On 6/13/2025 4:22 PM, Martin A. Flynn wrote:
The more I look at this, I'm leaning towards a recommendation of
scrapping the dipole and going to a 125' long wire and weatherproof /
outdoor tuner
For most configurations, it needs a counterpoise, and building could be
a good option. But antennas like this are generally poor at rejecting
noise, and most buildings are noisy.
What bands do you want to operate? Could your rig a fan dipole? Antennas
that are near resonance at frequencies of interest can be effectively
choked at the feedpoint, offering at least a bit of noise rejection.
There are some very effective symmetrically loaded dipole designs for
two bands with the second band being the second harmonic of the first,
and that dipole can be one element of the fan. I've used that to cover
160 and 80 with the loaded dipole, 40 and fifteen with the 40M dipole in
a two-element fan. For CQP (California QSO Party) county expeditions,
our club (NCCC) has built and loaned both wire and aluminum versions of
the loaded design for 20 and 40M. I did the electrical design, W6GJB did
the mechanical design and built them. Depending on mounting height and
ground conductivity, both designs provide a pretty good match to 50 or
75 ohm coax. SWR bandwidth of the un-loaded band is comparable to a half
wave dipole, while bandwidth on the loaded band is reduced by about
half. For the 40-20 combo, you want to tweak the design for the primary
operating mode.
These are NOT traps - their self resonance is well above the higher
band. Unlike trap antennas, the radiation efficiency of these loaded
antennas is comparable to an un-loaded half-wave.
Single-band elements of a fan are generally usable on the third
harmonic, but the loaded elements are not.
While there are a few multiband versions of the loaded designs
advertised in QST, they're much more complex to design and SWR bandwidth
suffers as the number of loading coils increases.
73, Jim K9YC
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