> In a message dated 3/20/02 5:37:37 PM Pacific Standard Time,
> n8de@thepoint.net writes:
>
> > When you raised the antenna to 80', did you measure the resonant
> > frequency AT THE ANTENNA? .. or at the end of a piece of coax?
> > The ONLY way to 'get it right' is to do ALL measurements AT
> the antenna,
> > NOT down at the shack!
>
> Well, you can measure all you want at the antenna but the only place
> where it MATTERS is at the end of the coax and what the
> transmitter 'sees'.
> Some antennas are wacky at the feedpoint but FB in the shack (I'm
> sure the
> feedline loss has a little to do with it). As long as the radio's
> happy - so
> am I.
You're easily pleased :-) It is not true that the only place it matters is
in the shack.
Assuming what is meant is resonance as opposed to match, quite a few
antennas have performance parameters that are extremely sensitive to having
the antenna properly adjusted for resonance at the operating frequency. Off
resonance and your gain and F/B can differ significantly from specs, even
though it might in fact be true that the match your rig sees is ok (probably
due to feedline losses!). Though not the antenna in discussion here, these
shortened 40 meter shorty beams are notorious for doing things like acting
reversed when operating off resonant frequency, such as on phone when the
antenna is tuned for CW. I'm sure a 205CA is not as grossly sensitive to
being operated off resonance, but the overall point is still true.
Bottom line: FIRST, get the antenna adjusted to resonate at the desired
frequency at the antenna. THEN worry about matching the feedline to that
feedpoint. This all done (ideally) in the position the antenna will be in
during service (or some simulation of it such as pointing straight up). Do
that and what the rig sees will take care of itself, assuming the feedline Z
is what the rig expects.
>
> I don't know why your antenna didn't change resonance when
> you got it up
> in the air. The 13.650 resonance on the sawhorses is about right.
> Sounds like
> you've got some interaction somewhere - guy wires? Wire antennas?
I've never seen such a deviance, although I must admit not to having tried
modern humongous arrays. Whenever I propped it up with the reflector on a
saw horse pointing straight up, what I got there was pretty much what I got
up top. Towers and wires when the antenna is down low can also do quite a
bit to screw it up. I just guyed the boom with ropes, pointing up, and did
the gross measurements close to the gnd. Worked every time for moderate
sized monobanders such as a 204BA. I don't think an extra element will
change the method.
>
> You only need to get it 30 feet or so off the ground to
> minimize ground
> capacitance effects.
Pointed straight up, my experience is you don't need more than a few feet,
assuming all metal is out of the way. Even with the reflector literally on
(dry) ground, it was a close approximation.
73,
Gary W2CS
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