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Author Topic: Demensions for 2 meter mobile whip  (Read 1475 times)
KE4BIW
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Posts: 41




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« on: May 13, 2009, 07:24:21 PM »

Just bought a pickup that has an almost new CB mount and whip on the tool box. The coax is very nicely routed into the cab so I'd like to cut down the 102" whip to an appropriate length for 2 meter operation. Can someone give me a good starting point for the length? I've got an MFJ antenna analyzer so I'll probably cut it just a shade too long and then check the SWR before making any fine tuning adjustments.

Thank you in advance for any guidance on this topic!!

Wade
K4WMN
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KB1LKR
Member

Posts: 1898




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« Reply #1 on: May 13, 2009, 08:33:17 PM »

Start about 20.5"? That might be 1/2" - 1 1/2" too long.

Larson's cut charts say 20" at 140 MHz 19 3/4" at 145 and 19 1/4" at 150 for their NMOQSPEC 1/4 wave commercial whips.
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K5LXP
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« Reply #2 on: May 13, 2009, 08:39:55 PM »

Rather than cut a 9' whip down to a foot and a half, if the mount is a standard 3/8 or other common mount, see about just getting a 2M antenna for it.

Mark K5LXP
Albuquerque, NM
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KI4SDY
Member

Posts: 1452




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« Reply #3 on: May 17, 2009, 08:51:01 AM »

I would pick up one of the inexpensive dual band 2 meter/70 cm whips that Val Pro makes. It has the correct thread and allows for the additional band later. I picked one up with a magnetic mount for $12.50 at a ham fest a few years ago and have it mounted on the tool box just like you are proposing. It works great, is very rugged and easy to garage.

Take the spring off the 9 foot CB whip and save it for 10 meters or several other HF bands with a tuner. In addition, you can still use it for CB with the spring removed. With what you have, you are on your way to multiband mobile enjoyment. I just talked to Oklahoma yesterday with a 25 watt Radio Shack transceiver and a 9 foot whip mounted on my truck bumper.  
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WB6BYU
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Posts: 13250




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« Reply #4 on: May 17, 2009, 09:15:47 AM »

Is your antenna bumper mounted?  The 2m antenna will do
much better when it is mounted on the roof of the vehicle.
While using the existing mount may seem convenient, it
isn't likely to give optimum performance at bumper level.
(I'm guessing it isn't a roof mount if it is being used
with a 9' whip.)  A fender or trunk mount will work
better, even if it isn't as good as the roof.

I think Hustler makes a tall 2m mobile antenna that
will screw into a bumper mount, but, while it may look
like it from the outside, internally it isn't just a
straight whip.

While it is possible to get a low SWR with a longer whip
(I've seen them cut to 5/4 wavelength, for example) the
radiation pattern isn't as efficient, with much of the
power radiated up into space.

But even a 19" whip on a bumper mount should allow you
to hit the local repeaters, even if it isn't optimum.
For a while I used a piece of brazing rod soldered into
the end of a 3/8-24 set screw for my 2m mobile antenna
in conjunction with a standard mobile mount.

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KI4SDY
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Posts: 1452




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« Reply #5 on: May 17, 2009, 10:15:35 AM »

You need to read KE4BIW's original post an re read mine. We both have CB screw post mounts on our tool boxes. I have installed a 2 meter antenna on mine and also have a bumper mounted 9 foot whip for HF. My 2 meter antenna is not mounted on the bumper, it is mounted on the tool box!

He and I both know a 2 meter antenna will perform better roof mounted, but are bowing to convenience. I have no regrets. Mine performs very well, as does the bumper mounted 9 foot whip. By the way, the 25 watt 10 meter mobile contact was from Florida to Oklahoma.
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KI4SDY
Member

Posts: 1452




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« Reply #6 on: May 19, 2009, 07:12:53 AM »

You will also find that open coil dual band stainless steel antenna as a "ProAM" brand. Originally, I used the magnetic base and then switched over to the threaded permanent tool box mount. It is very small, rugged and compact. Came to about $15.00 total, with tax. For the money, you cannot beat it!

Read my performance review on eHam about it, when I was using the magnetic mount. It performs as well, if not better, permanently mounted. I did add a stainless steel washer as a spacer at the base, because the permanent mount was a little shorter in height. That dropped the SWR down to 1.2 from 1.5. Small changes like that make a difference at these higher frequencies.    
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