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Author Topic: 2m whip question?  (Read 2647 times)

Posts: 2

« on: January 11, 2003, 07:50:58 PM »

I have a 102 inch whip from the good ole cb bands. I was thinking about taking it and cutting it down to a full wave length for 2 meter. I used an antenna cal, and it said 80.44 inches at 146.790. Will this work at full wavelength? I dont mind the large antenna i want the best performance i can get. I know ill give or take alittle trimming for swr but will this work??? Thanks, Jake "soon to be tech, next week"

Posts: 217

« Reply #1 on: January 12, 2003, 11:09:11 AM »

Jake, a full wave length whip will present a very high feed point impedance at the base.(possibly several thousand ohms.) You could cut it down to 3/4 wave (about 56 inches or so) and this should provide a usable match to 50 ohm coax. The radiation angle would be a little high, but should work OK. For mobile operation, sometimes a higher angle can be to an advantage, depending on surrounding terrain.
            John  K5CEY

Posts: 2

« Reply #2 on: January 12, 2003, 01:25:43 PM »

Thanks John, Im new to this 2 meter stuff and im learning every day. Three quater wave sounds good. but would i be better going half wave? I live in south louisiana and it is pretty flat around here. I just want to have the best performance i can get with this moble rig. Im gonna be on the outter fringes of our local repeters here in Baton rouge and i need all the help i can get. Im going to a hamfest to test this weekend and i was trying to get my radio set up so i can talk rite away! I was thinking of just buying a good mag mount and being done with it but i like to mess with this antenna stuff and it would be a great learning experince to get this ole whip to work!!     " and save me some bucks"  It sure is nice to have some good radios to talk on that are not 30 years behind in tech.   Thanks for anymore help. Jake

Posts: 217

« Reply #3 on: January 12, 2003, 03:13:45 PM »

Jake, a 1/2 wave would give you a lower radiation angle,best for flat terrain, but it would also be high impedance at the base and would require a tuning network to match it to your coax. Of course, you can do that, but then you may as well go to a 5/8 wave whip, which also requires a tuning network. The 5/8 is commonly made from base loaded CB antennas by modifying the loading coil arrangement. Since you have a full length 102 inch whip, I figured it was mounted with a standard heavy spring and coil mount. That's why I suggested a 3/4 wave. There's hundreds of two meter guys out there. You're bound to get more suggestions.
              John  K5CEY

Posts: 13019

« Reply #4 on: January 13, 2003, 03:58:12 PM »

Cutting the CB whip to 3/4 wave (about 60", start a bit
long for tuning, or course) should do.  If the bottom
1/4 of the whip is shielded by the car body in some
directions, it will probably work better!

Another approach is to turn the CB antenna into a
J-pole:  this takes a bit more mechanical work but
probably gives a more reliable radiator (unless the
whip bends significantly while driving.)  To do this
I would take a piece of rod or tubing (perhaps cut from
another whip?) and mount it in the middle of the
existing whip and parallel to it - perhaps spaced one
to 2 inches (2-5cm).  The top of the rod should be
38" (97cm) down from the top of the whip.  The main
mounting of the rod should be at the bottom, but an
insulating spacer about 2/3 of the way up the rod will
help.  The rod needs to be over 1/4 wave long, probably
2' (60cm) or more allows plenty of room for mounting.
I secure the rod to the whip at the bottom of the rod,
then make an adjustable (sliding) clamp to make good
electrical connection between the two (the "shorting
bar".)  Connect the shorting bar about 1/4 below the
top of the rod, and the coax about 1" above that (with
the center conductor to either the rod or the whip and
the braid to the other - it doesn't matter which.)
Then adjust the positions of the coax connection and
the shorting bar up and down the rod/whip combination
until you get the best match.

I've also converted a number of base-loaded CB mobile
antennas to 2m.  A 36 - 38" whip works as a half-wave,
and a 48" whip as a 5/8 wave.  (The distinction is
important, since they will require different matching
methods.)  Unwind the base loading coil and rewind to
suit - this may take some experimentation, since the
CB antennas vary in coil diameter, wire size, and other
characteristics that will affect the coil.  Generally
you will use one or two turns from the feedpoint to
ground, and another 3 or more from there to the antenna
whip.  With a half-wave whip, you will probably need
to connect a small variable capacitor across the coil
and tune it for resonance.


Posts: 528

« Reply #5 on: January 14, 2003, 05:52:36 AM »

Well, Jake , the first thing you need to do is learn
more about antennas. A fullwave vertical whip is not
usable due to the impedance difference. A quarter
wave is what you want but it would be absurd to cut
that 102 inch whip down to 18 inches. Put the 102
inch whip away for another project in the HF bands.
Do some more research and learning.
73 - Tim
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