Lacking the details and time to completely analyze Ed's modifications to
the TH3 antenna, I am assuming:
1. That the original 3 elements were placed on a 19 foot boom. (TH5)
2. That a Beta-match was no longer required for low VSWR because the
parasitics were not coupled as tightly to the DE, and the impedance rose
from 25-35 ohms up to 40-50 ohms.
3. The bandwidth increase is due mostly to the reduced coupling of the
parasitics to the DE rather than the off-center feed.
4. F/B and gain was reduced because of this reduction in coupling. F/B
may be more affected than gain because of the longer boom.
5. Some skewing of the radiation pattern likely occured from the offset
feed, however this may not be noticeable on a beam with such a wide
beamwidth. (60-70 degrees @ -3dB)
Conclusion: Yes, this may have more bandwidth at the expense of other
parameters, such as F/B and pattern symmetry.
The Beta-match (also called hairpin and stub match) works on the principle
of adding a shunt inductive reactance across the feedpoint. The amount of
reactance depends upon the frequency and the stub characteristic impedance.
The antenna is designed to have approx. 15-40 ohms real and 10-20 ohms
capacitive reactance (~ 25-j15) before matching. This real part of the
impedance also happens to coincide with that of a well-designed Yagi (good
gain and F/B). The imaginary part can be adjusted by the resonant length
of the DE. In a multiband trapped Yagi, the coupling is "usually" tighter
on 20m and less on 15 and 10m, so the Beta-match works well on all 3 bands.
73 Roger WB0DGF (Hy-Gain)
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com
Sponsored by: Akorn Access, Inc. & N4VJ / K4AAA