...."One thing that bothers me about this is that this doesn't guarantee
that the Z at the feedpoint is actually 50 j0."...
Who cares? The whole point of doing the adjustment was to get a better
match for the transmitter.
-------------------------------
..." The other question i have is, how do you know the element length
that gives you a match to 50 ohm feedline, is the antenna dimension that
gives you optimum gain and F/B? Is the element length for 50 ohm Z
always the length for best performance? "....
No, the 50 ohm point is not always the best for optimum performance.
The antenna works just like any other antenna in terms of feedpoint
impedance, gain and F/B. You can make small adjustments of the driven
element which will change the feedpoint impedance, without significantly
effecting the other parameters of the antenna. This is the only
adjustment you should do unless you know what you are doing. Length of
all the elements can be changed, but arbitrarily changing them will
destroy the performance of the antenna.
Jerry, K4SAV
Rob Atkinson, K5UJ wrote:
> Well, I must say, I have had some questions about the steppIR fluid
> motion antennas since I came back from Dayton.
>
> It seems that one of the overriding draws to the Fluid Motion brand
> antennas is that you can adjust the element lengths so that on any
> frequency above the low limit, you can achieve zero reflected
> power. There are two things about this that bother me, but I do not
> know if they matter or not since I have never owned one of these
> antennas.
>
> It's safe to assume that the feedline in a typical installation is at
> least 60 or 70 feet long, probably longer in most cases. This is
> usually a significant length in terms of wavelength above 30 m. I
> also assume that reflected power, or vswr, is being measured in the
> shack. One thing that bothers me about this is that this doesn't
> guarantee that the Z at the feedpoint is actually 50 j0. It's
> probably close enough on the lower bands but on 10 or 12 meters, where
> the feed is likely to be mulitple lambda in length, I wonder if using
> the antenna as a tuner gets the user into something like the "my
> feedline tunes my antenna!" type of situation i.e. the meter in the
> shack may show true unity along the line, or maybe you just happen to
> have adjusted the antenna so that the length of feedline you are using
> and your frequency result in a nice looking match at the shack but the
> feedpoint, if you could measure there, would tell a different story?
>
> The other question i have is, how do you know the element length that
> gives you a match to 50 ohm feedline, is the antenna dimension that
> gives you optimum gain and F/B? Is the element length for 50 ohm Z
> always the length for best performance?
>
> I essentially question giving no reflected power the highest priority
> which seems to be what happens with these antennas. Or maybe most of
> the time you get close enough to having your cake and eating it too so
> that my concerns are minor issues?
>
>
> 73,
>
> rob / k5uj
>
>
> ORIGINAL MESSAGE:
>
> On Mon, 11 Jun 2007 15:00:18 -0400, "Tom Rauch" <w8ji@contesting.com>
> wrote:
>
>> It isn't how an antenna really works that matters. The only thing
>> that really matters is how we feel about it.
>>
>> I've worked VK several times from my mobile on 160, and even Europe
>> on SSB 160. I've worked VK3IM while he was mobile a dozen times or
>> more on 160. In all cases antenna efficiency was less than 1% and
>> power less than 400W, only 100W for the VK3 who used to work me.
>>
>> :-)
>
>
> ------------ REPLY FOLLOWS ------------
>
> Spot on, as they say.
>
> SteppIR is becoming a religion, much like Collins.
>
> Bill W6WRT
>
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