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Author Topic: K40 CB Antenna to 10mtrs  (Read 16292 times)
KC3EF
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« on: February 14, 2010, 05:39:01 PM »

Has anyone converted a K40 to 10mts.  I got a free on this week and would like to use it on 10mtrs.
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K0BG
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« Reply #1 on: February 15, 2010, 06:23:36 AM »

You just have to shorten the whip a little. You won't get the SWR much below 1.5:1 due to the matching network in the base, but that's of little worry.
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KW6LA
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« Reply #2 on: February 18, 2010, 01:58:29 AM »

They work very well on CB and 10 mtrs. As Alan said, dont expect the VSWR to be much lower the 1.5:1 and band width of 600 Khz. A tuned 1/4 wave whip will beat it hands down, mounted the right way ~!

Enjoy

KW6LA
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AE7QT
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« Reply #3 on: December 04, 2010, 11:04:13 AM »

I agree with the 1/4 whip. Its hard to shake a stick at the 102" whip. But hey if it was free, just cut it down a little. Try to hard mount it up high, try to avoid mag mount unless you have too. Antennas are always a work in progress, so if the first thing doesn't work, mess with it, try several things, you will learn by trial and error and eventually get it right. have fun most of all and hope you get it figured out.
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KI4SDY
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« Reply #4 on: December 12, 2010, 10:13:31 AM »

You may not have to cut anything, so don't start out by doing that. You may ruin a good antenna. If you just want to tune it to 10 meters, push the radiator all the way in and tighten the set screw. Check the SWR. It may be 1.2 or less at that setting from my actual experience. If you want to use the antenna for 10, 11 and 12 meters, tune it to the middle of 11 meters and you will probably have less than a 1.5 reading on half of both the 10 and 12 meter bands. A regular full sized CB whip would be more broadbanded and give better performance. For 10 meters, just use it without the spring. For 10,11 and 12 meters install it with the spring.  Grin

Tune and check before you cut!  Wink   
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N1DVJ
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« Reply #5 on: December 15, 2010, 06:22:39 AM »

The BIG K40 antenna is an excellent unit to convert to 10M.  In a lot of cases, just shortening the whip SLIGHTLY is all that's required.  Check first, as has been pointed out, you may not have to trim it at all!  And if and when you do trim, trim by SMALL amounts, like 1/8 inch.

Don't try to cut the K40.  As the one I saw was 17-7 stainless, and will ruin almost anything you try to cut it with.  It's hard, stiff, and brittle.  So just take a file and notch it on both sides.  Then use a vice or good pliers to hold the nub, and force a bend where you notched it.  It should snap cleanly, with only a small amount of cleanup with a file needed.  A Dremel with a coated fibre cut-off disk makes it easy too.

You should get at LEAST 500khz usable bandwidth out of the thing.  And it should work fairly well.  Yeah, a full quarter wave would be better, but a 9 foot whip on the roof of your car?  Of course, we've all seen people that do that!  But realistically, who are we to poke fun when we have 50" whips?  It's just a matter of degree.

Another antenna that makes an EXCELLENT conversion to 10M is the old Radio Shack Deluxe Mag Mount.  This is NOT the same as their generic mag mount, and has been long discontinued.  It  used 12ga copper wire in the coil, and had a LONG whip on it.  Using it on 10M was usually just a minor tune and didn't take any cutting.

You can convert almost ANY of the base loaded CB antennas to 10M just by trimming.  Please note I said 'almost all', and their are no guarantees.  But the shorter (physically) the antenna is, the narrower the bandwidth.  Still, a bad antenna is better than no antenna.  To help with the conversion, just take the whip out of the base coil and put in a brass brazing rod.  Or even a straightened out wire coat hanger.  (Although wire coathangers are not nears as durable today as they were 20 years ago)  Make sure any paint or enamel is scraped away where the wire goes into the coil.  Set it as 'deep' into the adjustment as you can.  Check the tune.  Use simple wire cutters to trim off the antenna in 1/4" chunks until you get the tune you want or go past it.  You can tune it to length then.  Take it out of the base, and use it as a guide for how long to cut the stainless steel whip, but always cut about 1/4" long, just to be sure.  If you had to retune the coathanger by adjusting the length in the base, add the length you moved it there too to the length before you cut the stainless steel whip. 

If, by mistake, you go short, there's an easy (although dorky and geeky) solution.  A neon bulb.  Cut one lead off the bulb, then wrap the other lead around the tip of your antenna to hold the bulb up there.  Slip on a piece of clear heat shrink and shrink it down.  That will usually act to do the same as a capacitance hat and effectively 'add' length to your antenna.  The dorky geeky part comes in because the bulb will flicker on your transmissions.... 

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K6RMR
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« Reply #6 on: December 17, 2010, 06:41:33 PM »

I was given a K40 in the Late 1980's. I took off about 4 to 5 inches and I got a Good match
from 28.0 to 29Mhz. I bought the Mag Mount for it and Mounted it on a Steel Airconditioning
Box outside my Apartment Window in 1988 and with my Kenwood 430 I work the world.
The Steel Box made a Great Counter poise. I was also on the second Floor.
           
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K0BG
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« Reply #7 on: December 18, 2010, 03:57:56 AM »

Only problem is Stan, it isn't a counterpoise. Similar perhaps, but the wrong terminology. It is acting as a ground plane, albeit a bit inadequate.
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W5DXP
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« Reply #8 on: December 18, 2010, 08:55:20 AM »

Without changing the length of a 11m CB whip, a series capacitor at the feedpoint will resonate and improve the SWR on 10m. If I remember correctly, it took about 0.1 uF to neutralize the inductance and bring the feedpoint impedance closer to 50 ohms on 10m.
--
73, Cecil, w5dxp.com
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W8AAZ
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« Reply #9 on: December 30, 2010, 06:35:15 PM »

I used to modify cheapo base loaded mag mounts(not the ultra short types) for 10 meters and 2 meters.  Either removed a couple or three turns from the coil for 10 or if sealed, just cut the whip down.  Those work well enough when the band is hopping.  Ran one once with a FT-101 in the front seat.  DX but the band was hot.  THen I also had been known to redo the coils to make them into 5/8 2 meters antennas, a rather drastic coil change.  I am sure you could shorten it to resonate on 10 and some antenna makers for CB even give 10 meter adjustment data either to get ham sales or the bootlegger sales. 
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N1DVJ
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Posts: 530




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« Reply #10 on: December 31, 2010, 05:09:32 AM »

Some CB antennas were nothing but 10m antennas lengthened to make them work on CB.

When RS closed out the 3-element CB beam, I picked up 3 of them at local stores for less than $5 each.  I intended to just salvage the aluminum.  Then I sat down to design a 10m beam based on an antenna book I had.  When I checked, I discovered that the 'prime' example in the book was almost an exact match to the RS CB beam for boom length and spacing for use right in the middle of 10M CW.  Only the elements were lengthened to make it work on CB.

Since the elements were 2-piece, I just slotted the ends of the 1st piece and slid the second piece in and put a small hose clamp around it.  Set it up for 10M (28.400 at the time) aimed it skyward on the ground and fine tuned it.  Worked out great!
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KI4BDS
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« Reply #11 on: January 15, 2011, 01:52:12 PM »

According to the law you are not allowed to be over 13ft 6in tall. I heard they will give truckdrivers tickets for antenna over that so as many DOT cops as there are i'd be careful.
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WB6BYU
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« Reply #12 on: January 16, 2011, 06:01:48 AM »

Quote from: KI4BDS
According to the law you are not allowed to be over 13ft 6in tall...

Most of us are shorter than that.

Even with a K40 on our head.
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K0BG
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« Reply #13 on: January 16, 2011, 08:08:12 AM »

Karl, you need to add the phrase: in some states...

Interstate trucks operate under US DOT regulations, and the current DOT rules state 13' 6", except where regulated otherwise. For example, most western states set the maximum at 16 feet in interstate highways. In other words, you can't make a pat statement about heights over 13' 6"!
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W5DXP
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« Reply #14 on: January 16, 2011, 08:38:15 AM »

According to the law you are not allowed to be over 13ft 6in tall.

An AZ deputy sheriff actually tried to give me a ticket for having a top hat at 13.25 feet on my 75m shootout antenna. I asked him if he knew the exact legal limit and he said, "No, but I know your antenna is too tall." However, I got away with a 12.9' whip for 17m mounted to my trailer hitch which was already 2 feet above ground.
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