On Mon, 08 Dec 2008 00:23:16 -0800, John Smith
wrote:
One of my next projects is a GOOD VSWR meter for ghz ... unfortuantly,
on the list of priorities here, it is low on the list ... :-(
http://pe2er.nl/wifiswr/
I've built several. They work nicely and are a very important tool.
My main inspiration was blowing up my Wiltron Model something SWR
bridge and discovering the exhorbitant cost of a repair or
replacement. The designs are sufficiently similar that I can
substitute. Of course, it's not flat over many decades of bandwidth,
but for fairly narrow bands, it's quite adequate.
I have just tended to take the % signal readings on the chans as good
enough, and trimmed my antennas accordingly. I know, this does NOT take
into account the functioning/power-fluctuations/antenna(s)/etc. of the
AP over these ranges ...
I have a handy distant reference signal for 2.4GHz. It's line of
sight and creates no Fresnel Zone issues, so it's useful for antenna
testing. I start with my "standard" gain panel antenna and take a
measurement. I then take the antenna under test, and compare. I
rarely make an absolute level measurement. It's always relative to
some other antenna of known characteristics.
The only problem is that I have to climb onto my roof to do the
measuring. I'm currently at the tail end of radiation oncology (no
big deal), which makes me kinda tired. I find myself somewhat tipsy
on the roof, so no antenna measurements for a few more weeks. One
might also suggest that it affects my math and memory, but they're
currently just as bad as always.
But, excellent point, glad you mentioned it ... but you are right, my
elements end up being filed down a bit from the figures the calculator
spits out ...
Suggestion: I always build a 3 element yagi first. Just the
director, driven element, and reflector. With the antenna on the
bench (after clearing off the chain saw and bicycle repair debris), I
run a fairly wide sweep using a reflection coefficient bridge or VSWR
bridge to an oscilloscope. I then trim the various elements for
resonance. My sweeper has a marker generator, but an external
generator (or wi-fi trash generator) can also be used. Adding
additional elements does change the tuning a bit, but the really big
frequency determining elements are the first 3 elements.
http://802.11junk.com/jeffl/pics/home/slides/lab.html
Incidentally, don't forget the mounting bracket. I just hate it when
I prototype an antenna, and then find that the bracket, radome,
potting foam, serial number sticker, or other irrelevent hardware
detune the antenna.
Another trick that I use with wire rod type of antennas is to thread
the end of the rods. Attached to these are threaded standoffs. These
can be rotated to adjust the length of the elements. Also, don't
forget to round off or chamfer the ends of each element to improve the
bandwidth.
1AM. Time to give up before I start babbling nonsense.
Warm regards,
JS
Bah-Humbug (T'is the season).
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558