On Apr 8, 4:40 pm, Wimpie wrote:
On 9 abr, 01:26, K7ITM wrote:



Hi Wim (and lurkers),

On Apr 8, 1:56 pm, Wimpie wrote:

On 8 abr, 18:52, K7ITM wrote:

...

I just ran EZNEC on a frequency-scaled version of the 14MHz 5 element
Yagi included in the sample files, with the D.E. slightly shortened to
allow a decent hairpin match to 200 ohms at the design center
frequency.

Did you also scale the thickness of the elements?

Yes, EZNEC lets you scale everything in the same proportion in one
quick operation.

I did a frequency sweep, 49 to 51 MHz, in 0.25MHz steps.

...

I don't know whether this will give sufficient BW improvement for the
Yagi as the Q is also determined by the reflector en directors. I am
looking forward to your simulation results.

Yes, of course. If the antenna itself is not wide-band, there is
rather little you can do about it. It may be possible with a more-or-
less complicated network to make the SWR bandwidth somewhat greater
(assuming still low loss--if you have a lossy network, you can make
the SWR bandwidth very large -- ;-) But making the SWR bandwidth
large does not make the antenna's gain bandwidth large. That is, for
a given set of directors and reflectors, the pattern will change with
frequency without much regard for what you do to get power into the
antenna.

Thanks for the encouragement about running some simulations. I'll
report results as I have a chance, probably this evening (US Pacific
coast time).

Cheers,
Tom

Hello Tom,

I do not have experience with EZNEC, so I didn't know that it also
scales element diameter.

Fully agree with regards to VSWR BW, gain BW and wide band dummy
loads....

Here it is UTC+2, so it is time for me te visit my bed for some hours.
Tommorow VAT administration is waiting.

Best regards,

Wim

Hi Wim and lurkers,

Yes, EZNEC has some nice features. I exercised a couple of them
tonight, doing frequency sweeps with inductive and capacitive
matching. I did cheat: since I do not know the dimensions of the
hairpin match, I elected to just use a pure lumped inductance. I
suppose the error compared with a transmission line stub (hairpin)
won't be great. I also used for the first time ASCII file import for
the wire description, making use of Excel to generate the wires. (Why
did it take me so long?? ;-)

Somewhat to my surprise, I found that the SWR bandwidth of the
shortened D.E. (driven element) loaded with a shunt inductance and
that of the lengthened D.E. loaded with a shunt capacitance was not so
different.

The antenna I picked to model was one in the Orr and Cowan book, "The
Beam Antenna Book." It's a 6-element 1.20 wave long design, with 0.2
waves D.E. to reflector, and 0.25 waves D.E. to first director and
also between each adjacent pair of directors. For 6 meters it is a
little shorter than Tony's antenna, but not very much shorter. But
the elements (per Orr and Cowan's numbers) are 2 inches (about 50mm)
diameter. The large diameter almost certainly contributes to the
fairly broad bandwidth. All models below are done in freespace.

For the shortened D.E.: D.E. length = 95.4 inches. Feedpoint
impedance at 50.1MHz is 16.32-j54.63 ohms. Gain varies from 11.78dBi
at 49.5MHz to 11.98dBi at 50.1 to 11.68 at 50.5. f/b varies uniformly
from 19.43dB at 49.5 to 8.21dB at 50.5. Matched at 50.1MHz to 200
ohms with 189.4nH shunt at the feedpoint yields a 1.4:1 SWR bandwidth
of about 49.89MHz to 50.28MHz.

For the lengthened D.E.: length = 115.8 inches; impedance at 50.1 =
29.28+j70.7 ohms. Gain: , 12.05max, . f/b shows
same uniform degradation with increased freq; to
. 38.35pF shunt across feedpoint yields 1.4:1 SWR BW of
about 49.89 to 50.34MHz, just slightly greater than the shortened one.

(Neither of these includes frequency effects of the 4:1 balun to get
back to 50 ohms...)

I also tried a much-lengthened D.E., to get it up to 50+j204.1 ohms.
Then a series 15.57pF gives a 50 ohm match. Gain is similar to above,
11.83dBi -- 12.11dBi -- 11.93dBi, and f/b is also similar, 22.82 --
8.83. I notice both the gain and the f/b improve slightly as the D.E.
is lengthened, though it's small enough that you'd never notice it--it
would be swamped out by other effects. The 1.4:1 SWR BW for this
configuration is about 49.85MHz to 50.32MHz; this of course against a
50 ohm reference which is different from the first two examples above
which are against a 200 ohm reference.

OK--I think I got all the critical stuff in there...

Cheers,
Tom