As noted below, high gain designs usually have a low feedpoint impedance.
The lower the feed impedance, the greater the current in the driven
element,
which results in increased gain. Below 10 Ohms, efficiency may become
suspect, but above 10 Ohms or so, the gain seems to be real and
realizable.
The time honored solutions to this matching problem have been gamma
or hairpin matches.
Transmission line transformers can also provide a fairly wideband
transformation. At 33 Ohms, a single quarter wavelength (WL) of 50 ohm
cable will transfrom to 75 Ohms which is perfect for using CATV hardline.
Purists can then match back to 50 Ohms at the transmitter or just tune
their
amplifiers to match 75 ohms.
Lower impedance matching sections can be made by paralleling
transmission lines, just like paralleling resistors, i.e. two 72 Ohm
cables
(1/4 WL) in parallel will act as a 36 Ohm line, matching 18 ohms to 72
ohms
or 50 ohms to 26. Two 50 ohm 1/4 WL lines in parallel will act like a 25
ohm
line, matching 12.5 ohms back to 50 ohms.
Simple feedline matching transformers allow the use of higher gain
antenna
designs which present lower feed impedances.
de Tom N4KG
On Sun, 29 Mar 1998 10:15:34 -0700 "James R. Duffey" <ji3m@maxwell.com>
writes:
>Joe Reisert, W1JR, presents several nice Yagi designs in the Winter 98
>Communications Quarterly. He also has some good comments to make on
>Yagi
>design and construction in general.
>
>The designs have 50 Ohm feedpoint so matching is trivial-just hook up
>a
>choke balun to the split driven element, adjust the driven element
>length
>for resonance and go.
>
>They have large bandwidth, about 5%, and good front to rear ratio over
>the
>rear hemisphere, >20dB. The gain is modest at 5 dB(dipole) for the 3
>element and 6 dB(dipole) for the 4 element. This is somewhat lower
>than
>with designs optimized solely for gain. The high feedpoint impedance,
>however, makes it much easier to achieve the design gain. Many of the
>designs optimized for high gain have low feedpoint impedances which
>make
>matching difficult and tends to increase resistive losses.
>
>I intend to put up a few fixed wire beams this spring and summer.
>These
>designs seem ideally suited to that application. I will keep you
>posted as
>to how it all comes out.
>
>Although the price of ComQuart is pretty steep, $9.95 per issue, this
>may
>be worth the price.
>
>If you are looking at new Yagi designs you may wish to try these.
>
>The article has much the same flavor as his old Ham Radio VHF/UHF
>columns.
>- Duffey KK6MC/5
>
>James R. Duffey <ji3m@maxwell.com> (505) 764-3143
>Maxwell Technologies Inc. http://www.maxwell.com/
>2501 Yale Blvd SE Suite 300
>Albuquerque, NM 87106-4200
>
>
>
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