On Mar 17, 1:22 am, Ian White GM3SEK wrote:
Owen Duffy wrote:
"ve2pid" wrote in news:1174098010.745176.35010
:
Hi to all,
I read somewhere that, in the case of Yagis, ''in the range of 25-35
Ohm you get the
best balance between gain, pattern, bandwidth and element currents.'''
Is that true? And if so, I would like to have the theoretical
explanation behind this.
I don't understand why there is such a relationship, if there is.
I don't think there is such a relationship either. The feedpoint
impedance is part of the results from the design process, along with the
gain, pattern and beamwidth; but it does not determine any of those
other properties.
The so-called "best balance" between all these properties will depend
entirely on the personal priorities of the designer or the user.
(Antenna optimization software demands very clear instructions about
this, and forces you to think very hard about what you really mean by
"best".)
Hmmm. Interesting comments considering below.
Many good designs do have a feedpoint impedance in that 25-35 ohms
region; but so do some real lemons, too, so it isn't a reliable
indicator.
As Roy says, yagis with much lower feedpoint impedances tend to have
high internal (I^2*R) losses, which increase rapidly as the impedance
falls and element currents rise. Those can be classified as poor
designs, simply because there are plenty of better alternatives.
Also, it is possible with many designs to increase the feedpoint
impedance towards 50 ohms by adding a director at very close spacing
(about 0.05 wl). That director has relatively little effect on other
performance parameters, so it can be added fairly late in the design
process as a means of matching. (After construction, that close-spaced
director also allows final adjustment of the matching, by bending the
ends towards or away from the driven element.)
Yagis are often designed for a feedpoint impedance that is relatively
easily transformed to 50 ohms for the main transmission line.
A quarter wave transformer from 28 ohms to 50 ohms is relatively easily
made from two parallel 75 ohm lines. Perhaps that is the attraction to
the impedance range you mention.
That is a very reasonable strategy: if the impedance comes somewhere
close to a convenient value for matching, then optimize it to exactly
that value.
For example, DK7ZB has developed a range of yagi designs optimized for
28 ohms.
And there he says: "For the VHF-Bands (50-50.5MHz, 144-146MHz,
430-440MHz) a radiation resistance of 25-30Ohm has the best balance
for gain, back- and sidelobes, bandwidth and SWR at tenable losses."
We have come full-circle. [g]
Also, DK7ZB in describing what is clearly an unbalanced connection of
parallel lengths of coax ("Classic" match) says, "2. This line is a
simplified coaxial sleeve balun to avoid sleeve-waves on the braid of
the cable running to the station.", which it is clearly not.
There are links to these and several other designs from:http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)http://www.ifwtech.co.uk/g3sek
N7WS