Roy Lewallen wrote in
:
wrote:
Wrong, it just means you didn't read the whole posting. The issue
with a doublet of two full half waves is feed impedence is very high
and a very poor match for coax. Feed it properly (voltage feed) and
it will perform better than a 40m dipole. . .
A full wave dipole has less than 2 dB more gain than a half wave dipole
at its best angle. It has gain equal to or more than that of a half
wave
dipole over only 88 of the 360 degrees of coverage -- at all other
angles the full wave dipole has less gain. In fact, over 140 of the
total 360 degrees, the gain of a full wave dipole is more than 10 dB
less than that of a half wave dipole.
So in what way does the full wave dipole "perform better" than a half
wave dipole?
Roy,
We have had discussions about characterising baluns, and I understand the
complexity... however, let me try a rough analysis.
Following the discussion with one of the chaps I mentioned in the earlier
posting, he was using a full wave dipole, a RAK BL-50A balun and 25m of
foam RG58 (cellular) feedline, I have had a rough attempt at
characterising the balun (I have one) at 7MHz. It is a 1:1 voltage balun,
and o/c it appears to look like 3000 ohms resistance in shunt with 10uH
of inductance with very low applied voltage (at 7MHz).
This balun is rated at 1kW CW in 50 ohms, so the rated primary voltage
would be 223V RMS. If the apparent shunt resistance remained constant
(and it doesn't if flux is high... but flux should be kept low to avoid
excessive core loss / temperature), the core loss would be 16W. The
package could probably dissipate 40W of core and copper loss, so 16W+ for
core loss seems consistent with package size, so maybe 3000 ohms is about
right, possibly as low as half of that at full operating flux.
So, in the case of a full wave dipole low over ground, the modelled
feedpoint impedance is around 4200+j0 ohms. If this is shunted by
something in the range 1500 to 3000 ohms of core loss resistance,
efficiency ranges from 35% to 58%, and the antenna can absorb a max of
223^2/4200 or 12W before overheating the balun (perhaps a little more
since there is almost zero copper loss).
Then the feedline will have an efficiency from 20% to 10% due to the high
VSWR.
So this "more metal" antenna system, looks like it has some small
increase in directivity (1.6 from your posting), an efficiency of 7% to
6%, quite low gain (directivity * efficiency = -10dB compared to a
lossless half wave dipole+feed), a maximum input RF power rating of 50W
to 100W for a radiated power of around 12W. The EIRP is much lower
(almost 10dB lower) than a 100W transmitter feeding the same antenna of
half the length.
Gee, that all looks pretty complicated, no wonder people find "rules"
like "bigger is better" so appealing, it saves all that thinking and
understanding... but wait a minute, isn't that was ham radio was about?
73
Owen
PS: I am thinking about trying to characterise the balun at 223V applied,
I will see how time goes today.