On Sat, 23 Feb 2013 17:07:31 -0800, "Sal" salmonella@food
poisoning.org wrote:
"Jeff Liebermann" wrote in message
.. .
On Fri, 15 Feb 2013 23:31:52 -0800, "Sal" salmonella@food
poisoning.org wrote:
I'm building a yagi from the measurement tables the ARRL Antenna Book.
Which Antenna Book and which yagi?
snip
Jeff, I have the 18th Edition of the ARRL Antenna Book, copyright 1997.
I'm in Chapter 18, titled "VHF and UHF Antenna Systems." I'm using Table
16, titled "Dimensions for 16-element 3.9-wavelength 222-MHz Yagi."
I have the 19th edition. Same table on Pg 18-31.
I'm bugged by something else I'm seeing. Table 16 specifies a 664 mm driven
element (DE) It's longer than the DE of other construction articles. Most
articles call for something between 645 and 648 mm. (Example: On the same
page as Table 16, I see also Table 14, "Free-Space Dimensions for the
222-MHz Yagi Family." It gives a DE of 647 mm.)
That's easy. Both antennas have elements connected to the aluminum
boom (except for the driven element). Looks like the boom diameters
are different. When the elements are connected to the boom in this
manner, the boom becomes part of the element in that the conduction
path now goes around the boom. This tends to shorten the element
lengths. All the various yagi designer programs can include the boom
diameter in their calculations. I strongly suggest that unless you're
building EXACTLY what's in the article, that you use one of the
programs previous mentioned to grind the numbers correctly.
In addition, I have a suggestion. If you're using a solid rod for the
elements, round off the ends to a hemispherical radius. Use 1/2 the
distance between the base of the radius and the peak for the element
end point. If you're using hollow tubing, you can also cram a big
rivet or screw into the end to get the same rounded effect. This will
increase your usable bandwidth.
Something is indisputably okay. My prototype DE (664 mm) is hanging by
strings above my bench and I'm getting a 1.2:1 - 1.3:1 with a short length
of cable. I can hit a repeater 15 miles away with one watt without even
turning my DE vertical. Too bad neither of my analyzers works at 222.
Please note that you also get 1:1 with a dummy load. You can also
match just about anything with your gamma match. Tune the antenna for
gain, bandwidth, and/or F/B ratio. VSWR is easy to fix.
Also VSWR is not as important as pattern and gain. You can get
perfect VSWR, and have the signal sprayed all over the place. If you
look at the antenna patterns on the same pages as the tables, you'll
see that even with a properly designed antenna, you have a rather
strange and skewed pattern. Incidentally, T-match and balun should
fix the funny pattern.
Yes, I fully expect the match to need adjusting after I add parasitic
elements. The road from here-to-there has so many branches!
Not really. The first 4 or 5 elements are what provides the VSWR. Try
waving additional elements around the front of the antenna notice how
little the VSWR changes. However, the gain, directivity, and F/B
ratio certainly will be affected.
As for your lack of an antenna analyzer, this is actually a good
thing. What you really want to do is see what the antenna is doing
over a wide range of frequencies. For that, I suggest a return loss
bridge. I use a Telonic Rho-tector Model TRB-50 and a Texscan RCB-3.
Here's what's inside:
http://www.qsl.net/n9zia/rlb/texscan.png
Both will go from 0.5MHz to 1GHz. All you need is a decent RF sweep
generator, oscilloscope, terminators, and cables. You can see the
VSWR curve over the frequency range while you're tuning. You can also
do the same thing with a directional coupler and a diode detector, but
I think the return loss bridge is easier. I've also built a few for
microwave frequencies (2.4 and 5.7GHz).
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558