> I'd just put up an Inverted
> > V dipole, and be done with it. Or do something else less
> > risky.
> >
> Being such an advocate of vertical polarisaton Tom, I'm
really surprised to
> hear you say that.
Why Greg? A 1/4 wl slopper isn't much different than an
inverted V.
I can't think of a worse way to make a vertical!
> Apart from the (possible) problems of getting RF currents
where you don't
> want them, the sloper represents a really convenient way
of getting RF
> current high up in the mast, which is where you want it
for radiation.
Actually the height doesn't matter much at all. It is the
ampere-feet of area with unopposed currents that sets
radiation resistance. The slopper has as much current into
the tower at the feedpoint as it does in the thing people
call the antenna. Now if you had a huge plummers delight
Yagi up there and nearly all of the current terminated in
that Yagi's capacitance on 160 and very little flowed down
the tower, you'd have something only a little less effective
than shunt feeding the tower.
On the other hand if you don't have a low termination
impedance at the top of the tower, you have a real mess.
> use one here in Christchurch, and it represents the most
efficient way for
> me to launch a vertically polarised wave. I am about to
replace it with a
> true top feed arrangement (as per Radcom of a year or two
back) but my
> sloper has given me excellent service for 3 years. I
don't usually have a
> problem making you guys over there hear me, sometimes even
with 100W, so it
> can't be doing too badly!
I don't know. I've never heard you compare it to anything
else so I don't know how well it works. Besides, my point is
the slopper's main problem is results are random depending
on what else is on the tower.
> Surely with shunt feeding at the base, you have to be just
as careful not to
> get the RF where you don't want it?
Not at all. With a shunt fed tower you don't have current in
one wire going up and other coming down! You generally would
install a good ground with a shunt fed tower, and ALL of the
current would be vertically flowing except a tiny fraction
that loops around the reactance of the hairpin formed by the
shunt and the tower. It wouldn't be a so much a matter of
sheer luck if the system worked properly.
73 Tom
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