First homebrew antenna and a question
Owen Duffy wrote:
On Wed, 27 Sep 2006 23:36:33 -0400, jawod wrote:
My initial question related to a dipole 3 feet too long, no change in
feedline was/is contemplated...consider that a constant.
The length of feedline(s) may be constant, but the losses in the
feedline(s) depend on VSWR, and nothing you can do with an ATU changes
the loss in feedline(s) beyond the ATU.
I note that you have figured that the dipole is 3' too long, that is
about 3% too long, but the antenna resonance appears about 6% low at
14MHz. Are those two pieces of information consistent?
Should they be consistent?
Yes, the length of open wire section of the feedline influences the
frequencies at which the RG8 VSWR is low, near as much as the dipole
itself.
The band where the "tuning" of the radiator and openwire line section
(together) is most critical is 80m, the optimal bandwidth (from a feed
loss point of view) is narrowest, and has the steepest sides.
However, the place where the VSWR looking into your RG8 is most
predictable is at 20m (typically 14.2MHz) where the common form of the
G5RV should have a three half waves resonant dipole and half wave
electrical open wire section. The Z at the dipole centre will be
around 90+j0, and if the loss on the open wire line is low, the Z into
it will be 90+j0, for a VSWR on the RG8 of just under 2, a little
lower at the tx end.
If you were to find that the VSWR minimises higher or lower than 14.2,
it is a sign that the combination of the dipole length and open wire
section are too long or too short.
If you objective was resonance of the dipole + open wire section at
14.2 (and I now understand that is not your objective), you would be
6% low with your stated observations.
If you are happy with the location of the VSWR dip on 80m, leave it
all alone because the VSWR dip results in the least losses in your
RG8, and if that is of significant length, then the additional losses
are significant. Most other bands are less sensitive than 80m, but
note that a G5RV is not efficient on "all" bands, and so should not
qualify as an all band antenna.
Owen
--
Thanks Owen, for your good advice
John
AB8O
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