On Sun, 27 Jan 2013 10:53:41 -0500, "Tom" wrote:
But I want to hear some positive ideas. How can I make this work for 2m and
70cm. I don't want a 10m omni.
You would probably do better loading a step ladder. However, if you
insist, you can load just about anything if you can find a 50 ohm tap
point with a gamma match. Attach a connector ground to the bottom of
the element with a hose clamp. Put another hose clamp further up the
antenna, which will later be moved up and down. Solder a series
trimmer capacitor at the center pin of the connector. Add a length of
bare wire between the trimmer cap and the 2nd hose clamp. Connect a
directional coupler, SWR meter, or antenna analyzer to the connector.
Adjust the length of the wire and the trimmer cap for minimum VSWR.
The trimmer is used to resonate out the inductance of the length of
wire. Anything that works at 2m, will sorta work at 440Mhz, but you
can do better with a 2nd connector and tap. A coax balun is
recommended.
If you want to get clever, and the lower end of the vertical element
is insulated from ground, you can treat the antenna as a dipole,
exactly as you would with a T-match. Same idea, except the connector
is located at the midpoint of the antenna. A coax balun is
recommended. You'll need to deploy the coax cable perpendicular to
the antenna in order to prevent the coax cable proximity from wrecking
the match.
I've actually built and modeled such abominations and worse. They all
eventually tuned and matched. I've done flag poles, automobile
trunks, step ladders, towers, barbed wire, chain link fences, and
bicycles. Most were on commercial 30-50MHz/VHF/UHF frequencies. They
all worked but only if you didn't care where the RF went. I modeled a
few which showed truly bizarre radiation patterns and usable
bandwidth. I found it handy to use a directional coupler (or
reflection coefficient bridge), sweep generator, and scope instead of
the common SWR meter so that I could see unexpected resonances and
center the bandwidth over the desired operating frequency. I must
admit that I've never tried to do a dual band random length antenna,
but with two feeds, methinks it should be possible.
Now, on to something more practical. Use the CB antenna as a support
structure for a better antenna with some gain. I call your attention
to the AMOS/Franklin antenna, commonly used on 2.4GHz and cellular
panel antennas:
http://802.11junk.com/jeffl/antennas/ (see various AMOS models)
http://www.qsl.net/yu1aw/ANT_VHF/Amos_Ant/amos_antennas.htm
These are all 2.4 antennas, but scale nicely down to other
frequencies. The required sheet metal reflector will be the existing
CB antenna driven element. How you hang the added elements will be
your problem. The 4:1 balun is required. The 4NEC2 models above are
normalized to 2.4GHz. Find the GS card in the NEC file and change the
2442MHz to 146MHz and the model should work for 2 meters.
Now, if you're lazy and just want to get something on the air, use the
CB antenna as a 102 inch "tower". Find a suitable VHF/UHF antenna and
use a hose clamp to mount this antenna to the top of the CB antenna.
You now have the benefit of altitude, which methinks is usually worth
more than gain.
Good luck.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558