On Fri, 12 Oct 2001 firstname.lastname@example.org writes:
> Why do certain yagis used an insulated dipole (TA33) as the driven
> element, while others feed through some sort of matching
> (gamma, hairpin, etc)?
> What are the different mechanical issues that force the use of each?
GOOD questions John. (Please include your CALL in your signature)
I suspect the driving force behind NOT using split dipole driven
elements is that 3L Yagi's designed for higher gain tend to have
somewhat low feedpoint impedances, in the 10 to 30 ohm range,
and therefore require a matching system. It is possible to increase
the feedpoint impedance by moving the DE forward (towards the
director) but, you guessed it, as the feedpoint impedance increases,
gain decreases. Other methods involve playing with the parasitic
element tuning. Again, as the feedpoint impedance rises, gain
is reduced. The Split Dipole fed TA33 has a reputation for LOW
gain compared with other tribanders. The CC A3 seems to
have a better reputation as well as the HyGain TH3. The HyGain
EXPLORER is an even better design using open sleeves on 10M
with a pair of 15 meter traps in the 15/20M driven element for
improved bandwidth on all 3 bands. Too bad they didn't give
this antenna a TH number. I suspect it was always considered
a 'novice' antenna, but the design represents a true improvement
in the small tribander category.
TWO element Yagi's can obtain reasonable gain with a 30 ohm
feed impedance which is often just fed directly. A SIMPLE
match single band 2L Yagi's can be obtained by using a 1/4 WL
50 ohm line to step up to around 80 ohms and then simply use 75 Ohm
CATV hardline to come to the transmitter. This impedance (75 to 80 ohms)
is well within the tuning range of most pi-networks and only represents
a 1.5 to 1 SWR. Bottom Line: a split dipole feed for 2L Yagi's is
a good, simple, low cost solution.
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