. [:-) I'd have to destroy it to see what's
inside the box.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 1/9/2018 1:33 AM, Jim Brown wrote:
On 1/8/2018 8:19 PM, K9MA wrote:
I would be cautious about using a transformer above 40 meters, as the
stray capacitance may be too large. I haven't tried it, though, so
it might work with some transformers.
For years, we've been winding our transformers wrong. The windings
should be on opposite sides of the core, so that capacitive coupling
is minimized, NOT laying on top of one another. In a ferrite
materialthat is SUITABLE for use as a transformer, the mu is high
enough that a VERY high fraction of the flux in toroid is contained
within the core, with very little leakage flux. Windings that lay on
top of each other maximize capacitive coupling, and thus maximize
common mode current. Windings on opposite sides of the core minimize
common mode current.
Ferrites are semiconductors, so they act as a dielectric. When we
place windings on opposite sides of a core, we maximize magnetic
coupling and minimize capacitive coupling, which is from one winding
to the core and from the core to the other winding. That capacitance
is VERY small if the windings are widely spaced.
73, Jim K9YC
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